Functional effects of immune complexes formed between pembrolizumab and patient-generated anti-drug antibodies

The PD-1-targeting IgG₄ antibody pembrolizumab has significant anti-tumor activity in a proportion of stage IV melanoma patients. A subset of patients develop anti-drug antibodies (ADA) which can form immune complexes (IC) with pembrolizumab. Although IC can induce powerful, Fc-mediated, immune-regulatory effects, their functional impact during pembrolizumab treatment is unclear. The functional effects of IC generated in vitro using pembrolizumab and patient-derived ADA was, therefore, investigated. Screening identified a patient whose trough serum samples from three treatment cycles contained both ADA with neutralizing activity and low levels of pembrolizumab. This patient responded well to therapy over 2 years and had ongoing, infusion-related, hypersensitivity reactions despite the later absence of detectable ADA. The components of IC were mimicked by forming a complex of pembrolizumab by absorption onto a solid phase with or without subsequent exposure to the ADA⁺ patient sera. Complexes comprised of pembrolizumab alone significantly inhibited TLR4 (LPS)-driven IL-10 production by PBMC and stimulated the generation of reactive oxygen species by granulocytes. In contrast, soluble and solid-phase F(ab´)2 fragments of pembrolizumab had no effect demonstrating the requirement for cross-linked Fc regions. IC containing pembrolizumab and ADA could additionally induce complement and NK activation. The results of this study demonstrate that, when oligomerized, the Fc region of pembrolizumab alone can provide immuno-regulatory signals. Furthermore, IC containing both pembrolizumab and patient-generated ADA can induce additional signals. These Fc-mediated signals may modulate both hypersensitivity reactions and anti-tumor responses associated with pembrolizumab therapy.