Ectromelia-encoded virulence factor C15 specifically inhibits antigen presentation to CD4+ T cells post peptide loading

Abstract

Smallpox and monkeypox pose severe threats to human health. Other orthopoxviruses are comparably virulent in their natural hosts, including ectromelia, the cause of mousepox. Disease severity is linked to an array of immunomodulatory proteins including the B22 family, which has homologs in all pathogenic orthopoxviruses but not attenuated vaccine strains. We demonstrate that the ectromelia B22 member, C15, is necessary and sufficient for selective inhibition of CD4⁺ but not CD8⁺ T cell activation by immunogenic peptide and superantigen. Inhibition is achieved not by down-regulation of surface MHC- II or co-stimulatory protein surface expression but rather by interference with antigen presentation. The appreciable outcome is interference with CD4⁺ T cell synapse formation as determined by imaging studies and lipid raft disruption. Consequently, CD4⁺ T cell activating stimulus shifts to uninfected antigen-presenting cells that have received antigen from infected cells. This work provides insight into the immunomodulatory strategies of orthopoxviruses by elucidating a mechanism for specific targeting of CD4⁺ T cell activation, reflecting the importance of this cell type in control of the virus. Orthopoxviruses pose considerable threats to their hosts by producing a battery of proteins that disable the immune system at many levels through mechanisms that remain poorly understood. An essential part of most immune responses is the activation of CD4⁺ T cells by antigen-presenting cells through formation of a supramolecular structure termed the immunological synapse. We show here that the C15 protein of ectromelia, the cause of mousepox, inhibits CD4⁺ T cell activation through a novel immunoevasion mechanism that results in disruption of synapse formation. As many poxviruses encode C15 homologs, these studies could provide insights into the virulence of other family members including monkeypox and smallpox, both of great concern to human populations.