Autoantibodies against complement C1q in patients with Behcet's disease: association with vascular involvement

Anti-C1q antibodies in lupus nephritis children with glomerular microthrombosis

AIM

Glomerular microthrombosis (GMT) was a common vascular lesion in patients with lupus nephritis (LN). The objective of this study was to investigate the relationship between serum anti-beta2-glycoprotein I antibodies (a-β2GP1) and anti-complement 1q antibodies (a-C1q) antibodies and to investigate the possible mechanism of GMT in children with LN.

METHODS

The subjects were 191 children with LN diagnosed by renal biopsy in our hospital from January 2017 to January 2020. The patients were divided into GMT group and non-GMT group. The clinical manifestations, laboratory tests, renal pathology, prognosis of the two groups and the relationship between a-β2GP1 and a-C1q antibodies were observed.

RESULTS

In 191 children with LN, 52 cases (27.23%) presented with GMT. The value of C3, haemoglobin (Hb), estimate glomerular filtration rate (eGFR) and anticardiolipin antibody (ACA) in GMT group were lower than that of non-GMT group (p < .05, p < .01). The value of serum creatinine (Scr), 24 h proteinuria (PRO), urine red blood cells (RBC), N-acetyl-β-d-glucosidase (NAG) and retinol-binding protein (RBP), a-C1q, a-β2GP1, Systemic Lupus Erythematosus Disease Activity Index (SLEDAI) and renal histopathological activity index (AI) score in GMT group were higher than that of non-GMT group (p < .05, p < .01). The positive proportions of serum a-C1q and a-β2GP1 in GMT group were higher than those in non-GMT group (p < .05). According to Spearman correlation analysis, a-C1q was positively correlated with AI score, SLEDAI, a-β2GP1, GMT, LN-III and LN-IV. Hb, eGFR and a-C1q Ab were associated with the formation of GMT in children with LN. The complete proteinuria remission and renal survival in GMT group were significantly lower than those in non-GMT group (p < .05, p < .01).

CONCLUSION

LN children with GMT had more severe clinical manifestations and renal pathologic damages, and poor outcome. Serum a-C1q level was positively correlated with a-β2GP1, and a-β2GP1 may be involved in the formation of GMT in children with LN, which might involve in the activation of complement classical pathway.

The Immunopathology of Complement Proteins and Innate Immunity in Autoimmune Disease

The complement is a powerful cascade of the innate immunity and also acts as a bridge between innate and acquired immune defence. Complement activation can occur via three distinct pathways, the classical, alternative and lectin pathways, each resulting in the common terminal pathway. Complement activation results in the release of a range of biologically active molecules that significantly contribute to immune surveillance and tissue homeostasis. Several soluble and membrane-bound regulatory proteins restrict complement activation in order to prevent complement-mediated autologous damage, consumption and exacerbated inflammation. The crucial role of complement in the host homeostasis is illustrated by association of both complement deficiency and overactivation with severe and life-threatening diseases. Autoantibodies targeting complement components have been described to alter expression and/or function of target protein resulting in a dysregulation of the delicate equilibrium between activation and inhibition of complement. The spectrum of diseases associated with complement autoantibodies depends on which complement protein and activation pathway are targeted, ranging from autoimmune disorders to kidney and vascular diseases. Nevertheless, these autoantibodies have been identified as differential biomarkers for diagnosis or follow-up of disease only in a small number of clinical conditions. For some autoantibodies, a clear relationship with clinical manifestations has been identified, such as anti-C1q, anti-Factor H, anti-C1 Inhibitor antibodies and C3 nephritic factor. For other autoantibodies, the origin and the functional consequences still remain to be elucidated, questioning about the pathophysiological significance of these autoantibodies, such as anti-mannose binding lectin, anti-Factor I, anti-Factor B and anti-C3b antibodies. The detection of autoantibodies targeting complement components is performed in specialized laboratories; however, there is no consensus on detection methods and standardization of the assays is a real challenge. This review summarizes the current panorama of autoantibodies targeting complement recognition proteins of the classical and lectin pathways, associated proteases, convertases, regulators and terminal components, with an emphasis on autoantibodies clearly involved in clinical conditions.

The Diagnostic and Clinical Utility of Autoantibodies in Systemic Vasculitis

Considerable progress has been made in understanding the role of autoantibodies in systemic vasculitides (SV), and consequently testing for anti-neutrophil cytoplasmic antibodies (ANCA), anti-glomerular basement membrane antibodies (anti-GBM), and anti-C1q antibodies is helpful and necessary in the diagnosis, prognosis, and monitoring of small-vessel vasculitis. ANCA-directed proteinase 3 (PR3-) or myeloperoxidase (MPO-) are sensitive and specific serologic markers for ANCA-associated vasculitides (AAV), anti-GBM antibodies are highly specific for the patients with anti-GBM antibody disease (formerly Goodpasture’s syndrome), and autoantibodies to C1q are characteristic of hypocomlementemic urticarial vasculitis syndrome (HUVS; anti-C1q vasculitis). The results of a current EUVAS study have led to changes in the established strategy for the ANCA testing in small-vessel vasculitis. The revised 2017 international consensus recommendations for ANCA detection support the primary use PR3- and MPO-ANCA immunoassays without the categorical need for additional indirect immunofluorescence (IIF). Interestingly, the presence of PR3- and MPO-ANCA have led to the differentiation of distinct disease phenotype of AAV: PR3-ANCA-associated vasculitis (PR3-AAV), MPO-ANCA-associated vasculitis (MPO-AAV), and ANCA-negative vasculitis. Further studies on the role of these autoantibodies are required to better categorize and manage appropriately the patients with small-vessel vasculitis and to develop more targeted therapy.