ACA-1, a new differentiation antigen of activated lymphoid cells.

Rabbits were immunized with murine CBA T lymphocytes activated with C57BL/6 antigens (Tact). The immune rabbit sera were adsorbed with murine erythrocytes, serum, liver cells, and unstimulated T and B lymphocytes. Upon absorption, the antisera (ATactS) were not cytotoxic for unactivated thymus, lymph node, and spleen cells of different mouse strains in the cytotoxicity assay. ATactS did not inhibit the immune response of lymphocytes from unimmunized animals to SRBC or their proliferation in mixed lymphocyte culture. At the same time ATactS lysed 67% of actively proliferating T-lymphoma EL-4 cells, 31-56% of the T lymphocytes stimulated with allogeneic cells, and 53-56% of Con A- or LPS-stimulated splenocytes. ATactS also inhibited 90-100% of antigen-activated B cells, i.e., plaque-forming cells (PFC) producing 19 S and 7 S antibodies to serologically noncrossreacting antigens, such as sheep, rabbit, and rat RBC. It also decreased the intensity of the secondary immune response to SRBC. The effect of ATactS did not depend on the H-2 or Ala-I phenotype of target cells. Our experiments showed that ATactS activity fell after absorption with activated, but not quiescent lymphoid cells. These results suggest the presence of a special antigenic marker on activated murine T and B cells which differs from the generally recognized cell surface antigens, such as H-2, Ig, Ala-I, idiotype, Thy-I, MBLA, MTLA, Lyt-1,2,3, and others. It has been designated Aca-I (activated cell antigen).