Modulation of MHC-E transport by viral decoy ligands is required for RhCMV/SIV vaccine efficacy

Abstract

Strain 68-1 rhesus cytomegalovirus (RhCMV) vectors expressing simian immunodeficiency virus (SIV) antigens elicit CD8+ T cells that recognize peptide epitopes presented by major histocompatibility complex (MHC)-II and MHC-E molecules, instead of MHC-Ia, and are uniquely able to mediate stringent control and subsequent clearance of highly pathogenic SIV in ~50% of vaccinated rhesus macaques (RMs). We show that the MHC-E ligand VMAPRTLLL (VL9), encoded by the Rh67 gene (or its HCMV UL40 counterpart) is required for recognition of RhCMV-infected fibroblasts by MHC-E-restricted CD8+ T cells via its ability to promote intracellular MHC-E transport. Moreover, deletion of Rh67 from 68-1 RhCMV/SIV vectors, or mutation of its embedded VL9 ligand, abrogated induction of MHC-E-restricted CD8+ T cell responses, leaving responses that exclusively target MHC-II-restricted epitopes. These MHC-II-presented CD8+ T cell responses, though comparable in response magnitude and functional differentiation to responses arising from the efficacious 68-1 vector, did not protect RMs against SIV challenge, indicating that Rh67/UL40-enabled direct priming of MHC-E-targeted CD8+ T cells is a crucial element of RhCMV/SIV vaccine efficacy.

Cytomegaloviral determinants of CD8+T cell programming and RhCMV/SIV vaccine efficacy

“…Taken together, these data provide unequivocally evidence that RhCMV has evolved the ability to control the nature of its own recognition by CD8 + T cells -to our knowledge, the only virus with this capability, and thus the only virus-based vaccine vector that is genetically programmable with respect to CD8 + T cell epitope targeting. Importantly, the exclusively MHC-II-restricted SIV-specific CD8+ T cells responses elicited by the Rh67-deleted 68-1 RhCMV/SIV vectors, though also of similar (or higher) magnitude, longevity and effector memory differentiation as 68-1 RhCMV/SIV vector elicited responses, were, like the MHC-Iaand MHC-Ia + MHC-II-restricted CD8 + T cells elicited by 68-1.2 and 68-1.2 DRh157.5 vectors in this study, unable to provide protection against SIV challenge (21). CD4 + T cell responses are comparable across these differentially programmed RhCMV/SIV vaccines and antibody responses are largely absent in 68-1 RhCMV/SIV vaccinated RM (6)(7)(8)22), which makes the presence of MHC-E-restricted CD8 + T cell responses the only defined difference in adaptive (SIV-specific) immunity between efficacious and non-efficacious RhCMV/SIV vectors.…”

“…Although the specific immune functions mediating this efficacy in vivo remain to be elucidated (in particular, delineation of why MHC-E-restricted CD8 + T cell response are uniquely efficacious), these data provide the first documentation of a vaccine for CD8 + T cell immunity that includes programmability for unconventionally restricted CD8 + T cell responses with unique in vivo efficacy. The finding that the HCMV orthologs of the RhCMV genes that mediate CD8 + T cell response programming, including UL128/UL130, UL146/UL147 and UL40 (21), recapitulate the response programming function of their RhCMV counterparts when used to replace these counterparts in RhCMV strongly suggests the conservation of CD8 + T cell response programming in HCMV and supports the feasibility of translating this unique immunobiology to humans to create an effective HIV/AIDS vaccine. The flexible programmability of CMV vectored vaccines may also be exploitable in development of vaccines to other pathogens or to malignancies.…”

“…In a companion manuscript (21), we demonstrate that 68-1 vectors deleted for Rh67 (RhCMV ortholog of HCMV UL40), the gene product which is responsible for MHC-E upregulation in vector-infected cells, specifically fail to prime MHC-E-restricted CD8 + T cells, leaving CD8 + T cell responses which are only MHC-II-restricted, including MHC-II-restricted supertopes. Taken together, these data provide unequivocally evidence that RhCMV has evolved the ability to control the nature of its own recognition by CD8 + T cells -to our knowledge, the only virus with this capability, and thus the only virus-based vaccine vector that is genetically programmable with respect to CD8 + T cell epitope targeting.…”

Cytomegaloviral determinants of CD8 + T cell programming and RhCMV/SIV vaccine efficacy

“…In this regard, CMV is the epitome of sophisticated host modulation and immune evasion by a virus (21), and it is likely that the answer to this question lies in this host modulation/immune evasion biology. Indeed, in a companion manuscript (22), we demonstrate that 68-1 RhCMV vectors deleted for Rh67, the gene product which is responsible for intracellular transport of MHC-E in RhCMV-infected cells (UL40 in HCMV), specifically fail to prime MHC-E-restricted CD8 + T cells, leaving CD8 + T cell responses which are entirely MHC-II-restricted. Thus, MHC-E-restricted CD8 + T cell responses appear to be one consequence of intracellular transport with enhanced surface expression of MHC-E mediated by the Rh67 gene product.…”

“…To our knowledge, given the capabilities of this virus, this may the only virus-based vaccine vector system at this time that is genetically programmable with respect to CD8 + T cell epitope targeting. In the companion manuscript we document that exclusively MHC-IIrestricted SIV-specific CD8 + T cell responses, with MHC-II supertope recognition, elicited by the Rh67deleted 68-1 RhCMV/SIV vectors are unable to provide protection against SIV challenge (22). Thus, neither MHC-Ia-restricted CD8 + T cell responses, nor MHC-II-restricted CD8 + T cell responses (with or without supertopes) are able to mediate protection against SIV challenge, even though these responses are of similar magnitude and differentiation than the protective responses elicited by 68-1 RhCMV/SIV vectors.…”