An experimental model for the study of the opsonic activity of fibronectin in the clearance of intravascular complexes.

An experimental model is described which provides direct evidence of endocytosis of complexes of a charged colloid (dextran sulphate or DS) and of plasma fibronectin (FN) by reticulo-endothelial (RE) cells. This supports previous suggestions that plasma FN acts as a non-immunologically mediated opsonin which effects the clearance of various kinds of circulating colloids and particles by RE cells. DS of molecular weight 500 000 forms FN-containing complexes in rat plasma in vitro and lowers plasma FN levels acutely and in a dose-related fashion when given parenterally to rats. The plasma changes are accompanied by deposition of DS (shown histochemically as metachromatic material) and of FN (shown by specific immunofluorescence) in an identical distribution within RE cells in rat liver and spleen. The protein and polysaccharide components of the complex are disposed of by RE cells at markedly different rates. The model thus also offers a means of studying the dynamics of catabolism of plasma FN by this 'scavenger' pathway. Possible further extensions of the model for other purposes are also discussed.