Establishment and functional characterization of human herpesvirus 6-specific CD4+ human T-cell clones

In order to clarify the protective immune responses against a newly identified herpesvirus, human herpesvirus 6 (HHV-6), we established HHV-6-specific human T-cell clones and examined their functional properties. Five CD3+CD4+CD8- T-cell clones, which proliferated in response to stimulation with two different strains of HHV-6 in the presence of autologous antigen-presenting cells but not with herpes simplex virus type 1 or human cytomegalovirus, were established from peripheral blood lymphocytes of a healthy individual. The proliferative response of all T-cell clones to HHV-6 antigen was inhibited by addition of anti-HLA-DR monoclonal antibody, indicating that these clones were human leukocyte antigen (HLA) class II DR restricted. Of the five clones, two lysed HHV-6-infected autologous lymphoblasts, but not HHV-6-infected allogeneic cells or natural killer-sensitive K562 cells (group 1); one showed cytotoxicity against HHV-6-infected autologous lymphoblasts as well as HHV-6-infected allogeneic cells and K562 cells (group 2); and the remaining two showed no cytotoxic activity (group 3). The cytotoxic activity of group 1 was inhibited by addition of anti-HLA-DR monoclonal antibody to the culture, whereas this monoclonal antibody had no effect on the cytotoxicity of group 2 and did not induce the cytotoxicity of group 3. Perforin, which is one of the mediators of cytotoxicity, was abundantly expressed in group 1 and 2 clones. Moreover, all groups of clones produced gamma interferon after culture with antigen-presenting cells followed by HHV-6 antigen stimulation. These results suggest that HHV-6-specific CD4+ T cells have heterogeneous functions.