The interactions between plasma proteins of the complement and properdin systems and cells, especially formed elements of the blood, will be examined. In one line of experiments, cells will be used as targets in model systems to elucidate the mechanisms of interaction of the alternative pathway proteins. Erythrocytes coated with purified constituents of this pathway will be used to investigate further the function and mechanisms of the control proteins, C3b Inactivator and beta 1H globulin. The precise mode of interaction of these two proteins with the target substrate C3b will be determined. Erythrocytes coated with specific antibody immunoglobulins will be used to explore further the way in which IgG enhances the formation of the alternative pathway convertase, and the potential role of other immunoglobulin classes in alternative pathway activation will be studied. In the second line of experiments the biologic consequences of interaction between activated complement proteins and leukocytes will be examined. The recent finding that C3 receptors on mast cells enhance endocytosis by these cells will be extended, particularly in regard to the release of low molecular weight mediators. The capacity of lysosomal proteases from neutrophils to function in the degradation of C3b' will be extended, particularly in regard to the release of low molecular weight mediators. The capacity of lysosomal proteases from neutrophils to function in the degradation of C3b' will be studied and the fate of C3 bound to an immune complex and subsequently endocytosed by a neutrophil will be determined. Effects of the binding of activated complement components to receptors on monocytes will be sought.
