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Much of our focus in understanding Th1/Th2 development has been on the signals delivered by IL-12 and IL-4 as final determinants of terminal T cell differentiation. Because extinction of IL-12 signaling in early Th2 development could potentially be important in imprinting a more permanent Th2 phenotype on a population of T cells, we have also examined various parameters regulating the IL-12 signaling pathway. Whereas IL-4 appears to repress functional IL-12 signaling through inhibition of IL-12R beta 2 expression, IFN-gamma in the mouse, and IFN-alpha in the human appear to induce IL-12R beta 2 expression and promote IL-12 responsiveness. We propose that Th1 development can be considered in two stages, capacitance and development. Capacitance would simply involve expression of IL-12R beta 1 and beta 2 subunits, regulated by TCR, IL-4 and IFNs. The second stage, development, we propose is the true IL-12 induced developmental stage, involving expression of Stat4 inducible proteins. In the human, this may also occur via IFN-alpha, which is able to activate Stat4. It is perhaps possible that all of Stat4 actions on Th1 development may be exert directly by Stat4 at the IFN-gamma gene, however we suggest that, more likely, Stat4 may act to induce Th1 development through the induction of other non-cytokine genes, whose stable expression maintains the transcriptional state of a Th1 cell.

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Th phenotype development is controlled not only by cytokines but also by other parameters including genetic background. One site of genetic variation between murine strains that has direct impact on Th development is the expression of the IL-12 receptor. T cells from B10.D2 and BALB/c mice show distinct control of IL-12 receptor expression. When activated by Ag, B10.D2 T cells express functional IL-12 receptors and maintain IL-12 responsiveness. In contrast, under the same conditions, BALB/c T cells fail to express IL-12 receptors and become unresponsive to IL-12, precluding any Th1-inducing effects if subsequently exposed to IL-12. Previously, we identified a locus, which we termed T cell phenotype modifier 1 (Tpm1), on murine chromosome 11 that controls this differential maintenance of IL-12 responsiveness. In this study, we have produced a higher resolution map around Tpm1. We produced and analyzed a series of recombinants from a first-generation backcross that significantly narrows the genetic boundaries of Tpm1. This allowed us to exclude from consideration certain previous candidates for Tpm1, including IFN-regulatory factor-1. Also, cellular analysis of F1(B10.D2 x BALB/c) T cells demonstrates that Tpm1 exerts its effect on IL-12 receptor expression in a cell-autonomous manner, rather than through influencing the extracellular milieu. This result strongly implies that despite the proximity of our locus to the IL-13/IL-4 gene cluster, these cytokines are not candidates for Tpm1.

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Expression of IL-12Rs is one important checkpoint for Th1 development. BALB/c DO11.10 CD4+ T cells stimulated by Ag in neutral conditions lose expression of the IL-12R beta 2 subunit and become unresponsive to IL-12. In contrast, B10.D2 or F1 (BALB/c x B10.D2) DO11.10 CD4+ T cells maintain IL-12R beta 2 expression when stimulated similarly. Here we show that the loss of IL-12 responsiveness by BALB/c T cells involves the action of endogenous TGF-beta. BALB/c T cells stimulated in the presence of anti-TGF-beta specifically maintain IL-12 responsiveness, express IL-12R beta 2 mRNA, and can stimulate nitric oxide production in peritoneal exudate cells. Low concentrations of TGF-beta added exogenously during primary activation of B10.D2 or F1 T cells significantly inhibit their development of IL-12 responsiveness. These effects of anti-TGF-beta are dependent on endogenous IFN-gamma and are inhibited by exogenously added IL-4. Thus, at least one effect of TGF-beta on Th1/Th2 development may be the attenuation of IL-12R beta 2 expression.

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In this report, we examined the molecular basis underlying the genetic difference between BALB/c and B10.D2 T cells for T helper phenotype development in vitro. We found a strain-dependent difference in early maintenance of IL-12 responsiveness by T cells developing in vitro in unmanipulated (neutral) conditions. Thus, when activated without addition of exogenous cytokines or neutralization of endogenous cytokines, B10.D2, but not BALB/c, T cells remain responsive to IL-12 when activated for 7 days. The pattern of IL-12 responsiveness correlated with expression of the IL-12R signaling subunit, IL-12R beta2, and with IL-12-induced STAT4 phosphorylation. When activated under neutral conditions, BALB/c T cells rapidly lose IL-12R beta2 expression, STAT4 phosphorylation, and functional IL-12 responsiveness. More efficient maintenance of IL-12R beta2 expression by B10.D2 T cells activated under neutral conditions may explain the previously observed increase in IFN-gamma production relative to that of BALB/c. This difference could potentially provide greater protection from certain pathogens that do not immediately elicit strong Th1-inducing conditions via activation of the innate immune system.

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We examined the effect of genetic background on Th1/Th2 development. We discuss data demonstrating that genetic background is an important determinant of interleukin-12 (IL-12) responsiveness and the potential implications for disease progression in murine experimental leishmaniasis. Genetic analysis of the differential control of IL-12 responsiveness led to the identification of a controlling locus on the middle portion of murine chromosome 11. This genetic region (or its human counterpart, 5q31) has been associated with increased disease susceptibilities for several atopic, infectious, and autoimmune disorders. We discuss potential roles for genetic control of IL-12 responsiveness in the development of these diseases.

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Genetic background of the T cell can influence T helper (Th) phenotype development, with some murine strains (e.g., B10.D2) favoring Th1 development and others (e.g., BALB/c) favoring Th2 development. Recently we found that B10.D2 exhibit an intrinsically greater capacity to maintain interleukin 12 (IL-12) responsiveness under neutral conditions in vitro compared with BALB/c T cells, allowing for prolonged capacity to undergo IL-12-induced Th1 development. To begin identification of the loci controlling this genetic effect, we used a T-cell antigen receptor-transgenic system for in vitro analysis of intercrosses between BALB/c and B10.D2 mice and have identified a locus on murine chromosome 11 that controls the maintenance of IL-12 responsiveness, and therefore the subsequent Th1/Th2 response. This chromosomal region is syntenic with a locus on human chromosome 5q31.1 shown to be associated with elevated serum IgE levels, suggesting that genetic control of Th1/Th2 differentiation in mouse, and of atopy development in humans, may be expressed through similar mechanisms.

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The genetic background of T lymphocytes influences development of the T helper (TH) phenotype, resulting in either resistance or susceptibility of certain mouse strains to pathogens such as Leishmania major. With an in vitro model system, a difference in maintenance of responsiveness of T cells to interleukin-12 (IL-12) was detected between BALB/c and B10.D2 mice. Although naive T cells from both strains initially responded to IL-12, BALB/c T cells lost IL-12 responsiveness after stimulation with antigen in vitro, even when cocultured with B10.D2 T cells. Thus, susceptibility of BALB/c mice to infection with L. major may derive from the loss of the ability to generate IL-12-induced TH1 responses rather than from an IL-4-induced TH2 response.

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IL-12 and IL-4 direct T cell development toward Th1 and Th2 phenotypes, respectively. While IFN-gamma and IFN-alpha have been reported to regulate Th1 development as well, the mechanism and cellular locus of their effects are unclear. In this study, we use a TCR-transgenic system to examine the actions of these cytokines on CD4+ T cell phenotype development. We find that neither IFN-gamma nor IFN-alpha can induce Th1 development alone. However, IFN-gamma can significantly augment IL-12 priming for subsequent IFN-gamma production by T cells. Interestingly, lymphocyte endothelial cell adhesion molecule-1bright (naive) T cells require IFN-gamma during primary activation for maximal IL-12-induced Th1 development, whereas lymphocyte endothelial cell adhesion molecule-1dull (memory) T cells do not. IFN-alpha only partially substitutes for IFN-gamma in promoting IL-12-induced Th1 development. When the endogenous IFN-gamma present in primary T cell cultures is neutralized, IFN-alpha treatment augments IL-12-induced effects on inhibition of subsequent IL-4 production, but fails to significantly enhance IL-12 priming for subsequent IFN-gamma production. Thus, our data suggest that IFN-gamma provides a direct costimulatory signal to T cells to up-regulate IL-12-induced Th1 development and may operate by inducing IL-12 responsiveness in naive T cells.

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The experiments described above have allowed us to define the molecular events in IL-12 signalling. Within minutes after IL-12 treatment of responsive cells, Stat1, Stat3, and Stat4 are tyrosine phosphorylated. These molecules form nuclear DNA-binding complexes consisting of homodimeric Stat1 and heterodimeric Stat3-Stat4 complexes. In a murine in vitro phenotype development model, T cells rapidly and selectively lose their capacity to respond to IL-12 upon acquisition of the Th2 phenotype. This hyporesponsiveness is manifested by the inability of IL-12 to induce IFN gamma production in differentiated Th2 cells, as well as the inability of IL-12 to induce the activation of Stat4. Despite the functional defect of IL-12 signalling in Th2 cells, all known components of the IL-12 signal transduction pathway are present. We speculate that in Th2 cells, the second receptor chain may be absent or one of the other components may be modified. Genetic experiments in Balb/c and B10.D2 strains of mice have demonstrated several differences in T helper differentiation in vitro. Stimulation of T cells under neutral conditions results in a bias of Balb/c T cells toward the Th2 extreme and B10 T cells toward the Th1 extreme of cytokine production. Following stimulation under neutral conditions, B10 T cells retain the ability to respond to IL-12 while Balb/c T cells lose IL-12 responsiveness. Mating experiments have demonstrated that the B10 genetic effect is dominant in F1 mice. Analysis of backcrossed animals has suggested that the ability to respond to IL-12 in the secondary stimulation may be controlled by a single dominant B10 gene. The results we describe may have profound implications for allergy. Since allergic responses are largely due to the activation of the Th2 subset of T lymphocytes, a better understanding of T cell phenotype development may reveal multiple targets for therapeutic intervention. First, a better understanding of Th1 phenotype induction in response to IL-12 may allow prevention of in vivo allergic responses using pharmacological tools which bias allergen-specific responses to the Th1 subset. Second, a molecular explantation of why Th2 cells fail to reverse phenotype in response to IL-12 may allow treatment of atopic individuals to remove the disease-promoting T lymphocyte compartment. Finally, a better understanding of the basis for genetic differences in murine T helper cell differentiation may allow us to identify a causative genetic element in humans, yielding better diagnostic and therapeutic methods.

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