21.1.1, a novel activation marker of T and B cells.

A new rat mAb designated mAb 21.1.1 was raised against a T cell hybridoma of mouse origin, T2D4. This antibody, an IgG2b, immunoprecipitates from the membrane extracts of iodinated T2D4 cells a 56-kDa glycoprotein of apparent pI 4.6 which gives a 34-kDa polypeptide after treatment with endoglycosidase F. MAb 21.1.1 reacts with an antigen expressed on murine mitogen-activated thymocytes and T cells, and on B cells stimulated by anti-IgM antibodies. Cells isolated from the spleen, lymph nodes and bone marrow are negative, as are purified resting B cells or T cells. This antigen is strongly expressed on most day-16 fetal thymocytes whereas adult thymocytes are almost negative. mAb 21.1.1 may be useful for the study of activation and differentiation of T and B cells.