Constitutive immunoproteasome processing of cytomegalovirus antigenic peptides determines their capacity for memory inflation and immune protection

Abstract

Cytomegalovirus antigenic epitopes are marked by a stark duality of outcomes - some induce conventional responses, marked by immune contraction and maintenance of central memory cells; other induce a life-long inflation of antigen-specific CD8 T-cells with effector phenotypes. Similarly, only some epitopes induce protective responses. We set to identify the role of proteasomal processing in this phenomenon. We developed mouse cytomegalovirus (MCMV) mutants expressing the same antigenic epitope within the same viral gene, but in a position that is amenable to processing by the constitutive proteasome or in a position that can be processed exclusively by the immunoproteasome. We measured the capacity of T cells to recognize the antigenic epitope in co-culture with virus infected cells in vitro, or to respond to infection in vivo. Immune monitoring of LMP7-deficient mice was used to define the role of immunoproteasome processing in epitope specific responses. While both epitope localizations resulted in robust immune responses in wild-type mice, epitope positioning on the C-terminus of the same viral gene resulted in responses that were maintained in LMP7-deficient mice. The LMP7-independent responses showed an inflationary kinetics, both in wild-type and in LMP7-deficient mice. On the other hand, the LMP7-dependent response was marked by immune contraction and cells assuming a central memory phenotype. LMP7-independent responses depended on epitope processing by the proteasome, because MG132 blocked epitope recognition by cognate CD8 T cells in co-culture assays. Strikingly, only the immunoproteasome epitope provided a valid target for CD8 T cells in co-culture assays. Epitope processing by the constitutive proteasome might be a requirement for robust inflationary CD8 responses in vivo and for the ability of CD8 T cells to recognize the antigenic target in the context of virus infection and provide immune porotection. The link between memory inflation and immune protection will be discussed.