Immunomodulatory functions of murine CR1/2.

C3-fragments generated upon complement activation play an important role in the formation and regulation of immune responses. Receptors interacting with various activation fragments of this versatile complement component are expressed on a wide variety of cell types, such as lymphocytes, macrophages, dendritic cells, follicular dendritic cells, granulocytes, erythrocytes, consequently C3-products may influence several biological functions at different sites of the body, where complement activation takes place. In the last decade, genes, protein structure and functions played by murine complement receptors CR1 and CR2 (mCR1/2) have been deciphered. In this review, we wish to relate these properties, and fit it into the context of events following in vivo complement activation. We separately address the roles played by murine mCR1/2 as BCR coreceptor and as BCR independent structure, and propose a mchanism for the utilization of antigen-C3d conjugates bound on B cells. Finally, we raise some of the questions that remain to be elucidated in order to get a more precise picture of the functions of mCR1/2.