Properties of a second epitope of the murine Fc receptor for aggregated IgG.

The murine macrophage and lymphocyte Fc receptor for aggregated IgG (Fc gamma R) has previously been characterized by using the anti-Fc gamma R monoclonal antibody (mAb), 2.4G2. In the studies presented here, we describe a new mAb, 6B7C, that defines a second epitope of the Fc gamma R. The tissue distribution of the 6B7C epitope is coincident with the 2.4G2 epitope. However, only the 2.4G2 epitope is accessible to mAb binding on intact primary macrophages or lymphocytes. The 6B7C epitope is not detectable on primary macrophages or lymphocytes but is exposed on a portion of B lymphocyte Fc gamma R after activation by lipopolysaccharide and on some tumor cell lines. The expression of the 6B7C epitope on the surface of B lymphoblasts and tumor cell lines seems to correlate with their ability to release soluble Fc gamma R. The 6B7C mAb has the advantage that it reacts with native as well as denatured receptor and therefore can be used for techniques such as immunoblotting.