Impact of Distinct Poxvirus Infections on the Specificities and Functionalities of CD4⁺T Cell Responses

Abstract

The factors that determine CD4؉ T cell (T CD4 ؉) specificities, functional capacity, and memory persistence in response to complex pathogens remain unclear. We explored these parameters in the C57BL/6 mouse through comparison of two highly related (>92% homology) poxviruses: ectromelia virus (ECTV), a natural mouse pathogen, and vaccinia virus (VACV), a heterologous virus that nevertheless elicits potent immune responses. In addition to elucidating several previously unidentified major histocompatibility complex class II (MHC-II)-restricted epitopes, we observed many qualitative and quantitative differences between the T CD4 ؉ repertoires, including responses not elicited by VACV despite complete sequence conservation. In addition, we observed functional heterogeneity between ECTV-and VACV-specific T CD4؉ at both a global and individual epitope level, particularly greater expression of the cytolytic marker CD107a from T CD4؉ following ECTV infection. Most striking were differences during the late memory phase where, in contrast to ECTV, VACV infection failed to elicit measurable epitope-specific T CD4؉ as determined by intracellular cytokine staining. These findings illustrate the strong influence of epitope-extrinsic factors on T CD4 ؉ responses and memory. IMPORTANCE Much of our understanding concerning host-pathogen relationships in the context of poxvirus infections stems from studies of VACV in mice. However, VACV is not a natural mouse pathogen, and therefore, the relevance of results obtained using this model may be limited. Here, we explored the MHC class II-restricted T CD4؉ repertoire induced by mousepox (ECTV) infection and the functional profile of the responding epitope-specific T CD4؉, comparing these results to those induced by VACV infection under matched conditions. Despite a high degree of homology between the two viruses, we observed distinct specificity and functional profiles of T CD4 ؉ responses at both acute and memory time points, with VACV-specific T CD4 ؉ memory being notably compromised. These data offer insight into the impact of epitope-extrinsic factors on the resulting T CD4 ؉ responses.

Cowpox virus encodes a protein that binds B7.1 and B7.2 and subverts T cell costimulation

“…Interestingly, M2 or M2-like ORFs are found distributed among many poxvirus genomes that are not known to downregulate MHC class I antigen presentation, including most orthopoxviruses, leporipoxviruses, yatapoxviruses, and cervidpoxviruses (4). Ectromelia virus does not appear to encode an M2 protein, and infections provoke a considerably stronger virus-specific CD4 T cell response in comparison to VACV-WR, which does have an M2 protein (43), consistent with our experiments with ΔM2 CPXV compared with WT CPXV. Various strains of VACV have been used as vaccines or vectors to treat a number of infections, as well as cancers (44,45).…”

“…For CPXV-specific CD4 T cell IFN-γ production, DC2.4 cells were loaded with the indicated peptides (SI Appendix, Fig. S6) that share 100% sequence identity with identified VACV epitopes (43,54) for 2 h at 37°C. Next, 5 × 10 6 splenocytes were added, and GolgiPlug (BD Biosciences) was added 1 h later.…”

Cytotoxic CD4 T Cells—Friend or Foe during Viral Infection?

“…Defective control of ectromelia virus was found in mice with a specific deficiency of perforin in their CD4 T cells (157). Comparisons of ectromelia virus and vaccinia virus (the active constituent of the smallpox vaccine) infection in mice showed that ectromelia virus induced significantly more CD4 CTL than vaccinia with different epitope-specific CD4 cells exhibiting different cytotoxic frequencies (158). These results reveal that not all viral infections result in the generation of CD4 CTL with the same cytolytic ability.…”

Ectromelia virus accumulates less double-stranded RNA compared to vaccinia virus in BS-C-1 cells

“…While closely related, ECTV and VACV differ greatly in host range and in vivo pathogenesis and result in distinct levels of inflammation upon infection of mice via the footpad route (Siciliano et al, 2014). Additionally, ECTV – unlike VACV – does not express a functional K3 protein (Smith and Alcamí, 2002).…”