Vaccine-induced mucosal immune responses may be critical for protection against HIV infection, but may also result in short or long-term changes that enhance susceptibility to infection in some individuals, such as those with baseline seroreactivity to vaccine vector antigens. We examined cellular immune responses in blood and gut mucosal tissue roughly two years following vaccination with placebo or the Step study vaccine MRKAd5/HIV-1.
Participants vaccinated with either placebo or MRKAd5/HIV-1 during participation in HVTN 071, and HVTN 502/Merck 023 underwent phlebotomy and colonic mucosal biopsies via flexible sigmoidoscopy at two timepoints roughly six months apart. After isolation of mononuclear cells, we compared cellular phenotypes and intracellular cytokine responses in vaccine and placebo recipients with and without baseline serological reactivity to Ad5.
Surface expression of activation and gut-homing markers were elevated on CD4 + and CD8 + gut mucosal mononuclear cells (GMMC) in comparison with PBMC (p < 0.01), but were not significantly affected by baseline Ad5 serostatus or receipt of MRKAd5/HIV-1. ICS responses to stimulation with vaccine antigens were of low frequency and magnitude. Ad5 vector responses were seen in vaccinees and baseline seropositive individuals. CD4 + responses to vector antigens were more common in GMMC than PBMC (p < 0.01) and CD8 + responses to HIV gag insert antigens were more frequent in Ad5 seropositive than Ad5 seronegative individuals (p = 0.03).
Vaccination with the Ad5 vectored candidate HIV vaccine MRKAd5/HIV-1 does not lead to long-term changes in the activation state of mucosal CD4 + or CD8 + T lymphocytes regardless of baseline Ad5 serostatus. The findings of this study do not reveal a basis for enhanced susceptibility to HIV infection two years post vaccination.

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