Jean-Michel Bridon

Dendritic cells directly modulate B cell growth and differentiation

A cardinal feature of Langerhans cells or dendritic cells (DC) located within the mucosal epithelium is to capture foreign antigens after tissue injury and subsequently initiate immune responses. While migrating through the draining afferent lymph into the proximal secondary lymphoid organ, DC process the antigens. Within paracortical areas of the secondary lymphoid organs, DC [referred to in this localization as interdigitating dendritic cells (IDC)] select the rare antigen-specific T and B cells [1, 2]. IDC have the unique capacity to stimulate antigen-specific naive T cells to proliferate, secrete cytokines, and express CD40L [3–5]. In murine models, immunohistological studies have demonstrated that primary T cell-dependent B cell responses were initiated within the T cell/IDC-rich areas [6]. Activated T cells stimulate antigen-specific naive B cells to proliferate and to differentiate into germinal center founder cells or into short-lived plasma cells producing essentially immunoglobulin M (IgM) [7]. The germinal center reaction starts with the colonization of primary follicles by germinal center founder cells. Antigen transporting cells have been identified that trap immune complexes in the lymph and move these complexes onto the follicular dendritic cell network within the follicles [8]. The nature of such antigen transporting cells has not yet been formally identified. Within the follicles, follicular dendritic cells, which are probably not of hemopoietic origin, retain immune complexes, thus allowing B cells to endocytose, process, and present the antigen to CD4 T cells. After intense proliferation of centroblasts in the dark zone, irreversible events occur during the germinal center reaction that lead to isotype-switched B cells with high affinity for the antigen that eventually differentiate into either memory B cells or plasma cells. Apart from the critical role of DC in the initiation of cellular immune responses [4], several experiments strongly support their direct involvement in the regulation of humoral responses. Our studies now provide evidence that human DC directly interact with B cells in vitro and regulate mature B cell responses at various stages of their differentiation.

Interleukin 10 induces B lymphocytes from IgA-deficient patients to secrete IgA.

We have previously shown that human B lymphocytes cultured in the CD40 system, composed of an anti-CD40 mAb presented by a CD32-transfected fibroblastic cell line, proliferate but do not secrete antibodies. However, the addition of particles of Staphylococcus aureus Cowan (SAC) induces B cell differentiation even in the absence of exogenous cytokines (CD40/SAC system). Additionally, B lymphocytes cultured in the CD40 system in the presence of human IL-10, produce IgM, IgG, and IgA, and Ig levels are further increased by SAC. Here, we have studied the capacity of peripheral blood lymphocytes from patients with IgA deficiency (IgA-D) to secrete Igs, particularly IgA after CD40 triggering. Peripheral blood mononuclear cells (PBMNC) from IgA-D patients cultured in the CD40/SAC system produced IgM and IgG, but not IgA. The addition of IL-10 to the cultures, enhanced the production of IgM and IgG and most strikingly induced the production of high amounts of IgA. The addition of IL-10 to PBMNC from IgA-D patients activated through CD40 alone resulted in the production of IgA. Thus, SAC and anti-CD40 mAb stimulate B cells to differentiate into cells secreting IgG and IgM whereas IL-10 plays a central role in inducing B cells from IgA-D patients to differentiate into IgA secreting cells.

Virus overrides the propensity of human CD40L‐activated plasmacytoid dendritic cells to produce Th2 mediators through synergistic induction of IFN‐γ and Th1 chemokine production

Depending on the activation status, plasmacytoid dendritic cells (PDC) and myeloid DC have the ability to induce CD4 T cell development toward T helper cell type 1 (Th1) or Th2 pathways. Thus, we tested whether different activation signals could also have an impact on the profile of chemokines produced by human PDC. Signals that induce human PDC to promote a type 1 response (i.e., viruses) and a type 2 response [i.e., CD40 ligand (CD40L)] also induced PDC isolated from tonsils to secrete chemokines preferentially attracting Th1 cells [such as interferon‐γ (IFN‐γ)‐inducible protein (IP)‐10/CXC chemokine ligand 10 (CXCL10) and macrophage inflammatory protein‐1β/CC chemokine ligand 4 (CCL4)] or Th2 cells (such as thymus and activation‐regulated chemokine/CCL17 and monocyte‐derived chemokine/CCL22), respectively. Activated natural killer cells were preferentially recruited by supernatants of virus‐activated PDC, and supernatants of CD40L‐activated PDC attracted memory CD4+ T cells, particularly the CD4+CD45RO+CD25+ T cells described for their regulatory activities. It is striking that CD40L and virus synergized to trigger the production of IFN‐γ by PDC, which induces another Th1‐attracting chemokine monokine‐induced by IFN‐γ/CXCL9 and cooperates with endogenous type I IFN for IP‐10/CXCL10 production. In conclusion, our studies reveal that PDC participate in the selective recruitment of effector cells of innate and adaptive immune responses and that virus converts the CD40L‐induced Th2 chemokine patterns of PDC into a potent Th1 mediator profile through an autocrine loop of IFN‐γ.

CD40L activation of dendritic cells down-regulates DORA, a novel member of the immunoglobulin superfamily.

Using a cDNA subtraction technique, a novel member of the immunoglobulin superfamily was isolated from human Dendritic cells (DC). This cDNA which we named DORA, for DOwn-Regulated by Activation encodes a protein belonging to the CD8 family of receptors containing a single V type loop domain with an associated J chain region, a transmembrane region containing an atypical tyrosine residue and a cytoplasmic domain containing three putative tyrosine phosphorylation sites. The hDORA gene has been localised to chromosome 16. From database searches a rat cDNA was identified that encoded a polypeptide with 63% identity to hDORA. The expression of the human cDNA was studied in detail. Northern blot analysis revealed 1.0 kb and 2.5 kb mRNAs in peripheral blood lymphocytes, spleen and lymph node, while low levels were observed in thymus, appendix, bone marrow and fetal liver. No signal was noted in non-immune system tissues. By RT-PCR analysis of hDORA revealed expression in cells committed to the myeloid lineage but not in CD34+ precursors or B cells and low expression in T cells. Expression was also observed in DC, purified ex vivo or generated in vitro from either monocytes or CD34+ progenitors. This was down-regulated following activation both by PMA and Ionomycin treatment and also by CD40L engagement. In situ hybridisation performed on tonsil sections showed the presence of hDORA in cells within Germinal Centers. This structure and expression suggests a function as a co-receptor, perhaps in an antigen uptake complex, or in homing or recirculation of DC.

Transforming Growth Factor Beta (TGFβ) Directs IgA1 and IgA2 Switching in Human Naive B Cells

Until recently, there was no experimental system that permitted the study of soluble factors capable of inducing human B lymphocytes to switch isotypes in vitro. Unlike lipopolysaccharide which is currently used for murine B cells, most of human B cell activators are poorly efficient. However, two novel experimental procedures are now available: one is based on the capacity of activated T cells or their membranes to elicit a B cell response which involves interactions in surface molecules.1 The other one, which is referred to as the CD40 system was developed in our laboratory.2 In this “CD40 system”, polyclonal activation and sustained proliferation of human B cells3 are obtained by presentation of an anti-CD40 monoclonal antibody on an irradiated mouse fibroblastic L cell line that stably expresses CDw32 (FcγRII). Upon interaction with the CD40 molecule, the Fc portion of the antibody undergoes spatial rearrangement which allows high-affinity interaction with CDw32. One can further enhance the B cell response by coupling the anti-CD40 stimulation with surface Ig (slg) cross-linking agents such as anti-µ or S. aureus Cowan (SAC) particles (Fig. 1).