Specificity of T-cell lines established against TT or PPD. (A) Lack of cross-reactivity among TT, PPD, and C albicans antigens. To exclude cross-reactivity among the antigens used, 2 different T-cell lines (D1-TT and D2-PPD) were generated from PBMNCs of 2 healthy donors by repeated cycles of in vitro stimulation with TT and PPD, respectively, as described in “Generation of antigen-specific T-cell lines.” Their antigen specificity was then tested in a 3-day in vitro proliferative assay. Proliferative activity was measured by ³H-thymidine incorporation and is reported as counts per minute (cpm) in triplicate cultures ± SD. (B) Identification of HLA-DR as the restricting MHC molecule of the T-cell lines. To identify the restricting HLA isotype of the T-cell lines, antigen presentation by autologous antigen-presenting cells was assayed by repeating the proliferative assay after addition of mAbs against HLA-DR (L243), HLA-DP (B7/21.2), or HLA class I (W6/32) at the beginning of the culture time. The proliferative response of the D1-TT to TT was strongly inhibited only when the HLA-DR–specific mAb L243 was added to the culture.