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Monoclonal antibodies (MoAb) are useful therapeutic agents for the treatment of a variety of human disorders, although the effector mechanisms responsible for the outcome of an efficient immunotherapy remain unclear. This study was designed to address the early effects of MoAb on the migration patterns of lymphocytes in vivo. The clearance profiles and tissue distribution of 111In-labelled rat lymph node cells were examined in both normal and decomplemented allogeneic and semi-allogeneic recipients pre-injected with IgG2b (R3/13) or IgG2a (R2/15S) MoAb directed against the RT1Aa, the classical class I major histocompatibility complex antigen of the DA rat. Both MoAb were equally effective in not only augmenting the removal of DA and (DA x PVG)F1 cells from the circulation and promoting their subsequent localization within the liver but also causing a significant degree of cell lysis during the early phase of cell clearance, even in decomplemented recipients. Although R3/13 and R2/15S are known to target erythrocytes differently in normal and cobra venom factor (CVF)-treated animals, no differences were observed in the migration behaviour of lymph node cells in allogeneic or semi-allogeneic hosts pre-injected with the same MoAb. Since rat lymphocytes express a much higher level of the RT1Aa antigen as compared with erythrocytes, we could not exclude a possible role of residual complement components in the circulation of CVF-treated rats that may have accounted for the observed antibody-dependent effects on target lymphocytes. On the basis of these findings we believe that the design and methodology employed in our present experimental opsonization system were inadequate to define clearly the mechanisms responsible for antibody-mediated removal and destruction of target lymphocytes in vivo.

We describe studies aimed at maximizing the effector mechanisms responsible for eliminating target erythrocytes from the circulation in a fully homologous opsonization system in vivo. The effects on the subsequent fate of target erythrocytes were examined in both normal and decomplemented rats preinjected with a variety of rat IgG monoclonal antibodies (mAb) directed against different epitopes on the RT1Aa, the classical class I major histocompatibiliy complex antigen of the DA rat. In general, the clearance of both DA and (DA x PVG)F1 erythrocytes in normal rats preinjected with various pairs of noncompetitive mAb was very rapid when compared with the overall clearance patterns seen with individual antibodies. With all mAb combinations containing IgG2b or IgG2a, an intact complement system was an essential requirement for augmenting the initial clearance and promoting hepatic sequestration of these target cells. The removal of (DA x PVG)F1 erythrocytes, expressing half as much antigen, was considerably slower than the DA cells for each antibody pair tested although a notable degree of heterogeneity was observed in the overall behavior of both types of target cells with different mAb combinations. Our results suggest that the limiting effects of low antigen density on the target cells combined with the use of mAb of an isotype like the rat IgG2a can be overcome using pairs of mAb that recognize different epitopes on the same target antigen.

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.

The splenic component of the mononuclear phagocyte system (MPS) was investigated in the rat using N-ethylmaleimide-treated erythrocytes (NEM) and erythrocytes coated with a monoclonal IgG2b antibody (R3/13) directed against the rat RTIAa major histocompatibility antigen. Both cell suspensions were removed by the spleen, and their clearance times were significantly longer in splenectomized animals. The mean clearance times for the NEM-treated cells in both normal and cobra venom-treated rats were similar (19.1 ± 1.1 mm and 190± 10 mm, respectively) but differences were seen between the clearance of R3/13 antibody-sensitized cells in these two groups (normal rats 38.3 ± 2.8 mm and CVF-treated rats 51.7± 4.2 mm, P< 0.02). Different receptors were also involved in the removal of these cells; in normal animals recognition entailed interaction with complement receptors, whereas in CVF-treated animals this was implemented by Fc receptors. Complement activation prolonged the clearance rates of both R3/1 3 cells and NEM cells in normal animals, but the effect of complement activation on the clearance of NEM-treated cells was achieved via changes in splenic blood flow. When this was prevented from taking place no effect was seen on the clearance of NEM cells, although the clearance of R3/13 cells was inhibited by the complement fragments generated by complement activation. [ABSTRACT FROM AUTHOR]