(B10 X B10.D2)F1 mice were immunized with B10.A(5R) concanavalin A-stimulated thymocyte blasts. The genetic disparity between donor and recipient was restricted to the I-J and I-E subregions of the murine major histocompatibility (H-2) complex. A high-titered, T-cell-specific anti I-JkEk serum was obtained. The antiserum lysed 27-30% of haplotype k, q, or s lymph node cells, 5.3 +/- 2% of haplotype k spleen cells, and did not lyse thymocytes. Nylon wool-passed lymph node or spleen cells (H-2k) showed considerable reactivity with anti-I-JkEk serum (35-40% lysis); anti-Thy1.2 plus complement-treated spleen cells did not react (less than 5% lysis). I-Ek antibody was detected by B10.A(3R) lymph node cell reactivity (20% lysis), whereas reaction with H-2k lymph node cells after B10.A(3R) absorption demonstrated IJk antibody (12% lysis). Lymphocyte activation with alloantigen or mitogen led to increased anti-I-JkEk serum reactivity. These results, showing antibody production to at least two T-cell Ia antigenic determinants by concanavalin A thmocyte blast immunization, suggest that a group of I-region-encoded T-cell specificities may not have been detected using conventional immunization protocols because they would not have comprised a major antigenic component of the immunizing cell population. The existence of multiple Ia antigenic determinants unique to T lymphocytes would have important implications for serological and functional studies of T-cell subpopulations.

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