The
nucleocapsid protein (N) of morbilliviruses is not only a major structural
protein but also the most abundant
protein made in infected cells. We overexpressed the N
proteins of Rinderpest virus and Peste des petits ruminants virus in E. coli, which assemble into nucleocapsids in the absence of
viral RNA that resemble nucleocapsids made in the virus-infected cells. Employing these assembled structures resembling subviral particles, we studied the induction of both the antibody response and the cytotoxic T-lymphocyte (CTL) response in a murine model (BALB/c). A single dose of the purified recombinant nucleocapsids of both viruses in the absence of an adjuvant induces a strong CTL response. The CTLs generated are
antigen specific and cross-reactive with respect to each virus and, furthermore, this CTL response is MHC class I restricted. Based on the prediction for H-2(d)-restricted T-cell motifs we tested the lysis of transfected P815 (H-2(d)) cells expressing a nine
amino acid potential CTL
epitope, by splenic T cells in vitro restimulated with bacterially expressed RPV or PPRV N
proteins. We extended our study to the bovine system both to analyze the immunogenicity of these
recombinant proteins in the natural hosts and to show that PBMC from cattle vaccinated with
Rinderpest vaccine proliferate in vitro, in response to restimulation with soluble
nucleocapsid proteins. Furthermore, the murine CTL
epitope functions in the bovine system as a cytotoxic T-cell
epitope. This sequence, which is conserved in the N
proteins of morbilliviruses, conforms well to the predicted algorithm for some of the most common BoLA CTL antigenic
peptides.