Poor Antigen Processing of Poxvirus Particles Limits CD4+ T Cell Recognition and Impacts Immunogenicity of the Inactivated Vaccine

CD4 + T cells play critical roles in defending against poxviruses, both by potentiating cellular and humoral responses and by directly killing infected cells. Despite this central role, the basis for pox-specific CD4 + T cell activation, specifically the origin of the poxvirus-derived peptides (epitopes) that activate CD4 + T cells, remains poorly understood. In addition, because the current licensed poxvirus vaccines can cause serious adverse events and even death, elucidating the requirements for MHC class II (MHC-II) processing and presentation of poxviral Ags could be of great use. To address these questions, we explored the CD4 + T cell immunogenicity of ectromelia, the causative agent of mousepox. Having identified a large panel of novel epitopes via a screen of algorithm-selected synthetic peptides, we observed that immunization of mice with inactivated poxvirus primes a virtually undetectable CD4 + T cell response, even when adjuvanted, and is unable to provide protection against disease after a secondary challenge. We postulated that an important contributor to this outcome is the poor processability of whole virions for MHC-II-restricted presentation. In line with this hypothesis, we observed that whole poxvirions are very inefficiently converted into MHC-II-binding peptides by the APC as compared with subviral material. Thus, stability of the virion structure is a critical consideration in the rational design of a safe alternative to the existing live smallpox vaccine.