Complement activation in Type 1 human diabetes

Abstract

Complement activation was quantitated in serum and plasma of diabetic and normal subjects by sensitive competitive equilibrium radioimmunoassays (RIA) for C3a, C4a, C5a, Factor B, and a newly described C5 neoantigen (termed C5 activation antigen, and abreviated C5-AA) in a stable 54-kDa fragment of C5. Plasma C3a levels were significantly elevated in 8 of 16 patients with newly diagnosed Type 1 diabetes ( P < 0.0005) with the mean C3a concentration for these patients being more than 10-times greater than the mean value of normal controls. C4a levels were also elevated in 2 of these patients ( P < 0.02), but C5a levels, although higher than normal, were not significantly increased. In contrast, the levels of C5-AA in the serum of all patients ( 11 11 ) with chronic Type 1 diabetes were significantly higher than in control Type 2 patients (non-insulin-dependent diabetes) ( P < 0.0005) and 4 of 7 patients with new onset insulin-dependent diabetes mellitus also had significantly higher levels of C5-AA than the Type 2 patients ( P < 0.01). The levels of Factor B in the serum of 5 of 9 patients with new onset diabetes were significantly higher than normal ( P < 0.0025). Five recent onset Type 1 diabetes patients were evaluated longitudinally for C3a, C4a, and C5a: in 3 the levels of C3a were elevated during new onset disease decreasing into the normal range during remission; in 2 of these patients C4a was also significantly elevated and the levels decreased during remission; and in 3 patients the levels of C5a were not significantly elevated but they decreased during remission. Purified human complement proteins and complement hemolytic assays were used to measure complement activation in serum during incubation with rat pancreatic islet cells. With diluted normal human serum, less than 20% of C3 or Factor B were consumed during 30 min at 37°C, while with new onset Type 1 diabetic patient sera up to 90% of C3 and Factor B were consumed in 5 6 sera and 4 6 sera, respectively. These findings suggest (a) that complement activation fragments C3a, C4a, and C5a are generated in vivo in new onset Type 1 diabetes; (b) that both the classical and the alternative complement pathways may be activated; and (c) that this may result in a measurable activation of C5 generating biologically and immunologically active C5a and other C5 activation fragments. Elevated levels of C3a, C4a, Factor B, and C5-AA in plasma or serum (respectively) appear correlated with clinical disease activity in new onset Type 1 diabetes.