No abstract

The ability of rat monoclonal IgA, specific for 2,4-dinitrophenyl (DNA), 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 degrees 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)-conjugated 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.

No abstract

In the present study the clearance kinetics and tissue distribution of aggregated 125I-labelled monoclonal rat IgA [( 125I] AIgA) of different sizes were studied in rats. Soluble [125I]AIgA disappeared from the circulation in a biphasic manner with an initial rapid distribution half-life (T1) and a second slower half-life (T2). T2 was directly related to the size of the aggregates. High molecular weight [125I]AIgA, containing 10-12 IgA molecules per aggregate [( IgA]10-12), was cleared much faster than low molecular weight aggregates. The main site of clearance was the liver. The larger the size of the AIgA, the more degradation products were found in the circulation. After injection of [IgA]10-12, non-parenchymal cells (NPC) contained three times more radioactivity than parenchymal cells (PC) (NPC:PC ratio 3.06 +/- 0.96). Ratios of 0.82 +/- 0.03 and 0.62 +/- 0.12 were observed when [IgA]5-6 and [IgA]2 were injected respectively, suggesting a greater role for Kupffer cells in the clearance of large-sized IgA aggregates. Kupffer cells were shown to be the main cells for localization of large-sized AIgA established by immunohistochemical staining on liver cryostat sections.