1740 Unique Role of Cri in C

Receptors for C3b and C4b (CR1) 1 are found on the m e m b r a n e of phagocytes where they promote the b inding of C3bor C4b-bear ing particles and immune complexes (IC) (reviewed in 1). The majori ty o f CR1 in the vascular system, however, is present on erythrocytes in primates and platelets in other species. Furthermore, CR1 is found on B lymphocytes (2, 3) and on the epithelial cells of kidney glomeruli (4). The function of CR1 in these locations is unknown. In the present paper, we describe a new activity of CRa; that is, its part icipat ion in the breakdown of C3b associated with IC. This investigation was p rompted by previous observations that, when soluble IC were added to a mixture of normal h u m a n serum and autologous unseparated blood cells, they fixed complement and bound predominant ly to the erythrocytes. The binding was mediated principally via C3 incorporated into the IC (5, 6). After complement fixation, the b inding could not be prevented by preincubat ion of the IC with the serum enzyme C 3 b / C 4 b inactivator (I) and cofactor f l l H (H), but after binding, the IC could be released by treating the erythrocytes with I alone (7, 8). Dur ing this treatment, C3b in the IC was degraded beyond the iC3b stage, a fragment the size of C3c was released, and the IC became more reactive with cells bearing C3d receptors (CR2). This was unexpected, as previous reports indicated that the interaction of fluid phase C3b with I in the presence of H (9, 10) or o f purified CR1 (11) generated only the iC3b intermediate. Here, we study the nature of the h u m a n erythrocyte factor involved in the