Background/Purpose: Systemic lupus erythematosus (SLE) is a type I interferon (IFN-I)-driven autoimmune disorder with exaggerated B and T-helper (Th) cell responses.  Th17 cells, a recently identified T-helper cell subset, have been strongly implicated in the pathogenesis of SLE.  Since IFN-I suppress the generation and expansion of Th17 cells in an IL-27-dependent manner, it is unclear how pathogenic Th17 cells are generated in SLE in the presence of an environment characterized by high IFN-I levels.

Methods:   To test whether C5a has any impact on IFN-I-induced IL-27 production, bone marrow derived macrophages (BMDM) from wild type (WT) mice were treated with IFN-a in the presence or absence of C5a.  Supernatants of BMDM treated with IFN-a ± C5a or IFN-a ± anti-IL-27 Ab  were used to differentiate naïve CD4⁺ T cells in the presence of Th17 skewing conditions.  Similarly, BMDM from MRL.Fas^lpr mice were stimulated with a TLR7/8 agonist (IFN-I inducer) in the presence or absence of C5a.  To assess the in vivo consequence of C5aR activation on Th17 responses, we used the pristane induced lupus model and evaluated the development of lupus nephritis, number of Th17 cells and production of IL-27 in secondary lymphoid organs and kidney in WT and C5aR-/- mice (n=5).  We measured the expression of IRF-1 in MRL.Fas^lpr  macrophages in response to TLR7/8 agonist stimulation.  Finally, we measured serum C5a and IL-27(p28) levels by ELISA and the frequency of Th17 cells in peripheral blood from 22 SLE patients.  Results obtained from the serum ELISAs and Th17 assays were used to determine correlations between serum C5a versus IL-27, serum IL-27 and the percentage of peripheral blood Th17 cells and serum C5a and the percentage of Th17 cells.  To further validate our mouse model studies in human subjects with SLE, we treated macrophages from healthy donors with serum from SLE patients in the presence or absence of neutralizing anti-C5a Ab.

Results:  Activation of C5aR on macrophages blocked IFN-I-mediated IL-27 production (p < 0.0001) and permitted the differentiation of Th17 cells.  Similar findings occurred in lupus-prone mice and C5a exhibited a negative impact on TLR7 mediated synthesis of IL-27 (p < 0.001).  Consequently, C5aR^-/- mice were protected from lupus nephritis and showed increased IL-27 expression (p < 0.001) and reduced numbers of Th17 cells in the secondary lymphoid organs and kidney (p <0.001 for spleen and lymph nodes; p< 0.05 for kidney).  Furthermore, activation of the PI3K-Akt pathway was required for inhibiting IFN regulatory factor-1(IRF-1) mediated transcription of the IL-27(p28) gene.  In support of these mouse model studies, we found that serum from SLE patients significantly inhibited IFN-I-induced IL-27 production (p < 0.001), and the level of serum C5a directly correlated with the Th17 frequency in the SLE peripheral blood (r = 0.74; p = 0.001).

Conclusion:  In this report, we demonstrated the negative regulation of IFN-I induced IL-27 production by C5a via the C5aR on macrophages.  Our findings highlight a potential mechanism that explains how Th17 cells can develop despite strong IFN-I responses in SLE.  Thus therapeutic strategies to block C5aR activation may be beneficial for controlling pathogenic Th17-mediated inflammation in SLE.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/complement-component-c5a-permits-the-co-existence-of-pathogenic-th17-cells-and-type-i-interferon-in-lupus/