Identification of an epitope recognized by CD4+ Th clone A6. (A) Determination of HER2_16-30 recognition by clone A6. A series of synthetic peptides were added together with irradiated BALB/c spleen cells and clone A6. Incorporation of ³H-thymidine was examined after a 3-day incubation. A 15-aa peptide, HER2_16-30, was defined as the minimal T cell epitope. (B) MHC class II restriction of T cell clone A6 recognition. Clone A6 cells were co-cultured with irradiated BALB/c spleen cells and HER2_16-30 in the presence of various Abs and proliferation assays performed. Clone A6 recognition of HER2_16-30 was specifically blocked by anti-CD4 and anti-I-A^d mAb. (C) IFN-gamma production by clone A6. Culture supernatants of clone A6 cells stimulated by HER2_16-30 were tested for IFN-gamma and IL-4 release. Clone A6 is similar to Th1 type with regard to production of IFN-gamma, but not IL-4.