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The two outermost compartments of skin are populated by different Ag-presenting dendritic cell types. Epidermal Langerhans cells (LCs) are evolutionarily adapted to the continuous presence of harmless skin commensals by the selective lack of cell surface TLRs that sense bacteria. In this article, we analyze the ability of LCs and dermal dendritic cells (DDCs) to respond to virus infection. Live virus and intracellular TLR3-agonist dsRNA commit LCs more effectively than DDCs to stimulate naive CD8(+) T cell expansion and their differentiation into effector cells. This potent CD8(+) T cell-promoting capacity of LCs is causally related to high levels of virus-induced CD70 expression but not to IL-12 production. These data suggest a remarkable specialization of LCs in the induction of pathogen class-specific adaptive immunity. Whereas LCs ignore bacteria, they are superior to DDCs to initiate effective CD70-mediated CD8(+) T cells in response to virus stimulation.

Ligation of CD40 on dendritic cells (DCs) induces early production of inflammatory mediators via canonical NF-kappaB signaling, as well as late expression of the anti-inflammatory enzyme indoleamine 2,3-dioxygenase (IDO) via unknown signal transduction. By selective blocking of either the canonical NF-kappaB pathway using the NEMO-binding domain peptide or the noncanonical NF-kappaB pathway by small interfering RNA, we demonstrate that IDO expression requires noncanonical NF-kappaB signaling. Also, noncanonical NF-kappaB signaling down-regulates proinflammatory cytokine production in DCs. In addition, selective activation of the noncanonical NF-kappaB pathway results in noninflammatory DCs that suppress T-cell activation and promote the development of T cells with regulatory properties. These findings reveal an important role of the noncanonical NF-kappaB pathway in the regulation of immunity.

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.

Glucocorticoids (GC) are known to affect the immune response at several stages. However, little is known about how GC influence the initiation of the specific immune response at the level of dendritic cells (DC), the highly professional APC for T cells. Therefore, we studied whether GC modulate the cytokine production and T cell stimulatory function of DC. In LPS-stimulated DC, GC strongly reduced the secretion of the Thl-skewing factor IL-12p70 and, to a lesser extent, the production of the proinflammatory cytokines IL-6 and TNF-alpha. Regarding the T cell stimulatory function of DC, GC did not influence the cell surface expression of HLA-DR or the costimulatory molecules CD40 and CD80 and did not influence the ability of DC to take up Ag. Consequently, GC pretreatment of DC indeed did not affect their ability to stimulate CD4+ Th cell proliferation in response to superantigen. However, as a result of their defective production of bioactive IL-12, GC-pretreated DC have a reduced ability to promote the production of IFN-gamma in CD4+ Th lymphocytes, as shown by the observation that IFN-gamma production could be restored by exogenous IL-12. In contrast, GC treatment of DC enhanced the secretion of the antiinflammatory cytokine IL-10 and the type 2 cytokine IL-5 by the T cells. It is concluded that, in addition to their role as potent inhibitors of inflammation via the direct suppression of cytokine production in T cells, GC may further inhibit T cell-mediated inflammation indirectly via the suppression of IL-12 production by DC.

The capacity of APC to stimulate the proliferation of human peripheral blood T cells decreases upon ultraviolet-B (UVB) irradiation. The aim of this study was to investigate whether all T cell subsets are equally sensitive to this reduced APC function. Established human Th1, Th2, and Th0 clones were stimulated with monocytes in a soluble CD3 mAb-mediated assay that is dependent on the presence of APC. Monocytes were exposed to low nonlethal doses of UVB radiation before coculture with T cells. UVB irradiation inhibited the capacity of monocytes to stimulate the proliferation and IFN-gamma production of Th1 cells in a dose-related fashion. In contrast, UVB-treated monocytes induced normal proliferation and IL-4 production in Th2 cells. Stimulation of Th0 cell proliferation by UVB-irradiated monocytes was normal, but a preferential suppression of IFN-gamma production was observed, thus leading to a more Th2-like cytokine response. The loss of Th1 proliferation upon stimulation with UVB-irradiated monocytes could be overcome by rIL-2; however, IFN-gamma production remained suppressed. IFN-gamma production could be completely restored by rIL-12, whereas the addition of IL-1 beta, TNF-alpha, or indomethacin had no such effect, nor did the addition of mAb to CD28, added to compensate for the reduced B7 expression of UVB-irradiated monocytes. Monocytes exposed to UVB radiation exhibited reduced expression of mRNA for the IL-1 2 subunits p35 and p40 and suppressed production of the IL-12 p70 protein. Our results thus indicate that UVB irradiation of APC selectively impairs Th1-like responses, a phenomenon caused by the UVB-induced suppression of monocyte IL-12 production.

Several major pathological characteristics of atopic disease are causally related to CD4+ allergen-specific type 2 T-helper (Th2) cells with an aberrant cytokine secretion profile, comprising high levels of interleukin (IL)-4 and IL-5 and low levels of interferon (IFN)-gamma. Although the cytokine secretion patterns of CD4+ T-cells may be stable, they can be modulated by physiological factors which may be expected to be present during activation of these T-cells. In this review, we will focus on two secretion products of professional antigen presenting cells (APCs) and accessory cells with opposite modulatory effects on T-cell cytokine profiles, i.e. prostaglandin E2 (PGE2) and IL-12. PGE2 favours Th2-like cytokine secretion profiles by inhibiting the production of the Th1-associated cytokines, IL-2 and IFN-gamma, and in the presence of sufficient levels of IL-2, upregulating the production of the Th2-associated cytokines, IL-4 and IL-5, IL-12, on the other hand, induces and enhances IFN-gamma secretion in activated CD4+ T-cells, thereby promoting the generation of Th1 cells. PGE2 and IL-12 act via independent mechanisms and, therefore, do not mutually interfere with their modulatory effects. These data suggest that the relative contribution of PGE2 and IL-12 to the levels of secreted Th1- and Th2-associated cytokines are determined by their concentration ratio during T-cell activation.

IL-12 and PGE2 are two immunomodulators produced by accessory cells (AC) in response to various stimuli. IL- 12 enhances IFN-gamma production by activated CD4+ T cells, whereas PGE2 inhibits the secretion of this cytokine. Because these AC-derived factors exert clearly opposite modulatory effects on IFN-gamma production, we examined 1) the net-IFN-gamma production of CD4+ T cells, stimulated in the presence of both IL-12 and PGE2, 2) the susceptibility of activated CD4+ T cells in time by adding these modulators at different timepoints after stimulation, and 3) the relative contributions of AC-derived IL-12 and PGE2 to IFN-gamma levels by stimulating CD4+ T cells in the presence of LPS-activated monocytes and inhibitors of PGE2 or IL-12. Here, we demonstrate that 1) IL-12 and PGE2 do not abrogate the modulatory action of each other and that the net-IFN-gamma production is determined by their concentration ratio, 2) T cells become insensitive to PGE2, whereas susceptibility to IL-12 is retained after activation, and 3) activated monocytes potently modulate IFN-gamma levels of stimulated CD4+ T cells via release of IL-12 and PGE2. The relative contributions of these AC-derived factors shift in time, due to different production kinetics, from a dominant IL-12 effect to a mixed IL-12/PGE2 effect. Because the net IFN-gamma production of CD4+ T cells is largely determined by the ratio of IL-12 and PGE2 at the timepoint of T cell activation, an imbalance in the production of these immunomodulators may, therefore, lead to immunologic dysfunction.

Dendritic cells have been isolated from rabbit lymph nodes. Morphologically and phenotypically, they resemble dendritic cells from the mouse and rat. A comparison was made of the accessory cell function of dendritic cells and peritoneal macrophages in T-cell proliferation induced by phytohaemagglutinin (PHA) or Con A, or by a simultaneous treatment with the enzymes neuraminidase and galactose oxidase (NaGo). Dendritic cells seemed to be more effective than macrophages as accessory cells in these assays. However, macrophages suppress lymphocyte proliferation through the release of oxidating agents and production of prostaglandins. Elimination of this suppressive effect of the macrophages by addition of a combination of 2-mercaptoethanol (2-ME) and indomethacin in PHA-induced cell proliferation resulted in a much higher support by macrophages, giving results that were comparable to those obtained with dendritic cells, but in NaGo-induced proliferation, macrophages were still not as effective as dendritic cells in the presence of the drugs. Experiments in diffusion culture vessels and with interleukin-1-containing macrophage supernatants showed that support by accessory cells can be mediated by soluble factors in PHA-induced proliferation. In contrast, in NaGo-induced proliferation, lymphocytes and accessory cells have to interact directly.

In the majority of patients, allergic nickel contact dermatitis is associated with a proliferative response of peripheral blood T lymphocytes to nickel sulfate. Optimal proliferation was found in a concentration range of 1-2 X 10(-4) M nickel sulfate. Nickel-specific response of purified peripheral blood T cells requires the presence of antigen-presenting cells (APC). Both peripheral blood monocytes and skin-derived epidermal cells could function as APC, but epidermal cells were shown to be more potent than monocytes. By testing FcR+ monocytes and FcR- circulating dendritic cells for their antigen-presenting capacities, it was found that the critical APC within the fraction of monocytes is the circulating dendritic cell. Testing highly purified T6+ (CD 1) skin-specific dendritic cells (Langerhans cells, LC) and T6- epidermal cells as APC, the critical APC within the fraction of epidermal cells appeared to be the LC. The crucial role of LC was stressed in experiments using T cells from patients exhibiting a positive patch test to nickel but a low or absent proliferative response to nickel by unpurified peripheral blood cells. Whereas addition of peripheral blood APC was ineffective, addition of LC to purified peripheral T cells was shown to overcome this low responsiveness to nickel. These results indicate the crucial role of dendritic APC subsets in nickel-specific T-cell proliferation.

The impact of an improved culture medium (IMDM+), consisting of Iscove's modified Dulbecco's medium supplemented with albumin, transferrin, insulin, zinc, 2-mercapthoethanel, and 0.1% fetal bovine serum was investigated in phytohemagglutinin (PHA)-induced rabbit T cell proliferation. At the number of 2 X 10(5) cells/well purified T lymphocytes cultured in IMDM+ responded 3 to 10 times better than lymphocytes cultured in serum-supplemented RPMI 1640 medium. In these conditions, PHA-induced proliferation seemed not to require the presence of accessory cells. However, at lower cell numbers, T cell proliferation was more efficient when calculated on a per cell basis. At these low cell numbers, optimal proliferation required accessory cells like macrophages or dendritic cells. The apparent absence of this requirement for accessory cells at high T cell concentrations may be explained by the contribution of contaminating macrophages and dendritic cells in the purified T cell fractions.

The role of macrophages in mitogen-induced rabbit T cell proliferation has been investigated. The blastogenic response to the 3 mitogens, PHA, ConA and oxidative treatment by neuraminidase and galactose oxidase (NaGo) was tested. T cell proliferation was reduced by removal of low density or plastic adherent cells, including macrophages, and could be enhanced by the addition of peritoneal resident macrophages, indicating a macrophage requirement for rabbit T cell proliferation. However, PHA-induced proliferation could not be raised to the level expected. It was found that catalase and especially 2-ME could considerably enhance macrophage dependent proliferation, even at low macrophage concentrations. It is concluded therefore, that macrophages not only support but also suppress lymphocyte proliferation, namely by non-specific damage to lymphocytes through release of radicals and hydrogen peroxide. In addition, peritoneal, but not lymph node macrophages were found to suppress lymphocyte proliferation by prostaglandin production, although to a lesser extent. Experiments, done in the presence of blockers of macrophage-mediated suppression, showed that macrophages were able to magnify the PHA-induced T cell proliferation to the expected values. The experiments thus show that unactivated macrophages support and suppress lymphocyte proliferation at the same time.