A B S T R A C T Previous inThis resulted in an "erythrocyte fraction" beneath the percoll that contained the IC bound to erythrocytes, and a "plasma/buffy coat fraction" above the percoll that contained the IC in plasma and IC bound to buffy coat cells. Analysis of these data showed that the majority of the IC infused into the circulation rapidly became bound to erythrocytes. However, by 5 min after beginning the IC infusion, most of this IC load had been removed from the erythrocytes as they traversed the liver. In contrast, IC on erythrocytes did not deposit in kidney. The IC-bearing erythrocytes themselves were not trapped or detained by any organ. IC in the plasma/buffy coat fraction of blood were removed from the circulation but at a relatively low

Abstract. Binding of immune complexes (IC) to erythrocytes in vitro is the result of interaction between C3b sites on the IC, and complement receptors type I (CR1) expressed on primate erythrocytes. Recent evidence indicates that primate erythrocytes can also rapidly bind large, preformed IC in vivo. This study was undertaken to determine if the binding of IC to baboon erythrocytes in vivo is complement dependent and to examine the effect of complement depletion on IC clearance from the circulation. The results indicate that complement depletion in vivo reduced the binding of IC to erythrocytes. There was relatively little binding of IC to leukocytes in both the complement-depleted and complement-repleted condition. Thus, the majority of IC not bound to erythrocytes remained free in the plasma and, consequently, IC infusion during the complement-depleted state resulted in increased plasma IC concentrations. This was associated with a rapid disappearance of IC from the circulation. By contrast, in the normal or complement-repleted state, a large fraction of the IC became bound to erythrocytes during IC infusion, which resulted in lower plasma IC concentrations. Under these conditions, a more gradual rate of disappearance of IC from the circulation was observed. The relatively abrupt clearance of IC from the circulation in the complement-depleted state could not be accounted for by increased hepatic or splenic uptake. These data indicate that, in contrast to previous studies in nonpri-

Primate erythrocytes appear to play a role in the clearance of potentially pathogenic immune complexes (IC) from the circulation. This study was undertaken to compare the clearance from the circulation and tissue uptake of two monoclonal IC probes: one of which, IgGi-IC, was bound well by erythrocytes, the other of which, IgA-IC, was bound relatively poorly by erythrocytes. The IC probes were labeled with different iodine isotopes and infused either concommitantly or sequentially into the arterial circulation. The results indicate that, compared with IgGl-IC, IgA-IC bind less well to primate erythrocytes, are cleared from the circulation more quickly despite their smaller size, and show increased uptake in kidney and lung but decreased uptake in liver and spleen. Evidence is presented which suggests that this pattern of clearance from the circulation and systemic uptake of IgA-IC is the result of decreased binding of IgA-IC to circulating erythrocytes. These findings support the hypothesis that the primate erythrocyte-IC clearing mechanism may be critically important for the safe removal of IC from the circulation.

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