Fc Gamma Receptors and Complement Component 3 Facilitate Anti-fVIII Antibody Formation.

Patricia E Zerra,Amanda Mener,Courtney Cox,W Hunter Baldwin,Jerry William L Allen,Cheryl L Maier,Sooncheon Shin,Hans Verkerke,Satheesh Chonat,Shannon L Meeks,Sean R Stowell,Connie M Arthur,Christopher A Tormey,Ryan P Jajosky

doi:10.3389/fimmu.2020.00905

Abstract

Anti-factor VIII (fVIII) alloantibodies, which can develop in patients with hemophilia A, limit the therapeutic options and increase morbidity and mortality of these patients. However, the factors that influence anti-fVIII antibody development remain incompletely understood. Recent studies suggest that Fc gamma receptors (FcγRs) may facilitate recognition and uptake of fVIII by recently developed or pre-existing naturally occurring anti-fVIII antibodies, providing a mechanism whereby the immune system may recognize fVIII following infusion. However, the role of FcγRs in anti-fVIII antibody formation remains unknown. In order to define the influence of FcγRs on the development of anti-fVIII antibodies, fVIII was injected into WT or FcγR knockout recipients, followed by evaluation of anti-fVIII antibodies. Anti-fVIII antibodies were readily observed following fVIII injection into FcγR knockouts, with similar anti-fVIII antibody levels occurring in FcγR knockouts as detected in WT mice injected in parallel. As antibodies can also fix complement, providing a potential mechanism whereby anti-fVIII antibodies may influence anti-fVIII antibody formation independent of FcγRs, fVIII was also injected into complement component 3 (C3) knockout recipients in parallel. Similar to FcγR knockouts, C3 knockout recipients developed a robust response to fVIII, which was likewise similar to that observed in WT recipients. As FcγRs or C3 may compensate for each other in recipients only deficient in FcγRs or C3 alone, we generated mice deficient in both FcγRs and C3 to test for potential antibody effector redundancy in anti-fVIII antibody formation. Infusion of fVIII into FcγRs and C3 (FcγR × C3) double knockouts likewise induced anti-fVIII antibodies. However, unlike individual knockouts, anti-fVIII antibodies in FcγRs × C3 knockouts were initially lower than WT recipients, although anti-fVIII antibodies increased to WT levels following additional fVIII exposure. In contrast, infusion of RBCs expressing distinct alloantigens into FcγRs, C3 or FcγR × C3 knockout recipients either failed to change anti-RBC levels when compared to WT recipients or actually increased antibody responses, depending on the target antigen. Taken together, these results suggest FcγRs and C3 can differentially impact antibody formation following exposure to distinct alloantigens and that FcγRs and C3 work in concert to facilitate early anti-fVIII antibody formation.

Highlights

Undetectable levels of circulating factor VIII in most patients with severe hemophilia A results in impaired coagulation, and fails to induce immunological tolerance to fVIII during neonatal and early life [1, 2]
Given the possible role of Fcγ receptor (FcγR) in the developing immune response to fVIII, we first sought to define the role of FcγRs by leveraging mice completely deficient in the common γ chain used by all activating FcγRs (FcγRs I, III, and IV), a common approach to examine FcγR function [44]
In order to effectively understand risk factors that may predict the likelihood of inhibitor development and prevent this process in at-risk patients, key factors that regulate the development of anti-fVIII alloantibodies must be identified

Summary

Introduction

Undetectable levels of circulating factor VIII (fVIII) in most patients with severe hemophilia A results in impaired coagulation, and fails to induce immunological tolerance to fVIII during neonatal and early life [1, 2]. Therapeutic exposure to exogenous fVIII can induce the formation of inhibitory anti-fVIII antibodies (inhibitors), which render fVIII therapy ineffective [3,4,5,6,7,8,9]. One of the most common approaches to inhibitor eradication is immune tolerance therapy (ITT). Patients with inhibitors continue to be difficult to manage during acute bleeding episodes (e.g., trauma, surgery, etc.)

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Conclusion