Pedro L. Vieira

IL-10-Secreting Regulatory T Cells Do Not Express Foxp3 but Have Comparable Regulatory Function to Naturally Occurring CD4+CD25+ Regulatory T Cells

Regulatory T cells (TReg) control immune responses to self and nonself Ags. The relationship between Ag-driven IL-10-secreting TReg (IL-10-TReg) and naturally occurring CD4+CD25+ TReg is as yet unclear. We show that mouse IL-10-TReg obtained using either in vitro or in vivo regimens of antigenic stimulation did not express the CD4+CD25+ TReg-associated transcription factor Foxp3. However, despite the absence of Foxp3 expression, homogeneous populations of IL-10-TReg inhibited the in vitro proliferation of CD4+CD25− T cells with a similar efficiency to that of CD4+CD25+ TReg. This inhibition of T cell proliferation by IL-10-TReg was achieved through an IL-10-independent mechanism as seen for CD4+CD25+ TReg and was overcome by exogenous IL-2. Both IL-10-TReg and CD4+CD25+ TReg were similar in that they produced little to no IL-2. These data show that Foxp3 expression is not a prerequisite for IL-10-TReg activity in vitro or in vivo, and suggest that IL-10-TReg and naturally occurring CD4+CD25+ TReg may have distinct origins.

Microbial Compounds Selectively Induce Th1 Cell-Promoting or Th2 Cell-Promoting Dendritic Cells In Vitro with Diverse Th Cell-Polarizing Signals

Upon microbial infection, specific Th1 or Th2 responses develop depending on the type of microbe. Here, we demonstrate that different microbial compounds polarize the maturation of human myeloid dendritic cells (DCs) into stably committed Th1 cell-promoting (DC1) or Th2 cell-promoting (DC2) effector DCs that polarize Th cells via different mechanisms. Protein extract derived from the helminth Schistosoma mansoni induced the development of DC2s that promote the development of Th2 cells via the enhanced expression of OX40 ligand. Likewise, toxin from the extracellular bacterium Vibrio cholerae induced development of DC2s as well, however, via an OX40 ligand-independent, still unknown mechanism. In contrast, toxin from the intracellular bacterium Bordetella pertussis induced the development of DC1s with enhanced IL-12 production, which promotes a Th1 cell development. Poly(I:C) (dsRNA, mimic for virus) induced the development of extremely potent Th1-inducing DC1, surprisingly, without an enhanced IL-12 production. The obtained DC1s and DC2s are genuine effector cells that stably express Th cell-polarizing factors and are unresponsive to further modulation. The data suggest that the molecular basis of Th1/Th2 polarization via DCs is unexpectedly diverse and is adapted to the nature of the microbial compounds.

IL-10–producing and naturally occurring CD4+ Tregs: limiting collateral damage

Effective immune responses against pathogens are sometimes accompanied by strong inflammatory reactions. To minimize damage to self, the activation of the immune system also triggers anti-inflammatory circuits. Both inflammatory and anti-inflammatory reactions are normal components of the same immune response, which coordinately fight infections while preventing immune pathology. IL-10 is an important suppressive cytokine, produced by a large number of immune cells in addition to the antigen-driven IL-10-producing regulatory and the naturally occurring suppressor CD4+ T cells, which is a key player in anti-inflammatory immune responses. However, additional mechanisms have evolved to ensure that pathogen eradication is achieved with minimum damage to the host. Here we discuss those mechanisms that operate to regulate effector immune responses.

Glatiramer Acetate (Copolymer-1, Copaxone) Promotes Th2 Cell Development and Increased IL-10 Production Through Modulation of Dendritic Cells

Glatiramer acetate (GA; copolymer-1, Copaxone) suppresses the induction of experimental autoimmune encephalomyelitis and reduces the relapse frequency in relapsing-remitting multiple sclerosis. Although it has become clear that GA induces protective degenerate Th2/IL-10 responses, its precise mode of action remains elusive. Because the cytokine profile of Th cells is often regulated by dendritic cells (DC), we studied the modulatory effects of GA on the T cell regulatory function of human DC. This study shows the novel selective inhibitory effect of GA on the production of DC-derived inflammatory mediators without affecting DC maturation or DC immunostimulatory potential. DC exposed to GA have an impaired capacity to secrete the major Th1 polarizing factor IL-12p70 in response to LPS and CD40 ligand triggering. DC exposed to GA induce effector IL-4-secreting Th2 cells and enhanced levels of the anti-inflammatory cytokine IL-10. The anti-inflammatory effect of GA is mediated via DC as GA does not affect the polarization patterns of naive Th cells activated in an APC-free system. Together, these results reveal that APC are essential for the GA-mediated shift in the Th cell profiles and indicate that DC are a prime target for the immunomodulatory effects of GA.

Differential expression of inflammatory chemokines by Th1- and Th2-cell promoting dendritic cells: A role for different mature dendritic cell populations in attracting appropriate effector cells to peripheral sites of inflammation

Protective immunity to pathogens depends on efficient immune responses adapted to the type of pathogen and the infected tissue. Dendritic cells (DC) play a pivotal role in directing the effector T cell response to either a protective T helper type 1 (Th1) or type 2 (Th2) phenotype. Human monocyte‐derived DC can be differentiated into Th1‐, Th2‐ or Th1/Th2‐promoting DC in vitro upon activation with microbial compounds or cytokines. Host defence is highly dependent on mobile leucocytes and cell trafficking is largely mediated by the interactions of chemokines with their specific receptors expressed on the surface of leucocytes. The production of chemokines by mature effector DC remains elusive. Here we assess the differential production of both inflammatory and homeostatic chemokines by monocyte‐derived mature Th1/Th2‐, Th1‐ or Th2‐promoting DC and its regulation in response to CD40 ligation, thereby mimicking local engagement with activated T cells. We show that mature Th1‐ and Th1/Th2‐, but not Th2‐promoting DC, selectively express elevated levels of the inflammatory chemokines CCL2/MCP‐1, CCL3/MIP‐1α, CCL4/MIP‐1β and CCL5/RANTES, as well as the homeostatic chemokine CCL19/MIP‐3β. CCL21/6Ckine is preferentially expressed by Th2‐promoting DC. Production of the Th1‐attracting chemokines, CXCL9/Mig, CXCL10/IP‐10 and CXCL11/I‐TAC, is restricted to Th1‐promoting DC. In contrast, expression of Th2‐associated chemokines does not strictly correlate with the Th2‐promoting DC phenotype, except for CCL22/MDC, which is preferentially expressed by Th2‐promoting DC. Because inflammatory chemokines and Th1‐associated chemokines are constitutively expressed by mature Th1‐promoting DC and CCL22/MDC is constitutively expressed by mature Th2‐promoting DC, we propose a novel role for mature DC present in inflamed peripheral tissues in orchestrating the immune response by recruiting appropriate leucocyte populations to the site of pathogen entry.

ICOS expression by activated human Th cells is enhanced by IL-12 and IL-23: increased ICOS expression enhances the effector function of both Th1 and Th2 cells.

Previous mouse studies have shown that IL-4 increases the expression of ICOS on activated Th cells, resulting in enhanced ICOS expression on Th2 cells. In this study, we show that ICOS expression on human Th cells is not increased by IL-4, but by IL-12 and by IL-23 instead. Consequently, ICOS expression during IL-12-driven Th1 cell polarization was transiently increased compared with the levels on Th0 cells and IL-4-driven Th2 cells. Addition of IL-12 and/or IL-23 during restimulation increased ICOS expression to the same extent on pre-established Th1, Th2, and Th0 cells, indicating that ICOS levels are not stably imposed by prior polarization. In contrast to the findings in the mouse, IL-4 significantly suppressed the ICOS-enhancing effects of IL-12 and IL-23. The functional consequence of variable ICOS levels was shown in coculture experiments with cells expressing the ICOS-ligand B7-related protein 1 (either transfected Chinese hamster ovary cells or autologous dendritic cells). Ligation of ICOS on 2-day-preactivated effector cells increased their cytokine production to an extent proportional to their ICOS expression levels. As the ICOS-enhancing potentials of IL-12 and IL-23 were maintained for several days after stimulation, both on Th1 and Th2 cells, we propose the concept that local regulation of ICOS expression on activated Th cells by IL-12 and/or IL-23 may provide a powerful means to amplify effector T cell responses in peripheral tissues, independently of the polarized state of the Th cells.

ICOS-mediated signaling regulates cytokine production by human T cells and provides a unique signal to selectively control the clonal expansion of Th2 helper cells.

The CD28 homologue inducible costimulator (ICOS) has been demonstrated to regulate a number of T cell‐dependent immune responses in vivo. However, the expression and functional importance of ICOS during APC‐Th cell interaction in the human is not fully understood. Here, we demonstrate that ICOS‐mediated signaling plays an important role in the production of selective cytokines during both primary and subsequent Th cell responses upon allospecific or superantigen activation. In contrast, ICOS does not play a role in the differentiation of naive cells into Th1 or Th2 effector cells, nor does it determine the type of effector function of memory cells upon subsequent allogeneic challenge. In addition, our data demonstrate that ICOS provides a novel and unique role in regulating DC‐mediated Th2, but not Th1 cell clonal expansion. These data suggest that ICOS‐mediated signaling plays a discrete role in the regulation of human T helper cell responses.