Helper CD4+ T cells presumably play a major role in controlling cytomegalovirus (CMV) by providing help to specific B and CD8+ cytotoxic T cells, as well as through cytotoxicity-mediated mechanisms. Since CMV glycoprotein B (gB) is a major candidate for a subunit vaccine against CMV, we searched for gB-epitopes presented by human leukocyte antigen (HLA)-class II molecules.
Dendritic cells obtained from CMV-seropositive donors were loaded with a recombinant gB and co-cultured with autologous CD4+ T cells. Microcultures that specifically recognized gB were cloned by limiting dilution using autologous Epstein-Barr virus (EBV)-immortalized B cells pulsed with gB as antigen-presenting cells. To pinpoint precisely the region encoding the natural epitope recognized by a given CD4+ clone, we assessed the recognition of recombinant Escherichia coli expressing gB-overlapping polypeptides after their processing by autologous EBV-B cells.
We isolated several gB-specific CD4+ T-cell clones directed against peptides gB190-204, gB396-410, gB22-36 and gB598-617 presented by HLA-DR7, HLA-DP10 and HLA-DP2. While their precise role in controlling CMV infection remains to be established, gB-specific CD4+ T cells are likely to act by directly targeting infected HLA-class II cells in vivo, as suggested by their recognition of EBV-B cells infected by the Towne CMV strain.
The characterization of such gB-epitopes presented by HLA-class II should help to understand the contribution of CD4+ T-cell responses to CMV and may be of importance both in designing a vaccine against CMV infection and in immunomonitoring of subjects immunized with recombinant gB or with vectors encoding gB.