Tau passive immunotherapy efficacy is modulated by antibody Fc domain

Abstract

AbstractBackgroundPassive immunotherapy targeting tau is a promising strategy for the treatment of Alzheimer's disease. The proposed mechanisms of action for some tau antibodies is to engage with and clear extracellular pathogenic forms of tau, facilitated by microglial phagocytosis through antibody Fc receptor (FcR) binding. As FcR binding is mediated by the shape of the antibody Fc domain, this suggests that the antibody isotype affects the efficacy of these tau antibodies. We therefore aimed to directly compare the efficacy of a tau‐specific antibody cloned into a murine IgG1 framework, which has low‐affinity FcR binding, to that cloned into a murine IgG2a framework, which has high‐affinity FcR binding.MethodThe variable domains of our 2N‐tau specific antibody, RN2N (Nisbet et al., Brain 2017), were cloned into the murine IgG1 and IgG2a heavy chain frameworks. To investigate the ability of the two IgGs to stimulate microglial phagocytosis of recombinant tau, tau was labelled with pH‐Rodo and incubated with and without RN2N IgG, and then placed onto BV2 microglial cells. Phagocytosis was assessed by microscopy and flow cytometry. For the in vivo analysis, P301L tau transgenic pR5 mice were treated with 50 mg/kg RN2N IgG i.p. once a week for 10 weeks. Following treatment, brain pathology was assessed by western blot and immunohistochemistry.ResultAlthough the RN2N IgG1 and IgG2a had consistent binding affinities to tau, the RN2N IgG2a, which has a higher binding affinity to FcR, significantly increased the phagocytosis of extracellular tau compared to RN2N IgG1. In vivo, however, passive immunisation of pR5 mice with RN2N IgG1, but not RN2N IgG2a, reduced pathogenic tau levels in the brains of treated mice.ConclusionTaken together, our results demonstrate that although enhanced microglial activation via the RN2N IgG2a isotype increases extracellular tau phagocytosis in vitro, it is not required for antibody efficacy in vivo. This suggests that an antibody with a reduced affinity for FcRs may be beneficial in human clinical trials due to its low activation of pro‐inflammatory cytokine production. Furthermore, as RN2N‐mediated pathological tau reduction in this study was only minimal, targeting intracellular tau might be a better strategy.