The mystery behind membrane insertion: a review of the complement membrane attack complex

Complement in the Initiation and Evolution of Rheumatoid Arthritis

The complement system is a major component of the immune system and plays a central role in many protective immune processes, including circulating immune complex processing and clearance, recognition of foreign antigens, modulation of humoral and cellular immunity, removal of apoptotic and dead cells, and engagement of injury resolving and tissue regeneration processes. In stark contrast to these beneficial roles, however, inadequately controlled complement activation underlies the pathogenesis of human inflammatory and autoimmune diseases, including rheumatoid arthritis (RA) where the cartilage, bone, and synovium are targeted. Recent studies of this disease have demonstrated that the autoimmune response evolves over time in an asymptomatic preclinical phase that is associated with mucosal inflammation. Notably, experimental models of this disease have demonstrated that each of the three major complement activation pathways plays an important role in recognition of injured joint tissue, although the lectin and amplification pathways exhibit particularly impactful roles in the initiation and amplification of damage. Herein, we review the complement system and focus on its multi-factorial role in human patients with RA and experimental murine models. This understanding will be important to the successful integration of the emerging complement therapeutics pipeline into clinical care for patients with RA.

Association Between Promoter Polymorphisms in CD46 and CD59 in Kidney Donors and Transplant Outcome

Complement regulating proteins, including CD46, CD55, and CD59, protect cells against self-damage. Because of their expression on the donor endothelium, they are hypothesized to be involved in accommodation. Polymorphisms in their promoter regions may affect their expression. The aim of this study was to investigate if donor polymorphisms in complement regulating proteins influence kidney transplant outcomes. We included 306 kidney transplantations between 2005 and 2010. Five polymorphisms in the promoters of CD46, CD55, and CD59 were genotyped. A CD59 promoter polymorphism (rs147788946) in donors was associated with a lower 1-year rejection-free survival [adjusted hazard ratio (aHR) 2.18, 95% CI 1.12–4.24] and a trend toward impaired 5-year graft survival (p = 0.08). Patients receiving a kidney with at least one G allele for the CD46 promoter polymorphism rs2796267 (A/G) showed a lower rejection-free survival, though this became borderline significant after adjustment for potential confounders (aHR 1.87, 95% CI 0.96–3.65). A second CD46 promoter polymorphism (rs2796268, A/G), was also associated with a lower freedom from acute rejection in the presence of at least one G allele (aHR 1.95, 95% CI 1.03–3.68). Finally, the combined presence of both favorable genotypes of rs2796267 and rs147788946 had an additional protective effect both on acute rejection (p = 0.006) and graft survival (p = 0.03). These findings could help to identify patients who could benefit from intensified immunosuppressive therapy or novel complement inhibitory therapeutics.

Reduced Expression of Membrane Complement Regulatory Protein CD59 on Leukocytes following Lung Transplantation

Cellular protection against undesired effects of complement activation is provided by expression of membrane-bound complement regulatory proteins including CD59. This protein prevents membrane attack complex formation and is considered to be involved in graft accommodation. Also, CD59 downregulates CD4+ and CD8+ T-cell activation and proliferation. It is unknown whether CD59 expression is affected by transplantation. The aim of this study was to evaluate the quantitative CD59 antigen expression on distinct leukocyte subsets following lung transplantation (n = 26) and to investigate whether this differs from pretransplantation (n = 9). The results show that CD59 expression on leukocytes is significantly lower posttransplantation compared with healthy controls (p = 0.002) and pretransplantation (p < 0.0001). Moreover, the CD59 expression diminishes posttransplantation on all distinct lymphocyte subsets (p < 0.02). This effect appeared to be specific for CD59 since the expression of other surface markers remained stable or inclined following transplantation. The highest antigen expression posttransplantation was observed on CD4+ T cells and monocytes (p ≤ 0.002). These findings show that CD59 expression on leukocytes diminishes posttransplantation, which could result in decreased resistance against complement and enhanced T-cell activation. If such reduction in CD59 expression also occurs on endothelial cells from the transplanted organ, this could lead to a change into a prothrombotic and proinflammatory phenotype.