Gonorrhea is an acute purulent genital tract infection caused by the Gram-negative bacterium, Neisseria gonorrhoeae (N. gonorrhoeae) (45). It is estimated that there are 78 million new cases of gonorrhea which occur globally each year (46), and due to the lack of a viable vaccine and the emergence of multi-drug resistant strains, N. gonorrhoeae is considered a serious infectious threat (46, 47). Rates of infection are higher in women compared to men, and gonorrhea infection can lead to further negative health outcomes in women including pelvic inflammatory disease (PID), infertility and ectopic pregnancy (48). Furthermore, if left untreated, infection with N. gonorrhoeae can enhance the transmission and acquisition of HIV by up to 5-fold (49).

N. gonorrhoeae is primarily an extracellular bacterium that induces a pro-inflammatory response consisting of the cytokines IL-6, IL-1β and tumor necrosis factor-alpha (TNF-α), as well as an influx of neutrophils (47). Interestingly, these cytokines are also linked to Th17 differentiation. As such, IL-17 levels have been reported to be elevated in individuals infected with N. gonorrhoeae (50, 51). The mouse model of gonorrhea, which requires mice to be treated with E2 prior to infection (52, 53), also demonstrates a strong Th17 response following infection. In this model, infection persists in rodents for about 10-20 days, after which the bacteria are cleared. Over the past several years, M.W. Russell and colleagues have extensively used the mouse model to better understand the role of IL-17 during gonorrheal infection. In initial studies, Feinen et al. (45) showed that IL-17 was critical for controlling early gonorrheal infection in vivo, as blocking IL-17 or preventing IL-17RA signalling in mice resulted in prolonged infection, along with significantly diminished neutrophil recruitment. Further, Russell and Feinen (54) reported that unlike IL-17, IL-22 does not appear to impact N. gonorrhoeae infection in vivo, suggesting IL-17 specifically, but not other Th17-related cytokines, is critical for bacterial clearance.

Interestingly, N. gonorrhoeae infection does not elicit a strong Th1 or Th2 response in mice (45) and although N. gonorrhoeae induces local inflammation, there is no acquired immunity or immunological memory established (53, 55). These findings show that primary infection does not lead to substantial or sustained antibody responses, and mice can be re-infected with the same strain of bacteria without displaying enhanced resistance such as elevated antibodies or enhanced CD4+ T cell responses (53). These observations closely reflect known features of the human immune response to uncomplicated N. gonorrhoeae infection, where there is limited humoral and T cell immunity, even with recurrent infection (56, 57). This led to the theory that N. gonorrhoeae selectively elicits Th17-dependent innate responses that it can overcome, including neutrophil recruitment and upregulation of AMP production, while suppressing Th1/Th2-driven adaptive immunity that may be able to protect against subsequent infection (45, 58–60). This would imply that Th17 immunity is manipulated by N. gonorrhoeae and used to evade host mechanisms of protection. In this regard, further work by Liu et al. has shown that the absence of protective Th1 responses can be attributed to the production of TGF-β, which occurs following N. gonorrhoeae infection and skews immune responses towards Th17-mediated immunity (59, 60). Together, these studies suggest that N. gonorrhoeae actually suppresses adaptive immunity by upregulating the production of immunosuppressive cytokines, TGF-β and IL-10. Liu et al. demonstrated that by blocking TGF-β, it is possible to reverse this host immune response and enable the development of protective anti-gonococcal immunity consisting of Th1-driven responses, the presence of anti-gonococcal IgG and IgA antibodies, establishment of immunological memory and enhanced clearance of bacteria (59, 60). Additional work is needed to understand how to better direct immunity during infection with N. gonorrhoeae to maximize protection, as well as to better understand how to leverage IL-17 immunity in this effort.

Chlamydia trachomatis (C. trachomatis) is an intracellular human bacterium that causes the most common bacterial sexually transmitted disease worldwide, with over 250 000 new infections contracted daily (61). Like many other STIs, women have disproportionately higher rates of chlamydia infection prevalence globally (62). Although antibiotics can be used to treat chlamydial infection, more than 70% of women show no signs or symptoms of active infection (63). As a result, untreated infections can cause significant reproductive tract pathology in women leading to the development of conditions such as PID, chronic pelvic pain and infertility. C. trachomatis is also the cause of preventable blindness (trachoma) in developing countries (64, 65).

Although IL-17 has been shown to be protective against extracellular pathogens, the role of IL-17 in protection against intracellular bacterial pathogens, including chlamydia, is less clear. Multiple mouse models for chlamydia have been developed over the past two decades and studies have provided extensive information regarding chlamydia pathogenesis, immune response and vaccine design. As with most animal models of infectious disease, there are differences between human and murine chlamydia infections which should be considered when extrapolating findings (66). For instance, similar to other models involving genital pathogens, the hormonal microenvironment of the murine FRT has to be manipulated in order for infection to occur. In the chlamydia models, mice require pre-treatment with progesterone to enhance susceptibility to infection. It is well known that changing the hormonal microenvironment alters the structural physiology of the FRT epithelium and may also influence the function of the immune cells present (67). Another difference is that in mice, disease develops after a single exposure to bacteria, after which infection is often cleared. This contrasts what occurs in humans, where secondary infections are often required to drive significant pathology and long-term chronic infections are common. Furthermore, different species of chlamydia display tropism for specific hosts (68). While human urogenital C. trachomatis strains can be used to infect mice, the most common murine model of genital chlamydia infection uses the mouse-adapted C. muridarum pathogen. Intravaginal infection with C. muridarum evades murine cell-autonomous immune mechanisms and establishes a self-resolving genital infection, whereas infection with human C. trachomatis is rapidly cleared from the murine FRT and fails to establish productive infection or induce pathogenic immune responses. As such, the C. muridarum genital infection model is more amenable to the study of immune mechanisms and appears to replicate many aspects of human infection. Studies have shown that C. muridarum first infects the vaginal and cervical epithelial cells, after which it ascends the reproductive tract and causes upper reproductive tract pathology, similar to chlamydia-associated disease sequelae observed in women infected with C. trachomatis (69, 70). The infection in mice is resolved after approximately 4-6 weeks and results in long-lived adaptive immunity that protects against re-infection (71). It has been shown that Th1 cells and interferon gamma (IFN-γ) are critical for protection against primary genital C. muridarum infection (72), while CD8+ T cell responses and antibody responses are important for protection against re-infection (73–78).

The role of IL-17 during chlamydial infection has been extensively studied in the lungs, where it has been shown that in the absence of IL-17, there is greater replication of bacteria and decreased survival of infected mice (79). Further mechanistic studies have demonstrated that IL-17 appears to be necessary for DC priming of Th1 immunity in the lungs, without which there is compromised bacterial clearance (80). However, in the context of genital tract infection with C. muridarum, the role of IL-17 is less clear. For instance, some studies have shown that the importance of IL-17 in mediating protection against chlamydia is negligible. A study by Scurlock et al. (81) showed that although IL-17RA deficient (IL-17RA-/-) mice demonstrated reduced IFN-γ production in the lymph nodes and decreased neutrophil influx into the FRT, mice were still able to resolve primary C. muridarum infection normally and showed no differences in pathology compared to WT mice. Instead, both macrophage influx and TNF-α production was increased in the absence of IL-17, suggesting a compensatory mechanism to control infection. Likewise, Frazer et al. (82) showed that in the absence of IL-23, where chlamydia-specific Th17 responses were absent, mice exhibited normal susceptibility to genital infection and regular development of oviduct pathology.

In contrast, other findings suggest that IL-17 plays a pathogenic role during genital chlamydial infection. In a study by Andrew et al. (64), they found that the duration and magnitude of C. muridarum infection was significantly lower in IL-17 deficient (IL-17A-/-) mice compared to WT mice. In the absence of IL-17, they also noted decreased inflammatory pathology, which was related to significantly reduced recruitment of neutrophils and macrophages into the oviduct tissues. Interestingly, others have also shown that oviduct pathology observed following infection is associated with the infiltration of neutrophils into the FRT, as well as the production of inflammatory mediators and factors involved in tissue-remodeling such as matrix metalloproteases; all of which are mediated by IL-17 (83–86). Human studies have similarly associated greater neutrophil activation with increased disease progression in women infected with genital C. trachomatis (87) and suggested a role for IL-17 during infection, as cervical washes from infected women had 5-fold higher levels of IL-17 compared to uninfected controls (88). Altogether, studies regarding the importance of IL-17 during chlamydial infection are conflicting. This may be due to the fact that the strains of mice used in different studies vary, and it is well known that the mouse strain used for chlamydia infection may impact outcomes, including duration of infection, degree of upper genital tract infection, severity of infection-induced pathology and immune responses generated (71). As such, further work is required to better elucidate the role of IL-17 in the context of genital chlamydia infection.

HIV (human immunodeficiency virus) infection in the FRT leads to the rapid depletion of local CD4+ T cells, as well as viral dissemination throughout the body (89). It is a deadly virus if left untreated and can lead to the development of acquired immunodeficiency syndrome (AIDS). There are currently close to 38 million individuals living with HIV, with approximately 1.7 million new infections occurring each year (90). Women account for more than 50% of infections, and young women between the ages of 15–24 are particularly susceptible to infection (90). Although the risk of vaginal transmission is considered low, estimates indicate that 40% of HIV infections are initiated in the FRT (91). Additionally, infection with HIV is often associated with increased susceptibility to other sexually transmitted infections (STIs).

The role of IL-17 during HIV infection has not been comprehensively described, however, Th17 cells are important for HIV pathogenesis. HIV preferentially replicates in activated T cells, specifically CD4+ T cells (92), and the expression of the mucosal integrin α4β7 and HIV co-receptor CCR5 by activated T cells also increases susceptibility (93, 94). As such, Th17 cells, which are activated and terminally differentiated cells that express high levels of α4β7 and CCR5, are considered preferential target cells for HIV infection (95–100). In vitro and ex vivo studies using human cells and in vivo studies using a macaque model with the simian variant of the virus (SIV), have all shown that Th17 cells are target cells for HIV infection in mucosal tissues. For instance, Rodriguez-Garcia et al. (101) examined the phenotype and susceptibility of primary CD4+ T cells isolated from endometrium, endocervix, and ectocervix, to HIV infection ex vivo. They found that Th17 cells were the primary CD4+ T cell population which expressed HIV receptors CCR5 and CD90, and that these cells were the most susceptible to HIV infection in vitro. Likewise, several studies using samples collected from HIV-infected Kenyan women have also shown that Th17 cells are preferentially targeted by HIV, as Th17 cells were significantly depleted in infected individuals (93, 100, 102). Additionally, Boily-Larouche et al. (102) described a highly activated subset of CD4+ T cells in the FRT of HIV-infected female sex workers (FSWs) from Kenya, which expressed CD161 and differentiated into Th17 cells. These cells were found to express multiple HIV susceptibility markers and were severely depleted in HIV-infected FSWs, compared to uninfected FSWs. Similarly, McKinnon et al. (99) collected cervical cytobrush specimens from FSWs in Kenya and found that cervical IL-17+ CD4+ T cells preferentially co-expressed HIV receptors α4β7 and CCR5. Furthermore, these cervical Th17 cells were significantly depleted following HIV infection, suggesting they may serve as key target cells during HIV infection. Finally, in the macaque model of SIV infection, Stieh et al. (103) found that Th17 cells (CCR6+ CD4+) were also preferentially infected by SIV, and they showed that most SIV-infected cells expressed the master Th17 transcriptional regulator, ROR-γt.

Interestingly, Th17 and Th22 cells that make IL-17 and IL-122 play an important role in the renewal and maintenance of the mucosal epithelial barrier, which is known to be critical in protecting against HIV infection in both the gut and FRT (98, 104, 105). Several components of the mucosal epithelial barrier help mediate protection against HIV infection, including the physical composition of the barrier, the presence of immune cells, cytokines and antimicrobial factors, and the interactions between the barrier and the local microenvironment, including mucus and host microbiota (106). As such, the loss of Th17 cells may have a direct impact on the integrity of mucosal barrier and consequently impact further viral transmission and/or pathogenesis (107). This has been shown in the gut, where the depletion of Th17 and IL-22 producing T cells during chronic HIV infection has been associated with damage to the epithelial barrier and subsequent microbial translocation (108, 109). It is possible Th17 cells may be playing a similar protective role in the FRT during HIV infection, however more work is needed to elucidate this potential mechanism.

Herpes simplex virus type 2 (HSV-2), the primary virus causing genital herpes, is one of the most common STIs worldwide, with over 400 million individuals infected globally and approximately 17 million new infections occurring each year (110). Rates of infection are especially alarming in sub-Saharan Africa, where prevalence is as high as 80% amongst women between the ages of 15-49 (110). HSV-2 first infects genital epithelial cells, and then travels via retrograde transport along nerve axons to the dorsal root ganglia, where it establishes life-long latency (111). The neuronal cells act as a reservoir for the latent virus, which can be reactivated due to factors including stress and hormonal changes. Reactivation results in anterograde transport of the virus, resulting in productive replication in the FRT (112). Along with the development of painful genital ulcers, HSV-2 infection is associated with a 2- to 3-fold increased risk of HIV-1 acquisition and up to a 5-fold increase in transmission of HIV-1 (113, 114).

Unlike the role of IL-17 during bacterial and fungal infections at mucosal sites, very limited studies have examined the importance of IL-17 in viral infections, especially in the context of the FRT and HSV-2. As such, this has been an area of interest in our lab and we have focused extensively on the role of both innate and adaptive production of IL-17 in the FRT, along with the role it plays during viral infection with HSV-2. Additionally, we have examined how factors in the FRT microenvironment, such as female sex hormones and the vaginal microbiota, effect IL-17 production in the genital mucosa.

Our work over the past few years has provided significant insight regarding how IL-17 modulates critical anti-viral T cell responses in the FRT. While it is well established that anti-viral protection against HSV-2 in the mouse model of infection is largely mediated by Th1 responses (115–117), our findings are the first to show that Th17 responses are also important in the anti-viral immune response to HSV-2 infection (118–120). We directly examined the role of IL-17 in anti-viral protection against HSV-2 and found that compared to WT controls, IL-17A-/- mice immunized intravaginally or intranasally were more susceptible to HSV-2 challenge (119). IL-17A-/- mice had decreased survival, greater viral shedding, and more severe genital pathology post-challenge. Interestingly, we found that IL-17 played an important role in enhancing anti-viral Th1 responses in the FRT, as IL-17A-/- mice had impaired Th1 cells responses post-challenge (119) and mechanistically, this was associated with impaired Th1 priming by vaginal DCs (118). Taken together, these studies have demonstrated that IL-17 is critical for inducing an efficient Th1 immune response following HSV-2 immunization, resulting in effective protection against HSV-2.

While the presence of innate IL-17 has been implicated in the amplification of Th17 responses in other mucosal tissues (121–123), the significance of IL-17 produced by innate or innate-like sources in the FRT and its influence on Th17 immunity in the vaginal mucosa is less understood. Thus, we also investigated the how innate IL-17 in the FRT might induce adaptive Th17 responses (124). Our findings support results seen in other mucosa and demonstrate that innate IL-17 produced in the FRT is also important for inducing Th17 responses. Furthermore, our observation that vaginal DCs from IL-17A-/- mice produced lower amounts of IL-1β compared to WT DCs, suggested a mechanism in which innate IL-17 induces vaginal DCs to prime Th17 responses via IL-1β (124). Additionally, consistent with previous findings by Kim et al. (125), we showed that γδ+ cells are the primary source of innate IL-17 in the FRT under homeostatic conditions (124). We extended these findings to show that multiple factors found within the vaginal microenvironment, influence innate IL-17 production by γδ+ cells. We found that E2 treatment resulted in significantly greater proportions of IL-17-producing γδ+ T cells and that germ-free mice had significantly lower proportions (124). These results provide insight on how innate IL-17 can influence immune responses against infections in the FRT, as well as ways in which its production can be modulated.

As mentioned previously, the hormonal microenvironment can influence immune responses and disease outcomes in the FRT. For the past decade we have investigated the influence of E2 on immune responses in the FRT and found that E2 increases protection against HSV-2 infection, although the underlying immunological mechanisms remained unclear. Recently, we showed that better protection in E2-treated mice coincided with earlier recruitment and higher proportions of Th1 and Th17 cells in the FRT following either HSV-2 immunization (intravaginally or intranasally) (120) and/or challenge (118). This included greater establishment of tissue-resident memory CD4+ T cells in the FRT, which play a critical role against re-exposure to pathogens (120). Together with the reduced protection shown against HSV-2 challenge in E2-treated IL-17A-/- mice, these findings suggested that E2-mediated protection against HSV-2 is driven by the induction of Th17 responses, likely mediated by increased IL-1β production by vaginal CD11c+ DCs in the presence of E2. Thus, a novel and critical anti-viral role for IL-17 in the FRT has emerged through our work.