Dissection of factors contributing to HIV susceptibility in mucosal associated lymphoid tissues

Abstract

The mucosal and gut associated lymphoid tissues participate in all stages of HIV infection and pathogenesis from initial transmission to viral production, storage and persistence. In the investigations performed for this thesis, we used a multiparameter approach to define the potential for HIV transmission through the epithelium of the tonsil mucosal lymphoid tissue to enable a productive infection. First, we obtained tonsil tissues from adults with and without HIV infection to characterize HIV cellular targets and immune factors which might impact on susceptibility and resistance to infection. To explore these parameters in depth, we also established an ex vivo system to study mechanisms of adherence, entry and acute HIV infection and pathogenesis in this organ. Focusing on the tonsil epithelium as a primary site of transmission, we utilized laser capture microdissection (LCM) to isolate tonsil and oral epithelia in order to compare their gene expression profiles by micro-array analysis. These analyses revealed that an upregulation of gene expression for HIV entrapment molecules and viral co-receptors, particularly CXCR4, may facilitate entry through this site, while down-regulation of anti-viral responses may also act favorably for the virus to initiate its life cycle within this tissue. Once HIV is within the tonsil, whether through a primary or secondary infection, our studies of the mechanisms involved in acute infection of the tonsil cell population ex vivo reveal that the constitutive cytokine milieu of the tonsil is permissive to HIV infection and replication. This was further documented when we were able to demonstrate that treatment of relatively-resistant peripheral blood mononuclear cells with tonsil cell secreted products also enhances their ability to become infected and that this effect is abrogated when the TH2 cytokine IL-4 is inhibited. Importantly, despite increased secretion of the anti-viral cytokines interferon-alpha/gamma (IFN) in this environment, IFN signaling as monitored by STAT-1 activation and effective anti-viral and TH1 responses appears to be impaired through HIV-mediated alterations in host cell signal transduction. Collectively, these host defense pathways and virally influenced intracellular events further enable establishment of infection. However, other factors may exert a counter influence on viral susceptibility, including a population of regulatory T cells (Treg) which are not only relatively resistant to HIV, but may be beneficial to the host given our observations that, in culture, Treg suppress proliferation and viral replication of other CD4 T cells. Thus, the balance of opposing and supportive influences on HIV adherence, entry and replication contribute to launching the tonsil as a viral haven. This identification of underlying mechanisms responsible for susceptibility and resistance to HIV at mucosal lymphoid associated sites may aid in the development of vaccines and therapeutics targeted to these areas.