Abstract
Viruses, when used as vectors for vaccine antigen delivery, can induce strong cellular and humoral responses against target epitopes. Recent work by Hansen et al. describes the use of a cytomegalovirus-vectored vaccine, which is able to generate a stable effector-memory T cell population at the sites of vaccination in rhesus macaques. This vaccine, targeted towards multiple epitopes in simian immunodeficiency virus (SIV), did not induce classical CD8(+) T cells. However, non-canonical CD8(+) T cell induction occurred via major histocompatibility complex (MHC) class II and MHC-E. The MHC-E-restricted T cells could recognize broad epitopes across the SIV peptides, and conferred protection against viral challenge to 55% of vaccinated macaques. The human homologue, human leucocyte antigen (HLA)-E, is now being targeted as a new avenue for vaccine development. In humans, HLA-E is an unusually oligomorphic class Ib MHC molecule, in comparison to highly polymorphic MHC class Ia. Whereas MHC class Ia presents peptides derived from pathogens to T cells, HLA-E classically binds defined leader peptides from class Ia MHC peptides and down-regulates NK cell cytolytic activity when presented on the cell surface. HLA-E can also restrict non-canonical CD8(+) T cells during natural infection with various pathogens, although the extent to which they are involved in pathogen control is mostly unknown. In this review, an overview is provided of HLA-E and its ability to interact with NK cells and non-canonical T cells. Also discussed are the unforeseen beneficial effects of vaccination, including trained immunity of NK cells from bacille Calmette-Guérin (BCG) vaccination, and the broad restriction of non-canonical CD8(+) T cells by cytomegalovirus (CMV)-vectored vaccines in pre-clinical trials.