Characterisation of CD8 T cells in mucosa associated lymphoid tissue : Implications for immune control of HIV-1 infection

The role of CD8 T cells has been implicated in the partial immune control of HIV-1 infection. Ultimately these cells fail to control the infection and immune deficiency ensues in the absence of antiretroviral therapy. A primary focus of this thesis is the function of CD8 T cells in mucosa associated lymphoid tissue (MALT), a major site of HIV-1 replication, in particular in acute infection. In papers I and II, the expression of perforin in gut associated lymphoid tissue (GALT) was examined as an indicator of the cytotoxic CD8 T cell response during chronic HIV-1 infection and acute SIV infection. Considering the large size of the GALT, and the great number of potential CD4+ target cells for HIV-1, the contribution of CD8 T cells to control of viral replication at this site may be especially important. In paper I, it was found that the vast majority of CD8 T cells in rectal tissue, including HIV-1-specific cells, fail to express perforin at the protein level while perforin mRNA can be detected. However, rectal CD8 T cells express granzyme A, and are also capable of releasing IFN-γ upon stimulation with cognate peptide. The relative absence of lytic effector CD8 T cells in GALT may serve to protect the integrity of rectal mucosa under normal conditions, but might also provide an advantage to HIV-1 and other sexually transmitted viruses. For the examination of perforin expression at well-defined early time points post-infection, experimental SIV infection in rhesus macaques was studied (paper II). Significant increases in perforin protein and mRNA expression in the colon of macaques in acute SIV infection were observed. However, during chronic infection, despite ongoing viral replication, perforin expression returned to levels similar to those detected in naive animals. These findings demonstrate the presence of a robust perforin-positive response in GALT CD8 T cells during acute, but not chronic, SIV infection. In paper III, a subset of human CD8 T cells was described that expresses the chemokine receptor CXCR5, which enables chemotaxis in response to CXCL13, produced by stromal cells within B cell follicles. CXCR5+ CD8 T cells are enriched in tonsil and found scattered in the follicular mantle and dark zone of B cell follicles. The early effector memory phenotype of these cells suggests that this subset is well equipped to influence B cell differentiation through direct cell-cell contact and through the production of soluble cytokines. In line with this, CXCR5+ CD8 T cells support the survival of tonsil B cells in vitro. This suggests a novel role for MHC class I-restricted CD8 T cells and extends their repertoire of functions. In paper IV, the expression of CD7 on CD8 T cells in HIV-1 infected adults and children was investigated to determine the disease relevance of effector and memory cells distinguished by the level of CD7 expression. It was found that CD8 T cells from HIV-1 infected patients display profound down-modulation of CD7 expression as compared to healthy subjects, and there was expansion of both CD7low and CD7negative effector subsets, in particular in patients with rapid disease progression. Down-modulation of CD7 on CD8 T cells correlates directly with HIV-1 load, and cells specific for HIV-1, EBV and CMV are predominantly CD7low effector cells. Furthermore, recovery of CD4 counts on antiretroviral treatment is associated with reversion of the skewed CD7 profile in CD8 T cells. Thus, effector CD8 T cell subsets distinguished by lowered CD7 expression expand in a manner that correlates with the magnitude of antigenic challenge, and contract in response to successful antiretroviral treatment. In summary, the work presented in this thesis contributes to the understanding of CD8 T cells in MALT and peripheral blood in health, as well as in HIV-1 disease.