Background:  Gut-associated lymphoid tissue (GALT) is the major reservoir of lymphocytes and HIV-1 replication in vivo, yet little is known about HIV-1-specific CD8⁺ T-lymphocyte (CTL) responses in this compartment. We assessed the breadth and magnitude of HIV-1-specific CTL in the peripheral blood and sigmoid colon mucosa of infected subjects not on antiretroviral therapy.

Methods:  We studied 12 HIV-1 seropositive individuals who were not on therapy for at least 12 months, and 4 seronegative controls. Using interferon-γ (IFN-γ) ELISpot with a library of 53 peptide pools spanning all viral proteins, we analyzed the CTL responses of polyclonally expanded CD8⁺ T cells from peripheral blood mononuclear cells (PBMC) and GALT samples collected on 2 visits, 2 weeks apart.

Results:  Control assays on HIV-1-seronegative individuals yielded a false positive rate lower than 1.5%. Correlation between visits was very high, with concordance rates of 94.5%, 86.0%, and 98.6% for freshly isolated PBMC, expanded PBMC, and expanded GALT respectively. HIV-1-specific CTL in PBMC revealed a pattern of recognition with regions in Gag and Nef most frequently targeted. When we compared CTL responses of freshly isolated vs expanded CD8⁺ T cells from blood, we found that both were highly correlated across the group of infected subjects with a concordance rate of 76.1%. Expansion of CD8⁺ T cells from blood increased the sensitivity of detection (p = 0.0157). Comparison between CD8⁺ T cells expanded from blood and GALT revealed similar responses (10.2 ± 2.2 and 10.3 ± 1.8 recognized pools per person with mean magnitudes of 156 ± 3 vs 175 ± 4 SFC/10⁶ in blood and GALT respectively). In both expanded PBMC and GALT, CTL preferentially targeted Nef, followed by Gag, Vpu, and Vpr. The magnitudes of concordant responses in blood and GALT were similar, with values of 396 and 488 SFC/10⁶ respectively.

Conclusion:  This study provides a comprehensive analysis of the HIV-1 specific CTL responses in GALT, indicating that they are mirrored in peripheral blood during chronic infection. The pivotal role of this compartment as a portal of entry in acute infection and reservoir for replication in chronic infection underscores the importance of understanding the relationships between blood and gut mucosa immune responses. Elucidating such mechanisms may be important to optimizing vaccine efficacy by eliciting HIV-1-specific immunity in the GALT.

Keywords: HIV Pathogenesis; Mucosal Immunity; Immunology