Immunoglobulin G1 and immunoglobulin G4 antibodies in multiple sclerosis patients treated with IFNβ interact with the endogenous cytokine and activate complement

•IgG4 and IgG1 contributes to IFNβ-ADA profile•Neutralising IFNβ-ADA cross reacts and blocks endogenous IFNβ activity.•ADA-IFNβ results in IC formation and complement activation

A subset of patients with relapsing-remitting multiple sclerosis (RRMS) on therapy with interferon beta (IFNβ) develop neutralising anti-drug antibodies (ADA) resulting in reduced, or loss of, therapeutic efficacy. The aims were to characterise the relative contributions of anti-IFNβ antibody isotypes to drug neutralising activity, ability of these antibodies to cross-react with endogenous IFNβ, to form immune complexes and activate complement. IFNβ-specific ADA were measured in plasma from RRMS patients treated with IFNβ1a (Rebif®). Neutralisation of endogenous and therapeutic IFNβ by ADA was determined by IFNβ bioassay. IFNβ-ADA profile was predominantly comprised of IgG1 and IgG4 antibody isotypes. The contribution of IgG4-ADA towards neutralising activity was found to be minimal. Neutralising IFNβ-ADA blocks endogenous IFNβ activity. ADA interaction with therapeutic IFNβ results in immune complex formation and complement activation. In summary, IgG1 and IgG4 IFNβ-ADA have the ability to neutralise therapeutic and endogenous protein and to activate complement.