Search

SummaryDevelopment of inhibitory antibodies against factor VIII (FVIII) is a severe complication of replacement therapy in haemophilia A. Patients with inhibitors are treated with high FVIII doses in the context of immune tolerance therapy (ITT). Data from haemophilia A mouse model suggest that high FVIII concentrations prevent the formation of antibody secreting cells (ASCs) from memory B cells (MBCs) by inducing apoptosis. Fc gamma receptor 2B (CD32) is an important regulator of B cell function, mediating inhibitory signals after cross-linking with the B cell receptor. Here, the role of CD32 in the regulation of FVIII-specific MBCs was investigated using F8-/- and F8-/-CD32-/- knockout mice and monoclonal antibodies (mAbs). The initial immune response was similar between F8-/- and F8-/-CD32-/- mice, including concentration of anti-FVIII antibodies and number of FVIII-specific ASCs in spleen and bone marrow. In contrast, formation of ASCs from MBCs upon rhFVIII re-stimulation in vitro was abolished in F8-/-CD32-/- mice, whereas FVIII/anti-FVIII immune complexes significantly enhanced ASC formation in F8-/- mice. Inhibition of CD32 by mAbs or F(ab)2 fragments prevented ASC formation in a dose-dependent manner. Transfer of B cell-depleted splenocytes using CD45R (B220) depletion from CD32-competent mice did not restore ASC formation in F8-/-CD32-/- cells confirming that CD32 is required on B cells. We conclude that CD32 is a crucial regulator of FVIII-specific B cells and is required for the differentiation of MBCs into ASCs. Inhibition of CD32 could potentially improve the efficacy of FVIII in the context of ITT.

Mouse models with targeted disruptions of coagulation factor genes are used to study disorders of haemostasis such as haemophilia. Standard protocols to obtain biopsies for genotyping in breeding programmes are based on invasive sampling methods such as tail clipping. These procedures imply a high risk of fatal bleeding, especially in haemophilic mouse models. Here we used a non-invasive sampling method obtaining stool samples for DNA isolation in a breeding programme, aiming to introduce targeted disruptions of Fc receptor genes (Fc gamma receptor IIB and III) into the haemophilia A mouse model (factor VIII deficiency). Faecal pellets were reliably obtained from individual mice and high-quality DNA was extracted with a mean yield of 7.1 μg/pellet. Polymerase chain reaction amplification of wild-type and knockout alleles for Fc receptor and factor VIII genes was similar, comparing stool and peripheral blood as the source of genomic DNA. Definite genotype identification was achieved in a first attempt in 336 of 352 analyses (95%). Repeated analysis of homozygous knockout animals confirmed the first result in all cases. No animal was lost due to bleeding from the procedure. In conclusion, DNA isolation from stool is a preferable method for genotyping in laboratory animals, especially in models of bleeding disorders. Avoiding loss of animals due to bleeding implies a substantial improvement in animal welfare by reducing the number of animals used and may also advance the effectiveness of breeding programmes in these disease models.

Common variable immunodeficiency (CVID) is a heterogeneous disorder characterized by defective Ab production and recurrent bacterial infections. The largely unknown causes are likely to comprise a diverse set of genetic or acquired defects. In this study, we investigated terminal B cell differentiation in lymph nodes from CVID patients. Up to the germinal center B cell stage, B cell differentiation was normal but terminal plasma cell development was found to be impaired. Using differential Blimp-1 and Syndecan-1 expression in controls, we defined three different plasma cell subsets that correspond to progressive developmental stages locating to different sites in the lymph node. In the CVID patients, we could only detect one or two of these subsets indicating a defective differentiation. Thus, terminal plasma cell differentiation was found to be impaired despite normal expression of Blimp-1. B cells reaching only the first stage of plasma cell differentiation were further unable to undergo isotype switching and to up-regulate activation markers on B cells stimulated in vitro.

Abstract 204 The formation of inhibitory antibodies to factor VIII (FVIII) is the foremost complication of replacement therapy in hemophilia A. Patients with inhibitors are treated with very high doses of FVIII, over prolonged periods of time, to induce immune tolerance. Studies in a hemophilia A mouse model demonstrated that very high doses of FVIII can induce apoptosis in FVIII-specific memory B cells and prevent their differentiation into antibody-secreting cells. The Fc gamma receptor IIb (FcgRIIb) is expressed on B cells and mediates inhibitory signals after crosslinking with the B cell receptor. Here, we studied the potential role of this receptor in the regulation of memory B cell response to FVIII. FVIII knockout mice (B6;129S4-F8tm2Kaz/J) were crossed with FcgRIIb knockout mice (B6;129S4-Fcgr2btm1Ttk/J). Comparing F8−/− mice and F8−/−/FcgR2b−/− double knockout mice, the initial anti-FVIII antibody formation was similar after intravenous exposure to 4 weekly doses of 80 or 400 IU/kg. Similar numbers of FVIII-specific antibody-secreting cells were detected in the spleen and bone marrow by ELISPOT. As previously shown, in vitro re-stimulation of memory B cells from spleens of immunized F8−/− mice at doses of 1 to 200 ng/ml induced their differentiation into antibody-secreting cells. Higher doses of 400 to 800 ng/ml prevented differentiation. In F8−/−/FcgR2b−/− double knockout mice, however, formation of antibody-secreting cells was completely inhibited across all FVIII doses tested. Addition of B220-depleted splenocytes from F8−/− mice did not restore memory B cell function in F8−/−/FcgR2b−/− double knockout mice, indicating that the observed effect was not due to dysfunction of follicular dendritic cells or other antigen-presenting cells. Inhibition of FcgRIIb using a monoclonal antibody prevented the FVIII-specific memory B cell response in splenocytes from immunized F8−/− mice. Staining with propidium iodide, annexin V, or anti-caspase 3 indicated increased rates of apoptosis when FcgRIIb was blocked during re-stimulation. In summary, FcgRIIb plays a crucial role for the differentiation of FVIII-specific splenic memory B cells into antibody-secreting cells. Inhibition of FcgRIIb appears to sensitize B cells for apoptosis during re-stimulation with FVIII. This mechanism could potentially facilitate the eradication of FVIII-specific memory B cells during ITI. Disclosures: No relevant conflicts of interest to declare.