A Dominant Role for the Immunoproteasome in CD8+ T Cell Responses to Murine Cytomegalovirus

Abstract

Murine cytomegalovirus (MCMV) is an important animal model of human cytomegalovirus (HCMV), a β-Herpesvirus that infects the majority of the world's population and causes disease in neonates and immunocompromised adults. CD8+ T cells are a major part of the immune response to MCMV and HCMV. Processing of peptides for presentation to CD8+ T cells may be critically dependent on the immunoproteasome, expression of which is affected by MCMV. However, the overall importance of the immunoproteasome in the generation of immunodominant peptides from MCMV is not known. We therefore examined the role of the immunoproteasome in stimulation of CD8+ T cell responses to MCMV – both conventional memory responses and those undergoing long-term expansion or “inflation”. We infected LMP7−/− and C57BL/6 mice with MCMV or with newly-generated recombinant vaccinia viruses (rVVs) encoding the immunodominant MCMV protein M45 in either full-length or epitope-only minigene form. We analysed CD8+ T cell responses using intracellular cytokine stain (ICS) and MHC Class I tetramer staining for a panel of MCMV-derived epitopes. We showed a critical role for immunoproteasome in MCMV affecting all epitopes studied. Interestingly we found that memory “inflating” epitopes demonstrate reduced immunoproteasome dependence compared to non-inflating epitopes. M45-specific responses induced by rVVs remain immunoproteasome-dependent. These results help to define a critical restriction point for CD8+ T cell epitopes in natural cytomegalovirus (CMV) infection and potentially in vaccine strategies against this and other viruses.

Proteasome assembly defect due to a proteasome subunit beta type 8 (PSMB8) mutation causes the autoinflammatory disorder, Nakajo-Nishimura syndrome

“…Studies in animal models indicate that cells deficient in various immunoproteasome subunits show poor CD8 responses when challenged with epitopes (34,35) and may display alterations in the T-cell receptor (TCR) repertoire (36). In particular, β5i-deficient mice show increased susceptibility to pathogens, most likely due to the reduced efficiency of antigen presentation by β5i-deficient cells (12).…”

Priming of CD8+ T Cells against Cytomegalovirus-Encoded Antigens Is Dominated by Cross-Presentation

“…One requirement is the sporadic expression of the Ag during latent infection, which because of the use of the major immediate-early promoter (88) seemed to be guaranteed for the Ag encoded by our reporter mutants. Another criterion is the generation of the epitope by constitutive proteasome activity, rather than by the immune proteasome (89). It will be interesting to learn whether the differences observed between the E7 and NP epitopes of our reporter mutants in inducing the memory response depends on this property.…”

The (gradual) rise of memory inflation

“…There are a few pieces of data which support this. Firstly, inflationary responses appear to target epitopes which are generated independent of LMP7—ie, immunoproteasome‐independent 72. Immunoproteasome dependence is not possible to predict exactly from sequence alone but functional experiments show consistent differences between inflationary responses in both the MCMV and the adenovirus models 16.…”