Defining Homeostatic and Inflammatory Mononuclear Phagocytes in Human Blood and Tissue and Investigating Their Role in HIV Acquisition

Abstract

Tissue mononuclear phagocytes (MNPs) form the first line of cellular defence against pathogens breaching skin or mucosal barriers. As antigen-presenting cells, they sample and deliver antigens to CD4 T cells. These tissues contain Langerhans cells, dendritic cells (DCs), macrophages and monocytes. In HIV infection, this system is exploited: HIV accesses and destroys CD4 T cells, driving progression to AIDS. Genital inflammation is a major risk factor for HIV acquisition and can undermine pre-exposure prophylaxis (PrEP). Sexually transmitted infections and microbiota dysbiosis drive inflammation. Increased HIV susceptibility likely stems from impaired epithelial integrity, altered MNP phenotypes and recruitment of inflammatory HIV-target cells. Yet the role of inflammatory MNP subsets in early HIV capture remains poorly defined. Using human genital tissues and optimised digestion protocols, this thesis maps the MNP landscape of genital skin and mucosa under homeostasis and inflammation. We examined plasmacytoid DCs (pDCs), CD123⁺ Axl⁺ Siglec-6⁺ (AS)DCs and CD11c⁺ ASDCs. pDCs were confirmed as major type I interferon producers, while ASDCs were potent T cell activators capable of binding and transferring HIV. Notably, we identified ASDCs in genital tissue for the first time. We developed a 26-parameter flow cytometry panel to compare MNP populations across genital sites and assess HIV uptake. MNP composition varied across foreskin, labia, vaginal and cervical tissues. In vaginal mucosa: 1) cDC1s had phenotypes distinct from skin and intestine, 2) a transitional macrophage population bridged monocyte-derived and tissue-resident macrophages, and 3) CD14⁺ epithelial MNPs were abundant. Inflammation induced influxes of MDMs and Langerin⁺ cDC2s and broad marker shifts. Macrophages showed the highest HIV-binding efficiency. Defining inflammatory MNP populations supports development of PrEP strategies effective in inflamed mucosa and efforts to reduce HIV transmission.