A novel marker of murine bone marrow hematopoietic stem cells that is expressed on peripheral T cells and is associated with a functionally important molecule on activated cytotoxic T lymphocytes.

A MAb (R2/60) has been isolated that defines a novel lymphocyte marker of murine T cells. The determinant recognized by MAb R2/60 is present on a subset of bone marrow (BM) hematopoietic stem cells, on adult thymocytes, on peripheral T cells (both resting and activated), and on murine T cell tumor lines, although it is not expressed on mature B cells. In immunoprecipitation studies using radiolabeled membrane lysates from adult thymocytes, MAb R2/60 precipitated a 44-kDa membrane-bound dimer. Functionally, MAb R2/60 mediated antigen-independent cell lysis by activated CTLs, and by CTL clones, when bridged to Fc receptor-bearing target cells; however, binding of MAb R2/60 to effector cells prior to cytotoxic assays did not inhibit target cell lysis by CTLs, suggesting that the R2/60 determinant is involved in transmembrane signaling to already activated CTLs, but that it is not involved in target cell adhesion or antigen recognition. Moreover, direct stimulation of T cells by MAb R2/60 in the absence of additional stimuli did not induce cell proliferation, further implying that the R2/60 determinant is functionally involved in the effector rather than the inductive phase of the T cell response.