Demonstration of autoreactivity by a human monoclonal IgG anti‐Rh D antibody

Human IgG monoclonal antibodies (mabs) against the Rh D antigen have considerable potential for the prophylaxis of haemolytic disease of the newborn. We have carried out in vitro testing for cross-reactions with tissue components by screening two such mabs against animal tissues and a wide panel of human organs from nearly 50 individuals, most of whom were of known Rh D phenotype. Cryostat sections were studied by indirect immunohistochemical techniques. One of the mabs showed non-specific, dose-dependent binding to multiple tissue components whereas the other specifically and consistently reacted strongly with animal smooth muscle and human smooth muscle (vascular, in the walls of hollow viscera, in the respiratory tract) from both Rh D-positive and -negative donors. Immunoprecipitation experiments identified the probable smooth muscle antigen as actin or actin-associated. However, on the basis of inhibition experiments, and by direct estimation of the association constant, the affinity of this mab for smooth muscle was lower than that for Rh D. These results demonstrate autoreactivity by an IgG anti-D mab and show differences in the immunochemical characteristics of human anti-D mabs which may be clinically relevant.