The impact of macrophage inflammatory protein-3 alpha and other innate immune markers on susceptibility/resistance to HIV infection in the female genital tract mucosa using cellular and ex vivo tissue models

The distinctive feature of the Human Immunodeficiency Virus (HIV) epidemic in the 21st century is the burden it places on women. Scientists believe that the best opportunities for successful interventions to prevent sexual HIV transmission lie in the initial stages of infection at the portal of entry, the genital tract (GT), which offers the greatest host advantages and viral vulnerabilities. However, understanding of the correlates of protection/vulnerability and innate immunity at the portal of entry is poor. First and foremost, there is no agreement about which GT sub-compartment is the primary site of HIV/SIV infection. Second, the epithelium, previously studied solely for its function as a barrier, has hardly been investigated for its role in innate immunity in the context of SIV/HIV infection. MIP-3α, a chemokine secreted by epithelial cells, was previously proposed to have a role in amplifying the early Simian Immunodeficiency Virus (SIV) infection events in the GT of female macaques. Specifically, MIP-3α was shown to be secreted by epithelial cells of the endocervix, accumulating subepithelially within the first 24 hours post exposure, following deposition of an intravaginal inoculum of SIV. Similar studies in humans have not been reported. We hence undertook to study MIP-3α for its role in early HIV infection events in the endocervix of humans. In order to achieve this, we first characterised MIP-3α constitutive secretion patterns in different sub-compartments of the GT before proceeding to determine its induced secretion patterns, stimulating with HIV-1 and various Toll-like receptor ligands. For completeness we determined constitutive and induced secretion patterns of multiple soluble proteins (SPs) and antimicrobial peptides (AMPs) in the endocervices of humans and macaques. The GT being an immunohormonal system, we further studied the influence of endogenous hormonal changes on the stability of MIP-3α and that of other innate immune markers. We quantified MIP-3α with a sandwich Elisa, and SPs and AMPs with the Luminex multiplex bead assay. Our results showed that the GT is a rich source of MIP-3α with its levels being among those of the highest SPs in the GT. Constitutive levels were highest in the endocervical sub-compartment of all the sub-compartments studied. Further, the GT is an inflammatory environment, which would explain the high levels of MIP-3α. The primary driver of MIP-3α levels appears to be inflammation rather than hormonal levels. MIP-3α levels are significantly higher in the GT of humans than in macaques. There was no evidence that MIP-3α levels are elevated on exposure to HIV and SIV in humans and macaques, respectively. We therefore concluded that since the endocervix is unlikely to respond to HIV/SIV by secreting MIP-3α in vivo, contrary to the previous reports, MIP-3α is hence not a key player in amplifying early events in infection. And as such, it should not be a prime target for preventive therapy. Further, the human GT having a pre-existing inflammatory profile may explain the high rates of HIV sexual transmission. Lastly, we concluded that the infection mechanisms described in the macaque model (i.e. the 'outside-in' signaling) are likely not required for human infection.