A signalling accessory molecule revealed by a new anti-fibroblastoid L cell monoclonal antibody

Fibroblastoid mouse L-cells are widely used in immunological models because when transfected with class II-coding genes they become efficient antigen presenting cells. Little is known, however, about the cell surface markers borne by L-cells and their putative involvement/Interference with the experimental models studied. Rats were immunized against DAP.3 cells (subclone of L-cells) and monoclonal antibodies (mAbs) were prepared. One of them, 4D4, was studied in detail. It recognizes an epitope which is neither cell lineage- nor strain- nor species-restricted since, in addition to DAP.3 cells, it binds, as determined by flow cytometry and immunohistochemistry, to various cells such as CD8+ T cells from thymus, spleen, lymph node or intestinal epithelium, mouse peritoneal B cells and various tissues such as renal, pulmonary or intestinal epithelia. 4D4 mAb immunoprecipitates an undescribed 68 kDa protein. Functionally, this mAb inhibits the IL-2 secretion of a T cell clone in response to its peptide presented by appropriate class II-transfected L-cells and induces a negative selection of double positive CD4+CD8+ thymocytes. Since the 4D4 ligand is found on cells which are submitted to selection (T cells) and on cells which mediate selection (epithelial and antigen presenting cells), we conclude that 4D4 mAb defines a cell surface antigen involved, as an accessory molecule, in a cell selection process.