We analyzed dengue virus-specific CD4+ CD8- cytotoxic T lymphocytes (CTL) at the clonal level to further understand their role in dengue virus infections. Stimulation of peripheral blood mononuclear cells from two dengue virus type 4 (D4V)-immune donors with live D4V or noninfectious D4V antigen generated 17 HLA class II-restricted CD4+ CTL capable of specific lysis of dengue virus antigen-treated autologous lymphoblastoid cell lines. Thirteen clones were D4V specific, three clones were cross-reactive for D2V and D4V, and one clone was cross-reactive for D1V, D3V, and D4V. Antigen recognition by six D4V-specific clones and three D2V- and D4V-cross-reactive clones was restricted by HLA-DR7. Five D4V-specific CD4+ CTL clones lysed autologous lymphoblastoid cell lines infected with a dengue virus-vaccinia virus recombinant containing the E gene of D4V, whereas three serotype-cross-reactive CTL clones did not. These results indicate that E-specific clones are serotype specific and HLA-DR7 restricted in these two donors and suggest that a common epitope on E protein may be recognized. E protein-specific CD4+ CTL may be important mediators of virus clearance especially during reinfection with the same serotype as that in primary infection by providing help for virus-specific antibody production and lysis of virus-infected cells.