Evaluation of the mechanisms responsible for the reduction in erythrocyte complement receptors when immune complexes form in vivo in primates

Patients with immune complex-(IC) mediated diseases frequently have low levels of CR1 on E. The present study was undertaken to determine the role of circulating IC in causing low E-CR1 levels. E-CR1 were enumerated by measuring the binding of anti-CR1 mAb (E11) and rabbit anti-CR1 antibodies (RbaCR1) to E. In addition, the distribution of CR1 among E was assessed by flow cytometry of E stained with E11 and RbaCR1 and by evaluating the binding of E11-coated fluorescent beads (E11-beads) to E. E11-beads bind to clusters of CR1 on E. Five cynomolgus monkeys (CYN) were preimmunized to bovine γ-globulin (BGG). E-CR1 changes in these animals were assessed: 1) acutely, during the first 60 min after an infusion of BGG and 2) chronically, during daily administration of BGG infusions over 2 wk. Acutely, there was a decrease in the number of E-CR1 as measured by E11 binding to E (E11/ CR1). This decrease was not attributable to occupancy of CR1 by IC because the decrease in E11/ CR1 number persisted after the IC had been cleared from E. By comparing the E11/CR1 levels in arterial blood to hepatic vein blood (n = 5), or in pulmonary artery blood (n = 1), we determined that the acute decrease in E11/CR1 number did not occur whereas E circulated through liver, spleen, or lung. The decrease in E11/CR1 number required the binding of IC to E because it did not occur after BGG was infused into nonimmunized CYN (n = 2) or into a preimmunized complement-depleted CYN. The decrease in E11/CR1 number was not due to loss of CR1 from E because E11/CR1 number recovered 24 h after infusion of BGG and in addition, enumeration of E-CR1 with RbaCR1 and E11-beads did not reflect a decrease in E-CR1 number. After several daily BGG infusions there was a persistent decrease in E-CR1 levels and that decrease appeared to be mainly the result of loss of CR1 from E because the decrease was confirmed with all methods of E-CR1 measurement and because E-CR1 levels recovered only slowly after BGG infusions were discontinued. Both in vitro and in vivo IC bound preferentially to subpopulations of E, identified by their ability to bind multiple E11-beads and by their high intensity staining with the anti-CR1 antibodies E11 and RbaCR1. In conclusion, the decrease in E-CR1 levels seen when IC form in the circulation is the result of the binding of IC to E-CR1. These IC/E-CR1 interactions intially result in reversible changes in E-CR1 but after repeated interactions, destruction of E-CR1 occurs. The E-CR1 losses are relatively greater in the subpopulation if E that have a high density of CR1 and, presumably, a high capacity to bind IC.