Serum C3 and C4 levels and complement-dependent antibody-mediated cytotoxic activity of islet cell surface antibody in type 1 (insulin-dependent) diabetic children

Complement activation via the alternative pathway in a patient with insulin-dependent diabetes mellitus

A 41-year old male with insulin-dependent diabetes mellitus previously unsuccessfully treated with a controlled diet and glibenclamide, and subsequently with increasing insulin doses (5 and 20 IU/day) experienced polyuria, glycosuria and loss of weight. On admittance to hospital serum C3 concentrations were found to be depressed. The insulin dose was further increased to 30 IU/day and the patient was also treated with 20 mg nafamostat mesylate given intravenously twice daily for 6 days. On completion of nafamostat mesylate treatment serum C3 concentrations were increased but after 17 days they started to decrease again.

Does protease inhibitor inhibit complement activation caused by the immune complex associated with islet cell surface antibody?

Sera containing islet cell surface antibody were obtained from seven children with insulin-dependent diabetes mellitus soon after the onset of disease. After incubation of 51Cr-labelled rat islet cells with islet cell surface antibody, human AB-type serum with or without nafamostat mesylate was added before further incubation. Radioactivity in the supernatant was measured to determine complement-dependent antibody-mediated cytotoxicity. Cytotoxicity in untreated sera [mean (+/- SD) 19.4 +/- 4.0%] was significantly (P less than 0.001) inhibited by ethyleneglycoltetraacetic acid (EGTA) (7.1 +/- 4.9%), ethylenediaminetetraacetic acid (EDTA) (2.5 +/- 0.9%) and nafamostat mesylate (2.8 +/- 1.8%). Cytotoxicity of nafamostat mesylate-treated serum was significantly (P less than 0.05) lower than that of EGTA-treated serum but not significantly different from that of EDTA-treated serum. There was no difference in cytotoxicity between nafamostat mesylate-treated and untreated, inactivated human serum. The results indicate that the protease inhibitor nafamostat mesylate completely inhibited the complement activation of the immune complex associated with islet cell surface antibody by the classical and alternative pathways.

The principal site of glycation of human complement factor B

Accumulating amino acid sequence data have made it increasingly evident that many essential complement proteins have potentially modifiable lysine residues in putative critical functional regions. Evidence is now presented that glucose is covalently attached to lysine-266 of purified human complement Factor B as a result of glycation. Purified B was treated with NaB3H4, which reduces such bound glucose to a mixture of radiolabelled hexitols. Amino acid analysis revealed the expected radiolabelled hexitol-lysine epimers. In addition, fluorography of dried gels resolving the major high-molecular-mass h.p.l.c.-fractionated CNBr-cleavage peptides of NaB3H4-reduced B indicated that this radioactivity was specifically associated with the 15 kDa fragment derived from the N-terminal region of fragment Bb. Amino acid sequence analysis suggested that the C-terminal lysine (residue 266 of B) of the N-terminal Lys-Lys doublet of this peptide is preferentially modified. If such glycation can subsequently be shown to occur in vivo, then perhaps this modification might also be found to affect the functional activity of B and offer a potential explanation for some of the immunopathological complications of diseases exposing key plasma proteins, such as this active-site-containing proteinase of the multimeric alternative-complement-pathway C3/C5 convertases, to long-term high concentrations of glucose, such as the decreased resistance to infection and impaired chemotaxis and phagocytosis characteristic of diabetes.