Consisting of five exons and four introns, the ELANE gene is found on chromosome 19p13.3. Its DNA sequence consists of about 5,000 base pairs. The ELANE gene encodes neutrophil elastase (NE), a chymotrypsin serine esterase synthesized during the transformation of myeloblasts to promyelocytic granulocytes. NE is part of the neutrophil serine proteases (NSP) complex (NSPs), which also includes PR3 and histone G [1,2,3]. NSPs is only expressed in mature myeloid monocytes and plays a key role in promoting inflammatory responses and the destruction of pathogens. Activation of the misfolded NE protein product and subsequent unfolded protein responses in accelerated apoptosis during myeloid cell differentiation, followed by a maturation arrest of neutrophil precursors at the promyelocytic/granulocyte stage.

Intracellularly, NE and lysosomes participate in microbial decomposition under the synergistic effect of antimicrobial peptide and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase system [4]. Extracellularly, NE uses a variety of Gram-negative bacilli as substrates to split their pathogenic factors [5]. In inflammatory reactions, NE destroys bacteria and host tissues. Therefore, NE also plays an important role in non-infectious inflammatory responses [4]. Chromatin binds to positively charged NSPs or histones to form neutrophil extracellular traps (NETs) [6]. NETs capture pathogens through physical barriers, and NETs-related NSPs kill pathogenic microorganisms by degrading their virulence factors. Therefore, a lack of NE will weaken organism’s ability to clear pathogens.

Neutropenia is the main clinical manifestation of ELANE mutations. It has been reported that mutations in the ELANE gene are found in 80–100% of cyclic neutropenia (CN) cases [7,8,9] and 35–63% of severe neutropenia (SCN) cases [9,10,11]. Both spontaneous and germline mutations may occur. Neutropenia-associated ELANE mutations are usually frame shift and termination mutations that result in structural changes in protein.

CN is an autosomal dominant disease characterized by regular fluctuation of peripheral neutrophils from near normal to severe low levels, generally with a cycle of 21 days, but also can be 2–4 weeks [12]. Clinical manifestations include fever, lymphadenitis, oral ulcer and infections such as sinusitis, pharyngitis, cellulitis, pneumonia, acute peritonitis [10, 13, 14]. The pathogen of infection is usually Gram-negative bacilli. Symptoms usually recur, but severe infections, sepsis, and even death are rare. It can also be asymptomatic. SCN is a congenital phagocytic defect in primary immunodeficiency diseases, which can be autosomal dominant or recessive inheritance. Reported more than germline mutations, spontaneous mutations are characterized by repeated severe infections starting from first few months after birth. They may present with omphalitis at first, followed by otitis media, pneumonia, skin or liver abscess. Patients can have permanent teeth fall out at very early age due to stomatitis and gingival hyperplasia. Neutrophil absolute count (ANC) was always lower than 0.2 × 109/L [10, 15]. The most common pathogens are Staphylococcus aureus and Gram-negative bacilli. Because SCN patients cannot produce pus, the absence of pus at infected lesions is a characteristic change of SCN. Bone marrow biopsy shows neutrophil precursor maturation stop at the promyelocytic/myeloid stage. Unlike CN, SCN is characterized by transition to myelodysplastic syndrome (MDS) or acute myeloid leukaemia (AML) [1, 16].

In our study, case 1 presented with recurrent fever, infection and paediatric systemic lupus erythematosus. His SLE was characterized by blood and kidney involvement, hypocomplementemia, and many autoantibodies tested positive, including high ANA, anti-Sm, anti-SSA, anti-U1RNP, lupus anticoagulant and anti-β2 glycoprotein I-IgG antibodies. Case 2 presented with repeated infection and autoimmune haemolytic anaemia. Case 3 presented with recurrent infections and ANCA-associated small vasculitis. The main manifestations of his ANCA-associated small vasculitis were pulmonary involvement and positive p-ANCA. All three patients had recurrent fever and infections. Case 1 had recurrent episodes, his fever and infections present at the same time, along with neutropenia. Infections in all three cases were mainly bacterial, along with Epstein-Barr virus infection in cases 1 and 3. Different from previous reports, none of the three had oral ulcers. According to literature, ELANE mutations can be associated with arthritis and pyoderma gangrenosum, but all of such mutations were reported as individual cases. There were no reports of SLE, ANCA-associated small vasculitis and autoimmune haemolytic anaemia. It has been reported that SCN caused by ELANE mutations can lead to myelodysplastic syndromes (MDS) or acute myeloid leukaemia (AML). However, in our study, cases 2 and 3 were presented as SCN, but no MDS or AML on bone marrow biopsy. All three cases in this group had germline mutations, and case 1 also had the neurofibromatosis type 1 (NF1) gene mutation. The ELANE mutation of case 1 came from his father, who also presented with CN, but no recurrent infections. The parents of cases 2 and 3 had no clinical presentation of CN or SCN.

Recombinant human granulocyte colony-stimulating factor (G-CSF) can elevate the ANC level to normal or nearly normal in CN patients. Low-dose G-CSF treatment can shorten the duration of neutropenia, but it cannot change the periodic nature of neutropenia. G-CSF is the first-choice treatment for SCN. The Severe Chronic Neutropenia International Registry (SCNIR) reported that more than 95% of patients respond to G-CSF treatment and ANC can increase to 1 × 109/L. Most children with SCN respond to a dose between 3 and 10 µg/kg/day [17,18,19]. For patients who do not respond to G-CSF, hematopoietic stem cell transplantation is the only treatment at present [20]. The incidence of MDS or AML is 40% after 10 years in SCN patients treated with G-CSF greater than 8ug/kg/day. However, this complication is relatively rare in CN patients.

Although CN and SCN are mainly caused by ELANE mutations, studies reported that different sites of ELANE mutations can cause different clinical phenotypes. In other words, the clinical presentation of CN or SCN depends on the site of ELANE mutation and its effect on NE activity. The genotype and phenotype may have a one-to-one correlation. In this study, the three children had different genotypes, which led to different amino acid changes. Although two of them had SCN, they had different autoimmune diseases. The other case had CN, and the combined autoimmune disease was also different from the other two cases, which was consistent with the possible one-to-one correlation between genotype and phenotype reported previously.

Our literature review did not find clear pathogenesis of ELANE mutations combined with autoimmune diseases. However, it has been shown that dysregulation of innate immune pathways related to host defence has profound effects on various aspects of SLE pathogenesis, including disruption of immune tolerance, induction of interferon and other proinflammatory cytokines, abnormal adaptive immunity, and tissue damages. Evidence found in human-mouse models, both in vivo and in vitro, supports that neutrophil dysregulation plays a key role in the pathogenesis of SLE, including loss of immune tolerance, induction and amplification of inflammatory pathways, tissue damages, vascular diseases, and cardio metabolic dysfunction [21,22,23,24]. Studies reported that genetic diseases that change the cellular components of innate immunity have been shown to increase the risk and severity of SLE. ELANE mutations lead to neutropenia, which plays an important role in innate immunity, weakening anti-bacterial ability and effect in coagulation, angiogenesis, inflammation resolution and tissue repair. In addition, neutrophils can regulate innate and adaptive immune cells. The three children in this study were combined with SLE, AIHA and ANCA-related small vasculitis, respectively. These three diseases are all characterized by systemic damage caused by autoantibodies. Therefore, AIHA and ANCA-related small vasculitis may also have a mechanism similar to that of SLE.

Autoimmune symptom associated with ELANE mutations is rare. We may choose different prednisolone and immunosuppressive therapies for different diseases. Based on our study, we concluded that for CN patients, such as case 1, oral Leucogen and subcutaneous injection of G-CSF can improve neutropenia and reduce recurrent infections. For patients with CN and autoimmune diseases, we can give oral glucocorticoid and immunosuppressor, such as hydroxychloroquine and cyclosporine. For patients with autoimmune diseases and severe organ damages, intravenous cyclophosphamide may work. Allogeneic hematopoietic stem cell transplantation can cure immune deficiency and immune disorder completely for SCN patients such as cases 2 and 3, and is effective for patients with poor response to G-CSF or combined with MDS/AML.