UC Davis

Sexually transmitted infections (STIs) affect over 68 million people in the U.S.annually and are highly prevalent among young people. Antibiotic and antiviral drugs have widely been used to treat STIs in patients and this treatment strategy has led to the development of incurable multidrug resistant pathogens. Thus, there is an urgent need for the development of effective vaccines to control the transmission of STIs in the population. While vaccines have been developed to protect the public against some STIs such as Human Papilloma Virus, many other sexually transmitted pathogens such as Gonorrhea, Herpes Simplex Virus (HSV) and the intracellular bacterium Chlamydia Trachomatis, are uncontrolled in the population. Women are particularly vulnerable to such infections as these pathogens often wreak havoc on the female reproductive system. Indeed, Chlamydia trachomatis can cause pelvic inflammatory disease (PID), ectopic pregnancy, and even infertility in some women. Despite the striking threat of STIs to women’s health, our understanding of how immunity develops in the female reproductive tract (FRT) is incomplete. Much of our knowledge of immunity in the FRT derives from studies using mouse models of genital HSV and LCMV infections. These studies have highlighted an important role for memory T cells, especially those resident in the tissue (TRM) as well as local antibody secretion in the FRT for clearing viral infections. In addition, clusters of memory T cells and antigen presenting cells (APCs) termed memory lymphocyte clusters (MLCs) were shown to be vital for mounting secondary immune responses and clearance of HSV vi infection. Although there have been quite a few studies of viral FRT infections, very little is known about how immunity develops in the FRT after bacterial infections. Chlamydia muridarum (Cm) infects the upper female reproductive tract and is a great model to study intracellular bacterial infection and development of FRT pathology in mice. This pathogen replicates inside host epithelial cells and requires CD4 T cells for clearance of infection, complementing studies of human Chlamydia trachomatis infections in which resistance to infection correlates with interferon gamma production. Using Cm infection in mice, we sought to determine the immune requirements for secondary protection in the FRT. We first hypothesized that antigen-independent immunity could lead to local protection against FRT Chlamydia infection. To test this theory, we developed a pet shop mouse co-housing model at UC Davis based on previous work that established this method as a useful way to generate non-specific immunity against certain pathogens. This model consisted of co-housing inbred laboratory mice with “dirty” pet shop mice, which transmitted endemic pathogens to the lab mice. The goal of this model was to better recapitulate an experienced human immune system in laboratory mice. After the cohousing period, the laboratory mice were challenged with Cm and the severity of infection was measured. Using the pet shop co-housing model, we did not detect any advantage of non-specific immunity on resistance to Chlamydia infection. Therefore, to achieve effective immunity against genital Chlamydia infection, antigen-specific immunity was likely required. The development of TRM and MLCs in the female reproductive tract has been observed after genital Cm infection much like genital HSV infection. Hence, we vii hypothesized that these immune factors were required for antigen-specific immunity to Chlamydia infection in the FRT. We tested the requirement of resident immunity for secondary protection using parabiosis surgery and found that resident immune memory was completely dispensable for local FRT protection against Chlamydia. Furthermore, resident immunity was irrelevant to secondary protection whether this immunity was generated through local intravaginal immunization or distal intranasal immunization. Experiments examining protection after intranasal immunization demonstrated that circulating immunity was completely protective against bacterial burden and the development of pathology after intravaginal rechallenge infection. CD4 T cells were required to control vaginal infection in intranasal immune mice, demonstrating that these lymphocytes were likely the main mediators of circulating immunity. The findings in this study have greatly clarified the required immune components for secondary immunity to Chlamydia infection of the female genital tract. In summary, this work advances our understanding of how immunity to a sexually transmitted bacterial pathogen develops in the female reproductive tract, a previously unaddressed gap in knowledge in the field.