Functional aspects of the C1q receptors.
C1q, the recognition subunit of the classical complement pathway, has been shown to stimulate host defense mechanisms. It has become increasingly evident that C1q can bind to several different cell types resulting in a variety of functional consequences depending on the cell type. For example, the binding of C1q to monocytes enhances the ability of those cells to ingest pathogens, while interaction with neutrophils, eosinophils, endothelial cells and vascular smooth muscle cells triggers or enhances the generation of toxic oxygen species. While these latter oxygen products play a major role in the destruction of foreign pathogens, in some cases, such as immune-complex-induced vasculitis and myocardial infarction, these toxic oxygen molecules can cause excessive host tissue damage. The differential modulation of C1q-mediated activities by monoclonal antibodies suggest that there is more than one unique type of C1q receptor, which is not necessarily surprising given the diversity of responses reported. This review summarizes what is known about the functional consequences of the interaction of C1q with cells, and describes initial investigations of the biochemical mechanisms/signal transduction pathways involved in these C1q-induced responses. Further investigations of the C1q/C1q receptor system(s) should uncover basic mechanisms and modes of regulation of these defensive responses by the host, and potentially provide information useful for beneficial manipulation of these activities.