Jeannine S. Navratil

The Globular Heads of C1q Specifically Recognize Surface Blebs of Apoptotic Vascular Endothelial Cells

Complement protein C1q is required to maintain immune tolerance. The molecular mechanism responsible for this link has not been determined. We have previously demonstrated that C1q binds directly and specifically to surface blebs of apoptotic human keratinocytes, suggesting that it may participate in clearance of self Ags generated during programmed cell death. Here, we demonstrate that C1q also binds directly to apoptotic blebs of vascular endothelial cells and PBMC. These apoptotic cells are recognized by the globular heads of C1q, which bind specifically to the surface blebs, and deposition increases as the blebs mature on the cell surface. These observations suggest that C1q may participate in the clearance of apoptotic cells from the circulation and from the walls of the vascular lumen. The interaction of surface blebs with the globular heads of C1q suggests that surface blebs may be capable of directly activating the classical pathway of complement under certain circumstances, generating C4- and C3-derived ligands for receptors such as CR1, CR2, CR3, and CR4. Appropriate recognition of apoptotic cells by C1q and targeted clearance of the molecular contents of surface blebs to complement receptors may be critical for the maintenance of immune tolerance.

Erythrocyte C3d and C4d for monitoring disease activity in systemic lupus erythematosus

OBJECTIVE Disease activity in systemic lupus erythematosus (SLE) is typically monitored by measuring serum C3 and C4. However, these proteins have limited utility as lupus biomarkers, because they are substrates rather than products of complement activation. The aim of this study was to evaluate the utility of measuring the erythrocyte-bound complement activation products, erythrocyte-bound C3d (E-C3d) and E-C4d, compared with that of serum C3 and C4 for monitoring disease activity in patients with SLE. METHODS The levels of E-C3d and E-C4d were measured by flow cytometry in 157 patients with SLE, 290 patients with other diseases, and 256 healthy individuals. The patients with SLE were followed up longitudinally. Disease activity was measured at each visit, using the validated Systemic Lupus Activity Measure (SLAM) and the Safety of Estrogens in Lupus Erythematosus: National Assessment (SELENA) version of the Systemic Lupus Erythematosus Disease Activity Index (SLEDAI). RESULTS At baseline, patients with SLE had higher median levels of E-C3d and E-C4d (P < 0.0001) in addition to higher within-patient and between-patient variability in both E-C3d and E-C4d when compared with the 2 non-SLE groups. In a longitudinal analysis of patients with SLE, E-C3d, E-C4d, serum C3, and anti-double-stranded DNA (anti-dsDNA) antibodies were each significantly associated with the SLAM and SELENA-SLEDAI. In a multivariable analysis, E-C4d remained significantly associated with these SLE activity measures after adjusting for serum C3, C4, and anti-dsDNA antibodies; however, E-C3d was associated with the SLAM but not with the SELENA-SLEDAI. CONCLUSION Determining the levels of the erythrocyte-bound complement activation products, especially E-C4d, is an informative measure of SLE disease activity as compared with assessing serum C4 levels and should be considered for monitoring disease activity in patients with SLE.

Systemic lupus erythematosus and complement deficiency: clues to a novel role for the classical complement pathway in the maintenance of immune tolerance

Complete deficiency of C1q, the first component of the classical pathway of complement activation, is almost invariably associated with the development of systemic lupus erythematosus. Understanding why complement deficiency results in the specific autoimmune phenotype of SLE may provide valuable clues to the role of complement in the maintenance of immune tolerance. The following review will focus on the characteristics of complement-deficient SLE and the experimental evidence in support of our hypothesis that C1q may critically influence the immune response to self-antigen contained within surface blebs generated by apoptotic cells.

Platelet C4d Is Associated With Acute Ischemic Stroke and Stroke Severity

Background and Purpose— Platelets bearing complement C4d were recently reported to be 99% specific for a diagnosis of systemic lupus erythematosus (SLE) and associated with neuropsychiatric lupus. We compared the prevalence of platelet C4d and investigated the clinical associations of platelet C4d in patients with acute ischemic stroke. Methods— We recruited 80 patients hospitalized for acute ischemic stroke. Stroke severity was measured by the National Institutes of Health Stroke Scale (NIH-SS). Infarct volume was determined by MRI. Platelet C4d was measured by flow cytometry. Results— Mean age was 57.9 years (range: 24.6 to 86.8 years), 58% were male, and 91% were white. Eight patients (10%) with acute ischemic stroke were platelet C4d-positive, which was significantly higher in prevalence compared to healthy controls (0%, P<0.0001) and non-SLE patients with immune/inflammatory disease (2%, P=0.004). The median NIH-SS score and infarct volume for acute stroke patients were 6 (interquartile range [IQR]: 2 to 13) and 3.4 cc (IQR: 1.1 to 16.6), respectively. Platelet C4d-positive patients were more likely to have a severe stroke compared to those with negative platelet C4d (NIH-SS median: 17.5 versus 5, P=0.003). Positive platelet C4d was independently associated with stroke severity (P=0.03) after controlling for age, anticardiolipin antibody (aCL) status, and total anterior circulation of stroke involvement, and also with infarct volume (P=0.005) after controlling for age, aCL status, and old stroke by MRI. Conclusions— Platelet C4d is associated with severe acute ischemic stroke. Platelet C4d may be a biomarker as well as pathogenic clue that links cerebrovascular inflammation and thrombosis.

Apoptosis and autoimmunity.

Cell death by apoptosis plays a significant and seemingly contradictory role in the development and pathogenesis of autoimmune diseases. Apoptosis is integral to the assembly and maintenance of a healthy, self-tolerant immune system. However, many of the molecular and cellular events specific to apoptosis generate a reservoir of self-antigens with the potential to initiate and possibly perpetuate autoimmune conditions. Recent findings that support this latter, more sinister role for apoptosis have shed light on a mystery that is common to many systemic autoimmune diseases, namely, why the majority of autoantibodies produced in patients with these diseases target proteins that are normally found inside the cell, often within the nucleus. This review will discuss how autoantigens are specifically altered during the apoptotic process and how the complement system participates in recognizing and clearing these potentially immunogenic packages.

Apoptosis, Complement and Systemic Lupus Erythematosus: A Mechanistic View

Deficiencies in the classical pathway of the complement system have been implicated in the etiology and pathogenesis of systemic lupus erythematosus (SLE) for several decades. Recent advances have suggested that this link is due to a critical role of complement in the recognition and clearance of the cellular remnants of apoptosis. In this review, we summarize the role of apoptosis in generation of an autoimmune response, and we integrate recent advances that link apoptosis, complement activation and the etiopathogenesis of SLE.

Cell-Bound Complement Activation Products (CB-CAPs) as a Source of Lupus Biomarkers

Measurement of serum C3 and C4 has been used for several decades to monitor disease activity in patients with SLE. Despite the limited utility and recognized weaknesses of these assays, they have remained the gold standard during an era of unprecedented discovery in the complement field. The current urgent need for lupus biomarkers warrants efforts to mine the complement system for assays superior to serum C3 and C4. Recent studies of soluble and cell-bound complement activation products hold promise for achieving this goal.