Peptide Processing Is Critical for T-Cell Memory Inflation and May Be Optimized to Improve Immune Protection by CMV-Based Vaccine Vectors

Abstract

Cytomegalovirus (CMV) elicits long-term T-cell immunity of unparalleled strength, which has allowed the development of highly protective CMV-based vaccine vectors. Counterintuitively, experimental vaccines encoding a single MHC-I restricted epitope offered better immune protection than those expressing entire proteins, including the same epitope. To clarify this conundrum, we generated recombinant murine CMVs (MCMVs) encoding well-characterized MHC-I epitopes at different positions within viral genes and observed strong immune responses and protection against viruses and tumor growth when the epitopes were expressed at the protein C-terminus. We used the M45-encoded conventional epitope HGIRNASFI to dissect this phenomenon at the molecular level. A recombinant MCMV expressing HGIRNASFI on the C-terminus of M45, in contrast to wild-type MCMV, enabled peptide processing by the constitutive proteasome, direct antigen presentation, and an inflation of antigen-specific effector memory cells. Consequently, our results indicate that constitutive proteasome processing of antigenic epitopes in latently infected cells is required for robust inflationary responses. This insight allows utilizing the epitope positioning in the design of CMV-based vectors as a novel strategy for enhancing their efficacy.

The (gradual) rise of memory inflation

“…This is easily performed in the adenovirus model where minigene vectors can be readily generated. It is also nicely shown in the MCMV model by making recombinant viruses where the immunodominant M45 peptide is relocated to the C terminus of M45 43. In this case, there is inflation demonstrated against the same M45 epitope expressed in the same open reading frame.…”

“…However, they show normal inflationary responses to M38 and m139 epitopes (the subdominant IE3 epitope showed an intermediate phenotype). Further work to explore this phenomenon using bone marrow chimeric mice has been performed with similar conclusions 43, 50, 76. In mice where cross‐presentation is blocked by reconstitution of the bone marrow with cells lacking the relevant MHC or TAP, priming is impaired but inflation is not (and vice versa).…”

“…For the rhesus macaque‐based cytomegaloviruses, there are a series of additional modifications regarding immune evasion and, interestingly, cellular targeting which can have quite profound impacts on the specificities observed, some of which are unconventional (eg, HLA‐E restricted) and can have potentially potent antiviral properties 86, 87, 88. Overall—even in settings where more conventional CD8 + T‐cell responses are induced—there is evidence that these responses can provide protection against viruses and cancers 2, 43. Thus, understanding what the rules are that govern the induction and maintenance of these populations is very relevant to vaccines and so the models proposed have translational relevance.…”

Cytomegalovirus (CMV) Epitope–Specific CD4+ T Cells Are Inflated in HIV+ CMV+ Subjects

“…Potential explanations for DYS-specific inflation include the translation of gB mRNA without HCMV replication (69), gB colocalization to endosomes and endogenous presentation (28, 35), and the secretion of such gB epitope-loaded endosomes as immunogenic exosomes (36, 37). Endogenous epitope processing and presentation has been demonstrated recently to drive CD8 memory inflation (6, 27). However, the low pp65 EPD-specific responses might be due to pp65 polyprotein absence in immunogenic exosomes (37).…”

“…These epitopes are presented to CMV-specific T cells by latent HCMV-infected, non-hematopoietic reservoirs, including VECs, lymph node (LN) stroma cells, and cells in the bone marrow and lungs (1, 23–25). Maintenance of inflated CMV-specific T cell responses might also depend on their longer telomeres that positively correlate with persistence (26), or on epitope cleavage by constitutive proteasomes (6, 27). …”