C3 synthetic peptides support growth of human CR2-positive lymphoblastoid B cells

The nature of CR type 2 (CR2)-ligand interactions which leads to the activation of human B cells was analyzed by using synthetic peptides and CR2-positive cell lines. The third component of C (C3) supported the growth of human lymphoblastoid B cells in serum-free medium containing human transferrin. This effect was inhibited by an antibody to C3d (mAb 130) which specifically inhibits C3d binding to CR2, but not by other anti-C3 mAb. Synthetic peptides corresponding to the CR2-binding site on C3d, P28 (residues 1187-1214) or multivalent P13 [1202-1214)4-template), supported the proliferative response of CR2-positive human lymphoblastoid lines in a similar way as C3 and this response could be inhibited by the anti-CR2 mAb OKB7. The proliferative response to C3 or peptides was dose dependent and a 60-fold higher concentration of P28 peptide was required to induce the same level of proliferation as C3. This stimulation of growth was observed only on CR2 expressing cell lines Raji and Daudi, and not on the CR2-negative Burkitt lymphoma cell line Rael and the monocytic cell line U937. In contrast to the stimulatory effect of P28 and P13-template, monomeric P14 (1201-1214) was not able to support the growth of these cell lines. This peptide, however, inhibited the proliferative response of the CR2-positive lines to C3, P28, and multivalent-P13, thus indicating that cross-linking of the CR2 receptor is necessary for B cell proliferation. Another peptide, E12 (from glycoprotein (GP)350, the major EBV outer membrane GP) which shows a high degree of similarity with P14, also inhibited the proliferative response of Raji cells, suggesting that this segment on GP350 is involved in the interaction of EBV with CR2. The possibility of using the above peptides as well as other peptides with "tailor-made" structure in studying the multifunctional role of C3 is discussed.