Alternative pathway-mediated rebinding of immune complexes to human red blood cells.

Antigen-antibody complexes (Ag-Ab) prepared from ¹²⁵I-bovine serum albumin (BSA) and guinea-pig anti-BSA were (i) incubated at 37° for 4 min with undiluted normal human serum and autologous red blood cells (RBC) together, or (ii) incubated first at 37° for 30 min with serum diluted optimally for binding and then with RBC. Reactions were stopped by dilution and cooling, and RBC bearing antigen-antibody-complement complexes (Ag-Ab--C) were washed and resuspended in either untreated normal human serum (undiluted or diluted), serum treated with zymosan (SZYM), ethylenediamine tetracetic acid (SEDTA) or ethyleneglycol tetracetic acid-Mg⁺⁺ (SEGTA), serum heated at 56° for 30 or 120 min (SΔ30 or SΔ120), or buffer alone. The mixtures were placed at 37° and the percentage of Ag-Ab-C dissociated from RBC after progressively increasing times determined. (i) Ag:Ab:C bound to RBC with undiluted serum dissociated more rapidly following resuspension in SZYM, SEDTA, or SΔ30 than following resuspension in untreated serum. Rate of dissociation in SEGTA paralleled that in untreated serum. (ii) Ag-Ab-C bound to RBC with diluted serum dissociated following resuspension in SZYM, SEDTA, or SΔ30, but rebound and dissociated a second time following resuspension in untreated serum or SEGTA. Initial dissociation occurred in < min in undiluted serum, took place at 0° as well as 37°, and was diminished but detectable in >8- and > 64-fold- diluted serum, respectively. Rebinding required 37°, factors B and D and C3, and was maximal at 4-8 min. Subsequent dissociation had similar complement requirements to initial dissociation, but occurred only at 37° and was 90% complete at 15 min. No dissociation of Ag-Ab-C bound to RBC in (i) or in (ii) occurred following resuspension in SΔ120 or in buffer. These findings suggest that after initial binding, release of experimental immune complexes from RBC in whole serum involves concurrent dissociation and alternative pathway-dependent rebinding. [ABSTRACT FROM AUTHOR]