Anti‐Fc γ RIIB ( CD 32) Antibodies Differentially Modulate Murine FVIII ‐Specific Recall Response in vitro

Fc gamma receptors (Fc γ Rs) for IgG regulate adaptive immune responses by modulating activating and inhibitory signalling pathways within immune cells. Data from a haemophilia A mouse model demonstrate that genetic deletion or blockade of the inhibitory Fc γ R ( CD 32) suppresses the formation of antibody‐secreting cells ( ASC s) in vitro . Mechanisms preventing the FVIII ‐specific recall response, however, remain unclear. Here, the potential role of CD 32 inhibition was studied by differentially modulating receptor activity with selected anti‐ CD 32 monoclonal antibodies ( mA bs). Splenocytes from immunized FVIII ‐/‐ mice were restimulated with FVIII in the absence or presence of different anti‐ CD 32 mA bs over 6 days. At day 6, cytokine release was quantified from cell culture supernatant and the formation of FVIII ‐specific ASC s assessed. Binding of FVIII ‐containing immune complexes (F8‐ IC s) to bone marrow‐derived dendritic cells ( BM d DC s) was also investigated. The antagonistic CD 32 mA b AT 128 suppressed the formation of FVIII ‐specific ASC s and reduced secretion of IFN ‐ γ and IL ‐10. In contrast, the agonistic mA bs AT 130‐2 and AT 130‐5, and their F(ab’) 2 fragments, allowed the formation of FVIII ‐specific ASC s, even though the full IgG of AT 130‐2 reduced binding of F8‐ IC s to CD 32. Data suggest that an inhibitory signal is transmitted when F8‐ IC s bind to CD 32 and that this signal is required during memory B cell ( MBC ) activation to support formation of FVIII ‐specific ASC s. If the inhibitory signal is lacking due to CD 32 deletion or blockade with antagonistic anti‐ CD 32 mA bs, FVIII ‐specific T cell stimulation and  ASC formation are suppressed, whereas agonistic stimulation of CD 32 restores T cell stimulation and ASC formation.