LOCAL SYNTHESIS OF THE COMPLEMENT PROTEIN S - NEW FUNCTIONS FOR THE COMPLEMENT-SYSTEM

The complement system is primarily known as a blood killing system of pathogens, either directly or in combination with antibodies. Compleme nt is also involved in the regulation of the immune response, phagocyt osis of foreign particles and some or its fragments exhibit pro-inflam matory properties. This review wants to highlight new functions of the complement system. These functions are not connected with immunity an d are specific of the tissue where :complement is ex pressed. This new concept of complement functions has emerged from studies on so-called local expression of complement. Three different examples address this question. The adipose tissue expresses specifically the complement fa ctor D, that is not expressed by tile liver, the main source of comple ment in the blood, Factor D was first recognized as adipsin in the adi pose tissue. Besides factor D, adipose tissue also expresses the other components of the complement alternative pathway, suggesting that the activation of the endogenous alternative pathway can regulate the met abolism of adipocytes. In bone C3, the pivotal molecule of the complem ent system, exerts two different functions in bone resorption. First, it acts as a differentiating factor. for the commitment of macrophage- like cells into osteoclasts. C3 is produced by stromal cells in the bo ne marrow upon stimulation with derivates of vitamin D. Second, C3 can recruit osteoclasts in the bone matrix through activation of C3 by th e bone matrix and interaction with complement receptors expressed by o steoclasts. finally, the recent discovery of endogenous sources of com plement in the brain, microglia and astrocytes, and of the expression of some complement receptors by these cells argues for a role of compl ement in the physiopathology of the nervous tissue.