INTRODUCTION

Neutrophils are the most abundant leukocyte in the peripheral blood, and these cells are the first to arrive at inflamed sites. Although most investigators consider the involvement of neutrophils in the pathogenesis of SLE a relatively minor active role, this function of neutrophils in SLE was among the earliest experimental observations in patients with this disease. LE cells (neutrophils), neutropenia, and a role for neutrophils in the pathogenesis of immune complex-mediated vasculitis have been appreciated for many years. Intravascular neutrophil activation also plays a role in some cases of CNS lupus and pulmonary abnormalities observed in these patients. More recently, as the role for diminished clearance of immune reactants in the pathogenesis of SLE has been more widely appreciated, defective neutrophil-mediated clearance of apoptotic cells and immune complexes is thought to play a more basic role in the pathogenesis of SLE.

LE CELLS: A NEUTROPHIL-MEDIATED PHENOMENON

The discovery of the lupus erythematosus (LE) cell phenomenon by Hangraves and colleagues in 1948¹ was a pivotal milestone not only from a diagnostic aspect but from the standpoint of its pathophysiologic significance. As recent studies show, the LE cell represents a hallmark stage in the initiation and perpetuation of the autoimmune process that underlies the role of apoptosis (programmed cell death) in the pathogenesis of systemic lupus erythematosus (SLE). It has recently been shown that the LE cell results from phagocytosis of apoptotic bodies by neutrophils [polymorphonuclear cells (PMN)] induced by anti-double-stranded (ds) DNA autoantibodies.¹ Studies using flow cytometric analysis (FACS) of LE cells demonstrated the neutrophil as the phagocyte that engulfs apoptotic cells.¹ Anti-dsDNA antibodies are a serologic hallmark as well as a crucial factor in the pathogenesis of immune complex–mediated tissue damage in SLE.¹

The mechanism suggested to be involved in the phagocytic uptake of nuclei from apoptotic cells by neutrophils is induced by anti-dsDNA antibody-mediated inhibition of enzymatic cleavage of the nuclei.¹ The elucidation of the mechanism of production of the LE cells sheds light on the relevance and possible active role neutrophils play in the relationship between innate immune reaction presented by neutrophils and the specific immune response presented by antinuclear antibodies that underlie the pathogenesis of SLE.

NEUTROPHILS: THE ROLE OF INNATE IMMUNITY IN THE PATHOGENESIS OF SLE

Neutrophils (PMNs) are a key component of the primary host defense line against noxious pathogens (together with macrophages and dendritic cells). This host defense line comprises the cellular arm of the innate immunity. Impaired clearance of apoptotic cells has been suggested as a crucial factor in the pathogenesis of SLE.¹ To succeed with their physiologic reactions (such as chemotaxis, degranulation and phagocytosis) neutrophils become activated by various stimuli, including bacterial products, complement split products (such as C5a), immune complexes (ICs), chemokines, and cytokines [such as interleukin (IL)-1 and −8]. PMN activation is initiated upon recognition of antibody- or complement-opsonized particles as well as directly by microbial products via the toll-like receptors.^7,8 Table 23.1 summarizes the main neutrophil surface receptors and their expression regulation in SLE. Neutrophils constitutively express surface complement receptors (CR), CR1 (CD35), and CR3 (β₂-integrin, CD11b/CD18, Mac-1), which recognize C₃b/iC₃b split products of the complement system.

TABLE 23.1 NEUTROPHIL SURFACE RECEPTORS AND THEIR EXPRESSION IN SLE

IC are abundantly present in the serum as well as deposited in target organs affected in SLE. Neutrophil interaction with IC is mediated through binding to CR and Fcγ receptors (FcγR) present on these cells. Neutrophil activation in patients with SLE is mediated by IC binding to FcγR, as reflected by a unique cell adhesion expression. Neutrophils constitutively express β₂ integrins (CR3, CD11b/CD18) and L-selectin (LS, CD62L) on their surface, which play a role in the adhesion of neutrophils to endothelium and their egress to an extravascular site of inflammation. Upon neutrophil activation by chemoattractants [such as C5a, IL-8, and formylmethionyl-leucyl phenylalanine (FMLP)] there is an up-regulation of β₂-integrin with a concomitant shedding (down-regulation) of LS.

By contrast, in patients with active SLE there is a marked up-regulation of β₂ integrin surface molecules with no change in the expression of LS.¹ This discrepancy between the lack of LS shedding despite an up-regulation of β₂-integrin was shown to be related to FcγR activation by IC.^9,10 Other studies confirmed that neutrophils are activated in the circulation of patients with active SLE, as reflected by increased expression of CD11b/CD18 on their surface with no decrease in LS expression.¹ Moreover, the expression of β₂-integrin on the surface of circulating neutrophils correlated positively with activity scores in patients with SLE.^11,12 In contrast, there was no correlation between LS expression and disease activity.¹

Interaction between FcR expressed on phagocytic cells and antibodies plays a critical role in innate immune response. Neutrophils (the major type of phagocytic cell in the blood) express two types of FcR [FcγR-IIA (CD32) and FcγR-IIIB (CD16)], both of which bind effectively to IC. IC binding to FcR results in neutrophil activation.¹ In addition, FcR expressed on phagocytic cells are important in the process of IC clearance from the circulation. There is evidence suggesting that Fc-mediated clearance of IC is defective in patients with SLE.¹ Allelic variants of FcγR are common within the general population. However, certain polymorphism patterns have been linked to increased susceptibility to SLE.¹ Several studies addressing the association of FcγR-IIA and IIIA polymorphism and susceptibility for SLE in various ethnic groups yielded inconclusive results.

In a meta-analysis comprising 17 studies, the homozygosity for the R131 allele of the FcγR-IIA was associated with a 1.3-fold greater risk for developing SLE but not for lupus nephritis.¹ The evidence for an SLE susceptibility was not conclusive for patients of European descent compared to those of Asian and African descent. Another meta-analysis looking for the association of V/F158 genotype of FcγR IIIA demonstrated that the F158 allele poses a 20% greater risk for the development of lupus nephritis.¹ FF homozygotes were more prone to renal damage as compared to VV homozygotes (odds ratio 1.47).

These polymorphisms are associated with low-affinity FcγR, which have a lower capacity to clear ICs. The half-life of IgG-coated erythrocytes in the blood was prolonged in lupus patients expressing the FcγR-IIA-R/R131 genotype, whereas the homozygous genotype FcγR-IIIA-F (F158) was associated with arthritis and/or serositis in these patients.¹ This association of FcγR polymorphism with an increased risk of SLE and/or certain manifestations of the disease emphasizes the importance of neutrophils in the process of IC clearance, in that a delay in clearance results in greater IC deposition in tissues and organs such as the kidneys and blood vessels.

FcγRs play another role in the pathogenesis of SLE; namely, clearance of apoptotic cells. The half-life of circulating neutrophils is about 7 hours, and following egress to an extravascular tissue they die within 1 to 2 days. Under normal conditions, neutrophils are removed from the circulation as well as from inflamed tissue by apoptosis, genetically programmed cell death.¹ Delayed neutrophil apoptosis has been associated with autoimmune disorders, especially so in SLE where nuclear materials serve as autoantigens for the production of an array of antinuclear antibodies.¹ Nucleosomes are known as major autoantigens in SLE and are exposed at the cell surface in apoptosis. Thus, increased apoptosis and/or delayed clearance of apoptotic cells could contribute to an overload of autoantigens in the circulation that will eventually lead to an increased nucleosome-containing IC deposition in target tissues. Both increased neutrophil apoptosis and delayed removal of apoptotic neutrophils from the circulation have been described in lupus.^21,22

Increased neutrophil apoptosis was correlated with disease activity and could serve as a source for dsDNA, as suggested by a positive correlation between antibodies to dsDNA and increased neutrophil apoptosis found in lupus patients.¹ Moreover, delayed clearance of apoptotic neutrophils was observed in lupus and suggested as a major step in the pathogenesis of SLE.¹ A decreased ability of macrophages to ingest apoptotic neutrophils was found in SLE and inversely correlated with activity scores.¹ Taken together, these data indicate that increased neutrophil apoptosis together with decreased macrophage clearance of apoptotic cells yield a large nuclear-derived autoantigen burden that may induce or facilitate the formation of nucleosome-immunoglobulin complexes.

Various mechanisms have been suggested to explain the delayed clearance of apoptotic cells in lupus, such as complement deficiency and low pentraxin levels. With regard to delayed clearance of apoptotic neutrophils in SLE, reduced expression of CD44 and resistance to the apoptosis-inhibiting effects of granulocyte-colony stimulation factors (G-CSFs) were suggested as possible underlying mechanisms.^23,24 In conclusion, cumulative data suggest that neutrophil apoptosis plays a major role in the pathogenesis of SLE by serving as the largest source of nucleosomes and nuclear debris for autoantigens resulting in production of anti-dsDNA and other antinuclear antibodies in lupus.