Characterization of the humoral response to Orf virusvectored immunization

ABSTRACT

Recombinant Orf virus (rORFV) based vectors are under clinical development for COVID-19 vaccination. Little is known, however, about the cellular correlates of antibody responses to this poxviral vector platform. To monitor antigen-specific B cell responses to vaccination, we adoptively transferred to mice indicator populations of monoclonal B cells recognizing the glycoproteins (GPs) of either vesicular stomatitis virus or lymphocytic choriomeningitis virus and epitope variants thereof. Immunizations of mice with rORFV expressing the respective GPs stimulated the transferred B cells to engage in a protracted germinal center (GC) response, which was maintained longer-term when the delivered antigen was of lower affinity. GPspecific CD8 and CD4 T cells responses were also induced, and the latter included T follicular helper cells (Tfh). These T cell responses contracted over time but re-expanded upon homologous rORFV booster vaccination, alongside with an augmentation in antigen-specific memory B cells. Pre-existing rORFV-specific anti-vector immunity suppressed CD8 T cell responses to ORFV-vectored cargo whereas CD4 T cell and B cell responses were unaffected. Importantly, rORFV-based vaccination conferred long-term antibodymediated protection against VSV challenge. This study demonstrates the versatility of rORFV-vectored vaccination including its capacity to induce substantial GC B cell as well as Tfh responses. Limited interference by anti-vector immunity should facilitate the challenging task of maintaining protective antibody immunity by prime - boost vaccination.