Myeloid cell tropism enables MHC-E-restricted CD8⁺ T cell priming and vaccine efficacy by the RhCMV/SIV vaccine

The strain 68-1 rhesus cytomegalovirus (RhCMV)-based vaccine for simian immunodeficiency virus (SIV) can stringently protect rhesus macaques (RMs) from SIV challenge by arresting viral replication early in primary infection. This vaccine elicits unconventional SIV-specific CD8⁺ T cells that recognize epitopes presented by major histocompatibility complex (MHC)-II and MHC-E instead of MHC-Ia. Although RhCMV/SIV vaccines based on strains that only elicit MHC-II- and/or MHC-Ia-restricted CD8⁺ T cells do not protect against SIV, it remains unclear whether MHC-E-restricted T cells are directly responsible for protection and whether these responses can be separated from the MHC-II-restricted component. Using host microRNA (miR)-mediated vector tropism restriction, we show that the priming of MHC-II and MHC-E epitope-targeted responses depended on vector infection of different nonoverlapping cell types in RMs. Selective inhibition of RhCMV infection in myeloid cells with miR-142-mediated tropism restriction eliminated MHC-E epitope-targeted CD8⁺ T cell priming, yielding an exclusively MHC-II epitope-targeted response. Inhibition with the endothelial cell-selective miR-126 eliminated MHC-II epitope-targeted CD8⁺ T cell priming, yielding an exclusively MHC-E epitope-targeted response. Dual miR-142 + miR-126-mediated tropism restriction reverted CD8⁺ T cell responses back to conventional MHC-Ia epitope targeting. Although the magnitude and differentiation state of these CD8⁺ T cell responses were generally similar, only the vectors programmed to elicit MHC-E-restricted CD8⁺ T cell responses provided protection against SIV challenge, directly demonstrating the essential role of these responses in RhCMV/SIV vaccine efficacy.