Poxviruses encode a broad array of proteins that serve to undermine host immune defenses. Structural analysis of four of these seemingly unrelated proteins revealed the recurrent use of a conserved beta-sandwich fold that has not been observed in any eukaryotic or prokaryotic protein. Herein we propose to call this unique structural scaffolding the PIE (Poxvirus Immune Evasion) domain. PIE domain containing proteins are abundant in chordopoxvirinae, with our analysis identifying 20 likely PIE subfamilies among 33 representative genomes spanning 7 genera. For example, cowpox strain Brighton Red appears to encode 10 different PIEs: vCCI, A41, C8, M2, T4 (CPVX203), and the SECRET proteins CrmB, CrmD, SCP-1, SCP-2, and SCP-3. Characterized PIE proteins all appear to be nonessential for virus replication, and all contain signal peptides for targeting to the secretory pathway. The PIE subfamilies differ primarily in the number, size, and location of structural embellishments to the beta-sandwich core that confer unique functional specificities. Reported ligands include chemokines, GM-CSF, IL-2, MHC class I, and glycosaminoglycans. We expect that the list of ligands and receptors engaged by the PIE domain will grow as we come to better understand how this versatile structural architecture can be tailored to manipulate host responses to infection.

Viruses have evolved mechanisms of MHCI inhibition in order to evade recognition by cytotoxic CD8+ T cells (CTLs), which is well-illustrated by our prior studies on cowpox virus (CPXV) that encodes potent MHCI inhibitors. Deletion of CPXV viral MHCI inhibitors markedly attenuated in vivo infection due to effects on CTL effector function, not priming. However, the CTL response to CPXV in C57BL/6 mice is dominated by a single peptide antigen presented by H-2Kb. Here we evaluated the effect of viral MHCI inhibition on immunodominant (IDE) and subdominant epitopes (SDE) as this has not been thoroughly examined. We found that cross-priming, but not cross-dressing, is the main mechanism driving IDE and SDE CTL responses following CPXV infection. Secretion of the immunodominant antigen was not required for immunodominance. Instead, immunodominance was caused by CTL interference, known as immunodomination. Both immunodomination and cross-priming of SDEs were not affected by MHCI inhibition. SDE-specific CTLs were also capable of exerting immunodomination during primary and secondary responses, which was in part dependent on antigen abundance. Furthermore, CTL responses directed solely against SDEs protected against lethal CPXV infection, but only in the absence of the CPXV MHCI inhibitors. Thus, both SDE and IDE responses can contribute to protective immunity against poxviruses, implying that these principles apply to poxvirus-based vaccines.

Costimulation is required for optimal T cell activation, yet it is unclear whether poxviruses dedicatedly subvert costimulation during infection. Here, we report that the secreted M2 protein encoded by cowpox virus (CPXV) specifically interacts with human and murine B7.1 (CD80) and B7.2 (CD86). We also show that M2 competes with CD28 and CTLA4 for binding to cell surface B7 ligands, with stronger efficacy against CD28. Functionally, recombinant M2 and culture supernatants from wild-type (WT) but not M2-deficient (∆M2) CPXV-infected cells can potently suppress B7 ligand-mediated T cell proliferation and interleukin-2 (IL-2) production. Furthermore, we observed increased antiviral CD4 and CD8 T cell responses in C57BL/6 mice challenged by ∆M2 CPXV compared with WT virus. These differences in immune responses to ∆M2 and WT CPXV were not observed in CD28-deficient mice. Taken together, our findings define a mechanism of viral sabotage of T cell activation that highlights the role of CD28 costimulation in host defense against poxvirus infections.

Poxviruses encode a broad array of proteins that serve to undermine host immune defenses. Our group has been investigating a large sequence-diverse family of secreted poxvirus proteins that appear to share a conserved beta-sandwich fold, but differ in their binding specificities. Host proteins discovered to interact with this viral protein family include: chemokines, GM-CSF, IL-2, MHC class I, and GAGs. We have termed members of this superfamily; Poxvirus Immune Evasion (PIE) proteins, and there appears to be at least 20 distinct subfamilies. As it turns out, cowpox virus (CPXV BR) contains 10 PIE proteins, one of which is secreted from virally infected cells and specifically binds B7.1 (CD80) and B7.2 (CD86), two membrane proteins expressed on professional antigen presenting cells and activated T cells. We have termed this viral protein B7 Response Modifier (BRM) and demonstrated that it competes with the CD28 as well as CTLA4 receptors expressed on T-cells for binding to B7 proteins. However, at low concentration, BRM competes primarily with CD28 for binding to B7.2, consistent with the idea that cowpox uses BRM to undermine T cell activation but not checkpoint control. Quantitative interaction analysis reveals that BRM binds the ectodomain of B7 proteins with higher affinity than that reported for CD28 or CTLA-4. Functionally, BRM can potently disrupt T-cell proliferation and IL2 production co-stimulated by B7 proteins. Furthermore, supernatants of wild-type but not BRM deleted CPXV are capable of suppressing B7.2-mediated T-cell activation. In sum, our findings define a novel mechanism of viral immune evasion that highlights the role of CD28 co-stimulation in host defense of poxvirus infections.

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