Cytotoxic Molecule Expression and Ex Vivo Cytotoxicity of Rectal Mucosal CD8+ T-cells in HIV-1 Infection

We previously demonstrated that HIV-specific CD8+ T-cell responses are detected in gastrointestinal mucosa, and reflect clinical status. Intriguingly, however, the majority of rectal CD8⁺ T-cells do not express perforin, an effector molecule essential for killing infected cells. To elucidate effector functions of gastrointestinal CD8⁺ T-cells, we examined expression of perforin; granzymes (Grz) A, B, K; CD107a; and T-bet in rectal tissue and blood from HIV-infected and healthy adults. HIV-specific T-cells were stimulated with peptide pools or control stimuli; a redirected lysis assay was used to test cytotoxicity. Perforin production in response to TCR stimulation was reduced in mucosa compared to blood, as was expression of T-bet. Ex vivo killing was significantly reduced in rectal CD8+ cells compared to blood, regardless of HIV status or disease progression category. Although rectal CD8+ T-cells had predominantly ‘effector’; ‘effector memory’; or ‘transitional memory’ phenotypes, perforin expression within all subsets was reduced compared to blood. However, expression of cytotoxic effector proteins was elevated in HIV-infected subjects relative to controls. In summary, CD8⁺ T-cells in rectal mucosa expressed lower levels of perforin and GrzB and had reduced cytotoxic capacity relative to their counterparts in blood. These differences were not due to reduced frequencies of effector T-cells in mucosa, but may be related to lower expression of T-bet in rectal CD8+ T-cells. These data suggest that non-cytolytic effector functions rather than direct cytotoxicity may be the major role of CD8⁺ T-cells in rectal mucosa, and that antigen load plays a role in driving cytotoxic molecule expression.