SCARF1-induced efferocytosis plays an immunomodulatory role in humans, and autoantibodies targeting SCARF1 are produced in patients with systemic lupus erythematosus

Deficiency in the clearance of cellular debris is a major pathogenic factor in the emergence of autoimmune diseases. We previously demonstrated that mice deficient for scavenger receptor class F member 1 (SCARF1) develop a lupus-like autoimmune disease with symptoms similar to human systemic lupus erythematosus (SLE), including a pronounced accumulation of apoptotic cells (ACs). Therefore, we hypothesized that SCARF1 will be important for clearance of ACs and maintenance of self-tolerance in humans, and that dysregulation of this process could contribute to SLE. Here, we show that SCARF1 is highly expressed on phagocytic cells, where it functions as an efferocytosis receptor. In healthy individuals, we discovered that engagement of SCARF1 by ACs on BDCA1+ dendritic cells (DCs) initiates an interleukin-10 (IL-10) anti-inflammatory response mediated by the phosphorylation of signal transducer and activator of transcription 1 (STAT1). Unexpectedly, there was no significant difference in SCARF1 expression in SLE patient samples compared to healthy donor samples. However, we detected anti-SCARF1 autoantibodies in 26% of SLE patients, which was associated with dsDNA antibody positivity. Furthermore, our data shows a direct correlation of the levels of anti-SCARF1 in the serum and defects in the removal of ACs. Depletion of immunoglobulin restores efferocytosis in SLE serum, suggesting that defects in the removal of ACs is partially mediated by SCARF1 pathogenic autoantibodies. Our data demonstrate that human SCARF1 is an AC receptor in DCs and plays a role in maintaining tolerance and homeostasis.