Receptor-binding sites on C3 and C3b.

Human erythrocytes carry C3b receptors: Daudi lymphoid cells carry exclusively C3d receptors. With these two types of cells, it could be shown that isolated, uncleaved human C3 and soluble C3b possess two stable binding sites: SBS1-specific C3b receptors, and SBS2-specific for C3d receptors. Upon binding of freshly cleaved C3b (nascent C3b) via its labile binding site (generated on C3b through cleavage of C3) to the C3 acceptors on cell surfaces, the SBS2 becomes concealed.

Stimulation of murine B lymphocytes by isolated C3b.

Addition of isolated B3b to murine lymph node cell cultures induced increased DNA synthesis. The stimulation is dependent on the dose of C3b added and is potentiated by fetal calf serum present in the medium. Isolated C3 is less stimulatory than C3b; C3a and C3c had no effect on DNA synthesis in these cultures.

C1q deviation test for the detection of immune complexes, aggregates of IgG, and bacterial products in human serum.

This report describes a new, rapid, sensitive, and quantitative method for the detection of immune complexes, endotoxins, and other complement activating materials in patients sera utilizing the ability of these substances to react with isolated C1q. The procedure is based on the inhibition of radiolabeled C1q binding to sensitized sheep erythrocytes by C1q-reactive substances in pathological sera. The C1q deviation test may be performed on 50 mu1 of serum, using 1 mug of radiolabeled C1q per sample.

Interaction of peptidoglycans with anti-IgGs and with complement.

This report describes the interaction of peptidoglycan (Streptococcus group A, Staphylococcus epidermidis and Micrococcus lysodeikticus) with 2 serum mediator systems, namely with the anti-IgG system and with complement. The observation that the majority of rabbits hyperimmunized with A-variant streptococcal vaccine produced anti-group carbohydrate antisera containing anti-IgGs and antibodies directed to peptidoglycan suggested that the production of these 2 latter antibodies was related. This view was supported by the finding of a monoclonal 7S anti-IgG with antibody specificity for the pentapeptide of peptidoglycan as evidenced by inhibition of the coprecipitation of 7S anti-IgG with antigen-antibody complexes by the pentapeptide.

Third component of complement (C3): structural properties in relation to functions.

The third component of complement (C3) fulfills a pivotal role in the functions of the complement system. We have investigated the topological relationships among its polypeptide chains, physiologic fragments, enzyme attack regions, and functional sites. C3 consists of two chains (alpha and beta) which are linked by disulfide bonds and noncovalent forces and which have molecular weights of, respectively, 120,000 and 75,000.