Activation of rat complement by soluble and insoluble rat IgA immune complexes

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Abstract
The ability of rat monoclonal IgA, specific for 2,4‐dinitrophenyl (DNP), to activate the complement (C) system of the rat was investigated using aggregated IgA or IgA immune complexes (IC). IgA was coated onto a solid phase, and tested for its capacity to bind C3 upon incubation at 37°C in normal rat serum (NRS) in the presence of Mg‐EGTA. Binding of C3 was observed dependent on the dose of dimeric (d‐), polymeric (p‐) and secretory IgA tested. In contrast, little C3 fixation was observed in this system with monomeric (m‐) rat IgA or with mouse m‐ and d‐IgA (MOPC315). Soluble and insoluble rat IgA IC were prepared using dinitrophenylated rat serum albumin (DNP8RSA) as antigen (Ag), and assessed for C activation. It was shown that insoluble IC (immune precipitates; IP) containing m‐, d‐ or pIgA of rat origin activate the alternative pathway of rat C, as demonstrated by their capacity to induce C consumption in NRS in the presence of Mg‐EGTA. When p‐ and m‐IgA IP were compared for their capacity to activate C, it was found that p‐IgA activated C four times as efficiently as m‐IgA IP (at 2 mg/ml). Soluble rat IgA IC were prepared in an excess of DNP8RSA, fractionated by gel filtration on Sepharose 6B, and analyzed for C activation and antibody (Ab)/Ag ratio. In contrast to m‐IgA IP, soluble m‐IgA did not activate C. On the other hand soluble d‐IgA IC activated C dependent on their concentration and size: at a concentration of 0.1 mg/ml high‐molecular weight d‐IgA IC with a high Ab/Ag ratio were four times as efficient as low‐molecular weight IC with a low Ab/Ag ratio, and twice as efficient as IP prepared at equivalence. To demonstrate the induction by IgA of the assembly of the terminal membrane attack complex, trinitrophenyl (TNP)‐conjugugated rat red blood cells (TNP‐RRBC) coated with d‐ or p‐IgA were shown to be lysed in NRS in the presence of Mg‐EGTA. No lysis of m‐IgA‐coated TNP‐RRBC was observed. The results in this study demonstrate that both soluble and insoluble rat IgA IC activate the alternative pathway of homologous rat C. Alternative pathway activation by soluble rate IgA IC is dependent on the size of the IC. The degree of polymerization of the IgA Ab itself also influences C activation.