The 6C3 monoclonal antibody specifically recognizes CD249 which is also known as, 6C3/BP-1, Enpep, or Ly-51. CD249 is a glycoprotein cell-surface differentiation antigen, which was originally identified on pre-B cell lymphomas (spontaneous and chemical- or retrovirus-transformed, in vitro and in vivo). CD249 is a homodimeric cell-surface glycoprotein with 140-kDa subunits that has been identified to possess aminopeptidase A (APA) activity. The same antigen is expressed at high levels on bone marrow stromal cell lines which support in vitro B lymphopoiesis, on thymic dendritic cells and cortical epithelial cells, and on a wide variety of mouse and rat tissues known to possess APA activity. Subsets of normal bone marrow pre-B and B lymphocytes express low levels of CD249 which is rapidly up-regulated on the pre-B cells in the presence of IL-7. A role for the CD249 molecule in the IL-7-driven proliferation of B cell precursors has been postulated. However, B-cell abnormalities were not detected in CD249/Ly-51-deficient mice. Mature B lymphocytes, thymocytes, peripheral T lymphocytes, erythroid cells, and myeloid cells (with the exception of thymic dendritic cells) do not express CD249. The 6C3 antibody can be used to identify cortical epithelium in frozen sections of thymuses from normal, SCID, and TCR-transgenic mice. It is possible that the low level of CD249 antigen detected, by flow cytometry, on some thymocytes may be passively adsorbed from adjacent epithelial cells during preparation of the cell suspensions.
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