Targeting behavior of rat monoclonal IgG antibodies in vivo: role of antibody isotype, specificity and the target cell antigen density.

The studies described in this report were designed to investigate factors that could influence the behavior of erythrocytes following their interaction with monoclonal antibodies (mAb) in a fully homologous experimental opsonization system in vivo. The clearance profiles and tissue distribution of target erythrocytes were examined in both normal and decomplemented rats preinjected with rat IgG2a or IgG2b mAb directed against the same or different sites on RT1Aa, the classical class I major histocompatibility complex antigen of the DA rat. Complement played a major role in augmenting the clearance and promoting hepatic sequestration of target erythrocytes in rats preinjected with IgG2a mAb directed against the S site. In contrast, an intact complement system was not an essential requirement for erythrocyte clearance when S site-specific IgG2b mAb were used. With each antibody tested, (DA x PVG)F1 cells, expressing about half as much antigen, were removed significantly slower than DA erythrocytes, this finding being more pronounced when the animals had been preinjected with mAb of the IgG2a isotype. A comparison of the tissue distribution of DA and (DA x PVG)F1 erythrocytes indicated that hepatic uptake was greater for target cells expressing higher antigen density. A considerable degree of heterogeneity was observed in the in vivo behavior of the target erythrocytes with three groups of IgG2b mAb that recognized different sites on the class I molecule. The S site-specific IgG2b mAb were much more efficient in the hepatic Fc receptor-mediated clearance system than were the P site-directed mAb of the same subclass. Our results suggest that antibody specificity may also be a contributory factor, in addition to antibody isotype and target cell antigen density, in determining the fate of target cells in vivo.