Differential effects of interleukin-12 on the development of naive mouse CD4+ T cells

The influence of interleukin (IL)-12 and IL-4 on the differentiation of naive CD4⁺ T cells was studied in an accessory cell-free in vitro system. Dense CD4⁺ T cells were purified from unimmunized mice and activated using immobilized anti-CD3 monoclonal antibodies (mAb) in the presence of IL-4, IL-12, or a combination of both cytokines, and restimulated after 6 days by re-exposure to anti-CD3-coated culture wells. T cells initially activated in the presence of IL-4 produced substantial amounts of IL-4 and trace amounts of interferon (IFN)-γ after restimulation at day 6 with plate-bound anti-CD3 mAb. By contrast, T cells primed in the presence of IL-12 produced high levels of IFN-γ and only minimal amounts of IL-4, thus indicating that IL-12 and IL-4 by acting directly on stimulated naive CD4⁺ T cells support the development of T_H1 and T_H2 cells, respectively. When naive CD4⁺ T cells were stimulated in the presence of IL-12 together with IL-4 in comparable concentrations, the effect of IL-12 on T_H1 differentiation was largely inhibited by IL-4. On the other hand, IL-12 exerted no inhibitory effect on IL-4-induced T_H2 differentiation but rather enhanced the production of IL-4 after restimulation of the respective T cells. Decreasing amounts of IL-4 in combination with a high level of IL-12 led to an increasing production of IFN-γ by the emerging T cells and, simultaneously, to a relatively high production of IL-4. These data were confirmed by time-course experiments which revealed that the delayed addition of IL-4 to IL-12-primed T cell cultures resulted in a gradual restoration of IFN-γ production whereas in parallel the secretion of IL-4 was not reduced over a wide period of delay (6–72 h). These results, therefore, demonstrate that (a) IL-4 dominates the effect of IL-12, (b) IL-12 promotes the development of T_H1 cells; however, in the presence of IL-12 and relatively high levels of IL-4 also the development of T_H2-like cells is slightly but significantly enhanced by IL-12, and (c) high amounts of IL-12 in combination with relatively low levels of IL-4 give rise to a T cell population that upon rechallenge exhibited a cytokine profile resembling that of T_H0 cells.     

Human recombinant interferon-β influences T helper subset differentiation by regulating cytokine secretion pattern and expression of homing receptors

Type I interferons (IFN) are important regulators of both innate and acquired immunity. We have used an in vitro system of human CD4⁺ T cell differentiation to determine how IFN-β influences development of T helper (Th) subsets and homing receptor expression. IFN-β promoted differentiation of CD4⁺ T cells that produce low levels of both IFN-γ and lymphotoxin compared to interleukin (IL)-12-derived Th1 CD4⁺ T cells. IFN-β inhibited production of Th2 cytokines (IL-5 and IL-13) and augmented IL-12-mediated IL-10 secretion. In addition, IFN-β significantly enhanced L-selectin expression on CD4⁺ T cells and synergized with IL-12 to induce expression of cutaneous lymphocyte-associated antigen (CLA). This Th1 L-selectin⁺, CLA⁺ phenotype is characteristic of T cells found in normal human skin and suggests a role for type I IFN in the regulation of Th subset differentiation and tissue-specific homing receptors.     

Role of transforming growth factor-βthe preferential induction of T helper cells of type 1 by staphylococcal enterotoxin B

Stimulation of murine CD4⁺ T cells with staphylococcal enterotoxin B (SEB) results in the preferential development of T helper (Th) 1 cells [i.e. high interferon (IFN)-γ and low interleukin (IL)-4, IL-5 and IL-10]; whereas in response to plate-bound anti-CD3 or anti-T cell receptor-αβ, Th1 as well as Th2 cells develop. In the present study, we examined the mechanism which is responsible for the selective Th1 development in the SEB system. The addition of IL-4 resulted in a strong development of Th2 cells showing that SEB stimulation can result inTh2 differentiation. Co-stimulation with anti-CD28 was insufficient in this regard. Lack of Th2 development in the SEB system was in part due to the inhibitory effect of endogenously produced transforming growth factor-β (TGF-β), because anti-TGF-β allowed the development of Th2 cells. Similarly, TGF-β inhibited Th2 development and stimulated Th1 development in the anti-CD3 system. This shift was only partially prevented by also including IL-4 in the cultures. The effects of TGF-β could only partially be explained by stimulation of IFN-γ or inhibition of IL-4 as intermediatory cytokines: (1) TGF-β stimulated Th1 development even in the presence of anti-IL-4 and anti-IFN-γ, and (2) a strong inhibitory effect of anti-TGF-β on Th1 development was still observed when anti-IL-4 and IFN-γ were simultaneously added to the cultures. It is concluded that SEB favors Th1 development by stimulation of TGF-β production. Inhibition of Th2 development by TGF-β is due, in part, to inhibition of IL-4 and stimulation of IFN-γ, and, in part, to a direct effect of TGF-β on the responding T cells.     

IL-4-producing NK T cells are biased towards IFN-γ production by IL-12. Influence of the microenvironment on the functional capacities of NK T cells

NK T cells are an unusual T lymphocyte subset capable of promptly producing several cytokines after stimulation, in particular IL-4, thus suggesting their influence in Th2 lineage commitment. In this study we demonstrate that, according to the cytokines present in the micro environment, NK T lymphocytes can preferentially produce either IL-4 or IFN-γ. In agreement with our previous reports showing that their IL-4-producing capacity is strikingly dependent on IL-7, CD4⁻ CD8⁻ TCRα β⁺ NK T lymphocytes, obtained after expansion with IL-1 plus granulocyte-macrophage colony-stimulating factor, produced almost undetectable amounts of IL-4 or IFN-γ in response to TCR/CD3 cross-linking. However, the capacity of these T cells to produce IFN-γ is strikingly enhanced when IL-12 is added either during their expansion or the anti-CD3 stimulation, while IL-4 secretion is always absent. A similar effect of IL-12 on IFN-γ production was observed when NK T lymphocytes were obtained after expansion with IL-7. It is noteworthy that whatever cytokines are used for their expansion, IL-12 stimulation, in the absence of TCR/CD3 cross-linking, promotes consistent IFN-γ secretion by NK T cells without detectable IL-4 production. Experiments in vivo demonstrated a significant up-regulation of the capacity of NK T cells to produce IFN-γ after anti-CD3 mAb injection when mice were previously treated with IL-12. In conclusion, we provide evidence that the functional capacities of NK T cells, which ultimately will determine their physiological roles, are strikingly dependent on the cytokines present in their microenvironment.     

On the mechanisms by which transforming growth factor-β2 alters antigen-presenting abilities of macrophages on T cell activation

Peritoneal exudate cells (PEC) incubated with antigen in the presence of transforming growth factor-(TGF)-β2 selectively suppress delayed hypersensitivity and IgG2a antibody production when injected intravenously into naive syngeneic recipients. In this study, we have examined in vitro the effects of TGF-β2 on the antigen presenting abilities of PEC to activate DO11.10 T cells that express a transgenic T cell receptor that recognizes ovalbumin peptide fragment 323–339 in the context of I-A^d. PEC were pretreated overnight with TGF-β2, washed extensively, then co-cultured with DO11.10 T cells in the presence of native OVA or P323–339. We found that TGF-β2-treated PEC induced the production of the T helper type 2 (Th2) cytokine, interleukin-4 (IL-4), but unlike untreated PEC, were unable to stimulate the Th1 cytokines, IL-2 and interferon-γ (IFN-γ). Furthermore, TGF-β2 was produced in an autocrine fashion by TGF-β2-treated PEC and was responsible for this shift to a Th2 response. This conclusion was supported by the following results. First, TGF-β2-treated PEC were found to express much more TGF-β1 and TGF-β2 mRNA than untreated PEC. Second, TGF-β2-treated PEC secreted large amounts of TGF-β including its mature form. Third, addition of neutralizing anti-TGF-β2 antibodies, but not neutralizing anti-TGF-β1 antibodies, restored the ability of antigen-pulsed, TGF-β2-pretreated PEC to stimulate DO11.10 T cells to secrete IL-2 and IFN-γ. These results indicate that antigen-presenting cells that encounter antigen in a TGF-β-enriched environment (e.g., in the eye) shift responding naive T cells toward Th2 responses by producing TGF-β during antigen presentation.     

Regulation of IFN-γ production in granulocyte-macrophage colony-stimulating factor-deficient mice

Granulocyte-macrophage colony-stimulating factor-deficient (GM-CSF−/−) mice produce far lower serum levels of IFN-γ in response to LPS than GM-CSF+/+ mice. CD4⁺ and CD8⁺ T cells from LPS-injected GM-CSF−/− mice showed a deficiency in IFN-γ production and proliferative activity in response to IL-2 and IL-12, whereas IFN-γ production by NK cells was not compromised. These defects of T cells were reversed by administration of GM-CSF in vivo, but not by supplementation with GM-CSF in vitro. GM-CSF−/− mice do not have an intrinsic defect in IFN-γ production, because IL-12 injection induces the same high levels of IFN-γ in GM-CSF−/− and GM-CSF+/+ mice. To investigate the inhibitory effect of LPS on GM-CSF−/− T cells and the indirect restorative activity of GM-CSF, we tested the action of supernatants from cultured dendritic cells (DC). A factor or factors in the DC supernatant normalized serum IFN-γ levels and T cell responses in LPS-injected GM-CSF−/− mice. IL-18 reproduced some but not all of these in vivo and in vitro effects of DC supernatants. Our results indicate that GM-CSF is important in protecting T cells from inhibitory signals generated during immunization or exposure to LPS, and that this effect of GM-CSF is indirect and mediated by factors produced by DC.     

T cells made deficient in interleukin-2 production by exposure to staphylococcal enterotoxin B in vivo are primed for interferon-γ and interleukin-10 secretion

The superantigen staphylococcal enterotoxin B (SEB) induces a defect in interleukin (IL)-2 production by T cells expressing specific T cell receptor Vβ domains. The present study was undertaken to determine the capacity of T cells, made deficient in IL-2 production by exposure to SEB in vivo, to secrete interferon (IFN)-γ and IL-10 and to induce pathology upon SEB rechallenge. For this purpose, BALB/c mice received two intraperitoneal injections of 100 μg SEB with a 48-h interval. First, we compared peak serum levels of IL-2, IFN-γ and IL-10 after SEB rechallenge with those measured after a single SEB injection in control mice. The expected defect in IL-2 production in SEB-pretreated mice was associated with a major increase in IL-10 and IFN-γ levels which were about fivefold higher than in controls. Experiments in mice depleted of CD4⁺ or CD8⁺ cells as well as studies in which purified T cell populations were rechallenged with SEB in vitro indicated that both CD4⁺ and CD8⁺ cells from SEB-pretreated mice were primed for IL-10 and IFN-γ production. Furthermore, SEB-pretreated mice were sensitized to the toxic effects of the superantigen as indicated by a 30-70% lethality rate (vs. 0% in naive mice) within 48 h after SEB rechallenge. IFN-γ was involved in the lethal syndrome as it could be prevented by injection of neutralizing anti-IFN-γ monoclonal antibody. We conclude that SEB-reactive T cells made deficient for the production of IL-2 by exposure to SEB in vivo are primed for IFN-γ and IL-10 production, and that IFN-γ up-regulation is involved in the shock syndrome occurring upon SEB rechallenge.     

Cyclosporin A increases IFN-γ production by T cells when co-stimulated through CD28

Despite its calcineurin-inhibiting properties, cyclosporin A (CsA) can not inhibit IL-2 production when T cells are co-stimulated by CD80/CD86 on the antigen-presenting cells. We studied the in vitro effect of CsA on IFN-γ production. Anti-CD3 monoclonal antibody (mAb) was used as the primary stimulus for activation of purified human T cells. A stimulating anti-CD28 mAb, or CD80 or CD86 on stably transfected P815 cells, provided the co-stimulatory signal. IL-2 production was hardly affected by CsA under these stimulating conditions, while IFN-γ (at the protein and mRNA level) was markedly stimulated by CsA. The use of anti-CD3 or phorbol 12-myristate 13-acetate with ionomycin as the primary stimulus, together with co-stimulation through either CD28 or CD2 using transfectants with the appropriate ligands, allowed us to demonstrate that the resistance of IFN-γ production to inhibition by CsA required both CD3 and CD28 triggering. Inhibition of IL-10 production, and to a lesser degree of IL-4 production, by CD4⁺ cells was responsible for the enhancement of IFN-γ production in the presence of CsA. In conclusion, IFN-γ production by CD28-co-stimulated CD4⁺ T cells is resistant to inhibition by CsA and can even be facilitated by CsA as a result of removing a negative regulatory signal which is mainly IL-10 mediated. This finding might have implications for immunosuppressive strategies based upon the use of CsA.     

Mechanism of enhanced antigen presentation by B cells activated with anti-μ plus interferon-γ: Role of B7-2 in the activation of naive and memory CD4+ T cells

B cells activated with anti-γ antibody plus interferon (IFN)-γ exerted strong antigen presentation activity for T cell proliferation. The enhanced antigen presentation function was shown to be due to the increase in B7-2 expression. When B cells were stimulated with anti-μ, expression of MHC major histocompatibility complex class II, heat-stable antigen (HSA), ICAM-1 and B7-2 was increased. The presence of IFN-γ further augmented the expression of B7-2 on anti-μ-stimulated B cells. B7-1 was not expressed on B cells under these conditions. The participation of B7-2 in the elicitation of the proliferative response of T cells was confirmed by the inclusion of anti-B7-2 antibody in cultures. The enhanced expression of either HSA or ICAM-1 was shown not to play a major role in the increased B cell antigen presentation capacity. The major T cell population responding to this activated B cell antigen presentation was shown to be CD44^low naive CD4⁺ T cells, whereas CD45RB^low memory CD4⁺ T cells responded only weakly. The difference in proliferative responses between naive and memory CD4⁺ T cells was explained by the different efficiency in IL-2 production of these cell populations in response to antigen presentation by B cells activated by anti-μ plus IFN-γ. These results suggest that IFN-γ plays an important role in recruitment of naive T cells for an immune response.     

Expression of pro-inflammatory cytokines by TCRαβ+ T and TCRγδ+ T cells in an experimental model of colitis

An inflammatory bowel disease (IBD) comparable to human ulcerative colitis is induced upon transfer of T cell-depleted wild-type (F1) bone marrow into syngeneic T cell-deficient (tgε26) mice (F1 → tgε26). Previously we have shown that activated CD4⁺ T cells predominate in transplanted tgε26 mice, and adoptive transfer experiments verified the potential of these cells to cause disease in immunodeficient recipient mice. Using flow cytometry for the detection of intracellular cytokine expression, we demonstrate in the present study that large numbers of CD4⁺ and CD8⁺ TCRαβ⁺ T cells from the intraepithelial region and lamina propria of the colon of diseased, but not from disease-free mice, produced interferon-γ (IFN-γ) and tumor necrosis factor-α (TNF-α). Large numbers of T cells from peripheral lymphoid tissues of these animals also expressed IFN-α and TNF-α, but few expressed interleukin-4, demonstrating g strong bias towards Th1-type T cell responses in these animals. TCRγδ⁺ T cells, typically minor constituents of the inflammatory infiltrate of the colon in F1 → tgε26 mice, also expressed IFN-γ at a high frequency upon CD3 stimulation. In light of these findings we examined the potential involvement of TCRγδ⁺ T cells by testing their ability to induce colitis in tgε26 mice. We report here that tgε26 mice transplanted with T cell-depleted bone marrow from TCRα^null and TCRβ^null animals developed IBD. Furthermore, disease in these mice correlated with the development of peripheral and colonic TCRαδ⁺ T cells capable of IFN-γ production. These results suggest that IFN-γ may be a common mediator of IBD utilized by pathogenic T cells of distinct phenotype.     

Interleukin-12 but not interferon-γ production correlates with induction of T helper type-1 phenotype in murine candidiasis

By means of polymerase chain reaction-assisted mRNA amplification, we have monitored message levels of interleukin (IL)-12 in splenic macrophages and of interferon-γ (IFN-γ), IL-4, and IL-10 in CD4⁺ and CD8⁺ T cells using Candida albicans/host combinations that result either in a T helper type-1 (Th1)-associated self-limiting infection (“healer mice”) or in a Th2-associated progressive disease (“nonhealer mice”). The timing and pattern of message detection did not differ qualitatively by the expression of IFN-γ or IL-10 mRNA in CD4⁺ and CD8⁺ cells from healer (i.e. PCA-2 into CD2F1) vs. nonhealer (i.e. CA-6 into CD2F1 or PCA-2 into DBA/2) mice. In contrast, IL-4 mRNA was uniquely expressed by CD4⁺ cells from nonhealer animals. IL-12p40 was readily detected in macrophages from healer mice but was detected only early in infection in mice with progressive disease. Cytokine levels were measured in sera, and antigen-driven cytokine production by CD4⁺ and CD8⁺ cells was assessed in vitro, while IFN-γ-producing cells were enumerated in CD4^? CD8^? cell fractions. Overall, our results showed that (i) antigen-specific secretion of IFN-γ protein in vitro by CD4⁺ cells occurred only in healing infection; (ii) IL-4- and IL-10-producing CD4⁺ cells would expand in nonhealer mice in the face of high levels of circulating IFN-γ, likely released by CD4^? CD8^? lymphocytes; (iii) a finely regulated IFN-γ production correlated in the healer mice with IL-12 mRNA detection, and IL-12 was required in vitro for yeast-induced development of IFN-γ-producing CD4⁺ cells. Although the mutually exclusive production of IL-4/IL-10 and IFN-γ by early CD4⁺ cells may be the major discriminative factor of cure and noncure responses in candidiasis, IL-12 rather than IFN-γ production may be an indicator of Th1 differentiation.     

Different cytokine production profiles of γδ T cell clones: Relation to inflammatory arthritis

This study was performed to investigate whether γδ T cells could also be divided into subsets, identified by a cytokine profile, as described for αβ T helper (Th) cell subsets. Cytokine production was studied in 22 γδ T cell clones obtained from the synovial fluid and peripheral blood of one patient with inflammatory arthritis and compared to that of 26 αβ T cell clones of the same and different patients. Interferon-γ (IFN-γ) was produced by 18 (82%) and interleukin-4 (IL-4) by 17 (77%) out of 22 γδ T cell clones, respectively. In contrast, IL-10 was not produced, except at very low level in one case. The mean levels of IL-4 were lower for clones derived from synovial fluid. When considering the production of IFN-γ as an indicator of Th1 and that of IL-4 as an indicator of Th2, respectively, the most common pattern was a γδTh1-like pattern, with the combination of high levels of IFN-γ and low levels of IL-4. This pattern was found in Vδ1⁺ clones, all from synovial fluid. Additional patterns were also observed: a mixed, probably γδ Th0-like pattern with a more balanced production of both IFN-γ and IL-4; a γδTh1 pattern with the production of IFN-γ alone; a γδTh2 pattern with the production of IL-4 alone. These three patterns were also seen in blood γδ T cells which were all Vδ2, indicating that these patterns were independent of the γδ phenotype. γδ T cell clones produced lower levels of IFN-γ (p = 0.001) and higher levels of IL-4 than αβ clones (p < 0.02). These differences in cytokine production between αβ and γδ subsets and within these subsets may contribute to their respective role in chronic inflammation.     

Specific expression of surface interferon-γ on interferon-γ producing T cells from mouse and man

Interferon (IFN)-γ is a potent immunoregulatory protein secreted by CD4⁺ and CD8⁺ T cells and by natural killer cells. Here, we show that IFN-γ is specifically displayed at a low concentration on the cell surface of those activated T cells from mouse and man which express IFN-γ. It is transiently expressed on the cell surface with kinetics similar to those of intracellular IFN-γ expression. Detectable surface IFN-γ is not expressed by activated T helper (Th) cells producing other cytokines but which do not express IFN-γ. Thus, surface IFN-γ is the first available marker for live T lymphocytes expressing IFN-γ, e.g. Th1 cells.     

Modulation of interferon-γ receptor during human T lymphocyte alloactivation

Previous work has shown that neutralization of physiologically secreted interferon(IFN)-γ or blockade of its receptor during T lymphocyte activation inhibits both proliferation and cytotoxic T lymphocyte generation, suggesting that IFN-γ plays a crucial role in T lymphocyte induction and differentiation. In this study, the kinetics of the surface expression of the 90-kDa IFN-γ receptor (IFN-γR) was followed during human mixed lymphocyte reaction (MLR) to alloantigens. IFN-γR mRNA is constitutively expressed on resting peripheral blood lymphocytes emerging from nylon wood column (NW-PBL) and its expression increases two- to threefold on alloactivated NW-PBL. IFN-γR protein is poorly expressed on the membrane of resting CD3⁺ cells, but up-modulates after 3-day MLR and sharply down-modulates at day 6. Both the p55 and the p75 chains of interleukin-2 receptor (IL-2R) were shown to up-modulate in parallel with IFN-γR, whereas they were still highly expressed at day 6. After alloactivation, IFN-γ and IL-2 secretion starts at 24 h, peaks at day 3 and decreases just when IFN-γR and IL-2R begin to up-modulate. Proliferation peaks at day 6. Lastly, stimulation with distinct cell populations showed that the intensity of lymphocyte proliferation, IFN-γR membrane up-modulation, and IFN-γ and IL-2 secretion are regulated in a parallel manner, thus suggesting that they are interrelated. Taken as whole these results demonstrate that increased expression of IFN-γR on T lymphocytes can be a critical event during their activation, and strongly support the hypothesis that IFN-γ/IFN-γR interaction provides a signal for its progression.     

Interferon-γ-inducing factor enhances T helper 1 cytokine production by stimulated human T cells: synergism with interleukin-12 for interferon-γ production

The novel cytokine interferon-γ-inducing factor (IGIF) augments natural killer (NK) cell activity in cultures of human peripheral blood mononuclear cells (PBMC), similarly to the structurally unrelated cytokine interleukin (IL)-12. IGIF has been found to enhance the production of interferon-γ (IFN-γ) and granulocyte/macrophage colony-stimulating factor (GM-CSF) while inhibiting the production of IL-10 in concanavalin A (Con A)-stimulated PBMC. In this study, when anti-CD3 monoclonal antibody (mAb)-stimulated human enriched T cells were exposed to IGIF, the cytokine dose-dependently enhanced the proliferation of the cells and this could be completely inhibited by a neutralizing antibody against IL-2 at lower concentrations of IGIF. Neutralizing antibody against IFN-γ had only insignificant inhibitory effects on T cell proliferation at higher concentrations of IGIF. Enzyme-linked immunosorbent assays (ELISA) revealed that, like PBMC, T cells exposed to IGIF produced large amounts of IFN-γ; however, changes in the production of IL-4 and IL-10 were minimal. IGIF, but not IL-12, significantly enhanced IL-2 and GM-CSF production in T cell cultures, as determined by CTLL-2 bioassay and ELISA, respectively; however, both IGIF and IL-12 enhanced IFN-γ production by the T cells. When T cells were exposed to a combination of IGIF and IL-12, a synergistic effect was observed on the production of IFN-γ, but not on production of IL-2 and GM-CSF. In conclusion, IGIF enhances T cell proliferation apparently through an IL-2-dependent pathway and enhances Th1 cytokine production in vitro and exhibits synergism when combined with IL-12 in terms of enhanced IFN-γ production but not IL-2 and GM-CSF production. Based on structural and functional differences from any known cytokines, it was recently proposed that this cytokine be designated interleukin-18.     

Interferon-γ and interleukin-4 regulate T cell interleukin-12 responsiveness through the differential modulation of high-affinity interleukin-12 receptor expression

Interferon-γ (IFN-γ) and interleukin-4 (IL-4) are mutually antagonistic cytokines that stimulate CD4⁺ T cells to develop into either Th1 or Th2 cells. One feature of Th2 differentiation in mice is the loss of IL-12-induced Jak2 and Stat4 activation, which is accompanied by the inability to produce IFN-γ in response to IL-12. In this report, we show that freshly isolated human T cells activated with phytohemagglutinin (PHA) in the presence of IL-4 exhibit a greatly diminished response to IL-12, whereas the IL-12 response of T cells activated with PHA plus IFN-γ is enhanced. Radiolabeled IL-12 binding studies demonstrate that the impairment of T cell IL-12 responsiveness by IL-4 is associated with the down-regulation of high-affinity IL-12 receptor expression. In contrast, the enhancement of IL-12 responsiveness by IFN-γ is associated with the up-regulation of high-affinity IL-12 receptor expression. Through the use of a newly synthesized neutralizing antibody to the low-affinity IL-12 receptor β subunit (IL-12Rβ), we show that neither IL-4 nor IFN-γ affect the expression of IL-12Rβ, which we determine to be one of at least two low-affinity subunits required for high-affinity IL-12 binding. These findings suggest that IL-4 and IFN-γ exert opposite effects on T cell IL-12 responsiveness by differentially modulating the expression of low-affinity IL-12 receptor subunits that are distinct from IL-12Rβ and required, together with IL-12Rβ, for high-affinity IL-12 binding and IL-12 responsiveness. This provides a basis for understanding the interplay between different cytokines at the level of cytokine receptor expression, and offers insight into one of the mechanisms governing Th1 and Th2 development.