Molecular & Cellular Allergology

The activation of hematopoietic cells is positively and negatively controlled by membrane receptors. Among these are receptors for the Fc portion of antibodies (FcRs). Antibodies can both activate and inhibit cell activation when engaging activating and/or inhibitory FcRs. Activating FcRs contain or are associated with subunits which contain Immunoreceptor Tyrosine-based Activation Motifs(ITAMs). Inhibitory FcRs contain an Immunoreceptor Tyrosine-based Inhibition Motif(ITIM). Our unit studies 1) signals generated by activating and by inhibitory FcRs and their integration, 2) the roles of the various FcRs and the cells by which they are expressed in murine models of inflammatory diseases, particularly in mice the endogenous FcRs of which have been replaced by human FcRs, and 3) FcR-dependent activation and negative regulation in cells from normal donors and allergic patients. Results obtained in 2009-2010 are as follows.

1. The specificity and affinity of human Fcg Receptors for human IgG subclasses. The specificity and affinity of individual human FcgRs for the four human IgG subclasses is unknown. This information is critical for antibody-based immunotherapy which has been increasingly used in the clinics. We determined the binding specificity and measured the affinity of FcgRI, FcgRIIA, IIB and IIC, FcgRIIIA and IIIB and all their known polymorphic variants for human polyclonal and monoclonal IgG1, IgG2, IgG3 and IgG4. Wt and low-fucosylated IgG1 anti-CD20 and anti-RhD mAbs were also examined. These results document how FcgR specificity and affinity may account for the biological activities of antibodies. They therefore highlight the role of specific FcgR in the therapeutic and pathogenic effects of antibodies in disease. (P. Bruhns et al., Blood.2009; 113:3716-3725)

2. An unexpected IgG receptor on human basophils.Besides high-affinity IgE receptors (FceRI), human basophils express the low-affinity IgG receptors FcgRII. We found that they also express FcgRIIIB, that are GPI-anchored receptors and whose function is poorly understood. Noticeably, the proportion of FcgRIIIB+ basophils was significantly lower in atopic dermatitis patients than in normal donors or in patients suffering from an allergic respiratory diseases (allergic rhinitis and asthma), or a nonallergic skin disease (chronic urticaria).Our results challenge the two dogmas 1) that basophils do not express FcgRIII and 2) that FcgRIIIB is exclusively expressed by neutrophils. (N. Meknache et al. J. Immunol., 2009, 182: 2542–2550)

3. FcgRIV in K/BxN arthritis.The K/BxN serum-induced passive arthritis is a widely used mouse model of rheumatoid arthritis. It is thought to depend: 1) on IgG1 anti-glucose-6-phosphate isomerase antibodies that are present in K/BxN serum; 2) on FcgRIIIA that bind IgG1; and 3) on mast cells, monocytes/macrophages and neutophils that express FcgRIIIA.Using genetically modified multi-Fc receptor-deficient mice, we found that IgG2 autoantibodies could induce arthritis by engaging FcgRIV and that human transgenic FcgRI (CD64), which has the same specificity, tissue distribution and activating properties as FcgRIV, could replace FcgRIV.Our results unravel an unexpected complexity in the K/BxN model that is likely to apply to rheumatoid arthritis in humans. (D. Mancardi, F. Jönsson et al., J. Immunol. Cutting Edge, 2011, in press)

4. Neutrophils as inducers of systemic anaphylaxis. Classically, anaphylaxis is an IgE-dependent, FcεRI-dependent, mast cell-dependent, histamine-dependent life-threatening hyperacute immediate hypersensitivity reaction. We show that active systemic anaphylaxis involves primarily IgG antibodies, FcγRIIIA and FcγRIV, Platelet-Activating Factor (PAF), neutrophils and, to a lower extent, basophils. Neutrophil activation could be monitored in vivoduring anaphylaxis, and neutrophil depletion inhibited anaphylaxis. Importantly, mouse or human neutrophils restored anaphylaxis in anaphylaxis-resistant mice, suggesting that neutrophils can contribute to human anaphylaxis. (F. Jönsson, D. Mancardi et al., J. Clin. Invest., 2011, in press)

5. The dominant inhibitory effects of IgG receptors in human and murine basophils.Basophils express not only high-affinity IgE receptors (FceRI) whose engagement by IgE and antigen triggers the release and/or secretion of pro-inflammatory mediators and cytokines, but also low-affinity IgG receptors whose function is poorly known. We found that basophils express both activating (FcgRIIA in humans; FcgRIIIA in mice) and inhibitory (FcgRIIB in both species) IgG receptors, that FcgRIIA could hardly activate human basophils, whereas FcgRIIIA activated mouse basophils as efficiently as FceRI, that FcgRIIB inhibited IgG-induced FcgRIIA- and FcgRIIIA-dependent human and murine basophil activation and that FcgRIIB markedly decreased IgE-induced FceRI-dependent human and murine basophil activation. IgG receptors therefore function as a regulatory module which generates dominant inhibitory signals that can prevent IgG-induced and control IgE-induced basophil activation in mice and humans. (F. Jönsson, L. Cassard et al., submitted)

6. L. casei inhibits mast cell-dependent inflammation.The protective effect of probiotic bacteria against allergic and autoimmune diseases is thought to result from their interactions with dendritic cells, at the initiation of adaptive immune responses. We found that probiotics can affect allergic and autoimmune inflammation by acting at the effector phase of adaptive immune responses. Indeed, L. caseiprotected from IgE-induced passive systemic anaphylaxis and IgG-induced passive K/BxN arthritis, two in vivo models of acute allergic and autoimmune inflammation which bypass the induction phase of immune responses and depend on mast cells. L.casei also inhibited IgE- and IgG-induced mouse mast cell activation and IgE-dependent human basophil activation. Inhibition required a contact between mast cells and bacteria, was reversible, and selectively affected the Lyn/Syk/LAT pathway induced upon engagement of IgE receptors, leading to decreased MAP kinase activation, Ca2+ mobilization,degranulation and cytokine secretion. These results provide theoretical bases for a therapeutical use of probiotics to prevent inflammation in patients who have already synthesized specific IgE or autoantibodies. (C. Schiffer et al., submitted)

Keywords: Fc Receptors, Signal transduction, Immunoregulation, allergy, inflammation