RESUMO
The IgA producing murine B lymphoma, CH12.LX.C4.4F10 (4F10) and the IgM producing murine lymphoma, CH12.LX.C4.5F5 (5F5) were found to express substantial numbers of substance P (SP) receptors having dissociation constants equal to 0.69 nM. Binding of SP by these B lymphoma cells was via the tachykinin-specific C-terminus sequence, Phe-X-Gly-Leu-Met-NH2, because SP, SP antagonist (D-Pro2-D-Phe7-D-Trp9-SP), eledoisin, and substance K could effectively inhibit radiolabeled SP binding, whereas the SP N-terminus fragment, SP (1-4), could not. The functionality of these receptors could be demonstrated by the ability of subnanomolar concentrations of SP to induce Ig secretion in a dose-dependent fashion. However, the presence of a second stimulus in these cultures was required to obtain maximal increases. IgA secretion by 4F10 cells was elevated only 25 to 37%, and IgM secretion by 5F5 cells was not significantly increased in cultures in which nanomolar concentrations of SP were present. Conversely, coculturing 5F5 cells with a suboptimal concentration of LPS (50 ng/ml) and 10(-10)M SP resulted in an approximate threefold increase in supernatant IgM when compared to control cultures stimulated with LPS alone. While not as dramatic, 10(-10) M SP also enhanced IgA secretion of LPS-stimulated 4F10 cells by approximately 45%. This enhancement of Ig secretion was SP-specific, as evidenced by the ability of 1000-fold excess of SP antagonist to block SP-induced, but not LPS-induced, Ig production. Clearly, SP could act synergistically with LPS to enhance Ig secretion; therefore, we questioned whether this augmentation was also reflected at the level of H chain mRNA expression. 10(-9)M SP induced modest increases (50 to 60%) in mu-chain mRNA expression by LPS-stimulated 5F5 cells when compared with cells stimulated with LPS alone. The 4F10 cells did not display this magnitude of difference for alpha-chain mRNA expression. Thus, although SP-induced increases of mu-chain mRNA by 5F5 cells may contribute to the increased Ig secretion observed by these LPS-activated lymphocytes, it is unlikely that increased mRNA expression can totally account for the threefold increases in secretion that were observed.