Neutrophils in systemic lupus erythematosus

Introduction

Neutrophils are the most abundant immune-competent cells in the peripheral blood, and represent up to ∼70% of total white blood cells. They play an important role in innate immune responses and are often at the first line of defense against infections. Neutrophils are capable of phagocytosis and produce reactive oxygen species to kill invading microorganisms. They are short lived (half-life ∼7 hours in the circulation, though longer half-life of up to 5 days has been recently reported), granulated, and are characterized by the presence of multilobulated nuclei. Upon degranulation, neutrophils release immune-active peptides, such as α defensins, cathelicidin peptides (such as LL-37), and lactoferin, that can either directly participate in microorganism killing, or trigger other components of the innate or adaptive immune response. Neutrophils express most of the Toll-like receptor (TLR) family members, MHC class II, CD80, and CD86, and present antigens to T cells and can prime antigen-specific Th1 and Th17 differentiation. Indeed, neutrophils have been demonstrated to migrate towards inflammation sites, and then “reverse migrate” away from the inflammation site to the circulation, presumably to participate in eliciting a more generalized immune response. Neutrophils also produce a variety of cytokines such as BLyS (or BAFF) and APRIL and can therefore induce B cell stimulation. Neutrophils in the spleen can also produce IL-21 and have been shown to induce B cell activation, immunoglobulin production, and immunoglobulin somatic hypermutation and class switching.

A peculiar mechanism neutrophils possess within their defensive arsenal to hinder and kill invading microorganisms is forming neutrophil extracellular traps (NETs) ( Fig. 17.1 ). This process, called NETosis, allows for a rapid and robust externalization of the neutrophil micobicidal content and is a unique cell death mechanism initially described in neutrophils in 2004. During this process chromatin and neutrophil granular proteins are released, forming extracellular fiber-like structures that bind and “trap” invading bacterial pathogens. Importantly, NETosis appears to be dependent upon the presence of reactive oxygen species and requires NADPH oxidase activity. Neutrophils stimulated in the presence of an NADPH oxidase inhibitor do not generate reactive oxygen species and do not form NETs. NETosis also requires histone hypercitrulination, primarily mediated by peptidylarginine deiminase 4 (PAD4) in neutrophils. Histone hypercitrulination deconvolutes and unfolds chromatin to allow externalization and NET formation. Neutrophils from PAD4 deficient mice demonstrate inability to form NETs upon stimulation.

A role for innate immune dysfunction in the pathogenesis of lupus has been increasingly appreciated. While the LE phenomenon (neutrophil phagocytosis of a nucleus or apoptotic body) has been recognized for decades and previously used as a diagnostic tool in lupus, a pathogenic role for neutrophils in lupus has only been recently included in the paradigm of lupus pathogenesis ( Table 17.1 ).

Neutrophil dysfunction in SLE

Lupus patients can develop a number of hematological abnormalities. Neutropenia, which is usually mild in lupus, has been reported in up to 50% of patients. Similar to other “cytopenias” in lupus, the pathogenesis of neutropenia is likely to be immune-mediated destruction of neutrophils. This is supported by the detection of neutrophil-recognizing antibodies in lupus patients. Indeed, neutropenia in lupus patients is associated with anti-Ro antibody, which can bind and fix complement on the neutrophil surface.

The percentage of apoptotic neutrophils in lupus patients is increased compared to healthy controls, and the clearance of apoptotic neutrophils in lupus seems to be impaired. Phagocytic ability of lupus neutrophils is also defective, and purified anticardiolipin antibodies from lupus patients have been shown to inhibit phagocytosis in neutrophils. Furthermore, the lupus risk genetic variant in ITGAM (encoding for CD11b, a subunit of Mac-1) contributes to Mac-1 dysfunction in neutrophils and is associated with impaired phagocytosis. In addition, a missense genetic variant in NCF1 (neutrophil cytosolic factor 1), encoding a subunit of the phagocyte NADPH oxidase (NOX2), is associated with reduced reactive oxygen species production and increased susceptibility to lupus. Together, these data suggest that increased neutrophil apoptosis and defective apoptotic clearance and phagocytosis might play a role in the pathogenesis of lupus by increasing autoantigen exposure and the subsequent enhanced autoimmune response. Recent evidence suggests that lupus neutrophils produce higher levels of type-I interferon and that neutrophil-mediated type-I interferon production in the bone marrow can drive abnormal B cell differentiation in lupus. Furthermore, transcriptional profiling of whole blood samples from lupus patients revealed an association between a neutrophil transcriptional signature and the development of lupus nephritis.