Inhibitory IgG receptor FcgammaRIIB fails to inhibit experimental autoimmune myasthenia gravis pathogenesis

Deletion or inhibition of Fc gamma receptor 2B (CD32) prevents FVIII-specific activation of memory B cells in vitro

Development 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.

Rapid and reversible responses to IVIG in autoimmune neuromuscular diseases suggest mechanisms of action involving competition with functionally important autoantibodies

Intravenous immunoglobulin (IVIG) is widely used in autoimmune neuromuscular diseases whose pathogenesis is undefined. Many different effects of IVIG have been demonstrated in vitro, but few studies actually identify the mechanism(s) most important in vivo. Doses and treatment intervals are generally chosen empirically. Recent studies in Guillain-Barré syndrome and chronic inflammatory demyelinating polyneuropathy show that some effects of IVIG are readily reversible and highly dependent on the serum IgG level. This suggests that in some autoantibody-mediated neuromuscular diseases, IVIG directly competes with autoantibodies that reversibly interfere with nerve conduction. Mechanisms of action of IVIG which most likely involve direct competition with autoantibodies include: neutralization of autoantibodies by anti-idiotypes, inhibition of complement deposition, and increasing catabolism of pathologic antibodies by saturating FcRn. Indirect immunomodulatory effects are not as likely to involve competition and may not have the same reversibility and dose-dependency. Pharmacodynamic analyses should be informative regarding most relevant mechanism(s) of action of IVIG as well as the role of autoantibodies in the immunopathogenesis of each disease. Better understanding of the role of autoantibodies and of the target(s) of IVIG could lead to more efficient use of this therapy and better patient outcomes.

Selective recognition and elimination of nicotinic acetylcholine receptor-reactive B cells by a recombinant fusion protein AChR-Fc in myasthenia gravis in vitro

AChR-reactive B cells play a key role in the pathogenesis of myasthenia gravis (MG) by producing autoantibodies. Selective elimination of AChR-reactive B cells will be a promising way to treat MG. Thus, we generated a fusion protein (referred to as AChR-Fc) composed of the human extracellular domain of AChR α1 subunit and the Fc domain of the human IgG1 heavy chain, which could bind both to AChR-reactive BCR and FcγRIIB on the surface of AChR-reactive B cells. Our results showed that AChR-Fc inhibited the proliferation of AChR-specific hybridoma cells, promoted their apoptosis, and mediated cytotoxicity by cross-linking effector cells and complement. Likewise, AChR-Fc significantly reduced the number of AChR-reactive B cells from spleen of Lewis rats immunized with AChR ex vivo.