Restricted immune activation and clearance of anti-idiotype complexes between drug and anti-drug antibodies

Therapeutic antibodies can elicit an anti-drug antibody (ADA) response resulting in anti-idiotype immune complexes. Nevertheless, ADA-associated adverse events are usually rare, albeit with some exceptions, including infliximab. We investigated formation, clearance, and biological activity of these ADA-drug complexes using patient-derived monoclonal anti-infliximab antibodies. Ratio and concentrations of drug and ADA determined immune complex size, with large, irregularly shaped complexes (>6 IgG molecules) formed only in case of high ADA titers. Macrophages efficiently phagocytosed tetrameric and bigger complexes in vitro , but not dimers. Corroborating these results, ex vivo analysis of patient sera taken at trough demonstrated only dimeric complexes in circulation. Strikingly, none of the anti-idiotype complexes induced immune cell activation in a whole-blood assay, and only very large complexes activated the complement system. Notably, anti-idiotype hexamers did not activate complement, unlike Fc-linked hexamers that may form upon opsonizing a cellular target, influence complex size in patient sera. Although large immune complexes were formed in vitro , ex vivo these complexes were not detected in trough serum samples of infliximab treated patients. Further experiments with macrophages showed rapid phagocytosis of tetramers and larger complexes, suggesting that these complexes are cleared in vivo . Clearance by macrophages was already proposed in studies in humans 17 and cynomolgus monkeys, 27 where accumulation of radiolabeled ADA-infliximab immune complexes was detected in the liver and spleen. Type I hypersensitivity, we have recently shown that the vast majority of infliximab-induced infusion reactions is negative for IgE anti-infliximab. 23 Others also found no IgE anti-infliximab, elevated total IgE or elevated tryptase levels in IR+ patients, suggesting that the majority of infusion reactions are not IgE-mediated. The association of infusion reactions with ADA positivity 21 and high ADA titers 35 indicate that IgG ADA play a crucial role in these adverse events. Our study shows, for the first time, the mechanistic link between high ADA titers and the formation of large, irregularly shaped immune complexes that – under privileged circumstances – can activate complement. Our results thus suggest that, instead of an IgE mediated response, infusion reactions are more likely to be a Type III hypersensitivity reaction.