Eva V. Acosta-Rodríguez

Interleukins 1beta and 6 but not transforming growth factor-beta are essential for the differentiation of interleukin 17-producing human T helper cells.

Interleukin 17 (IL-17)–producing CD4+ helper T cells (TH-17 cells) have been linked to host defense and autoimmune diseases. In mice, the differentiation of TH-17 cells requires transforming growth factor-β and IL-6 and the transcription factor RORγt. We report here that for human naive CD4+ T cells, RORγt expression and TH-17 polarization were induced by IL-1β and enhanced by IL-6 but were suppressed by transforming growth factor-β and IL-12. Monocytes and conventional dendritic cells, but not monocyte-derived dendritic cells activated by microbial stimuli, efficiently induced TH-17 priming, and this function correlated with antigen-presenting cell production of IL-1β and IL-6 but not IL-12. Our results identify cytokines, antigen-presenting cells and microbial products that promote the polarization of human TH-17 cells and emphasize an important difference in the requirements for the differentiation of TH-17 cells in humans and mice.

Surface phenotype and antigenic specificity of human interleukin 17–producing T helper memory cells

Interleukin 17 (IL-17)–producing T helper cells (TH-17 cells) have been characterized in mice as a distinct subset of effector cells, but their identity and properties in humans remain elusive. We report here that expression of CCR6 and CCR4 together identified human memory CD4+ T cells selectively producing IL-17 and expressing mRNA encoding the human ortholog of mouse RORγt, a transcription factor, whereas CCR6 and CXCR3 identified TH1 cells producing interferon-γ and T helper cells producing both interferon-γ and IL-17. Memory T cells specific for Candida albicans were present mainly in the CCR6+CCR4+ TH-17 subset, whereas memory T cells specific for Mycobacterium tuberculosis were present in CCR6+CXCR3+ T helper type 1 subset. The elicitation of IL-17 responses correlated with the capacity of C. albicans hyphae to stimulate antigen-presenting cells for the priming of TH-17 responses in vitro and for the production of IL-23 but not IL-12. Our results demonstrate that human TH-17 cells have distinct migratory capacity and antigenic specificities and establish a link between microbial products, T helper cell differentiation and homing in response to fungal antigens.

Trypanosoma cruzi trans-sialidase initiates a program independent of the transcription factors RORγt and Ahr that leads to IL-17 production by activated B cells

We identified B cells as a major source for rapid, innate-like interleukin 17 (IL-17) production in vivo in response to Trypanosoma cruzi infection. IL-17+ B cells exhibited a plasmablast phenotype, outnumbered TH17 cells and were required for optimal response to this pathogen. Using both murine and human primary B cells, we demonstrate that exposure to parasite-derived trans-sialidase in vitro was sufficient to trigger modification of the cell surface mucin, CD45, leading to Btk-dependent signaling and IL-17A or IL-17F production via an ROR-γt and AHR-independent transcriptional program. Our combined data suggest that generation of IL-17+ B cells may be an unappreciated feature of innate immune responses required for pathogen control or IL-17-mediated autoimmunity.Here we identified B cells as a major source of rapid, innate-like production of interleukin 17 (IL-17) in vivo in response to infection with Trypanosoma cruzi. IL-17+ B cells had a plasmablast phenotype, outnumbered cells of the TH17 subset of helper T cells and were required for an optimal response to this pathogen. With both mouse and human primary B cells, we found that exposure to parasite-derived trans-sialidase in vitro was sufficient to trigger modification of the cell-surface mucin CD45, which led to signaling dependent on the kinase Btk and production of IL-17A or IL-17F via a transcriptional program independent of the transcription factors RORγt and Ahr. Our combined data suggest that the generation of IL-17+ B cells may be a previously unappreciated feature of innate immune responses required for pathogen control or IL-17-mediated autoimmunity.

Galectin-3 Mediates IL-4-Induced Survival and Differentiation of B Cells: Functional Cross-Talk and Implications during Trypanosoma cruzi Infection

The role of transcription factors in B cell survival and differentiation has been delineated during the last years. However, little is known about the intermediate signals and the intracellular pathways that control these events. In this study, we provide evidence both in vitro and in vivo, showing that galectin-3 (Gal-3), a β-galactoside-binding protein, is a critical mediator of B cell differentiation and survival. Although Gal-3 is not expressed in resting B cells from normal mice, its expression is markedly induced after activation with stimuli such as IL-4 and CD40 cross-linking. These signals promote survival and block the final differentiation of these cells, thus allowing the rising of a memory B cell phenotype. In addition, Gal-3 is expressed in B cells from Trypanosoma cruzi-infected mice, which received signals for activation and differentiation in vivo. By using an antisense strategy, we determined that Gal-3 is a critical signal mediating the effects of IL-4 on B cell fate. Blockade of intracellular Gal-3 in vitro abrogated IL-4-induced survival of activated B cells, favoring the differentiation toward a plasma cell pathway. Moreover, B cells with restrained endogenous Gal-3 expression failed to down-regulate the Blimp-1 transcription factor after IL-4 stimulation. Finally, inhibition of Gal-3 in vivo skewed the balance toward plasma cell differentiation, which resulted in increased Ig production and parasite clearance during T. cruzi infection. Thus, the present study provides evidence of a novel role for Gal-3 as an intracellular mediator of B cell survival and a checkpoint in IL-4-induced B cell commitment toward a memory phenotype.

Prostaglandin E2 enhances Th17 responses via modulation of IL-17 and IFN-γ production by memory CD4+ T cells

The contribution of Th1 and Th17 cells in chronic inflammatory conditions leading to autoimmunity remains highly controversial. In inflamed tissues, production of prostaglandins by COX‐2 has been proposed to favor Th17 responses indirectly by increasing IL‐23 and blocking IL‐12 release from APC. We report here that prostaglandin E2 (PGE2) can directly modulate cytokine production by human memory CD4+ T cells. TCR triggering in the presence of PGE2 increased IL‐17 and reduced IFN‐γ production by freshly isolated memory T cells or T‐cell clones. PGE2 triggered the EP2 and EP4 receptors expressed on T cells leading to a rapid increase of retinoic‐acid‐related orphan receptor‐γt (ROR‐γt) and decrease of T‐cell‐specific T‐box transcription factor 21 (T‐bet) mRNA. Moreover, PGE2 promoted the selective enrichment of IL‐17‐producing cells by differentially modulating the proliferation of memory T‐cell subsets in vitro. Taken together our results indicate that T‐cell effector function is a direct target for PGE2 modulation and suggest a novel mechanism by which inhibitors of prostaglandin synthesis, such as COX‐2 inhibitors, exert their anti‐inflammatory effect.

Polyclonal B cell activation in infections: infectious agents’ devilry or defense mechanism of the host?

Polyclonal B cell activation is not a peculiar characteristic to a particular infection, as many viruses, bacteria, and parasites induce a strong polyclonal B cell response resulting in hyper‐γ‐globulinemia. Here, we discuss the different roles proposed for polyclonal B cell activation, which can be crucial for early host defense against rapidly dividing microorganisms by contributing antibodies specific for a spectrum of conserved structures present in the pathogens. In addition, polyclonal B cell activation can be responsible for maintenance of memory B cell responses because of the continuous, unrestricted stimulation of memory B cells whose antibody production may be sustained in the absence of the antigens binding‐specific BCR. Conversely, polyclonal activation can be triggered by microorganisms to avoid the host‐specific, immune response by activating B cell clones, which produce nonmicroorganism‐specific antibodies. Finally, some reports suggest a deleterious role for polyclonal activation, arguing that it could potentially turn on anti‐self‐responses and lead to autoimmune manifestations during chronic infections.

BAFF and LPS cooperate to induce B cells to become susceptible to CD95/Fas‐mediated cell death

Microorganisms with pathogen‐associated molecular patterns (PAMP) activate B cells directly by binding to TLR and also indirectly by inducing APC to release cytokines such as BAFF that promote B cell survival. We found that murine B cells activated concomitantly with LPS (TLR‐4 ligand) and BAFF are protected from spontaneous apoptosis, but are more susceptible to Fas/CD95‐mediated cell death. This increased susceptibility to Fas‐induced apoptosis is associated with a dramatic coordinated up‐regulation of Fas/CD95 and IRF‐4 expression through a mechanism mediated, at least in part, by inhibition of the MEK/ERK pathway. Up‐regulation of Fas/CD95 by BAFF is restricted to B cells activated through TLR‐4, but not through TLR‐9, BCR or CD40. TLR ligands differ in the BAFF family receptors (R) they induce on B cells: BAFF‐R is increased by the TLR4 ligand, LPS, but not by the TLR9 ligand, CpG‐containing oligodeoxynucleotides, which, in contrast, strongly up‐regulates transmembrane activator and CAML interactor (TACI). This suggests the up‐regulation of Fas by BAFF is mediated by BAFF‐R and not by TACI. Consistently, APRIL, which binds to TACI and B cell maturation antigen but not BAFF‐R, did not enhance Fas expression on LPS‐activated B cells. Increased susceptibility to Fas‐mediated killing of B cells activated with LPS and BAFF may be a fail‐safe mechanism to avoid overexpansion of nonspecific or autoreactive B cells.

A Trypanosoma cruzi antigen signals CD11b+ cells to secrete cytokines that promote polyclonal B cell proliferation and differentiation into antibody-secreting cells.

Microbial‐induced polyclonal activation of B cells is a common event in several forms of infections, and is believed to play a crucial role both for enhancing the production of specific antibodies and for maintenance of B cell memory. Therefore, a major challenge in biomedical research is the identification of pathogen‐derived products capable of rapidly mounting B cell expansion and differentiation. Here we report that glutamate dehydrogenase (GDH) stimulates polyclonal proliferation and differentiation of naive B cells. This stimulation was found to be T cell independent, but to absolutely require CD11b+ cells. Moreover, we demonstrate that stimulation of CD11b+ cells by GDH leads to the production of IL‐6, IL‐10 and B cell‐activating factor (BAFF), all of which combine to powerfully induce B cell expansion. Importantly, IL‐6 and IL‐10 further drive B cell terminal differentiation into plasma cells by up‐regulating critical transcription factors and immunoglobulin secretion. Our data provide the first evidence that a protozoan antigen can induce BAFF production by accessory cells, which in concert with other cytokines trigger polyclonal B cell activation.

Depletion of immature B cells during Trypanosoma cruzi infection: involvement of myeloid cells and the cyclooxygenase pathway

The ability of a microorganism to elicit or evade B cell responses represents a determinant factor for the final outcome of an infection. Although pathogens may subvert humoral responses at different stages of B cell development, most studies addressing the impact of an infection on the B cell compartment have focused on mature B cells within peripheral lymphoid organs. Herein, we report that a protozoan infection, i.e. a Trypanosoma cruzi infection, induces a marked loss of immature B cells in the BM, which also compromises recently emigrated B cells in the periphery. The depletion of BM immature B cells is associated with an increased rate of apoptosis mediated by a parasite‐indirect mechanism in a Fas/FasL‐independent fashion. Finally, we demonstrated that myeloid cells play an important role in B cell depletion, since CD11b+ BM cells from infected mice secrete a product of the cyclooxygenase pathway that eliminates immature B cells. These results highlight a previously unrecognized maneuver used by a protozoan parasite to disable B cell generation, limiting host defense and favoring its chronic establishment.

FcγRIIb and BAFF Differentially Regulate Peritoneal B1 Cell Survival

B1 cells produce most natural Abs in unimmunized mice and play a key role in the response to thymus-independent Ags and microbial infection. Enlargement of B1 cell number in mice is often associated with autoimmunity. However, the factors that control peripheral B1 cell survival remain poorly characterized. Mice lacking the inhibitory receptor FcγRIIb exhibit a massive expansion in peritoneal B1 cells, implicating this receptor in B1 cell homeostasis. In this study, we show that peritoneal B1 cells express the highest levels of FcγRIIb among B cell subsets and are highly susceptible to FcγRIIb-mediated apoptosis. B1 cells upregulate FcγRIIb in response to innate signals, including CpG, and the B cell homeostatic cytokine BAFF efficiently protects activated B1 cells from FcγRIIb-mediated apoptosis via receptor downregulation. BAFF-transgenic mice manifest an expansion of peritoneal B1 cells that express lower levels of FcγRIIb and exhibit reduced susceptibility to apoptosis. Whereas both peritoneal B1 cells from wild-type and BAFF-transgenic mice immunized with CpG exhibit an increase in FcγRIIb levels, this change is blunted in BAFF-transgenic animals. Our combined results demonstrate that FcγRIIb controls peritoneal B1 cell survival and this program can be modulated by the BAFF signaling axis.