Newly Identified TLR9 Stimulant, M6-395 Is a Potent Polyclonal Activator for Murine B Cells

BACKGROUND: Toll-like receptors (TLRs) have been extensively studied in recent years. However, functions of these molecules in murine B cell biology are largely unknown. A TLR4 stimulant, LPS is well known as a powerful polyclonal activator for murine B cells. METHODS: In this study, we explored the effect of a murine TLR9 stimulant, M6-395 (a synthetic CpG ODNs) on B cell proliferation and Ig production. RESULTS: First, M6-395 was much more potent than LPS in augmenting B cell proliferation. As for Ig expression, M6-395 facilitated the expression of both TGF-beta1-induced germ line transcript alpha (GLTalpha) and IL-4-induced GLTgamma1 as levels as those by LPS and Pam3CSK4 (TLR1/2 agonist) : a certain Ig GLT expression is regarded as an indicative of the corresponding isotype switching recombination. However, IgA and IgG1 secretion patterns were quite different--these Ig isotype secretions by M6-395 were much less than those by LPS and Pam3CSK4. Moreover, the increase of IgA and IgG1 production by LPS and Pam3CSK4 was virtually abrogated by M6-395. The same was true for the secretion of IgG3. We found that this unexpected phenomena provoked by M6-395 is attributed, at least in part, to its excessive mitogenic nature. CONCLUSION: Taken together, these results suggest that M6-395 can act as a murine polyclonal activator but its strong mitogenic activity is unfavorable to Ig isotype switching.

Activin A Stimulates Mouse APCs to Express BAFF via ALK4-Smad3 Pathway

BACKGROUND: B cell-activating factor belonging to the TNF family (BAFF) is primarily expressed by macrophages and dendritic cells, and stimulates B cell proliferation, differentiation, survival, and Ig production. In the present study, we explored the effect of activin A on BAFF expression by APCs. METHODS: To investigate the effect of activin A on BAFF expression by mouse APCs, we measured the level of BAFF expression at the transcriptional and protein levels using RT-PCR and ELISA. RESULTS: Activin A markedly enhanced BAFF expression in mouse macrophages and dendritic cells at both the transcriptional and protein levels. SB431542, an activin receptor-like kinase 4 (ALK4) inhibitor, completely abrogated activin A-induced BAFF transcription. Furthermore, overexpression of DN-Smad3 abolished activin-induced BAFF expression at the transcriptional and protein levels. CONCLUSION: These results demonstrate that activin A can enhance BAFF expression through ALK4-Smad3 pathway.