Relevance of IgA in allergy: regulation by mucosal factors

Allergy is mediated by allergen-specific antibodies of various classes of which immunoglobulin E (IgE) produced by B cells upon stimulation by T helper type 2 cells is of crucial importance. Immunoglobulins have varying antigen and allergen specificity (introduced in chapter 1). IgE has to be specific for an allergen (sensitization) and bind simultaneously to several epitopes on the allergen to cause cross-linking of IgE immobilized on high affinity IgE receptors expressed on mast cells and basophils to induce degranulation (Chapter 2). IgE molecules can be specific for a protein of one species or bind to similar regions on allergens from different species, thereby causing cross-reactivity as discussed for shrimp and mussel tropomyosin in chapter 2 and fig and ficus in chapter 3. In most cases of cross-reactivity the primary sensitizing allergen can be identifying by inhibition assays (Chapter 3). The presence of allergen-specific IgE in serum of allergic patients does not necessarily cause clinically relevant symptoms of allergy when blocking immunoglobulins of non-IgE isotypes are present, like IgG4, which is induced by specific immunotherapy. Until now, serum-derived IgA has not been shown to be able to block IgE-allergen interactions and the potential protective role of allergen-specific IgA is still debated. The aim of this thesis was to study the potential protective role of the mucosal immune system and especially IgA production by B cells against allergy. The mechanisms underlying differential regulation of the two IgA subclasses IgA1 and IgA2 production characteristic for humans are largely unknown. Most likely factors in the local mucosal tissue are involved in differentiating between IgA1 and IgA2 production. Evidence for the role of dendritic cells (DC) in IgA production is increasing. We generated and characterized two DC types not previously described in literature by using the mucosal factors retinoic acid (RA) and transforming growth factor (TGF)-ß (Chapter 5). Those RA- and TGF-ß-derived dendritic cells have tolerogenic characteristics, as shown by reduced inflammatory cytokine production (IL-12 and TNF-α), but non-significant reduction in IL-10 production, and reduced expression of activation marker CD86 and maturation marker CD83 after stimulation with bacterial ligands. To study the role of epithelial cell- and DC-derived molecules in induction of production of IgA by B cells and plasma cells, the mucosa-related cytokines APRIL, BAFF, IL-10, TGF-ß, and vitamin A-derived RA were added to B cells. Addition of RA resulted in differentiation of B cells into plasma cells (CD38+) and enhanced secretion of IgA1 and IgA2 when also IL-10 and APRIL or BAFF was present. Our data indicate that production of IgA1 is increased in the presence of BAFF (probably by plasma cells), whereas IgA2 production is increased in the presence of APRIL which is produced by either DCs or epithelial cells in vivo (probably by de-novo induction). When RA and IL-10 were added, B cells upregulated homing integrin ß7 on the membrane, which is believed to preferably direct homing to the small intestine. In literature, increased levels of APRIL have been associated with decreased prevalence of eczema, whereas increased levels of TSLP have been associated with presence of eczema. In chapter 6, it is shown that APRIL is involved in induction of IgA2, and that TSLP potently inhibits IgA1, and to a lesser extent also IgA2 production, which is consistent with reports showing that increased levels of IgA correlate with less eczema. Also in chapter 4, an association was found between reduced levels of allergen-specific IgA2 in serum and presence of eczema and asthma. Also the tissue (systemic, e.g. serum versus mucosal, e.g. saliva) where IgA is determined is differentially linked to the clinical status in house dust mite-allergic patients. In addition, allergen-specific levels of IgA2 rather than IgA1 appeared to be associated with absence of allergy. Within house dust mite-allergic patients, the ratio between serum-IgE and saliva-IgA2 was associated with severity of local (mucosal) clinical symptoms. IgA production is diminished in the absence of intestinal bacteria and colonization of the gut induces production of IgA. The ontogeny of the bacteria-specific repertoire of intestinal IgA in relation to composition of the intestinal microbiota has not been studied before. In chickens we analysed the CDR3-repertoire development in the first ten weeks post hatch. No association between bacterial composition and IgA CDR3 repertoire was found, indicating that bacteria may induce IgA production but not cause extensive modification in the specificity of IgA (Chapter 7). Transitional changes in the composition of the microbiota were restored once IgA production was initiated, suggesting that IgA is directly involved in regulation of the intestinal microbiota composition. Diet-derived RA is a key regulator of tolerance and IgA production in the mucosa. In addition, dietary ingredients can directly interact with cells of the intestinal immune system. In chapter 8 we show that bovine IL-10 binds to the human IL-10 receptor and thereby modulates bacterial ligand induced activation of monocytes and DCs. Bovine milk also contains immunoglobulins that are specific for bacterial ligands and potential allergens that are also encountered by the human mucosal immune system. Bovine IgG efficiently binds to human IgG receptors and can modulate myeloid cell activation by LPS (Chapter 9). Immunoglobulin from bovine milk may therefore provide potent opportunities to either induce tolerance or antigen-specific immune responses resulting in the production of protective IgA, thereby assisting in providing protection against pathogenic infections. This protection may be mediated by bovine IgG alone, or in cooperation with IL-10, TGF-ß and vitamin A-derived RA. The findings from chapters 2-9 are discussed and applied for the field of allergy, both for the clinic and experimental research. The finding that human IgA1 and IgA2 are differentially regulated by innate factors and differentially correlated with the presence or absence of allergy and severity of clinical symptoms is a promising finding. This finding may provide new opportunities for allergen-specific immunotherapy. In addition, efficacy of other mucosal vaccination strategies could be affected by the innate mucosal mechanisms of regulation of IgA production.