Background/Purpose: SLE is characterized by the inappropriate activation of type I Interferon (IFN) and increases in apoptosis and NETosis by neutrophils, which in combination with defective apoptotic cell and NET clearance provides an ongoing source of self-antigen. IFN further propagates the disease process by promoting B cell activation, survival, and differentiation into plasma cells (PC). PC produces autoantibodies that form immune complexes (IC) to further stimulate IFN production creating a vicious cycle. Important questions that remain unclear are the site and mechanisms of IFN activation. We have recently demonstrated a prominent IFN signature in the bone marrow (BM) of SLE patients that is more pronounced than paired peripheral blood and correlated with higher serum autoantibodies and disease activity. We hypothesize that BM is a key site of IFN activation in SLE.

Methods: BM supernatant (BMS) and serum were obtained from SLE patients (n=11) and healthy controls (HC, n=4). Plasmacytoid dendritic cells (pDC) were purified from healthy donor blood. To determine if BMS and serum induce IFN production, pDC was cultured with 5% BMS or serum with and without necrotic cell material. Necrotic cell material was generated by repeat freeze-thawed U937 cells. Culture supernatants were collected and IFNα was measured by ELISA. Blocking experiment was performed using 10ug/ml Hydroxychloroquine and 5ug/ml anti-CD32.

Results: We found that BMS from 27% of SLE patients was able to induce pDC to produce IFNα even without adding necrotic cell material (BM: 7.9±2.5, BM + necrotic: 7.7±2.1 ng/ml, n=3). The serum from the same patients also induced pDC to produce IFNα, but this effect was greatly enhanced by necrotic cell material (serum: 5.0±2.3, serum + necrotic: 256.46±153.54 ng/ml, n=3). BMS from 36% of SLE patients did not induce IFN production even with necrotic cell material (BM+necrotic: 0.014±0.007 ng/ml, n=4) although paired serum was able to do so with added necrotic cell material (serum+necrotic: 19.25±9.24 ng/ml, n=4). In contrast, neither BMS nor serum from 36% of SLE was able to induce IFNα production by pDC (serum+necrotic: 0.003±0.001, BM+necrotic: 0.0028±0.0018 ng/ml, n=4). BMS and serum from HC did not induce pDC to produce IFN (serum+necrotic: 0.0007±0.0007, BM+necrotic: 0.0045±0.0042 ng/ml, n=4). Serum autoantibodies were higher in the IFN inducer SLE subjects (average number of autoantibodies present for non-IFN inducer= 1 vs. 3.57 for IFN inducers, p=0.01). The ability of SLE BMS and serum to induce IFN was dependent on endosomal toll-like receptors as treating pDC with hydroxychloroquine inhibited IFN production. The induction was also blocked by anti-CD32 suggesting dependency on FcγRIIa. The relationship to the IFN signature and NETosis is under evaluation. Additionally, we are examining ICs as interferogenic factors in the BM.

Conclusion: These data suggest that the SLE BM contains factor(s) that promote type I IFN production by pDC that correlates with the presence of serum autoantibodies. The mechanisms of IFN induction in the BM appear to be dependent on TLR and FCR signalling and may relate to the presence of immune complexes generated in situ.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/sle-bone-marrow-contains-factors-that-promote-type-i-interferon-activation/