Monoclonal Antibodies Define the Characteristics and Cellular Distribution of an Ia-Associated Protein

Abstract

In a recent report we described the identification of physical associations between Major Histocompatibility Complex (MHC) Class II (Ia) antigens and other structures of Mr 67,000, which were significantly enhanced following brief T-B cell co-culture (1). To further investigate this 67K Ia-associated product, monoclonal antibodies (MAb) were produced against isolated 67K material and their reactivity examined. Cell surface binding by these MAb was detected only after perturbation of the membrane by cellular adherence or following aldehyde fixation, which indicates that the determinant recognized by these mAb is retained in the plasma membrane in a covert fashion. All lymphoid cells tested showed reactivity with the MAb as determined by immunofluorescence and by ELISA, but no binding was detected on bone marrow or peritoneal macrophages. Expression of the antigen reactive with these antibodies followed a similar pattern with established murine cell lines, with T and B cell lines and a pre-B cell line showing reactivity, while no antigen was detected on macrophage-like and fibroblast cell lines. The intensity of antigen expression by normal lymphoid cells was ordered: thymocytes greater than splenic T cells greater than or equal to bone marrow lymphocytes greater than splenic B cells. No correlation was observed between expression of Ia antigens by non-lymphoid cells and expression of the 67K molecule. These observations suggest that this antigen is primarily a marker of lymphoid cells, with the highest expression on cells of the T lymphocyte lineage. Finally, inhibition of antigen-specific, MHC-restricted T-cell activation by the MAb directed against the 67K structure suggests an important functional role for this interesting molecule originally identified by its physical association with Ia following T-B cell interactions.