Generation of Inhibitory NFκB Complexes and Phosphorylated cAMP Response Element-binding Protein Correlates with the Anti-inflammatory Activity of Complement Protein C1q in Human Monocytes

The interaction of C1q with specific cells of the immune system induces activities, such as enhancement of phagocytosis in monocytes and stimulation of superoxide production in neutrophils. In contrast to some other monocyte activators, C1q itself does not induce pro-inflammatory cytokine production, but rather inhibits the lipopolysaccharide (LPS)-stimulated induction of certain pro-inflammatory cytokines and induces expression of interleukin-10. To investigate the molecular mechanism by which C1q exerts this effect on gene expression, the influence of C1q on the activation of transcription factors of the NFkappaB family and cAMP response element-binding protein (CREB) was assessed. C1q treatment increased kappaB binding activity in freshly isolated human monocytes in a time-dependent fashion as assessed by electrophoretic mobility shift assays. In antibody supershift experiments, anti-p50 antibody supershifted the C1q-induced NFkappaB complex, whereas anti-p65 antibody had little effect, suggesting that C1q induced the translocation of NFkappaB p50p50 homodimers. This is in contrast to the dominant induction of p65 containing complexes in parallel monocyte cultures stimulated with LPS. C1q treatment also induced cAMP response element (CRE)-binding activity as demonstrated by electrophoretic mobility shift assay, increased phosphorylation of CREB, and induction of CRE driven gene expression. In contrast, CREB activation was not detected in LPS-treated monocytes. These results suggest that C1q may modulate the cytokine profile expressed in response to inflammatory stimuli (e.g. LPS), by triggering inhibitory and/or competing signals. Because C1q and other defense collagens have been shown to enhance clearance of apoptotic cells, this regulatory pathway may be beneficial in avoiding autoimmunity and/or resolving inflammation.