Human B cells in peripheral tolerance

Abstract

Abstract English Tight regulation of immune responses is essential to prevent immune disorders such as allergies. A healthy response to innocuous environmental antigens is characterized by immunological tolerance while allergies are the result of an exaggerated immune response towards such antigens (also referred to as allergens). The exact mechanisms that determine whether an individual develops allergies are still not fully understood. Some of the key events that underlie the development of an allergic response are the differentiation and expansion of allergen-specific T helper (Th) 2 cells that mediate the activation of allergen-specific B cells, leading to the production of allergen-specific IgE antibodies. Healthy individuals that are exposed to an allergen frequently mount an active immune response without developing allergies. Such a response is characterized by the development of allergen- specific inducible interleukin (IL)-10-producing regulatory T (Tr1) cells. IL-10 is an immunosuppressive cytokine that inhibits antigen-specific T cell proliferation and cytokine production and is essential in peripheral tolerance to allergens, autoantigens and tumor antigens. Furthermore, IL-10 inhibits IgE production by B cells while augmenting the production of non-inflammatory IgG4 antibodies. Recent studies have identified IL-10-producing B (regulatory) cells as important regulators of immune responses primarily in autoimmune diseases. This thesis is focused on the role of B cells in the regulation of immune responses, particularly in relation to induction and maintenance of peripheral tolerance to allergens. We characterized the phenotype and function of human inducible IL-10-producing regulatory B cells (Br1 cells) from peripheral blood (described in chapter 6.1). We identified Br1-related surface markers (high expression of CD25 and CD71 and low expression of CD73), demonstrated that Br1 cells potently suppress antigen-specific CD4+ T cell proliferation and represent precursors of IgG4-producing plasma cells. We looked at the frequencies of allergen-specific Br1 cells from beevenom allergic patients before and after receiving allergen-specific immunotherapy (SIT), and from highly beevenom-exposed non-allergic beekeepers. We found that the frequency of allergen-specific Br1 cells was low in allergic individuals and increased upon SIT to levels comparable to those observed in beekeepers. Next, we studied the effect of IL-10 overexpression on the function and phenotype of B cells (described in chapter 6.2). We found that IL-10 overexpression induces a regulatory phenotype in B cells, and arms them to suppress antigen-specific proliferation of PBMC, production of inflammatory cytokines and maturation and differentiation of monocyte-derived dendritic cells. Furthermore, IL-10-overexpressing B cells suppressed TLR2-L or TLR4- L-induced proinflammatory cytokines and chemokines from PBMC. Finally, we investigated activation of B cells by different TLR-ligands, cytokines, and CD40L stimulation and the modulation of these responses by mammalian suppressive telomeric oligodeoxynucleotide (ODN) sequences (described in chapter 6.3). We found that direct and indirect stimulation of human B cells through endosomal associated TLRs plays a pivotal role in polyclonal B cell activation and immunoglobulin production to maintain long-term serological memory in humans. Furthermore, we observed a direct and general suppressive effect of mammalian telomeric ODN on the activation of human B cells at the level of proliferation, class-switch recombination, plasma cell differentiation and immunoglobulin production. Suppression of B cell responses by telomeric ODN may represent a natural mechanism to control B cell activation under chronic inflammatory conditions. The findings described in this thesis provide new insights into the role of B cells in the regulation of immune tolerance. Human B Cells in Peripheral Tolerance Willem D. van de Veen