Background

Sexually transmitted infections (STI) remain major societal and economic burden despite public health initiatives, vaccinations and development of antibiotics (O’Connell and Ferone, 2016). There is estimated to be 357 million new infections annually and STIs have been identified by the WHO as a global priority for elimination (WHO Global Health Sector Strategy on Sexually Transmitted Infections 2016-2021). Among the various bacterial STI, Chlamydia trachomatis is the most common STI (Starnbach, 2018) and stands first with a 131 million new infections annually worldwide (Poston et al., 2017). High reinfection in the genital tract is exacerbated by chronic pelvic pain, pelvic inflammatory disease, and fallopian tube scaring (O’Connell and Ferone, 2016) resulting in infertility and other malignancies. The major hurdle in controlling chlamydial spread is that the infection remains asymptomatic, thus leading to chronic, recurrent and persistent infections, with no vaccines developed so far.

Chlamydia undergoes a unique, biphasic developmental cycle that generally modulates between two morphological forms. Extracellular, infectious elementary bodies (EBs) attach to host cells within 15 min of infection, after which they are internalized into a membrane bound vesicle called an inclusion. EBs then differentiate into metabolically active, non-infectious, reticulate bodies (RBs) in 12 h that undergo binary fission, followed by secondary differentiation into EBs after 36 h post infection (hpi). Within/after 48 hpi most of the bacteria is in the EB form and are released upon host cell lysis or extrusion (Abdelrahman and Belland, 2005).

Chlamydia trachomatis being an obligate human pathogen lacks an in vivo model. To evaluate specific vaccine antigens that can be used in humans, and to study the host pathogen interaction to understand the pathogenicity of Chlamydia infection, a suitable in vivo model need to be established (De Clercq et al., 2013). Unlike in Chlamydia muridarium, a natural pathogen in mouse, intravaginal inoculation of the bacteria only leads to a mild genital infection and does not cause ascending infection or tubal pathology. The innate immune response in the lower genital tract of the mouse can rapidly eliminate human chlamydial organism (Sturdevant and Caldwell, 2014). Thus a higher number of infectious particles are required to establish infection, which yields a 2 log units lower bacterial load in the uterine horns (Darville et al., 1997; Ramsey et al., 2000). To address this limitation an intra-bursal mode of infection was established (Carmichael et al., 2013), via a survival surgery and directly inoculating the bacteria into the bursal region. This method bypasses both the cervical barrier and endometrial lumen, thus do not resemble the natural path of vaginal infection (see Figure 1).

The primary site of Chlamydia trachomatis infection is the cervical epithelium (Buckner et al., 2016). Subsequently, the infection ascends to the upper genital tract tissues, the uterine horns and oviducts, which frequently lead to hydrosalpinx, fibrosis, and infertility, which are also common post-infection sequelae in women (Morrison and Caldwell, 2002; Shah et al., 2005). Mice have a bicornuate uterus with two lateral horns and are lined with ligaments carrying blood and lymphatic vessels. The cervix has cranial, fundal and caudal segments. The caudal segment or cervix consists of a single cavity that protrudes into the vaginal opening. To establish an ascending infection, which closely resembles infection in human uterus and fallopian tube, we need to bypass the cervix and inoculate the pathogen into the uterine fundus. This leads to an easy establishment of ascending infection in mouse. This method has been made used in various study (Gondek et al., 2012; Pal et al., 2018; Rajeeve et al., 2018).