The murine B cell line CH12.LX.C4.5F5 (CH12 (5F5) expresses adrenocorticotropin (ACTH) receptors, which can modulate IgM secretion by these cells. Interestingly, the response to ACTH was concentration dependent, inducing IgM secretion at subnanomolar amounts and suppressing secretion at micromolar amounts. With the use of an enzyme-linking immunospot assay it was possible to demonstrate that the ACTH-induced increase in IgM secretion by CH12 (5F5) cells was caused at least in part by an increase in the number of cells secreting IgM. CH12 (5F5) cells activated with suboptimal concentrations of LPS demonstrated a similar biphasic response. ACTH at concentrations of 10(-13) to 10(-9) M augmented IgM secretion in LPS-activated cells as much as sixfold, whereas 10(-6) M ACTH slightly decreased LPS-induced IgM secretion. At the mRNA level, subnanomolar concentrations of ACTH increased microH chain mRNA expression up to twofold in unstimulated or LPS-stimulated CH12 (5F5) cells. Taken together, these studies show that physiologically relevant concentrations of ACTH can interact directly with receptors on these B lymphocytes to enhance IgM secretion and microH chain mRNA expression. Although ACTH does increase intracellular cAMP levels in CH12 (5F5) B cells, it is unlikely that the induction of this second messenger pathway is by itself responsible for the ACTH induced B cell differentiation. The concentration of ACTH necessary to stimulate significant intracellular cAMP increases was 10- to 100-fold higher than that required to increase IgM secretion. Furthermore, CH12 (5F5) cells treated with varying concentrations of 8-bromo cAMP or cholera toxin were inhibited in their ability to secrete IgM. These results strongly suggest that the enhancing effects of ACTH on CH12 (5F5) IgM secretion are via mechanisms independent of those mediated by cAMP.

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