Background/Purpose: Efficient clearance of apoptotic cells, known as efferocytosis, plays a pivotal role in maintaining self-tolerance. Dysfunction in efferocytosis is implicated in the pathogenesis of inflammatory conditions such as systemic lupus erythematosus (SLE), where uncleared apoptotic cells introduce potential autoantigens and inflammatory products like cell-free DNA. The accumulation of post-apoptotic cell remnants owing to inefficient clearance can be a consequence of disease intrinsic and extrinsic factors, such genetic predisposition and inflammatory environment respectively. Little is known, however, what components in SLE patient circulation impact efferocytosis. This study aims to investigate the influence of SLE patient serum and known SLE-associated danger signals on efferocytosis in human macrophages.

Methods: Primary human monocytes were differentiated into macrophages ex vivo to assess macrophage phagocytosis of apoptotic cells. Inflammatory conditions were modeled using interferon (IFN)-β or serum collected from SLE patients or healthy controls. Efferocytosis was quantified through imaging and flow cytometry. Macrophages were stimulated with Poly I:C, R848, hydroxychloroquine, or IFN-β for 24 hours prior to the efferocytosis assay.

Results: Macrophage uptake of apoptotic cells was diminished in the presence of IFN-β and serum from SLE patients compared to healthy controls. Blocking IFNAR1/2 with Anifrolumab restored defective efferocytosis induced by IFN-β but only partially rescued efferocytosis impairment in the presence of SLE serum. This suggested that other factors in the serum could contribute to reduced clearance in SLE serum. We then tested how SLE-associated danger signals such as single- and double-stranded RNA mimics, R848 and Poly (I:C), respectively, impact macrophage clearance of abundant apoptotic cells. Both Poly (I:C) and R848 are potent inducers of Type-I IFN and have been shown to accelerate disease progression in preclinical models of SLE. R848, but not Poly I:C, reduced efferocytosis in a dose dependent fashion. We also tested the effect of hydroxychloroquine, an extensively used FDA-approved drug in SLE patients, and no measurable modulation on apoptotic cell clearance was observed.

Conclusion: These findings highlight an additional mechanism contributing to defective efferocytosis in SLE. We demonstrate that macrophages cultured in the presence of SLE serum and IFN-β exhibit compromised apoptotic cell clearance, thereby exacerbating accumulation of inflammatory stimuli in SLE. Interestingly, only single-stranded RNA mimics (but not double-stranded RNA or hydroxychloroquine) reduced apoptotic cell clearance. Further discoveries defining the molecular machinery involved in efferocytosis can open avenues for therapeutic intervention in SLE.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/sle-patient-serum-and-sle-associated-danger-signals-impair-efferocytosis-in-human-macrophages/