No abstract

An abnormal neutrophil subset has been identified in the PBMC fractions from lupus patients. We have proposed that these “low density granulocytes” (LDGs) play an important role in lupus pathogenesis by damaging endothelial cells and synthesizing increased levels of proinflammatory cytokines and type I interferons. To directly establish LDGs as a distinct neutrophil subset, their gene array profiles were compared to those of autologous normal density neutrophils and control neutrophils. LDGs significantly overexpress mRNA of various immunostimulatory bactericidal proteins and alarmins, relative to lupus and control neutrophils. In contrast, gene profiles of lupus normal density neutrophils do not differ from those of controls. LDGs have heightened capacity to synthesize extracellular traps (NETs) which display increased externalization of bactericidal, immunostimulatory proteins and autoantigens, including LL-37, IL-17, and double-stranded DNA (dsDNA). Through NETosis, LDGs have increased capacity to kill endothelial cells and to stimulate IFN-α synthesis by pDCs. Affected skin and kidneys from lupus patients are infiltrated by netting neutrophils, which expose LL-37 and ds-DNA. Tissue NETosis is associated with increased anti-dsDNA in sera. These results expand the potential pathogenic roles of aberrant lupus neutrophils and suggest that dysregulation of NET formation and its subsequent responses may play a prominent deleterious role.

The early events leading to the development of rheumatoid arthritis (RA) remain unclear but formation of autoantibodies to citrullinated antigens (ACPA) is considered a key pathogenic phenomenon. Neutrophils isolated from patients with various autoimmune diseases display enhanced extracellular trap formation (NETs), a phenomenon that externalizes autoantigens and immunostimulatory molecules. We investigated whether aberrant NETosis occurs in RA, determined its triggers and examined its deleterious inflammatory consequences. Enhanced NETosis was observed in circulating and synovial fluid RA neutrophils, compared to neutrophils from healthy controls and from patients with osteoarthritis. Further, netting neutrophils infiltrated RA synovial tissue, rheumatoid nodules and skin. NETosis correlated with ACPA presence and levels and with systemic inflammatory markers. RA sera and immunoglobulin fractions from RA patients with high levels of ACPA and/or rheumatoid factor significantly enhanced NETosis, and the NETs induced by these autoantibodies displayed distinct protein content. During NETosis, neutrophils externalized citrullinated autoantigens implicated in RA pathogenesis, whereas anti-citrullinated vimentin antibodies potently induced NET formation. The inflammatory cytokines IL-17A and TNF-α induced NETosis in RA neutrophils. In turn, NETs significantly augmented inflammatory responses in RA and OA synovial fibroblasts, including induction of IL-6, IL-8, chemokines and adhesion molecules. These observations implicate accelerated NETosis in RA pathogenesis, through externalization of citrullinated autoantigens and immunostimulatory molecules that may promote aberrant adaptive and innate immune responses in the joint and in the periphery, and perpetuate pathogenic mechanisms in this disease.

Material Supplementary 9.DC1http://www.jimmunol.org/content/suppl/2010/02/15/jimmunol.090219

IL-17 and IL-23 are absolutely central to psoriasis pathogenesis as drugs targeting either cytokine are highly effective treatments for this disease. The efficacy of these drugs has been attributed to blocking the function of IL-17-producing T cells and their IL-23-induced expansion. However, we demonstrate that mast cells and neutrophils, not T cells, are the predominant cell types that contain IL-17 in human skin. IL-17+ mast cells and neutrophils are found at higher densities than IL-17+ T cells in psoriasis lesions and frequently release IL-17 in the process of forming specialized structures called extracellular traps (MCETs and NETs, respectively). Furthermore, we find that IL-23 and IL-1β can induce MCET formation and degranulation of human mast cells. Release of IL-17 from innate immune cells may be central to the pathogenesis of psoriasis, representing a fundamental mechanism by which the IL-23-IL-17 axis mediates host defense and autoimmunity.