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. 2018 Mar 15;154(4):604–612. doi: 10.1111/imm.12908

The role of neutrophils in skin damage induced by tissue‐deposited lupus IgG

Xuanxuan Guo 1, Xiang Fang 1, Guodan He 1, Muhammad Haidar Zaman 1, Xibin Fei 1, Wei Qiao 1, Guo‐Min Deng 1,2,3,✉
PMCID: PMC6050218  PMID: 29450882

Summary

Skin injury is the second most common clinical manifestation in patients with systemic lupus erythematosus (SLE). Neutrophils are crucial effector cells in the immune system but the significance of neutrophils in the pathogenesis of SLE is not clear. This study is to explore the role of neutrophils in the skin damage of SLE. We used lupus‐prone mice and a C57BL/6 mouse model of lupus serum IgG‐induced skin inflammation to investigate the role of neutrophils in skin damage of SLE. We found that a few neutrophils infiltrated the inflammatory sites of skin in lupus‐prone mice and the lupus‐IgG‐induced skin damage mouse model. Depletion of neutrophils did not affect the development of skin inflammation caused by lupus IgG, and lupus IgG can induce apoptosis of neutrophils. The apoptosis of neutrophils induced by lupus IgG is related to FcγRIII and Fas/Fas ligand pathways. Our study indicates that neutrophils are not major contributors in the skin damage caused by tissue‐deposited lupus IgG but death of neutrophils caused by lupus IgG may provide a resource of a large amount of autoantigens in SLE.

Keywords: apoptosis, lupus‐IgG, neutrophils, systemic lupus erythematosus

Abbreviations

FasL

Fas ligand

FcR

Fc receptor

H&E

haematoxylin & eosin

LDG

low‐density granulocyte

Mono

mononuclear cells

NAS‐DCE

naphthol AS‐D choloracetate

NET

neutrophils extracellular trap

Poly

polymorphonuclear cells

SLE

systemic lupus erythematosus.

Introduction

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease that predominantly affects women and is typically accompanied by high levels of autoantibodies and multi‐organ tissue damage.1, 2 The exact pathogenesis remains unclear. Skin injury is the second commonest manifestation in patients with SLE.3, 4

Neutrophils are crucial effector cells in innate immune responses and also act as important regulators of adaptive immunity.5 They are the most abundant leucocytes in human blood and have a short lifespan.6, 7, 8 To maintain homeostasis, neutrophils are continuously released into the blood circulation after differentiation and maturation in bone marrow.9

Neutrophils are powerful effector cells against infectious threats. After recruitment in the inflammatory foci, activated neutrophils destroy the invaders through phagocytosis, degranulation, reactive oxygen species and neutrophil extracellular traps (NETs) with high efficiency.10, 11 However, the excess infiltration and activation of neutrophils at sites of tissue damage required to fight with pathogenic factors, can also cause chronic inflammation, impair injury repair and lead to loss of organ function.7, 12

As an important component in the immune system, neutrophils express various Fc receptors that are primarily involved in the recognition of IgG‐opsonized pathogens and also participate in immune complex‐mediated inflammatory processes.13 Among various kinds of Fc receptors in neutrophils, the low‐affinity Fcγ receptor(FcγR) is the most important.14 Mouse neutrophils express FcγRIII and FcγRIV,15 which are transmembrane adaptor proteins, the Fc‐receptor γ‐chain. They carry an immunoreceptor tyrosine‐based activation motif in their intracellular tail for expression and signalling.

High levels of autoantibodies in serum are a feature in patients with SLE.1, 2 Deposition of IgG occurs in tissue lesions of skin, and lupus IgG is a major contributor to the development of skin inflammation induced by lupus serum.3 In skin inflammation induced by lupus IgG, large amounts of mononuclear cell infiltration were observed.3 Although lymphocytes play an important role in the pathogenesis of SLE,16, 17 it is monocytes not lymphocytes that are essential in the development of skin inflammation induced by lupus serum IgG.3 The role of neutrophils in skin inflammation induced by lupus IgG is not clear.

In this research we investigated the role of neutrophils in the development of skin tissue damage in lupus‐prone mouse and skin tissue damage caused by lupus IgG in a C57BL/6 mouse model. Our data indicate that neutrophils do not exert a crucial role in the development of skin damage induced by skin‐deposited lupus IgG, even though lupus IgG can induce the apoptosis of neutrophils. But neutrophils may exert a key role in the production of high levels of autoantibodies in SLE by providing a resource of autoantigens. Our data impart another clue to explain the neutropenia found in the blood of patients with SLE.18

Materials and methods

Mice and reagents

FcγRIIb−/− (002848), FcγRIII−/− (003171) and MRL/lpr (000485) mice were obtained from the Jackson Laboratory (Bar Harbor, ME). B6.MRL/lpr(J000482)mice and C57BL/6 mice were purchased from the Model Animal Research Centre of Nanjing University. All mice were maintained in specific pathogen‐free conditions at the Animal Centre of Nanjing Medical University. The animals had access to food and water ad libitum. The protocol of the animal experiments was approved by the Nanjing Medical University Institutional Animal Care and Use Committee.

For IgG purification, protein G agarose beads were purchased from Sigma‐Aldrich (St Louis, MO). Lupus serum was kindly provided by the First Affiliated Hospital of Nanjing Medical University. Serum from patients fulfilling at least four of 11 revised criteria of the American College of Rheumatology for the classification of SLE was studied. All patients had SLE disease‐activity index scores ranging from 0 to 18, with one normal individual (from X.G.) serving as controls within this study. Detailed information of SLE patients is shown in Table 1. For flow cytometry assay, LY‐6G peridinin chlorophyll protein‐eFluor 710 (BD, Franklin Lakes, NJ), allophycocyanin‐conjugated anti‐mouse/human CD11b (BioLegend, San Diego, CA) and phycoerythrin‐conjugated anti‐mouse CD178 (Fas ligand) (eBioscience, San Diego, CA) were used. HISTOPAQUE‐1119 (Sigma); Percoll (Solarbio, Beijing, China) and lysing buffer (BD) were applied for neutrophil purification. For detection of apoptosis, FITC Annexin V Apoptosis Detection Kit I (BD) and an In Situ Cell Death Detection Kit, POD (Roche, Basel, Switzerland) were used.

Table 1.

Clinical information of patients with systemic lupus erythematosus used in this study

Sample Gender (F/M) Age (years) SLEDAI Treatment Antibody status
SLE 1 F 43 8 Methylprednisolone Chloroquine anti‐dsDNA(+) anti‐histone(+) anti‐nucleosome(+) anti‐SSA/Ro60kD(+) anti‐SSA/Ro52kD(+) anti‐SSB/La(+)
SLE2 F 54 19 Chloroquine anti‐dsDNA(+) anti‐SSA/Ro52kD(+) anti‐nRNP/Sm(+)
SLE3 F 45 4 Prednisolone Chloroquine anti‐Sm(+) anti‐RNP(+)
SLE4 F 47 8 N/A anti‐dsDNA(+) anti‐Sm(+) anti‐SSA(+) anti‐SSB(+)
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SLE, systemic lupus erythematosus; SLEDAI, systemic lupus erythematosus disease‐activity index.

Isolation of neutrophils from murine bone marrow and human blood

Neutrophils were isolated from murine bone marrow according to the protocol described previously.19 The procedure was as follows: murine bone marrow cells were collected by flushing both ends of the bone shafts with phosphate‐buffered salein (PBS) (sterile). Cells were separated by density gradient centrifugation in a 15‐ml conical tube: overlaying 3 ml of Histopaque 1119 (density, 1·119 g/ml) at the bottom, and then adding 3 ml of Histopaque 1077 (density, 1·077 g/ml), finally adding 1 ml cell‐containing PBS in the upper layer. After centrifugation for 30 min at 800 g, neutrophils were collected from the interface of Histopaque 1119 and Histopaque 1077. Neutrophils from human peripheral blood were isolated by using Percoll: 80% and 60% density gradients of Percoll were prepared for centrifugation at 800 g. Neutrophils were gained from the interface of the 80% and 60% Percoll gradients.

Flow cytometry

After culturing, the neutrophils were washed three times and suspended with PBS or binding buffer. Then, the cells were incubated with antibodies (LY‐6G, CD11b, CD178) in the dark for 40 min at 4°. The cells were washed again before the flow cytometry test. For apoptosis detection, they were analysed within 1 hr after incubation with Annexin V and propidium iodide for 15 min at room temperature. Annexin V staining can identify apoptosis at an earlier stage, propidium iodide is used to distinguish viable from non‐viable cells.

Isolation of IgG from lupus serum

Lupus IgG was isolated from lupus patient serum collected at the First Affiliated Hospital of Nanjing Medical University, using protein G agarose beads. Protein G can bind the Fc region of IgG with high affinity at neutral to basic pH. We used 100 mm glycine HCl buffer (pH 2·7) to elute the protein G agarose beads with bound IgG. The IgG remained in the glycine HCl and separated from protein G agarose beads. Finally the pH of the eluted IgG in glycine HCl buffer was adjusted to pH 7·0.

Histopathological staining and immunohistochemical examination

Tissue sections of skin were collected from mice at 3 days and 1 day after intradermal injection of lupus IgG. All the tissue sections were made after routine fixation and paraffin embedded. Tissue sections were cut and stained with haematoxylin & eosin (H&E). All slides were coded and evaluated in a blinded manner. The severity of skin lesion inflammation was judged on the following score from 0 to 4. Grade 0 for normal tissues, grade 1 for hyperplasia of the epidermis, grades 2–4 for different amounts of infiltrating inflammatory cells in the skin.3

Neutrophil depletion procedure

Neutrophils were depleted by injecting 1 mg anti‐Ly6G antibody (clone 1A8) (Bio X Cell, West Lebanon, NH) intraperitoneally, 24 hr before intradermal injection of lupus IgG, and with 0·5 mg anti‐Ly6G antibody 24 hr after lupus IgG injection. The efficiency of cell depletion was detected by flow cytometry.

Statistical analysis

Statistical analyses were performed using the Student's t‐test; P‐values ≤ 0·05 were considered statistically significant.

Results

Neutrophils are not the major inflammatory cells in skin damage of lupus‐prone mice and lupus IgG‐induced mouse models

To investigate the role of neutrophils in the development of skin tissue damage in SLE, we used lupus‐prone mice that spontaneously develop SLE‐like clinical manifestations.20 Histopathological examination showed skin inflammation in B6.MRL/lpr mice. To further confirm these data, we used skin sections of MRL/lpr mice (Fig. 1a) and observed the skin inflammation. Based on the neutrophil property of having a polymorphic nucleus, we analysed the ratio of mononuclear and polymorphonuclear cells in inflammatory sites of skin tissue sections using H&E staining. We found that there were very few polymorphonuclear cells in the inflammatory sites of skin from lupus‐prone mice (Fig. 1b). To further ensure the low participation of neutrophils, we used naphthol AS‐D chloroacetate(NAS‐DCE) staining, which is specific staining for neutrophils.21 After analysing the ratio of mononuclear cells and neutrophils in inflammatory sites, we found few infiltrating neutrophils in the damaged skin site (Fig. 1c). These data indicate that neutrophils are not the main inflammatory cells in the development of skin injury in lupus‐prone mice.

Figure 1.

Figure 1

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Involvement of neutrophils in skin injury of lupus‐prone mice and lupus‐IgG‐induced model mice. (a) Representative pictures of histopathological examination of skin in female B6.MRL/lpr(about 30 weeks old), MRL/lpr (about 25 weeks old) and C57BL/6 mice. Original magnification ×1000. (b) The number of mononuclear cells and polymorphonuclear cells infiltrating skin of female MRL/lpr mice. (c) Representative pictures of neutrophil‐specific staining and counts of neutrophils in tissue sections of skin in MRL/lpr mice using naphthol AS‐D chloroacetate (NAS‐DCE). Red arrow refers to neutrophils. Original magnification ×1000. (d) Representative pictures of histopathological examination and evaluation of mononuclear cells and polymorphonuclear cells in skin inflammation induced by lupus IgG. Original magnification ×1000. (e) The number of mononuclear cells and polymorphonuclear cells infiltrating skin inflammation induced by lupus IgG. (f) Representative pictures of neutrophil‐positive staining and evaluation of neutrophils in skin inflammation induced by lupus IgG. Red arrow refers to neutrophils. Original magnification ×1000.

As previously observed, skin inflammation was induced by intradermal injection of lupus IgG from patients with SLE and lupus‐prone mice.3 (Fig. 1d) Hence, we further investigated whether neutrophils are involved in the development of skin inflammation induced by lupus IgG. We found that there was very little multinuclear cell infiltration in sites of skin inflammation caused by lupus IgG (Fig. 1e). To further confirm the result obtained from H&E staining, we performed NAS‐DCE staining and found very few NAS‐DCE staining positive cells in inflammatory sites induced by lupus IgG (Fig. 1f). So these data also indicate that neutrophils are not major inflammatory cells in skin inflammation induced by lupus IgG.

Depletion of neutrophils did not affect the development of skin inflammation caused by lupus IgG

As an important immune system response, neutrophils migrate rapidly towards the inflammation foci.7 However, we did not find large amounts of infiltrated neutrophils at sites of tissue damage in either lupus‐prone mice or lupus‐IgG‐injected mouse models. To investigate whether neutrophils contribute to skin inflammation induced by lupus IgG, we depleted neutrophils in experimental mice by anti‐Ly6G antibody.22 Neutrophil depletion was confirmed by flow cytometry (Fig. 2a). The experimental mouse skin was collected for histopathological examination on day 3 after intradermal injection of lupus IgG. The results showed that severity of skin inflammation was not affected in mice by depleting the neutrophils compared with mice without the depletion of neutrophils (Fig. 2b). As neutrophils are recruited rapidly during the immune response, neutrophils may still have a certain role in the early stage of inflammation induced by lupus IgG. Hence, we determined whether neutrophils are important in early stages of skin inflammation induced by lupus IgG. We collected experimental mouse skin on day 1 after intradermal injection of lupus IgG in neutrophil‐depleted mice and control mice and found that the severity of skin inflammation was not significantly different between mice with and without neutrophil depletion (Fig. 2c). Those studies suggest that neutrophils did not play a crucial role in the development of skin inflammation induced by lupus IgG.

Figure 2.

Figure 2

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The effect of neutrophil depletion in the development of skin inflammation induced by lupus IgG. (a) Flow cytometry was used to evaluate the depletion efficiency of neutrophils from peripheral blood in C57BL/6 mice following anti‐LY6G antibody treatment. (b) Representative histopathological picture and severity of skin inflammation 3 days after intradermal inoculation of lupus‐IgG in C57BL/6 mice with or without depletion of neutrophils (n = 5 per group). Original magnification ×200. (c) Representative histopathological picture and severity of skin inflammation 1 day after intradermal inoculation of lupus‐IgG in C57BL/6 mice with or without the depletion of neutrophils (n = 5 per group). Original magnification ×200.

Neutrophils may undergo apoptosis, which might be the reason for their absence in the development of skin inflammation induced by lupus IgG

According to the experiments in Figs 1 and 2, we know that neutrophils are not the major inflammatory cells in SLE‐derived skin injuries and have no meaningful contribution to the skin inflammation induced by lupus IgG. Therefore, we suspect whether neutrophils are recruited into the site of damage during the whole process of skin inflammation induced by lupus IgG. So we investigated the infiltration of neutrophils during the pathological process of lupus‐IgG‐induced skin inflammation and found that the infiltration of neutrophils peaked at 24 hr, and dropped after that (Fig. 3a). The results showed that neutrophils were actually involved in the skin inflammation induced by lupus IgG. According to the features of skin inflammation induced by lupus IgG, the lesions appeared at 3 hr, peaked at 3 days after injection, and lasted for at least 14 days. So, where did the neutrophils go after being recruited to the foci?

Figure 3.

Figure 3

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Neutrophils may undergo apoptosis after infiltrating the lupus skin lesion. (a) Counts of neutrophils in the pathological process of skin inflammation induced by lupus IgG. The results were from three independent experiments. **P < 0·01. (b) Representative photograph of positive fragments stained by NAS‐DCE in skin inflammation of female MRL/lpr mice, lupus‐IgG‐induced model mice and normal tissues of C57BL/6 mice. Original magnification ×1000. (c) Representative photomicrograph of TUNEL staining in skin inflammation of female MRL/lpr mice, lupus‐IgG‐induced model mice and normal tissues of C57BL/6 mice. Original magnification ×400. (d) Representative photomicrograph of IgG deposition stained through immunohistochemistry in skin inflammation of female MRL/lpr mice, lupus IgG‐induced model mice and normal tissues of C57BL/6 mice. Original magnification ×400.

Although we did not find large numbers of positive NAS‐DCE‐stained neutrophils, we observed fragments of positive NAS‐DCE staining in inflammatory sites of skin from MRL/lpr mice and lupus‐IgG‐induced model mice (Fig. 3b). There were many positive cells by TUNEL staining in skin lesions from MRL/lpr mice and the lupus‐IgG‐induced model mice (Fig. 3c). We also observed the IgG deposition in those skin sections (Fig. 3d). Those data suggest that neutrophils may undergo apoptosis in inflammatory sites induced by lupus IgG.

Lupus IgG directly leads to apoptosis of neutrophils

To determine whether lupus IgG induces apoptosis of neutrophils, neutrophils were isolated from mouse bone marrow, and the purity of neutrophils was detected by flow cytometry (Fig. 4a). Then, to confirm apoptosis of neutrophils induced by lupus IgG, we assessed the apoptosis of neutrophils by flow cytometry after incubation with lupus IgG in vitro for 18 hr. The results showed that lupus IgG significantly enhanced apoptosis of neutrophils and the ratio of apoptosis depends on the dose of lupus IgG (Fig. 4b). TUNEL staining confirmed the apoptosis intensity of neutrophils induced by lupus IgG (Fig. 4c).

Figure 4.

Figure 4

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Apoptosis of neutrophil induced by lupus IgG. (a) The purity of neutrophils separated from mouse bone marrow measured by flow cytometry. (b) Apoptosis of mouse neutrophils induced by various doses of lupus IgG detected by flow cytometry. The results were from three independent experiments. (c) TUNEL‐staining of apoptotic mouse neutrophils induced by lupus IgG in vivo. Original magnification ×400. (d) Human neutrophils isolated from peripheral blood demonstrated by haematoxylin & eosin staining. Original magnification ×400. (e) Apoptosis of human neutrophils after incubation with lupus IgG for 12 hr detected by flow cytometry.

We also further investigated whether lupus IgG triggers apoptosis of neutrophils from human blood and found that it did (Fig. 4d,e). Hence lupus IgG can trigger apoptosis of neutrophils from human and mouse.

FcγRIII and Fas/FasL pathways play a role in the apoptosis of neutrophils induced by lupus IgG

To understand the mechanism of apoptosis of neutrophils induced by lupus IgG, we first investigated the role of Fc receptor (FcR) in neutrophil apoptosis induced by lupus IgG. Neutrophils express various FcRs that bind to IgG in cell surfaces.23 The different IgG subclasses bind with varying affinities and specificities to the different FcγRs.24 To determine the effect of FcγR on neutrophil apoptosis induced by lupus IgG, we used neutrophils isolated from FcγRIII and FcγRIII‐deficient mice. We found that apoptosis induced by lupus IgG decreased in neutrophils with FcγRIII deficiency(Fig. 5a) but not with FcγRII deficiency(data not shown). These data indicate that FcγRIII may play an important role in neutrophil apoptosis induced by lupus IgG.

Figure 5.

Figure 5

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The apoptosis of neutrophils induced by lupus IgG is related to FcγRIII and Fas/FasL pathways. (a) Apoptosis of neutrophils from C57BL/6, FcγRIII−/− mice induced by lupus‐IgG for 18 hr detected by flow cytometry and the comparison of relative apoptotic ratio with each control group of neutrophils from C57BL/6 and FcγRIII−/− mice. (b) Representative picture and numbers of NAS‐DCE positively stained neutrophils in skin lesions caused by lupus IgG in C57BL/6 mice and in skin lesion of Faslpr−/− mice. Red arrow refers to neutrophils. Original magnification ×1000. (c) Apoptosis of neutrophils separated from C57BL/6 and lupus‐prone mice (Faslpr−/−) induced by lupus IgG for 18 hr measured by flow cytometry and the comparison of relative apoptotic ratio with each control group of neutrophils from C57BL/6 and Faslpr−/− mice. (d) The expression of FasL in neutrophils separated from C57BL/6 mice after culturing with lupus IgG for 18 hr analysed by flow cytometry. (e) FasL expression of neutrophils separated from C57BL/6 and Fcγ RIII −/− mice after incubation with lupus IgG for 18 hr analysed by flow cytometry.

According to the results of NAS‐DCE staining of both MRL/lpr and lupus‐IgG‐induced mouse skin sections, we found that apoptosis of neutrophils decreased in MRL/lpr mice compared with lupus IgG‐induced mice (Fig. 5b). Lupus‐prone mice have Fas gene deficiency so these data suggest that Fas/FasL is involved in apoptosis of neutrophils induced by lupus IgG. To further investigate the effect of Fas/FasL on apoptosis of neutrophils induced by lupus IgG, neutrophils isolated from lupus‐prone mice were incubated with lupus IgG in vitro. Flow cytometry analysis showed that apoptosis of neutrophils from lupus‐prone mice was decreased compared with neutrophils from normal mice (Fig. 5c). We also found that lupus IgG increased FasL expression on neutrophils (Fig. 5d). These data indicate that Fas/FasL may play a role in the apoptosis of neutrophils induced by lupus IgG. To explore the effect of FcγRIII in neutrophil apoptosis, neutrophils were isolated from FcγRIII‐deficient mice and flow cytometry was performed. As a result we found that expression of FasL induced by lupus IgG decreased in neutrophils with FcγRIII deficiency (Fig. 5e). Hence the results suggest that activation of FcγRIII contributes to the Fas/FasL pathway of neutrophil apoptosis induced by lupus IgG.

Discussion

Our studies demonstrate that there is little involvement of neutrophils in skin damage in lupus‐prone mice and lupus‐IgG‐induced mouse models, depletion of neutrophils did not affect the development of skin inflammation induced by lupus IgG.

We were surprised by the results obtained from our studies. It is known that neutrophils are major immune system leucocytes and are always first to be recruited to and reach the inflammatory sites,7 and play a crucial role in innate immunity5 and autoimmune arthritis.25 However, several lines of evidence and the results of our studies strongly indicate that neutrophils do not play an important role in the skin inflammation induced by lupus IgG. The presence of high‐level autoantibodies in serum is a feature of SLE, and IgG deposition in different organs is related to those organs’ tissue damage, such as damage of skin, brain and kidney tissues.1, 3 IgG is also a major contributor to skin inflammation induced by lupus serum.3, 26 Then, we found that there was very little or almost no polymorphonuclear cell infiltration and neutrophil‐positive staining in inflammatory sites of skin tissue. Next, depletion of neutrophils did not affect the development of skin inflammation induced by lupus IgG. These findings suggest that neutrophils are not important in the development of the organ tissue damage induced by deposited lupus IgG.

The infiltration of neutrophils during the pathological process of lupus‐IgG‐induced skin inflammation peaked at 24 hr and then dropped. Specific staining of neutrophil fragments and apoptotic cells was found in inflammatory sites where lupus IgG was deposited. The lupus IgG led to apoptosis of neutrophils directly. Lupus IgG induced activation of FcγRs in neutrophils and caused the death of neutrophils through increased FasL expression, indicating that lupus IgG leads to apoptosis of neutrophils directly and this effect is related to the FcγRIII and Fas/FasL pathways.

Our studies demonstrate that mononuclear cells are major inflammatory cells in the damage of skin in lupus‐prone mice. Monocytes/macrophages, but not T and B cells, play important roles in the development of skin inflammation induced by lupus IgG.3 Patients with recalcitrant skin injury treated with the monocyte‐differentiation blocker azathioprine can respond well.27 These data strongly suggest that monocytes/macrophages are major inflammatory cells in the damage of organ tissue in SLE.

It is known that neutropenia in blood is common in patients with SLE.18 Moreover, we did not find large numbers of infiltrating neutrophils in the tissue inflammation from lupus‐prone mice and lupus‐IgG‐injected mouse models. Hence, it is not clear where neutrophils disappear and which factor makes neutropenia in SLE.

Clinical study shows an increase of apoptotic polymorphonuclear cells in patients with SLE.28 In our in vitro experiments we found apoptosis of neutrophils induced by lupus IgG and our in vivo experiments confirmed the presence of NAS‐DCE‐stained fragments of neutrophils around inflammatory sites of tissue. Apoptosis of neutrophils induced by lupus IgG is associated with activating FcγR and Fas/FasL. These data indicate that apoptosis of neutrophils induced by lupus IgG may lead to deficiency of neutrophils in inflammatory lesions induced by deposited lupus IgG in organ tissue.

According to previous studies, monocytes and tumour necrosis factor‐α play an important role in lupus skin inflammation. Hence, the existence of large amounts of tumour necrosis factor‐α in the foci could also make a contribution to the neutrophil apoptosis.3, 29 This might explain why few neutrophils infiltrated skin injuries of lupus‐prone mice, which have an apoptosis defect through the Fas/FasL pathways.

Apoptosis of neutrophils induced by lupus IgG or immune complexes may be the source of autoantigens in patients with SLE. Recent studies reported a distinct subset of neutrophils30, 31 – low‐density granulocytes isolated from the peripheral blood of patients with SLE – representing an abnormal kind of neutrophil.32, 33 They also have high capacity to form NETs and are easily activated for apoptosis by microorganisms and cytokines when compared with normal density SLE‐derived neutrophils and neutrophils from control groups.34 Apoptosis of netting neutrophils are found in skin lesions of patients with SLE.34 This suggests that apoptosis of neutrophils might be a source of autoantigens in SLE.

Although neutrophils are not major contributors to the damage of organ tissue caused by deposition of lupus IgG but neutrophils may play an important role in high‐level production of autoantibodies in the initiation of SLE. Our data recognize the role of neutrophils in SLE‐derived skin damage and help to understand that apoptosis of neutrophils induced by immune complexes may provide a source of autoantibodies in SLE.

Disclosures

The authors report no conflicts of interest.

Acknowledgements

Financial support was provided by the Research Initiating Fund of Nanjing Medical University (Ky101RC071203) (Deng GM) and National Natural Science Fund of China (81472111)(Deng GM).

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