About this Research Topic
The complement system participates in host defense against a variety of bacterial and viral infections. These actions of complement are either independent of adaptive immunity or form an important regulatory element of humoral and cellular immunity. Undue or misdirected complement activity causes many of the pathologic changes associated with autoimmune diseases. Epidemiological studies, utilizing large cohorts of individuals from different ethnic backgrounds, have shown strong associations between deficiency of the early components of the classical pathway of complement and autoimmune disease, particularly in those diseases well-defined by autoantibodies, such as systemic lupus erythematosus (SLE). The etiology of SLE is poorly understood, involving both genetic predisposition and environmental factors contributing to an impaired tolerance. Thus, considerable effort has focused on identifying genetic factors and lymphocyte defects contributing to this disease.
The disease is a multifactorial process with both T and B lymphocytes playing important roles at all stages, multiple T and B-cell abnormalities’ with atypical cytokine profiles, loss of central tolerance, as well as defects in antigen presenting cells have all been implicated. Among the candidate genes identified, the strongest association exists with complete deficiency of C1q and C4, 90% and 80%, respectively with weaker associations seen with deficiencies of C1r, C1s, and C2. Forming an interface between activated complement components and various cell phenotypes are complement regulators and receptors. Despite the fact that no other known genetic deficiency confers such a high penetrance of human systemic autoimmune disease as complete C1q and C4 deficiency, our knowledge of the role played by these proteins and their receptors in cellular homeostasis and in patients with SLE is incomplete, especially in regards to the effects of C4 isotypes and their various alleles. Complement fragments bound to each other and within an immune complex could simultaneously ligate more than one receptor thus altering that cell’s behavior or serve as a bridge to initiate crosstalk between cells.
Hence, this Research Topic will highlight the multiple links existing between the early components of complement and immune cell responses as they might relate to systemic autoimmune disease. Since the mechanism underlying the relationship between these complement deficiencies and SLE is still unclear we will also entertain novel hypothetical papers in this area.
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