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Invest Clin 61(4): 349 - 375, 2020 https://doi.org/10.22209/IC.v61n4a05
Trichomonas vaginalis: pathogenesis and its role in cervical cancer
José Núñez-Troconis
Department of Obstetrics and Gynecology, Faculty of Medicine, Universidad del Zulia, Maracaibo, Venezuela.
Key words: Trichomonas vaginalis; cancer of the cervix; inflammation mechanisms; carcinogenesis; sexual transmitted infection; Trichomonas vaginalis virus.
Abstract. The objective of this article was to review and to analyze the possible role that Trichomonas vaginalis has as a co-factor in the origin and development of cervical cancer. For that purpose, the Latin-American and inter- national bibliography was reviewed using the Pub-Med, Google Scholar, Spring- er, the Cochrane Library, Embase, Scielo, Imbiomed-L, Redalyc and Latindex web sites. The searches included the key words: Trichomonas vaginalis, epi- demiology of Trichomonas vaginalis, epidemiology of cervical cancer, inflam- mation mechanisms, Trichomonas vaginalis and inflammation mechanisms, Trichomonas viruses, carcinogenesis, cervical cancer and co-factors, sexually transmitted infections and cervical cancer, cancer and inflammation mecha- nisms, Trichomonas vaginalis and cervical cancer. Publications from 1970 to June 2020 were reviewed and analyzed. This review article analyzes the possible mechanisms that Trichomonas vaginalis could play in the carcinogenesis of the cervical cancer as a co-factor with the human papilloma virus or as an indepen- dent factor.
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Corresponding author: José Núñez-Troconis. Department of Obstetrics and Gynecology, Faculty of Medicine,Universidad del Zulia, Maracaibo, Venezuela. E-mail: jtnunezt@gmail.com
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Tricomonas vaginalis: patogénesis y su papel en el cáncer cervical.
Invest Clin 2020; 61 (4): 349-375
Resumen. El objetivo del artículo fue revisar y analizar el posible papel que el protozoario flagelado Tricomonas vaginalis tiene como co-factor en el origen y desarrollo del cáncer del cuello uterino. Para dicho propósito, se revi- só la bibliografía latino-americana e internacional en las páginas electrónicas de Pub-Med, Google Scholar, Springer, la biblioteca Cochrane, Embase, Scielo, Imbiomed-L, Redalyc and Latindex. La búsqueda incluyó las palabras claves: Tricomonas vaginalis, epidemiología de Trichomonas vaginalis, epidemiología del cáncer cervical, mecanismos de la inflamación, Trichomonas vaginalis y mecanismos de la inflamación, virus de Tricomonas vaginal, carcinogenesis, cáncer cervical y co-factores, infecciones de transmisión sexual y cáncer cervi- cal, mecanismos de la inflamación y cáncer cervical. Se revisaron y analizaron las publicaciones desde 1970 hasta junio 2020. Este artículo de revisión analizó los posibles mecanismos que la Tricomonas vaginalis pudiera jugar en la car- cinogénesis del cáncer del cuello uterino tanto como co-factor con el virus del papiloma o como un factor independiente.
Received: 04-08-2020 Accepted: 24-10-2020
Vaginal discharge is the most common cause of gynecological consultation (1). The most common vaginal discharges are bacte- rial vaginosis (BV) produced by Gardnerella vaginalis (Gv), Candida albicans (CA) and Trichomonas vaginalis (Tv) (2,3). Table I shows the most common causes of vaginal discharge (4). Tv is the most common, prev- alent, curable non-viral sexually transmitted infection (STI) worldwide (5,6).
Literature searches were performed electronically in PubMed, Medline, ISI,
Springer, Embase, Web of Knowledge, DOAJ, Google Scholar and the Cochrane Library for original articles written in the English language and in Scielo, Latindex, Imbiomed- L, Redalyc and Google Scholar for original articles written in the Spanish language. Se- lection criteria included randomized clinical trials, observational trials, open-label non- randomized trials, and case reports related to Tv and cervical cancer. The Cochrane Li- brary was searched for reviews. Publications from 1970 to June 2020 were reviewed. Three hundred and ninety references were found and 150 met the inclusion criteria.
The searches included the key words (Mesh): Trichomonas vaginalis, epidemiol- ogy of Trichomonas vaginalis, epidemiology
Non-Infective | Non-STI | STI |
Physiological | Bacterial Vaginosis | Chlamydia trachomatis |
Cervical Ectopy | Candida Albicans | Neisseria gonorrhea |
Foreign Bodies | Trichomonas vaginalis | |
Vulvar Dermatitis | ||
CAUSE OF VAGINAL DISCHARGE.
GLOBAL ESTIMATES OF NEW CASES OF CURABLE STIs IN 2016.
Sexual Transmitted Infection | No. (millions) |
Chlamydia trachomatis | 127 |
Gonorrhea | 87 |
Syphilis | 6 |
Trichomonas vaginalis | 156 |
Total | 376 |
STI: Sexual Transmitted Infection.
STIs: Sexual Transmitted Infections.
of cervical cancer, Trichomonas vaginalis and infection, inflammation mechanisms, Trichomonas vaginalis and inflammation mechanisms, Trichomonas viruses, carcino- genesis, cervical cancer and co-factors, sex- ual transmitted infections and cervical can- cer, cancer and inflammation mechanisms, Trichomonas vaginalis and cervical cancer. The electronic search and eligibility of the studies were evaluated by the author. The author reviewed, analyzed and discussed the Trichomonas and its role in cervical cancer.
According to World Health Organiza- tion (WHO) (2), there are more than one million curable STIs that occur each day. Globally, it is estimated that for 2016, there were roughly 376 million new infections of the four curable STIs: Clamydia tracomatis (Ct), gonorrhea, syphilis and trichomoniasis as seen on Table II (2,6). According to dif- ferent authors (2,6) the prevalence of Tv for 2016 was estimated in 5.3% (95% IC: 4.0–
7.2) in women and 0.6% (95% IC: 0.4–0.9) in men, with regional values ranging from
1.6 to 11.7% in women and from 0.2 to 1.3% in men. Rowley et al (6) reported a Tricho- moniasis incidence of 40 per 1000 in women (95% IC: 27–58) and 42 per 1000 men (95%
IC: 23–69). Although Tv is a worldwide par- asite, prevalence and incidence rates differ or depend on the part of the world, the cul- ture, the religion and the population studied
(5,7). Tv is considered as the second most frequent STIs and the second most common cause of lower genital tract infection world- wide (8). WHO (2) reported that the African Region had the highest incidence rates for trichomoniasis in women and men. In the America region, the Pan-American Health Organization (9) reported an incidence of
18.8 million new cases of trichomoniasis among females, and 13.6 million new cases among men and a prevalence of 18.8 million existing cases of trichomoniasis among fe- males, and 3.2 million existing cases among men in 2012. According to the Center for Disease Control and Prevention (10), the prevalence in USA of trichomoniasis is 2.1% in women between 15-49 years old and they estimate that 3.7 million people have tricho- moniasis. Studies in Latin America revealed prevalence values of 7.6% in Argentina (11), 7.8% in Chile (12) and 9.1% in Peru (13). In Brazil, prevalence ranged from 2.6% to 20% among women (14-16). In Venezuela, Núñez et al (17) reported a prevalence of 7%. The incidence of Tv infection depends on several risk factors including age, sexual activity, number of sexual partners, sexual partner promiscuity, menstrual phase, pros- titution, imprisionment, use of illicit drugs, race, diagnostic techniques, low social and economic conditions, as well as other STIs such as Herpes Virus 1 and 2(HVS), Ct, CA, Gv, Gonorrhea (G), Human Papilloma Vi- rus (HPV), Human Immunodeficiency Virus
(HIV) (10,18). The prevalence is high among low social income patients from gynecologic and STI clinics (18). Patel et al. (19) using urine samples found that Tv infection preva- lence was 0.5% and 1.8% among males and fe- males, respectively. Tv infection prevalence was 4.2% among black males, 8.9% among black females, and 0.03% and 0.8%, respec- tively, among males and females of other races/ethnicities. Tv infection prevalence was positively associated with female sex (OR: 6.1; confidence interval 95%(IC):3.3– 11.3); black race (vs other races/ethnicities OR:7.9 IC 95%:3.9–16.1); older age (vs 18– 24 years, OR: 3.0 IC 95%:1.2–7.1) for 25- to
39-year-olds and OR: 3.5, IC 95%: 1.3–9.4) for 40 to 59-year-olds); having less than a high school education (vs completing high school or more, OR: 2.0, IC 95%:1.0–4.1); being below the poverty level (vs at or above the poverty level, OR: 4.0, IC 95%:2.1–7.7); and having ≥2 sexual partners in the past year (vs 0–1 sexual partners, OR:3.6, IC 95%: 2.0–6.6).
Tv infection occurs in the female and male urogenital tract and humans are the only natural host for the parasite (18,20). As all STIs, Tv is transmitted by sexual in- tercourse and the evidences that corrobo- rate for the classification of trichomoniasis as STI are: 1.- high frequency of infection in urethra and/or prostate of male partners of infected women; 2.- the prevalence of in- fection is higher among female attending in STD clinics and among prostitutes than in postmenopausal women and virgins; and 3.- Tv die outside of the human body, unless they are protected from desiccation (18,21). Tv infection results in a variety of clinical manifestations in most cases the patients are asymptomatic, but some may develop signs typically associated to the disease. Moreover, beyond the symptoms, the main issue concerning trichomoniasis is its rela- tionship with serious health consequences such as cancer (22,23), adverse pregnancy outcomes (24,25), infertility (26,27), and HIV transmission and acquisition (28). The
infection was traditionally known as symp- tomatic in women and asymptomatic in men (18), however, this currently is changing and recent data have mentioned that around 80% of Tv infections are asymptomatic in both men and women (29). Symptomatic women ranged between 50-85% (21,30-32) while in
men the percentage was 15-77% (30,31).
Tv was discovered in 1896 by the French medical doctor and microbiologist, Alfred F. Donné; in 1916, Hoehne showed the Tv was responsable for producing vaginitis (33). Tv belongs to the Trichomonadidae family. Four different types of Trichomonas (T) can be found in humans: T. hominis, T. tenax, T. vag- inalis y Pentatricomonas hominis (Ptv). The
T. hominis is found in the intestines, the T. tenax in the mouth, the Ptv in the intestines and Tv in the vagina and uretra. As it was mentioned, they parasite the urogenital area of women and men (5). Tv is a flagellated parasitic protozoan, typically pyriform but occasionally amoeboid in shape, extracellu- lar to genitourinary track epithelium with a primarily anaerobic lifestyle (34). The indi- vidual organism is 8–20 μm long and 2–14 μm wide. Four flagella project from the anterior portion of the cell and one flagellum extends backwards to the middle of the organism, forming an undulating membrane. Tv has a foamy, cyanophilous cytoplasm (greenish grey on a Pap smear) with red granules, and eccentric, possibly red-coloured nucleus. An axostyle extends from the posterior aspect of the organism. Parallel to this membrane, a bundle of microtubules called the costa, is arranged inside the cell. It has a Golgi ap- paratus associated with microfilaments (the parabasal filaments) that, together, form the so-called parabasal body. Traversing the cy- toplasm as an axis and protruding from the posterior end, it presents a structure also made up of microtubules called the axostil. This axostil, in its anterior part, widens and partially covers the nucleus. As a continua- tion of the axostil towards the anterior part,
there is another microtubule structure, the pelta, which partially covers the basal struc- tures of the flagella. The nucleus, which has an endosome, is arranged in the ante- rior zone, near the point of insertion of the flagella. In stains, the parabasal nucleus- body-axostil (anterior part) flat assembly is usually stained as a single mass. It lacks mi- tochondria and instead has organelles called paracostal bodies (because it is close to the costa) and paraxostillary bodies (because it is close to the axostil), which are hydrogeno- somes, whose function is to produce energy (ATP) under anaerobic conditions (5). This hydrogenosome, is a mitochondrion-like or- ganelle that generates hydrogen (35). Due to its microaerophilic lifestyle, Tv does not have the ability to generate ATP by oxidative phosphorylation but depends on substrate level phosphorylation. Originally, it was as- sumed that the hydrogenosome is an ances- tral form of the mitochondrion. It is now ap- parent, based on phylogenetic studies, that the hydrogenosome constitutes a reduced form of fully developed mitochondria. Nev- ertheless, the hydrogenosome remains an intriguing organelle, and the extraordinary size of the Tv genome, exceeding 160 Mb, will certainly provoke further research in the years to come (36,37). Tv has a large genome (strain G3, 176,441,227 bp) with ~60,000 protein coding genes organized into six chro- mosomes (38). Tv is a highly predatory ob- ligate parasite that phagocytoses bacteria, vaginal epithelial cells and erythrocytes, and is itself ingested by macrophages. Tv uses carbohydrates as its main energy source via fermentative metabolism under aerobic and anaerobic conditions (31,32).
Its incubation time is generally between 4 and 28 days (21). Tv primarily infects the squamous epithelium of the female and male urogenital tract. Tv resides in the female lower urogenital tract (vagina, urethra) and the male urethra and prostate, where it rep- licates by binary fission. Tv is transmitted among humans, its only known host, pri- marily by sexual intercourse. Infection may
persist for long periods, possibly months or even years, in women but generally persists less than 10 days in males (39). The parasite does not appear to have a cyst form and does not survive well in the external environment, but can survive outside the human body in a wet environment for more than three hours (5,31). However, there may be a pseudocyst form. Tv pseudocyst has been found to be more virulent in animals and could have rel- evance for humans, particularly in the case of cervical neoplasia (40,41). Tv can be in- fected with double-stranded RNA (dsRNA) viruses that may have important implica- tion for trichomonal virulence and disease pathogenesis (41-43). As it was mentioned before, Tv is transmitted during sexual in- tercourse; men also become infected (as- ymptomatic hosts), but most commonly ex- hibit urethral infections. Tv has to deal with constant stress in its physiological niche of the urogenital tract that includes changes in pH, fluctuation in iron balance and other nutrients and desquamation of vaginal epi- thelial cells associated with menstrual cycle (44). In addition, the parasite may reside in the urethra of women from where it re- infects the cervico-vaginal segment upon the pH increase. When the lactobacilli have reinstated, the vaginal pH level returns to normal and the parasite disappears from the area since it cannot survive in an acid en- vironment. Hence, the conclusion of some specialists according to which, some women heal by themselves. In fact, the parasite pres- ent in the urethra reinvades the vagina in the period prior to menses, when the pH in- creases to 7.5 and after the menstrual cycle, when the pH decreases again to 4.0-4.5, Tv moves back into the urethra. Thus, in the ab- sence of a sustained therapy, reinfection oc- curs. The parasite adheres to the wall of the cervicovaginal cells and, by a very effective enzyme system, it interferes in the metabo- lism of the host generating toxins that may lead to cell necrosis. In addition, for sur- vival, the parasite needs very little energy, which it acquires by the glycolysis of cellular
glycogen (5,45). It is known that parasitic protozoans have a variety of complex carbo- hydrates on their surfaces, e.g., glycolipids, glycoproteins, and lipid-anchored glycosyl- ated phosphatidylinositol (GPI). These gly- coconjugates have been reported to play important roles in host cell invasion and evasion of host immune responses by sev- eral protozoa (46,47). Fichorova et al. (48) have demonstrated that Tv express lipophos- phoglycans (LPGs) (2 106 to 3 106 copies/ parasite) anchored on the cell surface via an inositolphosphoceramide (49,50).
The high density of lipophosphoglycans (LPGs) on the parasite surface suggests that they have important roles in parasite biology and in the infections they cause. The same au- thors (49,50) previously showed that pretreat- ment of trichomonads with periodate abol- ished adhesion of specific parasites to their respective host cells, implying the involvement of carbohydrate molecules in these processes (51). This is of a fundamental importance to understand the pathobiology of trichomonia- sis and to define the role that this molecular component(s) play(s) in the infection of host tissues, in particular vaginal epithelial cells (VECs). Fichorova et al. (48) confirmed the role of surface carbohydrates in the adhesion of Tv to host epithelial cells in the lower female genital tract and strengthen the evidence of LPG involvement in this process.
In order to survive as an extracellular parasite, Tv must adhere to the epithelial lining or extracellular matrix components of the urogenital tract especially to VECs (52,53). Attachment to cells, microorgan- isms or other surfaces, drives a transition of the ovoid free-swimming parasite into an amoeboid form which it is highly adherent
(53) (Fig. 1). In addition to VECs, Tv can bind diverse cellular and non-cellular struc- tures upon which a similar transition to an amoeboid form is observed. This suggests that the parasite utilizes either an adapt- able non-specific or multiple specific bind-
ing mechanisms to mediate attachment and signal its morphological remodelling (52).
Multiple molecules have been postulat- ed to be involved in attachment of T. vagi- nalis to target cells (Fig. 1). These adhesion molecules can be separated into three cat- egories: 1. a controversial set of metabolic proteins that appear to have secondary ad- hesive properties; 2. a lipophosphoglycan; and 3. a collection of membrane proteins, many of which have been recently identified through genomics and proteomics (52).
The class of proteins designated as ad- hesion proteins (APs) consisting of AP23, AP33, AP51, AP65 and AP120 are the most controversial of the adhesion molecules (54). However, it has also been reported that these proteins are present on the sur- face of the parasite (55,56). Finding pro- teins with multiple functions and localiza- tions is reasonable; Alderete et al. (57) have reviewed about this dual function of these enzymes as metabolic proteins and adhes- ins. Other research groups, however, have provided immunofluorescent microscopic evidence that these molecules are located exclusively in the hydrogenosome, deny- ing the dual localization (54). It has been shown that AP23, AP33, AP51 and AP65 ad- here to several cell types and bind to these targets in the absence of membrane pro- teins (54,58). These results suggest a non- specific membrane binding, a property that would argue against a precise role in patho- genesis. Recent research shows that AP51 and AP65 bind to hem and hemoglobin, a property that paradoxically supports their promiscuous nature and lack of adherence specificity and implies a role in binding to a key nutrient (59).
It has recently been proposed that glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a glycolytic enzyme, is involved in adherence of Tv to the host extracellular matrix protein fibronectin and is considered with dual-functional: adhesins/enzymes (60).
The ovoid free-swimming form of Trichomonas (upper left-hand side) transitions to an amoeboid form upon binding to a host epithelial cell (upper right-hand side). Lower panel: various T. vaginalis membrane molecules: LPG, hypothetical proteins (Hyp Prot), membrane proteins with conserved domains (Cons Memb Prot) and dual-functional proteins (Dual Fxn Prot) implicated in the interac- tion with ECM and host cell PM receptors. Galectin-1 (Gal-1) is the only reported host cell receptor. Adapted from: https://www.google.com/search?q=Trichomonas+vaginalis:+current+understandin g+of+host%E2%80%93parasite+interactions&sxsrf=ALeKk01yqd1vojQZy74KRaq3mf_VB1SNVA:1 596481107780&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjpgsfK2 qAhUB66QKHUfaDoIQ_ AUoAXoECA4QAw&biw=1440&bih=701#imgrc=u0vsGUaBogCy0M.
Tv is coated by a complex structure of glycoconjugates known as a glycocalyx. Stud-
ing to laminin and adhesion to human VECs (51). The first, and only, surface glyconjugate isolated from Tv is the most abundant. With
6
ies of this glycocalyx began almost 40 years
2–3×10
molecules per cell, Tv LPG was
ago with looking lectin at the parasite surface (61). This investigation has been narrowed to a single surface glycan: LPG. Studies have sug- gested that surface carbohydrates, specifically terminal N-acetylgalactosamine or galactose, are important in determining the virulence of Tv. It was found that treating parasites with glycosylases caused a more than 20-fold reduc- tion in binding of parasites to human VECs (62). In addition, exposure of Tv to periodate to oxidize the glycocalyx reduced parasite bind-
thought to be a probable candidate to play a role in host–parasite interactions (49). This complex molecule is composed of an inositol- phosphoceramide anchor (50) and a polysac- charide core composed of glucose, galactose, N-acetylglucosamine, N-acetylgalactosamine, rhamnose and xylose at a composition of 4.2%, 14.6%, 35.8%, 2.4%, 27.1%, 15.9%, respective-
ly. Several important biological functions have been attributed to Tv LPG. Several lines of evi- dence demonstrate that Tv LPG is an adhesion
molecule: 1. when LPG is added, the Tv biding to the epithelial cells are inhibited to epithelial cells (48,49,63); 2. LPG binds to human galec- tin-1 and this interaction results in host cell at- tachment (64). Although it is unclear whether galectin-1 binding causes alteration in host cell gene expression, LPG exposure to several epithelial cell types up-regulates interleukin-8 and macrophage inflammatory protein 3a ex- pression (48). Furthermore, LPG activates the pro-inflammatory transcription factor nuclear factor κB (65). These activities support a role for LPG in parasite attachment, specificity of binding and manipulation of host cell-gene ex- pression. Even though several functions of Tv LPG have been defined, important questions remain unanswered. It is unclear if galectin-1 is the only receptor for LPG, as several other ga- lectins could function as receptors, particularly galectin-3, which is expressed by VGEs (48).
When attempting to identify proteins involved in the attachment of a parasite to a host cell, an obvious starting place is pro- teins anchored to the parasite’s surface. Sev- eral observations suggest that Trichomonas surface proteins are involved in attachment to host cells and structures. Trypsinization of the surface of Tv decreases binding of the parasite to laminin, fibronectin and several human cell lines (66). It has been found a membrane-localized protein is involved in the attachment of Tv (67). The comple- tion of the Tv genome (38) has launched investigations of protein families that con- tain transmembrane domains and domains shared with surface proteins implicated in pathogenesis in other organisms (54). Four families encompassing 128 genes, encod- ing serine, cysteine and metalloproteases were identified. These proteases may play a role in degrading host proteins or extra- cellular matrix to clear attachment sites (54). Alternatively, they may be involved in degrading parasite–host bonds allowing the parasite to be released and transmitted to the next host. Additionally, three gene fami-
lies encoding 47 proteins similar to surface proteins expressed in other mucosal patho- gens, chlamydial polymorphic membrane proteins, immunodominant variable surface antigens and giardial VSP (variant surface protein)-like proteins, were identified (54). These bioinformatic analyses also revealed the presence of genes encoding 11 surface lectins that may bind to carbohydrates in the extracellular matrix or on the host cell (54). This possibility is strengthened by a previous observation that treatment of host cells with periodate to oxidize carbohydrates reduced parasite binding (68). Finally, the largest family identified through these stud- ies was the BspA-like group named for their similarity to leucine-rich proteins of muco- sal bacteria known to mediate adherence to host cells (54). This gene family was initially found to encode 658 proteins, but further analysis revealed 911 proteins with 191 con- taining predicted transmembrane. Further- more, indirect immunofluorescence shows that at least one family member, TvBspA-625 (where TvBspA is Tv BspA), is expressed on the surface of the parasite (54,69). Future experiments are needed to determine the expression patterns of predicted membrane proteins and to provide direct evidence for their involvement in the interaction of Tv with its host. Understanding both the func- tion and expression patterns of these pro- teins will be critical to determine whether any, may be valid targets for rational drug or vaccine design.
The importance of maintaining an in- tact mucosal layer is crucial to avoid the occurrence of pathological disorders asso- ciated with inflammatory disease, in which disruption of the epithelial barrier leads to severe inflammation of the mucosal tissue compartments (70-73). The epithelial bar- rier is maintained by connections between adjoining epithelial cells, which are the first sites of contact with the host. These cells are responsible for separating potentially harm-
ful luminal content from the underlying tis- sue. This function of acting as a physical bar- rier is accomplished by junctional complex, which comprises a plethora of membrane-as- sociated and transmembraneous proteins or- ganized in discreet, spatially restricted com- plexes (72,73). For establishing infection, Lin et al. (70) findings provide solid evidence to support claims that the damages of Tv in- fection to host cells and cell junctions is ini- tiated by the attachment of live parasites. Tv requires to break through the mucus layer and adhere to epithelial cells of urogenital tract (74). It also penetrates into basement membrane and binds to extracellular matrix proteins (75). Previous reports have shown that host cells infected by Tv presented dis- ruption before being lysed (76,77).
Epithelial cells act in a manner simi- lar to that of a connector in a host-microbe communications network, building signal transduction connections between luminal microbes and the host (76,77). Tv virulence depends on two factors: cytoadherence and cytotoxicity (78). Studies using the scan- ning electronic microscope and transmission electronic microscope on samples collected from patients with Tv vaginitis showed that the capacity to adhere to VECs lead Tv to development of pseudopodial and filopo- dial processes giving a distinct form which is spherical or ovoid (79-82). Furthermore, there is a reason to suspect that the adhe- siveness and amoeboid form of Tv contrib- ute in an important way to the pathogenesis and virulence. Health (81) found that the damage to VEGs is related to the adhesion of this amoeboid form to the cells. The ad- hesiveness is a mechanism for increasing the efficiency of parasite-derived cytotoxic and cytolytic factors since they could act at short range between the VEC and the adher- ing amoeboid Tv (81). Also, several products from the parasite’s metabolism such as lac- tic acid, hydrolytic enzymes: hyalurodinase, glicosidases, mucinases, adhesins, and acid phosphatase contribute to the cytotoxitciy (74). The amoeboid movements of Tv, act-
ing alone or together with the sectored fac- tors, and provide a mechanism for injuring the epithelial cells. Heath (81) showed that this amoeboid movements of Tv are impor- tant in disrupting the epithelial cells. The au- thor (81) concluded that both mechanisms, mechanical and chemical, work in the hu- man Tv infection. The adhesion of Tv to the vaginal and exocervical epithelia could be an important role in disrupting the superficial layers of squamous epithelium causing the necrosis and increasing the rate of desqua- mation which are very common features of severe Tv vaginitis. Also, some authors have mentioned that there is an increased in the sub-epithelial vascularity of the vagina and cervical walls, in other words an acute in- flammation (80,82). Trichomonas requires to break through the mucus layer and ad- here to epithelial cells of the urogenital tract (74). It also penetrates into the basement membrane and binds to extracellular matrix proteins (75). Although pathogenesis and virulence in human trichomoniasis is not fully understood, progress has been made in identifying parasite products that can dam- age host cells and tissues.
Adhesion is thought to play an impor- tant role in the pathogenesis of trichomonia- sis, and investigations of the molecular basis of adhesion of Tv to human cells have identi- fied several adhesion molecules (Ad) on the surface of the parasite. Much of the evidence for the role of Ad in pathogenesis has come from co-culture experiments in which anti- bodies to Ad were shown to reduce parasite adhesion and subsequent cytopathic effects [CE] on host cells (83). Additional studies had shown that contact of Tv with mamma- lian targets caused up-regulation of Ad and that the parasite assumed a flattened shape, essentially laminating itself to the host cell (75). Cysteine proteinases seem to be nec- essary for efficient Ad-mediated adhesion of parasites to targets (84). Control of the ex- pression of Ad also seems to be under the
influence of iron, since transcription induc- tion of ap65-1 (adhesion protein 65,000) was reported to be regulated, via certain iron-responsive DNA elements (e.g., AG- ATAACGA) (85). The Ad family appears to be regulated at several levels. These findings suggest that adhesion of Tv facilitates effi- cient cytotoxicity toward mammalian cells and probably involves complex interactions similar to the situation for other cell-cell contacts such as leukocytes (86). Additional parasite molecules with functional carbohy- drate groups, distinct from Ad, appear to be involved in the adhesion of Tv to VEC (86), although the precise roles of the molecules in parasite adhesion and CE are not known. Little is known about the host cell receptors to which parasite adhesion molecules bind; although there is some evidence that lam- inin may be a target for trichomonad adhe- sion (87).
A variety of hydrolases have been de- scribed in Tv, with cysteine proteinases be- ing particularly prevalent. Ranging from 20 to 110 kDa, several of these lower mo- lecular mass proteinases are released from the Trichomonads (88-90). There has been described other Tv products such as cell- detaching factors which are released by the parasite (91,92); it is known that some of them do have trypsin-like activity. These fac- tors are active on human cells, causing them to detach. The release of cell-detaching fac- tors and proteinases from Tv clearly implies that these parasite products could degrade proteins such as laminin, vitronectin, and other components of the extracellular ma- trix, thus effecting the release of host cells from tissue. In addition, the levels of secre- tory leukocyte proteases inhibitor in patients with Tv infections are significantly lower than those in uninfected patients (93,94), The substrate specificity and structure of some of the proteinases of Tv are now be- ing determined (88). Proteinases with low- er molecular masses are released from the
parasite; proteinases of 25, 27 and 34 kDa specifically hydrolyzed synthetic substrates with arginine-arginine residues, whereas other proteinases have activity over a wide substrate range (88,90).
Evidence suggests that Tv may produce molecules that are delivered to target cells and mediate cytotoxicity through damage of the target cell plasma membrane. One of these molecules creates pores in erythrocyte membranes as detected by electron micros- copy (95), thus displaying perforin-like activ- ity (96). An additional membrane-attacking molecule has recently been detected in Tv, a lytic factor (LF) is released by Tv that can destroy nucleated cells and erythrocytes and specifically degrades phosphatidylcholine, suggesting that it is a phospholipase A2 (97).
The importance of the thioredoxin sys- tem as one of the major antioxidant defense mechanisms in trichomonads, was confirmed by showing that the parasite responds to en- vironmental changes, resulting in increased oxidative stress by up-regulating thioredoxin and thioredoxin peroxidases levels (88).
Another factor that could increase its virulence is the presence of concomitant vi- ruses (98). In 1985 it was first observed that a virus-like particles infected the protozo- ans, and were later recognized as being com- ponents of a double-stranded RNA (dsRNA) virus infecting the protozoans (99-101)]. Tv virus (TvV) is a non-segmented, 4.5–5.5 kilo-base pair (kbp), They are a dsRNA virus belonging to the genus Trichomonaviruses and to the viral family Totiviridae (102). TvV is encased within an 85 kilodalton major vi- ral protein capsid arranged in the shape of a 120-subunit icosahedral and is closely associ- ated with the Golgi complex (103). TvVs use a viral RNA-dependent RNA polymerase to replicate its genetic material. TvVs affect the
total protein expression of Tv, and especially the expression of cysteine proteinases and the immunogenic protein P270, which are linked to cytotoxicity, cytoadherence, degra- dation of basement membrane of epithelium and host immune evasion (78,98,104,105). TvVs can be divided into four different viral strains that have the ability to co-infect Tv at the same time: TvV1, TvV2, TvV3, and TvV4 (106). Each TvV strain has different ef- fects on various aspects of Tv pathogenesis. TvV1 and TVv2 have been linked to genital symptom severity, while TvV2 and TvV3 are involved in the up-regulation of the surface expression of P270, cysteine proteinases and other virulence factors of Tv that help the Tv evasion from the host’s immune response. The role of TvV4 has yet to be elucidated (98,106,107). However, TvVs are considered to be well adapted to Tv such that they cause few if any deleterious effects in the proto- zoan and maintain a largely non-cytopathic and persistent infection (102).
Despite their non-lytic life style, infec- tion of Tv by the dsRNA viruses causes the up-regulation of synthesis and surface ex- pression of a highly immunogenic protein, P270 (104,108). The presence of dsRNA vi- ral infection of Tv is also associated with dif- ferential qualitative and quantitative expres- sion of cysteine proteinases (78). Cysteine proteinases are linked with Tv cyto-adher- ence to vaginal epithelium, cytotoxicity, and degradation of basement membrane com- ponents (109). Two Tvs surface proteinases bind to host epithelial cells and are related to levels of cytoadherence and cytotoxicity (109,110). Thus, dsRNA viruses induce vari- ous phenotypic changes that may impact Tv virulence (78). Type 1 is the termed used to name virus negative Tv and type 2 for Tv positive to virus (108). The biological signif- icance of Tv infection by TvVs remains poor- ly understood. It appears that TvV infection is generally a noncytopathic phenomenon that leads to stable, persistent infections of the protozoan host without an extra-cellular transmission phase (111). Transmission of
TvV occurs during cell division and possibly also during mating (112). Infection of Tv by TvVs could modulate the pathogenicity and virulence of Tv infections of the human geni- tal mucosa (106). The percentage of TvVs- infected-Tvs ranged from a low of 30% (100) to a high of 100% (113). Graves et al. (99) reported a 40% of Tvs infected by TvVs. Wen- del et al. (108) and Graves et al. (99) found that the Tvs infected by TvVs were older (me- dian age 38 years). These authors (99,108) hypothesized that older women with Tv have less immune surveillance against TvV. How- ever, Heidary et al. (114) did not find an as- sociation between TvV+ isolates and older age. Thus, the relationship between TvV pos- itivity and older age remains controversial, and additional studies are needed.
Epithelial cells act as a connector in a host-microbe communications network, building signal transduction connections between luminal microbes and the host. The epithelial cells act as sensors for the microbes and as providers of signal to im- mune cells to activate inflammatory and immune responses (115). Twu et al. (116) demonstrate that Tv exosomes fuse with and deliver their contents to host cells and modulate host cell immune responses. In ad- dition, they showed that (116) parasite exo- somes act to modulate host:parasite interac- tions and promote exosomes in promoting parasite:parasite communication and host cell colonization. Lin et al. (115) showed that there are seven cytokines expressed in the Tv infection; among them, is IL -8. A few days after the human become infected by Tv, there is a degeneration of the vaginal epithe- lium, followed by leukocytic neutrophil infil- tration that is associated with abundant vag- inal discharge and intensely inflamed vaginal tissues. However, little is known about the early mechanism of how neutrophils accu- mulate or how epithelial cells mediate the initial inflammatory response upon Tv infec- tion. Fichorova et al. (48) have provided evi-
dence for an early involvement of chemokine production by epithelial cells that occurs in response to nontoxic doses of LPG and pre- cedes cytotoxic effects. Interleukine-8 (IL -
8) is known to govern polymorphonuclear leukocytes localization and function (115) and is the major chemokine responsible for neutrophil recruitment to sites of tissue in- sult and inflammation (117-119), Shaio et al. (120,121) reported the interleukin-8 re- sponse to infection with Tv in human mono- cytes and neutrophils. Also, the same authors (120,121) found the presence of IL -8 in the vaginal discharge from patients with symp- tomatic trichomoniasis, providing evidence for involvement of IL -8 in the inflammatory response to Tv. Production of IL -8 by mono- cytes and neutrophils stimulated by live tro- phozoites (122) or undefined Tv fragments has been reported (121). They (121,122) have reported species-specific IL -8 produc- tion by human vaginal, ectocervical, and en- docervical epithelial cells, in response to a chemically defined T. vaginalis constituent, purified T. vaginalis LPG. Increased IL -8 lev- els in genital tract secretions and amniotic fluid have been correlated with upper geni- tal tract infections and preterm labor (123), suggesting that Tv IL -8 up-regulation may underlie the mechanisms linking trichomo- niasis with preterm labor.
It has been hypothesized that the pro- duction of IL-8 during Tv infection may be secondary to the cytopathic effects and the release of the early response pro-inflamma- tory cytokines: Tumor Necrosis Factor-alfa (TNF-a) and Interleukine-1beta (IL-1β), by damaged epithelial cells. Fichirova et al. (48) demonstrated that IL-8 is secreted by the cervical and vaginal epithelial cells, with no storage of significant IL-8 amounts in the epi- thelial cytoplasm. On the other hand, large intracellular stores of IL-1 and some small amounts of TNF-a are present in these cells and released once the membrane rupture and cell death (48). TNF-a and IL-1β induce the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB) activation via TNF
receptor I and IL-1 receptor I, respectively, and also may act as toll-like receptor ago- nists in the reproductive tract epithelial cells (119,124). The notion of the primary role of IL-1β and TNF-a in the Tv-induced inflam- matory response has found indirect support in the evidence of increased cervico-vaginal levels of these cytokines under pathological conditions commonly associated with tricho- moniasis, e.g., vaginitis, bacterial vaginosis, HIV-1 infection, and preterm labor (125). However, Fichirova et al. (48) showed that LPG stimulates a significantly increased IL-8 production in the absence of cell toxicity and at low baseline levels of endogenous IL-1β and TNF-a. These findings suggest a direct recog- nition rather than a secondary cytopathic or host factor-mediated signaling effect of Tv LPG and support the hypothesis that the che- mokine response to Tv may be initiated via LPG interaction with VEC surface receptors. Although these results indicate that LPG can trigger responses directly in VECs, and that this effect does not discard LPG interaction with leukocytes, such as macrophages, during the subsequent inflammatory response that may occur by other mechanisms and have a potent contribution to the pathogenesis of trichomoniasis. Fichorova et al. (48) men- tioned that the MIP-3 production could play an important role in the immune response to Tv infection in the VECs. MIP-3a is known to recruit Langerhans-like CD34 progenitor dendritic cells (DCs) to the site of inflam- mation, by guiding them as they traverse the lamina propia on their way to the epithelial surface, a journey which precedes their matu- ration (126). Also, Fichorova et al. (48) found that LPG-induced IL-8 and macrophage in- flammatory protein-3 (MIP-3) a production by the VECs is, at least, partially MyD88 in- dependent. This cytoplasmic adaptor protein MyD88 is essential for rapid NF-kB activation and cytokine production in response to mul- tiple TLR ligands including LPS (127). The MyD88-independent activation via TLR is im- portant for DC maturation (128). Fichorova et al. (48) have presented evidences of the
major role of Tv LPG in the pathogenesis of the mucosal inflammatory reaction and sug- gested its modulatory role in the innate host immune response through selective species- specific cytokine up-regulation and multiple signaling pathway. Lin et al. (115) found that the vascular endothelial growth factor Beta (VEGF-β) is induced in human vaginal and ectocervical cells infected with Tv. Ac- cording to these results, it is reasonable to assume that the disruption of host cell patho- genesis may be caused by these cytokines. Another cytokine which increased expres- sion level, BMP-2 is a member of the TGF-β superfamily, playing an important role in the embryonic development, suppression of the immune response, and differentiation and proliferation of tissues and cells. Fichorova et al (48) have provided evidence that the major role of Tv LPG in the pathogenesis of the mu- cosal inflammatory reaction and suggests its modulatory role in the innate host immune response through selective species-specific cytokine up-regulation and multiple signal- ing pathways.
The actual understanding of immuni- ty to Tv has come from observations of re- sponses in human patients and experimenta- tion using in vitro models and animal models of the related species, T. foetus. Natural in- fection seems to produce immunity that is only partially protective, since reinfection of patients can be 30% on follow-up (86). As it was mentioned before, most of Tv infec- tion is often asymptomatic and recurrent, despite the presence of specific antibodies, suggesting the importance of the innate im- mune defense. Tv adhesion proteins, cyste- ine proteases, and the major parasite LPG play distinct roles in the pathogenesis and evasion of host immunity. LPG plays a key role in the parasite adherence and signal- ing to human vaginal and cervical epithelial cells, which is at least in part mediated by galectins. The epithelial cells respond to Tv infection and purified LPG by selective up-
regulation of pro-inflammatory mediators. At the same time, Tv triggers an immuno- suppressive response in monocytes, macro- phages, and dendritic cells (117).
Antibody responses during Tv infection have been shown to occur in infected patients by several types of immunoassays (129). In- fection in humans produces parasite-specific antibodies in the reproductive tract and, in most instances, circulating antibodies in the serum. There is also evidence of lymphocyte priming as detected by antigen-specific pro- liferation of peripheral blood mononuclear cells (130). Natural infection with Tv results in priming of acquired immune responses. The precise protective effects of parasite- specific immunity has not been so simple. In vitro analyses of the effects of serum-de- rived antibodies and monoclonal antibodies to surface antigens have identified anti-ad- hesin antibodies that block the adhesion of Tv to various human cell lines, such as HeLa (131), and to VECs (51). However, proof of the protective nature of antibodies in elimi- nating infection or limiting pathogenesis in vivo has been difficult because of lack of an adequate experimental-animal model for vaginal infection studies.
The presence of parasite-specific im- munoglobulin G (132) and immunoglobu- lin A (133) responses, also indicates prim- ing of helper T cells, although the relevant antigens and the exact effects of antibodies on the parasites are unknown. One obvious target of protective antibody could be the adhesin molecules used by the parasite to fa- cilitate close contact to host cells, a process previously shown to lead to efficient host cell destruction (134). The molecular ba- sis of adhesion of Tv has been investigated, and four antigenic surface molecules have also been implicated in the adhesion of Tv to VECs; their expression is being up-regu- lated during attachment to host cells (75).
Antibodies to these molecules protected tar- get cells from parasite-mediated cytotoxic- ity (75), suggesting that anti-adhesion im- mune responses could be important in vivo protection against the pathogenic effects of Tv. However, actual understanding of immu- nity to Tv remains unclear. It is not known whether acquired immune responses are re- quired for protection and, if it is true what role is played by acquired immunity in con- taining or eliminating infections (88). There is not a strong protective immunity after a natural infection in humans (88).
The majority of women (85%) (135) and men (77%) (136) with Tv are asymptomatic. One third of asymptomatic women become symptomatic within six months (21). Among symptomatic men, symptoms include ure- thral discharge and dysuria. Among symp- tomatic women, common sites of infection include the vagina, urethra and endocervix. Symptoms include vaginal discharge (which is often diffuse, malodorous, yellow-green), dysuria, vaginal and/or vulvar itching, vulvar irritation, vaginal odor and abdominal pain. The normal vaginal pH is 4.5, but with Tv infection increases this markedly, often to
>5 (21,31,32). Colpitis macularis or straw- berry cervix (microscopic, punctate haem- orrhages in the cervix) is seen in about 2-5
% of women, though with colposcopy this rises to nearly 50% (137,138) and the cer- vix becomes erythematosus, edematous, fragile and bleeds easily (139). Other com- plications include infection of the adnexa, endometrium, Skene and Bartholin glands. In men, it can cause epididymitis, prostati- tis, and decreased sperm cell motility (140). When symptoms arise, the most common complaint among women diagnosed with Tv is vaginal discharge, presented in more than 50% of cases, followed by pruritus or dysuria (139). The vaginal discharge is a classical signal of trichomoniasis and it is due to intense leukocytic infiltration within the genital tract because of the death of epi-
thelial cells, which promotes inflammation and leads to an increased number of poly- morphonuclear leukocytes in the vaginal fluid (140). It is important to highlight that the infection symptoms are cyclic and more intense around the menses period because of the effect of iron on parasite pathogen- esis (21). Studies have shown an association between Tv and vaginitis, cervicitis, urethri- tis, bacterial vaginosis, candidiasis, herpes simplex virus type-1 and type-2, chlamydia, gonorrhea, syphilis, human papillomavirus (HPV) and human immunodeficiency virus [32,37). In addition, endometritis, adnexi- tis, pyosalpinx, and atypical pelvic inflam- matory disease are related to Tv infection (117). Trichomoniasis may also affect the pregnancy course, causing low birth weight, premature rupture of membranes and pre- term delivery (141). There are some evi- dence that Tv infection can be transmitted vertically leading to cases of vaginal and re- spiratory infections in neonates (142,143). Mann et al. (144) showed an association be- tween maternal Tv infection and intellectual disability in children.
Over the past two decades, our under- standing of inflammation in tumorigenesis has led to the identification of a number of molecules that are strongly linked to the development of human cancers (145-147). Like tumorigenesis, tumor-promoting in- flammation and tumor-associated inflam- mation (TAI) are the phenotypic product of a complex set of cellular and molecular interactions that result in an imbalance in the local microenvironment that is most analogous to an unresolved ‘wound-healing’ response (147).
A number of the cellular and molecu- lar mechanisms involved in inflammation-in- duced tumor initiation, promotion and pro- gression are now well described ( Table III). In recent years, strong evidence from many epidemiological and experimental studies have demonstrated that inflammation also
MOLECULE, CELLS AND TISSUE ALTERATIONS OBSERVED WITH CHRONIC INFLAMMATION AND TUMOR PROMOTING CONSEQUENCE.
Genomic instability, chromosome remodeling, epigenetic changes and altered gene and miRNA expression. |
Altered post-translational modification, activity and localization of cell proteins. |
Altered cell metabolism. |
Induction of cell growth and anti-apoptotic signals→ uncontrolled cell growth and retention of cells with damaged genomes. |
Vasodilation, leakage of the vasculature and infiltration of leukocytes → disrupted tissue integrity and altered microenvironment and immunosuppression and recruitment of myeloid suppressor cells. |
Altered cell polarity → disturbance in stroma/epithelial tissue matrix and loss of differentiation signals. |
Tissue necrosis → neovascularization and hypoxia. |
Induction of matrix metalloproteinases → invasiveness and spread. |
plays an important role in the development of CIN and cervical cancer (148). Cervical inflammation may be associated and may be a cofactor in high-grade cervical lesions in women infected with oncogenic HPV. In oth- er cancer types, independent of the causal agent, chronic inflammation exposes the tis- sue to constant genotoxic damages. In cer- vical cancer, chronic inflammation added to the initial changes caused by high-risk HPV, may contribute to viral persistence and dis- ease progression (149,150).
Acute inflammation has two balanced and biologically opposing effector arms or mechanisms: 1. a pro-apoptotic or tumori- cidal and 2. a wound healing or pro-tumor- igenic relationship model, where immune cells participate with the non-immune cells in the local environment (e.g. epithelial, vasculature and neuronal) (151,152). Lo- cal or systemic adaptive immune responses (cell-mediated and humoral immunity) are activated and mobilized by selective signal-
ing between the activated innate immune effector cells (e.g. macrophages and mast cells) and their counterparts in the adaptive immune system (e.g. T and B lymphocytes). In the acute inflammation, immune cells possess properties specialized that allowing them to recognize and eliminate intrinsic or extrinsic foreign elements and that injure or damage host tissue (acute phase). In the intermediate or resolution phase response, the immune cells function to resolve inflam- mation and repair the damaged tissue. Unre- solved and persistent inflammation has been described as the loss of, or deregulation in the balance between both mechanisms. The role of persistent inflammation as a con- tributing factor in tumorigenesis is well ac- cepted and, in many cancers, thought to be a necessary component. Examples include a causal relationship between inflamma- tion and infectious agent-associated cancers [e.g. hepatitis B and C virus (liver), human papilloma virus (e.g. cervix, anal) and the bacterium Helicobacter pylori (stomach)]. The relationship between cancer and inflam- mation is also supported by the elevated risk
of cancer in chronic inflammatory condi- tions, such as colitis-associated colorectal cancer. Importantly, the cause-effect rela- tionship between inflammation and cancer is a challenging concept as it implies that inflammation precedes the processes. How- ever, current evidence widely suggests that in the case of cancer, which is a multi-step and complex process, inflammation is an integral component of the overall pathogen- esis of disease at the microenvironment level that not only contributes in a causal way but also supports a permissive state for tumors to grow (145). It is important to recognize that TAI in solid tumors is itself a complex pathologic process, with contributions from classic immune cells as well as poorly charac- terized, cancer-associated fibroblasts and the epithelial tumor cell compartment (151). As it was mentioned before, Tv is infected by four different types of dsRNA viruses. When the parasite dies, releases its viral load with- in the human vagina. This could explain why symptoms can worsen upon successful anti- biotic treatment and why Tv infected by the virus contribute to increase the inflamma- tory response and the virulence (153).
Trichomonas vaginalis and cervical cancer
Epidemiology of cervical cancer
In 2018, according to the International Agency for Research on Cancer/Globocan (IARC), the cervical cancer (CC) was the ninth more frequent type of cancer world- wide, including both sexes, with 569,847 new cases, representing an incidence of 3.2% among all the cancers, and caused 311,365 deaths (incidence:3.3%), being the ninth deadliest cancer worldwide (IRC). CC rep- resents the second most common female cancer, after breast cancer, worldwide (154). The Pan-American Heath Organization/ World Health Organization (PAHO/WHO)
(155) reported 72,000 new cases of CC and almost 34,500 deaths in the American con- tinent during 2018. The mortality rate was three times higher in Latin-America and the Caribbean area than in North-America (155).
It is well known that the development of pre-malignant and malignant cervical le- sions is multi-factorial. One of this most important factor is HPV, especially the high risk-HPV (hr-HPV) type 16 and 18, as the main etiological agent. Other risk factors have been identified in various cross-section- al and prospective cohort studies, including number of sexual partners (lifetime and re- cent), early onset of first intercourse, pro- miscuos partner, smoking, long-term use of oral contraceptives, multiparity, vaginal infection, malnutrition, certain dietary de- ficiencies, immunosuppressive conditions such HIV, and chronic inflammation due to co-infection with others microorganisms Ct, HSV-2, Ca, gonorrhea, BV, Ureaplasma urealyticum, Ureaplasma parvum, and Tv (156-158). Different authors (159,160) have mentioned that the association among these STIs and cervical lesions may be related to their inflammation effects. Infection with hr- HPV plays a central role in cervical carcino- genesis. Epidemiological and molecular in- vestigations have shown unequivocally that hr-HPV infection is the main causal factor in initiating the progressive transformation that leads to cervical intraepithelial neopla- sia and to cervical cancer (161-163). Many reports have demonstrated that HPV infec- tions are mostly transitory and that only a small percentage of infections persist and may progress to dysplastic lesions: type-spe- cific HPV persistence is the strongest risk factor for cervical cancer (164). The reasons for the variable natural history remain poor- ly understood, but it has been generally ac- cepted that different cofactors are involved in the development of dysplasia in HPV in- fected women. Factors, such as specific HPV genotype, viral load, and co-infection with more HPV types, are likely to be involved in the progression to precancerous cervical lesions (164-167). In addition, environmen- tal co-factors mentioned before have also been investigated as potentially involved in the disease process (168-170). The sexually transmitted pathogens were reported to in-
teract with HPV as cofactors in the progres- sion of cervical neoplasias. For instance, Ct and Tv were proved to be positively corre- lated with hr-HPV persistent infection (171).
Trichomonas vaginalis/HPV/cervical intraephitelial neoplasia
Another important issue regarding complications of trichomoniasis in women is its involvement with an increased risk of cer- vical cancer. Some studies have pointed Tv as a predictor for cervical neoplasia since there is a high relative risk of preinvasive lesion and invasive cancer (CIN) (8,172). As it was mentioned before, the cervical epithelium is a protective barrier which is disrupted by in- flammation produced by Tv infection, allow- ing the penetration of hr-HPV to the base- ment membrane (8,159,163,172,173). As it was mentioned in this manuscript before, Goshi et al. (172) reported that the associa- tion of Tv with high grade CIN is because of the epithelial alteration and damage that characterize these conditions facilitate the proliferation of the organism. The cell medi- ated immunity generated against the invad- ing organism involves recruitment of leuco- cytes in large numbers, which are associated with findings on cytology of smears. Further, the parasite often imbibes nutritious ele- ments like fatty acids and iron by destruc- tion of host red blood cells, which is caused by cytotoxic trypsin like substances called cell detaching factor, CDF and N-nitrosa- mines liberated during infection, which also promote the process of epithelial atypia and dysplasia. Vaginal pH rises during Tv infec- tion, which allows the growth of the organ- ism. Tv thrives on tissue debris and serous exudate and produces tissue damage that is extensive and atypical. Disease pathogen- esis of Tv infection necessitates multitude of cross-communications involving viruses, bacteria, eukaryotes and human host (174). Gram et al. (175) mentioned that the infec- tions by HPV and Tv play an important role in the pathogenesis or CIN and CC. Lazenby et al. (8) had found Tv using nucleic acid am-
plification test (NAAT) to be associated with an increased risk of acquisition of high risk HPV (OR 4.2, 95% C.I. 1.7-10.3). Donders
et al. (176) demonstrated that HSIL cases where Tv was discovered, hr-HPV was pres- ent. Theses authors (176) found that women with ASC-US with Tv had no HPV (36%). Although, the presence of Tv was higher in patients HPV negative/ASC-US compared to the group HPV positive (low and high risk)/ ASC-US patients. Tao et al. (177) found that TV infection is associated with CIN espe- cially to high degree. They (177) mentioned that there is some epidemiological evidence to suggest that genital tract disease such as cervical inflammation might be linked to cervical cancer or high-grade lesions, and a significant association between cervical in- flammation and HISL was identified in this study. Tv may act as a cofactor facilitating the development of cervical HPV infection to high-grade lesion and cervical cancer. Multiple studies have demonstrated an as- sociation between previous and current Tv infection and cervical dysplasia and human papillomavirus (172,178). Fem et al. (179) found that women with HPV are more likely to be infected by other types of STIs. Also, they said Tv+ women had an increased risk to get hr-HPV infection compared to nega- tive Tv women. Yap et al. (180) found anti- bodies against Tv in 41.3% of patients with invasive CC compared with 5% in control group.
Tv is considered as the second most frequent STIs and the second most com- mon cause of lower genital tract infection worldwide. Because Tv infection is highly prevalent in sexually active populations, it is now gaining greater recognition as an important source of reproductive morbid- ity and is clearly associated with significant public health problems. Also, Tv infection has the potential to increase the possibil- ity for the acquisition and transmission of
HIV, possibly HSV-2 and HPV. It has been independently associated with cancer, so it has been considered as a co-factor or factor for CC. Trichomoniasis is clearly associated with significant public health problems. The disease presentation is variable and diagno- sis can be difficult with the most commonly used methods. Certain socio-demographic and behavioral risk factors may assist in pre- dicting infection. A majority of women and a significant proportion of men with tricho- moniasis are asymptomatic; these patients would thus escape detection and treatment under syndromic management recommenda- tions. Control programs must work towards increased screening in order to have any ef- fect on the burden of this disease. In recent years, many advances have been made in the epidemiology, diagnosis, and treatment of Tv. The focus of these efforts, however, has largely been on women. Although there are still many open questions regarding Tv’s epidemiology, particularly in the context of facilitated HIV contagion and cancer, our understanding of Tv’s pathogenesis made a large leap forward and the picture is becom- ing ever more complete. Finally, the Tv ge- nome has remained as a fascinating field of study. So far, there have been four dsRNA vi- ruses identified. This area opens a great and broad field for research about the virulence and its possible role as co-factor or factor in CC in the next years to come.
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