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The Journal of Immunology, 1999, 162: 807-812.
Copyright © 1999 by The American Association of Immunologists

Differential Requirement of IFN Consensus Sequence Binding Protein for the Production of IL-12 and Induction of Th1-Type Cells in Response to IFN-{gamma}

Chang-You Wu*, Haruko Maeda*, Cristina Contursi{dagger}, Keiko Ozato{dagger} and Robert A. Seder1,*

* Clinical Immunology Section, Laboratory of Clinical Investigation, National Institute of Allergy and Infectious Diseases; and {dagger} National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892


    Abstract
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
IFN-{gamma} exerts multiple biological activities in the modulation of immune responses by the induction of transcription factors. One transcriptional factor of the IFN regulatory factor family found to be critical in regulating IL-12-dependent IFN-{gamma} production in vivo following infectious challenge has been designated IFN consensus sequence-binding protein (ICSBP). In this study, the role of ICSBP in regulating type 1 responses to T cell-specific stimulation in vitro was assessed. Total splenocytes from ICSBP-/- mice stimulated with soluble anti-CD3 were markedly impaired in the production of IFN-{gamma} compared with similarly stimulated cells from ICSBP+/+ mice. Consistent with the decrease in IFN-{gamma} production, splenocytes from ICSBP-/- mice stimulated with anti-CD3 in the presence or absence of IFN-{gamma} or a soluble CD40 ligand agonist failed to produce IL-12 p40 and IL-12 p70 protein; however, the deficient production of IFN-{gamma} from ICSBP-/- mice could be restored by the addition of anti-CD28 Ab in an IL-12-independent manner. In contrast to the previous data, production of IFN-{gamma} from naive CD4+/LECAM-1high cells of ICSBP-/- mice that had been primed in vitro with anti-CD3 was similar to or greater than that of ICSBP+/+ controls. In addition, the presence of IFN-{gamma} in priming cultures enhanced both priming for IFN-{gamma} and IL-12 responsiveness from ICSBP-/- CD4+ T cells. Overall, these results provide evidence that ICSBP is differentially required for the ability of IFN-{gamma} to regulate type 1 cytokine responses from APCs and CD4+ T cells.


    Introduction
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Type 1 immune responses characterized by IL-12 induction of IFN-{gamma} is the principal mechanism linking innate and adaptive immunity to a variety of intracellular infections (1). In this regard, IFN-{gamma} has two important roles. First, IFN-{gamma} is the critical effector molecule leading to activation of macrophages and subsequent intracellular killing. Second, IFN-{gamma}, by its ability to enhance IL-12 (2) and to inhibit IL-10 production (3), leads to the maintenance of a Th1-type response. Due to the diverse and important functions of IFN-{gamma}, there has been great interest in studying the transcription factors that control IFN-{gamma} activity.

The ability of IFN (type I and type II) to regulate transcription is controlled in large part by a group of proteins such as IRF-1, IRF-2, and IFN consensus sequence-binding protein (ICSBP),2 which belong to the IFN regulatory factor (IRF) family of transcription factors (4, 5, 6, 7, 8). These proteins function by binding to the IFN-stimulated response element found in promoters of IFN-inducible genes. ICSBP is unique among these factors in that it is expressed exclusively in cells of the immune system (9) such as activated T cells (10) and macrophages (11, 12). In addition, ICSBP is notable for its regulation by IFN-{gamma} as well as of IFN-{alpha}ß.

Recently, several studies have used ICSBP-/- mice to examine its role in regulating the immune response to a variety of infectious pathogens (13, 14, 15, 16). In murine models of Listeria monocytogenes (14), Leishmania major (15), and Toxoplasma gondii (16), the fact that ICSBP-/- mice had enhanced susceptibility provided clear evidence for its importance for infections requiring IL-12-dependent production of IFN-{gamma}. In these studies, it was directly shown that IL-12 production was markedly reduced (15, 16). Moreover, IL-12 production could not be restored from cells of ICSBP-/- mice in response to microbial stimuli even in the presence of exogenous IFN-{gamma} (15, 16). These results provided clear evidence for the importance of ICSBP in IL-12 induction in response to intracellular pathogens as well as for the ability of IFN-{gamma} to enhance IL-12 transcription (2). The role of ICSBP in regulating IFN-{gamma} production in response to T cell stimuli has also been studied. Initial work showed that IFN-{gamma} production was reduced from total spleen cells of ICSBP-/- mice compared with that of the controls (13) following stimulation with Con A; however, these data contrasted with a recent report showing that IFN-{gamma} production was similar for cells of ICSBP-/- and ICSBP+/+ mice in response to Con A (16). In addition, a recent study showed that purified CD4+ T cells from ICSBP-/- mice stimulated in vitro with a polyclonal T cell mitogen and exogenous IL-12 still had decreased production of IFN-{gamma}. Thus, based on the aforementioned data, the role of ICSBP in regulating type 1 immune responses following T cell stimulation is still not entirely clear. Furthermore, while the ability of IFN-{gamma} to induce IL-12 from APCs is ICSBP dependent, whether ICSBP is also required for IFN-{gamma} to regulate T cell differentiation has not been examined.

In the studies reported here, we first established the role of ICSBP in regulating type 1 cytokine responses from spleen cells in response to anti-CD3 stimulation. In addition, since IFN-{gamma} can influence CD4+ T cell differentiation (17) and IL-12 responsiveness (18), we determined whether ICSBP was required for the effects of IFN-{gamma} on these functions. Our results are consistent with previous reports showing that ICSBP is required for IL-12 production in response to a T cell stimulus. In addition, they confirm that IFN-{gamma} requires ICSBP to enhance IL-12 production from APCs. By contrast, the presence of IFN-{gamma} in priming cultures resulted in enhanced priming for IFN-{gamma} and IL-12 responsiveness from naive CD4+ T cells of ICSBP-/- mice. Overall, these data suggest that the requirement of ICSBP for type 1 cytokine responses is cell specific, restricted to IL-12 induction by APCs in response to microbial or T cell stimuli. Moreover, ICSBP does not appear to be required for IFN-{gamma} to influence CD4+ T cell differentiation.


    Materials and Methods
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Mice

Female ICSBP wild-type (ICSBP+/+) and ICSBP-deficient (ICSBP-/-) mice were generated as previously described (13) and used at the seventh generation of back-crossing to the C57BL/6 strain. Animals were kept under pathogen-free conditions. Mice used were between 6 and 8 wk of age.

Media and reagents

HBSS (Biofluids, Rockville, MD) was used as wash medium. Complete culture medium consisted of RPMI 1640 supplemented with 10% heat-inactivated FCS, penicillin (100 U/ml), streptomycin (100 µg/ml), sodium pyruvate (1 mM), L-glutamine (2 mM), and 2-ME (50 µM), all of which were purchased from Biofluids. Staphylococcus aureus Cowan strain (SAC) was purchased from Sigma (St. Louis, MO).

Recombinant cytokines and Abs

rIL-2 was purchased from Genzyme (Cambridge, MA). Mouse rIL-4 and purified monoclonal rat antimouse IL-4 (11B11) were prepared as previously described (19). One unit of IL-4 is equivalent to ~0.5 pg. Mouse rIFN-{gamma}, purified rat anti-mouse IFN-{gamma}, purified rat anti-mouse IL-10, anti-CD4, and anti-lymphocyte endothelial cell adhesion molecule-1 (anti-LECAM-1) were purchased from PharMingen (San Diego, CA). Anti-CD3 (2C11) was a generous gift of Dr. Jeffrey Bluestone (University of Chicago, Chicago, IL). Anti-CD28 ascites was a generous gift of Dr. James Allison (University of California, Berkeley, CA). Mouse rIL-12 and Abs to mouse IL-12 were generous gifts of Dr. Maurice Gately (Hoffman-La Roche, Nutley, NJ). A soluble CD40 ligand (CD40L) agonist (CD40 ligand/trimer (CD40LT)) was a generous gift of Immunex (Seattle, WA).

Preparation of splenocytes and culture condition

Total splenocytes were prepared from pooled spleens on Ficoll gradient (Sigma). Cells were cultured in 96-well plates at 4 x 105 cells/well in a total volume of 200 µl in complete culture medium. Cultures were incubated in the presence or absence of stimuli as indicated in the figure legends. The supernatants from these cells were used to measure the induction of cytokines by ELISA.

Preparation of CD4+ T cells

Lymph node CD4+ T cells from ICSBP+/+ and -/- mice were prepared in the following manner. Pooled lymph node cells were removed from ICSBP+/+ and ICSBP-/- mice and passed over a CD4 subset column (R&D Systems, Minneapolis, MN). Cells were then stained with phycoerythrin-labeled anti-CD4 and FITC-labeled LECAM-1 (Mel 14). These cells were subjected to FACS with a FACStarPlus (Becton Dickinson, Sunnyvale, CA). Post-sort analysis revealed >99% CD4+/LECAM-1high T cells.

Primary and secondary stimulation of CD4+ T cells

Sorted CD4+/LECAM-1high cells were plated at a density of 5–7 x 105 cells/well in a total volume of 1.5 ml in 24-well plates precoated with anti-CD3 Ab at 5 µg/ml in coating buffer (borate-buffered saline; pH 8.5). Cytokines and Abs were added to wells at the initiation of cultures as indicated in the figure legends. After 4 days, cells were harvested and washed three times, and 1–2 x 105 cells were restimulated with immobilized anti-CD3 plus soluble anti-CD28 Abs in a total volume of 200 µl/well in 96-well plates. Forty-eight hours later, supernatants were collected and assayed for lymphokine.

Measurement of cytokine production

Measurement of IFN-{gamma} and IL-4 were assessed by specific ELISA as previously described (19). The lower limit of detection of IFN-{gamma} and IL-4 was 30 pg/ml and 2 pg/ml, respectively. IL-12 p70 ELISA was performed with a kit from Genzyme with a sensitivity of 10 pg/ml. Mouse IL-12 p40 ELISA was performed as described (16). The lower limit of p40 was 20 pg/ml. In all experiments, serial dilutions of supernatants were used to measure cytokine content in each separate ELISA. Supernatants were assayed in triplicate for all experiments.


    Results
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Impaired IFN-{gamma} production from splenocytes of ICSBP-/- mice is independent of IL-4 and IL-10

Previous studies have shown that ICSBP is required for IL-12 production in response to various microbial stimuli (15, 16). In contrast, the role of ICSBP in regulating IL-12 production in response to T cell stimuli is less well defined. In this regard, several reports have shown that production of IFN-{gamma} from spleen cells of ICSBP-/- and ICSBP+/+ mice in response to Con A was similar (16). As IFN-{gamma} is often a sensitive bioassay for IL-12 activity, these data raise several possibilities. First is that there is an ICSBP-independent/IL-12-independent pathway for IFN-{gamma} production in response to a T cell mitogen. Alternatively, since IL-12-dependent production of IFN-{gamma} in response to T cell stimulation is CD40L/CD40 dependent (20, 21), the possibility exists that this costimulatory pathway is mediating IL-12-dependent/ICSBP-independent induction of IFN-{gamma} in response to Con A. Finally, it has been previously reported that mRNA expression for IL-4 and IL-10, cytokines known to inhibit IFN-{gamma} production, is increased in ICSBP-/- mice (15, 16). The following series of experiments was initiated to assess these possibilities.

As shown in Fig. 1Go, production of IFN-{gamma} was markedly inhibited from spleen cells of ICSBP-/- mice compared with cells from the control ICSBP+/+ mice in response to anti-CD3 alone. Addition of neutralizing Abs to IL-4 and/or IL-10 to cultures did not restore the production of IFN-{gamma} from cells of ICSBP-/- mice. In contrast, addition of anti-IL-12 to cultures strikingly inhibited IFN-{gamma} produced from cells of control ICSBP+/+ mice. Taken together, these data suggest the decrease in IFN-{gamma} from ICSBP-/- mice in response to a T cell stimulus is likely due to deficient IL-12 production and not to the inhibitory effects of IL-4 or IL-10.



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FIGURE 1. Cytokine production from splenocytes of ICSBP-/- and ICSBP+/+ mice. IFN-{gamma} production is markedly impaired independent of IL-4 and IL-10. Total splenocytes from ICSBP-/- and ICSBP+/+ mice were plated at 4 x 105 cells/200 µl in 96-well plates with soluble anti-CD3 Ab (2 µg/ml) in the presence or absence of anti-IL-12 (10 µg/ml), anti-IL-4 (10 µg/ml), anti IL-10 (10 µg/ml), or anti-IL-4 + anti-IL-10. After incubation for 3 days, IFN-{gamma} from supernatants was assessed by ELISA. Results are expressed as mean ± SD of triplicate cultures and are representative of four experiments.

 
Anti-CD28 stimulation restores IFN-{gamma} production from cells of ICSBP-/- mice in response to anti-CD3

In the previous figure, while IFN-{gamma} was substantially diminished from cells of ICSBP-/- mice or ICSBP+/+ mice stimulated in the presence of anti-IL-12, there was still an appreciable amount induced in response to anti-CD3. As IFN-{gamma} produced in response to T cell stimuli is controlled in large part by CD40L/CD40 induction of IL-12 and/or CD28/B7 stimulation (21), the role of these costimulatory interactions in regulating IFN-{gamma} production was assessed. As shown in Fig. 2Go, consistent with the results seen above, IFN-{gamma} produced from ICSBP-/- mice stimulated with anti-CD3 was significantly less than that from ICSBP+/+ mice and was not affected by addition of anti-IL-12 to the cultures; however, addition of anti-CD40L Ab completely inhibited the production of IFN-{gamma}. Furthermore, addition of a stimulatory anti-CD28 mAb to the cultures strikingly enhanced production of IFN-{gamma} from cells of ICSBP-/- mice similar to that induced from the ICSBP+/+ mice. The CD28-mediated enhancement of IFN-{gamma} was not inhibited by the presence of anti-IL-12 or anti-CD40L mAb in the cultures. These data are consistent with the ability of CD40L/CD40 costimulation to enhance IFN-{gamma} production through a CD28-dependent/IL-12-independent mechanism and provide strong evidence that the capacity of total spleen cells from ICSBP-/- mice to produce IFN-{gamma} in response to a T cell mitogen and exogenous CD28 stimulation is relatively normal.



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FIGURE 2. Anti-CD28 restores IFN-{gamma} production from cells of ICSBP-/- mice in response to anti-CD3. Total splenocytes from ICSBP-/- and ICSBP+/+ mice were plated at 4 x 105 cells/200 µl in 96-well plates with soluble anti-CD3 Ab (2 µg/ml) in the presence or absence of anti-IL-12 (10 µg/ml), anti-CD40L (3 µg/ml), anti-CD28 (2 µg/ml), anti-CD28 plus anti-IL-12, or anti CD40L. After incubation for 3 days, IFN-{gamma} content was assessed from supernatants by ELISA. Results are expressed as mean ± SD of triplicate cultures and are representative of three experiments.

 
The fact that exogenous CD28 stimulation was required to normalize the IFN-{gamma} response from cells of ICSBP-/- mice was further evaluated. First, unstimulated total splenocytes from ICSBP+/+ and ICSBP-/- mice had similar expression of B7.1 and B7.2 (data not shown). Of interest, while expression of B7-1 was also similar from both groups of mice following stimulation with anti-CD3, B7.2 expression was increased 2-fold on ICSBP+/+ cells. Finally, the proliferative response of ICSBP+/+ cells in response to anti-CD3 was also increased compared with that of ICSBP-/- cells. Overall, these data suggest that in response to anti-CD3, there is less activation of the cells from the ICSBP-/- mice.

ICSBP is differentially required for the regulatory effects of IFN-{gamma} and IL-10 on IL-12 p40 production

To directly show that ICSBP is required for IL-12 production in response to anti-CD3, IL-12 p40 and p70 production was assessed following stimulation with various mitogens. As shown in Tables I and II, cells from ICSBP-/- mice stimulated with anti-CD3 in the presence or absence of anti-CD28 produced no detectable IL-12 p40 or p70, consistent with the results shown above. In addition, there was no detectable IL-12 p70 from ICSBP-/- mice stimulated with a potent inducer of IL-12 such as SAC (Table IGo). Furthermore, while the addition of IFN-{gamma} caused an increase in IL-12 p40 (Tables I and II) and IL-12 p70 (Table IGo) from cells of wild-type mice stimulated with SAC or LPS, addition of IFN-{gamma} did not enhance IL-12 p40 or p70 from cells of ICSBP-/- mice. While these results are consistent with previous data showing that ICSBP is required for the ability of IFN-{gamma} to enhance IL-12 (2), we were also interested in how ICSBP may effect negative regulators of IL-12 production. In this regard, the addition of anti-IL-10 mAb to cultures caused a striking enhancement in IL-12 p40 from cells of both ICSBP-/- and ICSBP+/+ mice in response to SAC. Taken together, these results suggest that IL-12 p40 transcription may be differentially regulated, in that ICSBP does not affect IL-10-mediated inhibition of IL-12 expression while it critically affects IFN-{gamma}-mediated induction of IL-12.


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Table I. Experiment 1: IL-12 p40 and p70 production by splenocytes from ICSBP+/+ and ICSBP-/- mice (pg/ml)1

 
IFN-{gamma} enhancement of Th1 priming is independent of ICSBP and IL-12

Since ICSBP is essential for IFN-{gamma} to enhance IL-12 production from macrophages (15, 16) and IFN-{gamma} has been shown to be a potent regulator of Th1 differentiation (17), it was of interest to study whether ICSBP was also required for IFN-{gamma} to influence CD4+ T cell differentiation. To this end, highly purified naive CD4+ T cells from ICSBP-/- and ICSBP+/+ mice were primed in vitro under a variety of conditions, and IFN-{gamma} production was determined following in vitro restimulation. As shown in Fig. 3Go, sorted CD4+/LECAM-1high cells from ICSBP-/- stimulated with immobilized anti-CD3 plus soluble CD28 for 4 days and restimulated produced amounts of IFN-{gamma} comparable with that of the control mice. Addition of IFN-{gamma} to priming cultures caused a three- to fourfold increase in IFN-{gamma} production following restimulation from cells of both ICSBP-/- and ICSBP+/+ mice. Of note, the presence of IFN-{gamma} in priming cultures also caused a decrease in IL-4 production (Fig. 3B). Moreover, the fact that addition of anti-IFN-{gamma} to priming cultures caused a decrease in production of IFN-{gamma} further supports a role for endogenous IFN-{gamma} in regulating Th1 differentiation. Thus, these data clearly show that naive CD4+ T cells from ICSBP-/- mice are not deficient in their production of IFN-{gamma} when stimulated in an IL-12-independent system. Moreover, it should be noted that in a separate experiment, sorted CD4+ T cells from ICSBP-/- mice stimulated with immobilized anti-CD3 in the absence of CD28 stimulation produced comparable amounts of IFN-{gamma} to similarly stimulated wild-type mice. This suggests that ICSBP is not required for production of IFN-{gamma} from naive cells even under these more limiting (absence of CD28) stimulatory conditions. Finally, the fact that IFN-{gamma} is able to influence Th1 differentiation in the absence of ICSBP provides strong evidence that the major biologic effects of ICSBP on type 1 immune responses are relatively cell specific.



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FIGURE 3. IFN-{gamma} enhancement of Th1 priming is independent of ICSBP and IL-12. Sorted CD4+/LECAM-1high T cells from ICSBP-/- mice were plated at 5–7 x 105 cells/ml in 24-well plates previously coated with anti-CD3 (5 µg/ml) in the presence or absence of IFN-{gamma} (500 U/ml), anti-IL-4 (10 µg/ml), anti-IFN-{gamma} (10 µg/ml), or IL-4 (500 pg/ml). After incubation for 4 days, cells were harvested, washed, and restimulated with immobilized anti-CD3 (5 µg/ml) plus anti-CD28 (2 µg/ml). Two days later, IFN-{gamma} (A) and IL-4 (B) were assessed by ELISA. Results are expressed as mean ± SD of triplicate cultures and are representative of three experiments.

 
IFN-{gamma} augments IL-12 responsiveness in ICSBP-/- mice

Another important mechanism by which IFN-{gamma} affects T cell priming is through its ability to enhance IL-12 responsiveness by increasing IL-12 Rß2 expression (18). To evaluate the role of endogenous IFN-{gamma} on IL-12 responsiveness, sorted CD4+/LECAM-1high T cells from ICSBP-/- mice were stimulated in primary cultures with anti-CD3 in the presence or absence of anti-IFN-{gamma} or IL-4. After 4 days, cells were restimulated with or without IL-12, and production of IFN-{gamma} was determined. As shown in Fig. 4A, addition of IL-12 to the secondary cultures caused a three- to fourfold increase in production of IFN-{gamma} from cells primed with anti-CD3 alone; however, addition of anti-IFN-{gamma} or IL-4 in primary cultures strongly inhibited the ability of IL-12 to enhance production of IFN-{gamma} in secondary cultures. By contrast, addition of IFN-{gamma} or IL-12 to primary cultures caused a striking increase in IFN-{gamma} production in the presence or absence of IL-12 in secondary cultures (Fig. 4B). Taken together, these data underscore that the ability of IFN-{gamma} to regulate T cell differentiation in CD4+ T cells is maintained in ICSBP-/- mice.


    Discussion
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Recent work by us and others has shown that the requirements for IL-12 induction are different following stimulation with an infectious or T cell-specific stimulus (20, 21). While infectious pathogens can induce IL-12 in a CD40L/CD40- or IFN-{gamma}-independent manner, T cell stimuli require CD40L/CD40 stimulation. Recently, a better understanding of the molecular basis for IL-12 regulation has come from studies using ICSBP-/- mice. These studies convincingly showed that induction of IL-12 in response to a variety of microbial stimuli is markedly impaired in ICSBP-/- mice, resulting in substantially diminished production of IFN-{gamma} (15, 16). It should be noted, however, that in some of the previous reports, production of IFN-{gamma} from spleen cells of ICSBP-/- mice is only modestly affected, suggesting that induction of IL-12 by T cell stimuli may be intact and not require ICSBP. Alternatively, as CD40L/CD40 stimulation induced by Con A can also enhance expression of B7/CD28, it is possible that production of IFN-{gamma} was independent of IL-12. In the studies reported here, we show directly that IL-12 p40 production from spleen cells of ICSBP-/- mice in response to anti-CD3 is markedly inhibited. Moreover, addition of a CD40 ligand agonist to cultures still did not enhance IL-12 production. Since levels of ICSBP expression in unstimulated APCs are low, our results raise the possibility that CD40L/CD40 stimulates induction of ICSBP in these cells, similar to that by IFN-{gamma} stimulation (11). Finally, our results showing that addition of a stimulatory anti-CD28 Ab strikingly enhanced production of IFN-{gamma} from spleen cells of ICSBP-/- mice in response to anti-CD3, even in the presence of anti-IL-12, suggest that IL-12-independent production of IFN-{gamma} is mediated by CD40L/CD40 enhancement of B7. We conclude that ICSBP is essential for IL-12 p70 induction and for IL-12-dependent production of IFN-{gamma} in response to both microbial and T cell stimuli.

A second major focus of this study was to determine whether ICSBP was essential for the ability of IFN-{gamma} to regulate type 1 cytokine responses. Previous studies have shown that upstream events such as STAT 1 activation and production of nitric oxide are intact in ICSBP-/- mice (16), suggesting that the effects of ICSBP act downstream of the STAT 1 pathway and affect a more limited number of IFN-{gamma}-inducible genes. In this regard, our results are consistent with recent reports showing that ICSBP is required for the ability of IFN-{gamma} to enhance IL-12 p40 production (15, 16). Of additional interest was the finding that spleen cells from ICSBP-/- mice stimulated with SAC and anti-IL-10 had a demonstrable increase in IL-12 p40 but not in p70. Thus, ICSBP is not likely to regulate IL-10-mediated inhibition of IL-12 production. Finally, there is evidence showing that IFN-{gamma} can directly influence the differentiation of naive CD4+ T cells toward a Th1-type cell (17). Moreover, IFN-{gamma} has also been shown to enhance Th1 differentiation by enhancing the expression of the IL-12 Rß2 chain on CD4+ T cells (18). In these studies, we first show that, in an IL-12-independent system, the absence of ICSBP does not affect Th1 differentiation. In addition, the presence of IFN-{gamma} in priming cultures both enhanced the priming for IFN-{gamma} itself and increased IL-12 responsiveness. Overall, these results show that the role of ICSBP in regulating type 1 cytokine responses is cell specific and is limited to its effects on IL-12 p40 production but is not required for IFN-{gamma} to influence CD4+ T cell differentiation. This latter observation may have biologic and clinical relevance in the following manner. It was recently reported that a small number of individuals with a mutation in the IL-12R ß1 chain have increased susceptibility to intracellular pathogens such as Mycobacterium avium, BCG, or Salmonella enteritidis infection (22, 23). In these studies, it was of interest that these patients still produced appreciable amounts of IFN-{gamma} (>1 ng/ml) in response to mitogenic stimulation. Furthermore, it was noted that IL-12Rß1-/- patients had a milder clinical course than did similarly infected patients with IFN-{gamma}R deficiency (24, 25). Based on these observations, the authors concluded that this milder clinical course might be due to the existence of IL-12-independent pathways for IFN-{gamma} production. Moreover, it was speculated that this IL-12-independent production of IFN-{gamma} may have had an immunological effect, insofar as they were not able to detect IL-4. These data further underscore the fact that for most intracellular infections, while IL-12 affects the magnitude of the Th1 response, IFN-{gamma} is the critical effector cytokine in mediating intracellular killing. Furthermore, the findings reported here that IFN-{gamma} can still affect the differentiation of Th1 and Th2 responses by CD4+ T cells in the absence of ICSBP leaves open the possibility that in settings in which IL-12 or IL-12 responsiveness is markedly diminished, the relatively small amount of IFN-{gamma} produced may still exert a biologic and immunologic effect as demonstrated in these patients.

To conclude, we speculate that the differential requirement for ICSBP in regulating type 1 cytokine production observed in APCs and naive T cells may be attributable to distinct contributions of other IRF family proteins such as Pip/IRF4 in these respective cell types. In this regard, Pip/IRF-4 is highly homologous to ICSBP in structure and is induced by similar stimuli in lymphoid cells (26, 27). Work is under way to determine whether compensatory mechanisms exist in T cells to obviate the need for ICSBP in controlling Th1 differentiation.



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FIGURE 4. IFN-{gamma} augments IL-12 responsiveness in ICSBP-/- mice. Sorted CD4+/LECAM-1high T cells from ICSBP-/- mice were plated at 5–7 x 105 cells/ml in 24-well plates previously coated with anti-CD3 (5 µg/ml) in the presence or absence of anti-IFN-{gamma} (10 µg/ml) or IL-4 (500 pg/ml) (A) and IFN-{gamma} (500 U/ml) or IL-12 (2 ng/ml) (B). After incubation for 4 days, cells were harvested, washed, and restimulated with immobilized anti-CD3 (5 µg/ml) + anti-CD28 (2 µg/ml). Two days later, IFN-{gamma} content was assessed by ELISA. Results are expressed as mean ± SD of triplicate cultures and are representative of three experiments.

 

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Table II. Experiment 2: IL-12 p40 production by splenocytes from ICSBP+/+ and ICSBP-/- mice (pg/ml)a

 

    Acknowledgments
 
We thank Dr. Maurice Gately (Hoffman-La Roche) for IL-12 and anti-IL-12 Abs, and Brenda Rae Marshall for editorial assistance.


    Footnotes
 
1 Address correspondence and reprint requests to Dr. Robert A. Seder, CIS, LCI, NIAID, Building 10, Room 11C215, NIH, Bethesda, MD 20892. E-mail address: Back

2 Abbreviations used in this paper: ICSBP, IFN consensus sequence binding protein; SAC, Staphylococcus aureus Cowan strain 1; CD40L, CD40 ligand; IRF, IFN regulatory factor; LECAM-1, lymphocyte endothelial cell adhesion molecule-1; CD40LT, CD40 ligand/trimer Back

Received for publication July 23, 1998. Accepted for publication October 7, 1998.


    References
 Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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