Abstract
Neutrophil extracellular traps (NETs) have been implicated in the pathogenesis of systemic Lupus erythematosus (SLE), since netting neutrophils release potentially immunogenic autoantigens including histones, LL37, human neutrophil peptide (HNP), and self-DNA. In turn, these NETs activate plasmacytoid dendritic cells resulting in aggravation of inflammation and disease. How suppression of NET formation can be targeted for treatment has not been reported yet. Signal Inhibitory Receptor on Leukocytes-1 (SIRL-1) is a surface molecule exclusively expressed on phagocytes. We recently identified SIRL-1 as a negative regulator of human neutrophil function. Here, we determine whether ligation of SIRL-1 prevents the pathogenic release of NETs in SLE. Peripheral blood neutrophils from SLE patients with mild to moderate disease activity and healthy donors were freshly isolated. NET release was assessed spontaneously or after exposure to anti-neutrophil antibodies or plasma obtained from SLE patients. The formation of NETs was determined by microscopic evaluation using DNA dyes and immunostaining of NET components, as well as by live cell imaging. We show that SLE neutrophils spontaneously release NETs. NET formation is enhanced by stimulation with antibodies against LL37. Inhibition of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity and MEK-ERK signaling prevents NET release in response to these antibodies. Signaling via the inhibitory receptor SIRL-1 was induced by ligation with anti-SIRL-1 specific antibodies. Both spontaneous and anti-neutrophil antibody-induced NET formation is suppressed by engagement of SIRL-1. Furthermore, NET release by healthy neutrophils exposed to SLE plasma is inhibited by SIRL-1 ligation. Thus, SIRL-1 engagement can dampen spontaneous and anti-neutrophil antibody-induced NET formation in SLE, likely by suppressing NAPDH oxidase and MEK-ERK activity. Together, these findings reveal a regulatory role for SIRL-1 in NET formation, potentially providing a novel therapeutic target to break the pathogenic loop in SLE.
Highlights
Systemic lupus erythematosus (SLE) is a chronic relapsingremitting autoimmune disease with pleiotropic, at times lifethreatening, clinical manifestations
Quantification of MPO- and neutrophil elastase (NE)-stained areas confirmed that significant release of MPO and NE occurred in response to anti-LL37 antibodies (Figure 1D), similar to the release of extracellular DNA stained with Sytox Green (Figure 1B)
We describe for the first time a role for Signal Inhibitory Receptor on Leukocytes-1 (SIRL-1), an immunoreceptor tyrosine-based inhibitory motif (ITIM)-bearing inhibitory receptor, in the regulation of immunogenic neutrophil extracellular traps (NETs) release by primary human neutrophils in SLE
Summary
Systemic lupus erythematosus (SLE) is a chronic relapsingremitting autoimmune disease with pleiotropic, at times lifethreatening, clinical manifestations. The disease is characterized by a permanent state of immune stimulation, leading to the accumulation of autoantibodies targeting doublestranded DNA (dsDNA) as well as other nuclear antigens. The presence of type I interferon-producing plasmacytoid dendritic cells is a hallmark of SLE [1]. SLE patients produce autoantibodies against antimicrobial peptides present in NETs such as human neutrophil peptide (HNP) and the antimicrobial peptide LL37 [2]. Exposure to these autoantibodies in turn stimulates neutrophils from SLE patients to release NETs which gives the immune system access to antigenic DNA resulting in perpetuation or even aggravation of disease.
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