It was recently demonstrated that synoviocytes (FLS) from rheumatoid arthritis (RA) patients express BAFF transcripts that are up-regulated by tumor necrosis factor alpha (TNFalpha) and interferon-gamma (IFNgamma). Thus, BAFF increases in RA target cells might be related to activation of the receptors of innate immunity. The purpose of this study was to determine whether ligands of Toll-like receptor 2 (TLR-2), TLR-4, TLR-9, and alpha5beta1 integrin are able to induce BAFF synthesis by RA FLS.
Quantitative reverse transcription-polymerase chain reaction analyses and enzyme-linked immunosorbent assays were performed to evaluate BAFF messenger RNA induction and BAFF release from FLS after stimulation by ligands for TLR-2, TLR-4, TLR-9, alpha5beta1 integrin (bacterial lipopeptide [BLP] palmitoyl-3-cysteine-serine-lysine-4, lipopolysaccharide [LPS], CpG, and protein I/II, respectively), TNFalpha, and IFNgamma.
In contrast to IFNgamma, neither TNFalpha, LPS, BLP, nor CpG induced the de novo synthesis and release of BAFF by FLS. Priming of cells with IFNgamma did not have a synergistic effect on BAFF synthesis by FLS stimulated with bacterial products known as pathogen-associated molecular patterns. Moreover, we found that IFNgamma-induced BAFF synthesis is inhibited by simultaneous stimulation with either TLR ligands or TNFalpha. We also showed that interplay between TLRs, TNF receptors, and IFNgamma signaling induces the expression of suppressor of cytokine signaling 1 (SOCS-1) and SOCS-3 and reduces IFNgamma-dependent STAT-1 phosphorylation, which might explain this inhibition. In contrast, we demonstrated that stimulation of alpha5beta1 integrin can induce BAFF synthesis and release per se and that stimulation of this pathway has no inhibitory effect on IFNgamma-induced BAFF synthesis.
Our findings indicate that BAFF secretion by resident cells in target organs of autoimmunity is tightly regulated by innate immunity, with positive and negative controls, depending on the receptors and the pathways triggered.