Biologic functions of the IFN‐γ receptors
Authors: G. Tau, P. Rothman
Journal: Allergy (1999)

Interferon-gamma (IFN-γ) is a cytokine that plays an important role in inducing and modulating an array of immune responses. Cellular responses to IFN-γ are mediated by its heterodimeric cell-surface receptor (IFN-γR), which activates downstream signal transduction cascades, ultimately leading to the regulation of gene expression. In order to study the role of IFN-γ in a number of immune responses and pathways, researchers have generated mice with altered patterns of IFN-γR gene expression. These studies, together with analyses of naturally occurring mutations of the IFN-γR in man, have been instrumental in elucidating the diverse functions of IFN-γ, and are the subject of this review. Originally identified 30 years ago as an agent with antiviral activity, IFN-γ has since been characterized as a homodimeric glycoprotein with pleiotropic immunologic functions ( 1–3). IFN-γ is primarily secreted by activated T cells and natural killer (NK) cells, and can promote macrophage activation, mediate antiviral and antibacterial immunity, enhance antigen presentation, orchestrate activation of the innate immune system, coordinate lymphocyte–endothelium interaction, regulate Th1/Th2 balance, and control cellular proliferation and apoptosis ( 1, 2). It was not until 20 years after the identification of IFN-γ that its cell-surface receptor was discovered ( 4–9). The α chain of the IFN-γR, also known as IFN-γR1 or CD119, was the first component of the receptor to be identified and cloned ( 10–14). Although it binds IFN-γ with relatively high affinity, IFN-γR1 alone is unable to mediate the biologic responses to this cytokine ( 12, 15–17). Subsequent complementation studies led to the identification and cloning of an accessory factor (AF-1), also known as the β receptor chain or IFN-γR2, as the protein required, in addition to IFN-γR1, to endow a cell with the ability to respond to IFN-γ ( 15, 18–21). Specific residues within the cytoplasmic domains of both the α and β chains of the IFN-γR are critical for transducing the IFN-γ signal from the cell surface to the nucleus through the activation of intracellular signaling pathways ( 22–25). Mutations in either component of the IFN-γ receptor that impair or alter the ability of cells to respond to this ligand have global consequences for IFN-γ-mediated immunity, and therefore serve as an important tool for analyzing the pleiotropic effects of this cytokine ( 26, 27). The receptor complex that mediates the full biologic function of IFN-γ consists of at least two species-matched chains: IFN-γR1, a 90-kDa glycoprotein (which is 472 amino acids (aa) long in man, and 451aa in mice) encoded on human chromosome 6 and mouse chromosome 10, and IFN-γR2, a 60–67-kDa glycoprotein (which is 316aa in man and 314aa in mice) encoded on human chromosome 21 and mouse chromosome 16 ( 4, 18, 20, 28–35). IFN-γR1 is the major ligand-binding subunit, binding IFN-γ with a Ka of 109–1010 M−1 and a receptor-to-ligand ratio of 2:1, as inferred from the crystal structure of the occupied receptor and other studies ( 4, 34, 36, 37). IFN-γR2 increases the affinity of IFN-γR1 for its ligand, presumably by enhancing the stability of the complex, but plays only a minor role in direct ligand binding ( 38). The β chain is, however, obligatory for transducing the IFN-γ signal ( 21, 38, 39). Both chains of the IFN-γ receptor are members of the class II family of cytokine receptors that includes tissue factor, the IL-10 ligand-binding component, and both chains of the IFN-α receptor (IFN-αR) ( 40, 41). Like other family members, the IFN-γR α and β chains lack intrinsic kinase activity. Signaling through the IFN-γR is mediated through JAK1 and JAK2, members of the Janus family of protein tyrosine kinases, which are constitutively associated with specific membrane-proximal residues on the cytoplasmic domains of IFN-γR ( 25, 34, 39, 42–44). JAK1 binds the 266LPKS269 motif (also known as the box 1 motif) on IFN-γR1, while JAK2 binds the 263PPSIPLQIEEYL274 motif (or the box 1, box 2 motif) on IFN-γR2 ( 23, 25, 44–46). Ligand binding leads to receptor oligomerization, with two IFN-γR1 chains bound to one IFN-γ homodimer, and the subsequent recruitment of two IFN-γR2 chains to the complex ( 25, 37, 38, 45–48). IFN-γ-mediated aggregation of its receptor components brings the inactive JAKs associated with the cytoplasmic tails of the α and β chains into close proximity with one another ( Fig. 1). Once clustered, the JAKs are reciprocally activated through sequential auto- and transphosphorylation events ( 42, 49). Activated JAKs phosphorylate a specific tyrosine residue near the C-terminus of the IFN-γR1 (Y440 in man) ( 24, 42, 45, 50) ( Fig. 1). This phosphorylated tyrosine residue pair (one on each IFN-γR1 chain) is embedded within a recognition sequence (440YDKPH444) to which STAT1 (a member of the Signal Transducers and Activators of Transcription family of latent cytoplasmic proteins) binds through its SH2 (src homology 2) domain ( 24, 45, 51, 52). The docking of STAT1 molecules at their target sequences on the IFN-γR complex is followed by their phosphorylation on tyrosine residue Y701 by the receptor-associated JAKs ( 53–55) ( Fig. 1). Once phosphorylated, two STAT1 proteins homodimerize via reciprocal SH2-phosphotyrosine interactions, forming a protein complex first identified as GAF (gamma-activated factor) ( 51, 56). The STAT1 homodimer then translocates to the nucleus, where it binds a nine-nucleotide consensus sequence, TTNCNNNAA, known as a GAS (gamma-activated site) element ( 57–59). This binding site has been identified in the regulatory regions of over 200 genes; therefore, recognition of this element by STAT1 homodimers can modulate the expression of a vast array of genes, thereby mediating the biologic functions of IFN-γ ( 2). IFN-γ signaling cascade. A) IFN-γR is composed of α and β chains. JAK1 is constitutively associated with IFN-γR1 while JAK2 is constitutively associated with IFN-γR2. B) IFN-γ binding to its receptor leads to aggregation of receptor components. Subsequently, JAKs are activated through auto- and transphosphorylation events. Activated JAKs then phosphorylate tyrosine residue near C-terminus of IFN-γR1. C) STAT1 molecules dock at phosphorylated receptor, and are then phosphorylated by activated JAKs. D) Phosphorylated STAT1 proteins homodimerize via reciprocal SH2-phosphotyrosine interactions, and translocate to nucleus, where they regulate gene transcription. Mechanistically, it has been suggested that after ligand binding, IFN-γ signaling is initiated by JAK2 autophosphorylation, followed by phosphorylation of JAK1 ( 34, 60). Activated JAK1 is then thought to phosphorylate IFN-γR1, providing a docking site for STAT1. After binding to its receptor site, STAT1 is believed to be activated through phosphorylation by JAK2. Studies with kinase-negative JAK1 and JAK2 mutants have demonstrated that while the mutant JAK2 cannot support IFN-γ-stimulated gene transcription, the kinase activity of JAK1 is not fully required for its role in transducing the IFN-γ signal ( 49). Signaling through the IFN-γ receptor may be regulated at several points along the pathway. One mechanism is the modulation of receptor expression. Control of IFN-γR1 levels on the cell surface has been proposed to be a mechanism through which a cell alters its sensitivity to IFN-γ ( 61–63). Attenuation of the IFN-γ signal by this mechanism has been linked to differences in the biologic responses it elicits in cells expressing different levels of IFN-γR1. Moreover, downmodulation of IFN-γR2 expression in IFN-γ-producing T cell subsets renders these cells unresponsive to IFN-γ ( 64–66). Cells appear to have intracellular mechanisms to regulate the IFN-γ signal as well. Members of the recently identified SOCS ( Suppressors Of Cytokine Signaling) family of proteins may negatively regulate cytokine signaling through a number of potential mechanisms ( 67–69). For example, SOCS1 has been shown to inhibit IFN-γ signaling, presumably by preventing JAK kinase activation ( 70, 71). In fact, the expression of SOCS-1 is induced by IFN-γ, suggesting that a negative feedback pathway exists in which cells may attenuate their sensitivity to this cytokine after the activation of IFN-γ signaling. In contrast, the transcriptional activity of STAT1 (and therefore the IFN-γ signal) has been shown to be enhanced through serine phosphorylation ( 72, 73). Additional intracellular mechanisms thought to regulate IFN-γ-mediated signal transduction may include the dephosphorylation of IFN-γR1, which would prevent STAT1 docking; the dephosphorylation of STAT1, which would prevent its homodimerization; and the ubiquitination of STAT1, which would lead to its degradation by targeting it to the proteosome pathway ( 42, 51, 74, 75). These mechanisms could limit the availability of activated STAT1, although their importance in the regulation of IFN-γ signaling is unclear. To study the role of IFN-γR1 in mediating IFN-γ signaling and biologic responses, researchers have disrupted its endogenous locus to generate IFN-γR1-null (-\-) mice ( 26). These mice have not only proved to be a useful tool to study IFN-γ signal transduction, but also constitute one of the first physiologic systems available to study IFN-γ biology. IFN-γR1-deficient mice develop normally to adulthood with no phenotypic anomalies. They appear to have normal leukocyte populations in lymphoid organs and normal baseline levels of MHC class I and II molecule expression. However, cells derived from these mice are unable to initiate signaling in response to IFN-γ. They are therefore insensitive to any of the biologic effects of IFN-γ, including its antiviral effects and antitumor activity ( 26, 76). As described below, these mice have severe defects in their immune responses. IFN-γ has long been thought to be the most important activator of macrophages; in fact, it was, for a time, known as macrophage-activating factor (MAF) ( 77). Nitric oxide (NO) production and MHC class II upregulation are two key effector mechanisms of macrophages, both critically dependent on IFN-γ, and are essential for cell-mediated and Th1-type immune responses ( 1, 2, 77, 78). Therefore, it was not surprising that the IFN-γR1 (-/-) mice exhibited severely compromised macrophage functions, such as granuloma formation ( 79–85). The IFN-γ dependence of protective and self-destructive immune responses mediated, in part, by macrophages was examined in several infectious and autoimmune models that are discussed in later sections of this review. IL-12 is a cytokine secreted primarily by activated macrophages ( 2, 86–88). It induces IFN-γ production by NK cells and Th1 cells, while IL-12 production itself is induced by IFN-γ ( 89). IL-12 also primes naive T cells (Thp) to differentiate along the Th1 pathway ( 90–94). It is, however, unclear whether the biologic functions of IL-12 are mediated principally by IFN-γ ( 95). A number of groups utilized IFN-γR1-deficient mice to study the dependence of the effector functions of IL-12 on IFN-γ. Systemic administration of IL-12 is known to inhibit hematopoiesis, leading to a decrease in the number of peripheral blood lymphocytes (PBLs) and a diminished bone-marrow cellularity ( 96). Administration of IL-12 also increases splenic cellularity, secondary to an influx of activated macrophages and NK cells. In contrast, IFN-γR1 (-/-) mice injected with IL-12 mainly develop a severe, lethal lung disorder characterized by interstitial and perivascular infiltrates and diffuse pulmonary edema. Therefore, it appears that, in the absence of IFN-γ responses, many effects of IL-12 are attenuated. On the other hand, IL-12 appears to induce a different set of immune disorders that are independent of, or normally suppressed by, IFN-γ. Other experiments also suggest that IL-12 may have IFN-γ-independent effects. For example, in one study of collagen-induced arthritis (CIA), the severity of this autoimmune disease was attenuated in IFN-γR1-deficient mice after treatment with anti-IL-12 neutralizing antibodies, which are thought to suppress inflammatory disease by, in part, indirectly inhibiting IFN-γ secretion ( 97, 98). Therefore, while many of the effects of IL-12 (such as cross-regulation of type 2 inflammatory responses) are abrogated in IFN-γR1 (-/-) mice, other effector functions of IL-12 (such as direct Th1 priming, which may explain the CIA phenotype) may be independent of IFN-γ ( 90, 97, 99, 100). Naive CD4+ cells, or Thp cells, are believed to have the potential to develop into either of the two major subsets of CD4+ T helper cells: Th1 and Th2 cells ( 100, 101) ( Fig. 2). Th1 cells are primarily defined by their ability to secrete IFN-γ, IL-2, and tumor necrosis factor (TNF)-β, and are clinically associated with an ability to orchestrate cell-mediated immune responses and organ-specific autoimmunity ( 102). Differentiation of Thp cells toward the Th1 phenotype is dependent on the presence of IL-12 in their microenvironment during stimulation throughout the T-cell antigen receptor (TCR) ( 92, 103). Th1-polarizing effects similar to those of IL-12 have been ascribed to IFN-γ ( 104). On the other hand, Th2 cells arise when IL-4 is present during antigenic stimulation, and produce IL-4, IL-5, IL-10, and IL-13 ( 105). These cells are clinically associated with phagocyte-independent, antibody-mediated host defense and allergic immune responses ( 102). Because of the potential role of IFN-γ in generating Th1 cells, mediating their effector function and regulating Th1/Th2 balance, T-cell responses were examined in IFN-γR1 (-/-) mice ( 94, 99, 106). Effects of IFN-γR on T helper subset differentiation. In some experimental models utilizing IFN-γR1 (-/-) mice, there appeared to be a shift in the Th1/Th2 balance in response to stimuli that normally induce Th1 immunity (Fig. 2). For example, as described above, a predominant Th2 response ensued in response to systemic IL-12 administration in these mice ( 96). A similar Th2-directed immune disorder was also seen in one model of schistosomiasis, where not only were Th1-dependent immune responses impaired, but also aberrant Th2 responses led to a severe lung disorder ( 107). These observations support previous work that suggested that, while IFN-γ is normally considered an effector of inflammation, it may have an important inhibitory role in certain immune responses ( 108). In another study, Th2-mediated pulmonary inflammation was induced by rechallenge of previously sensitized mice with nasally administered OVA ( 109). This inflammatory lung disease persisted in IFN-γR1 mutant mice long after it was resolved by wild-type mice. Allergic pulmonary inflammatory reactions are often mediated by a type 2 response characterized by eosinophils, IgE antibodies, and cytokines such as IL-4 and IL-5 ( 110–112). Therefore, one explanation of the observed lung phenotype in IFN-γR1 mutant mice is that, in the absence of the cross-regulation normally seen between Th1 and Th2 responses (with Th1-derived IFN-γ presumably suppressing the Th2 response), a Th2-mediated immune response can proceed unhindered ( 99, 102). Alternatively, there may be other mechanisms by which IFN-γ suppresses inflammation ( 108, 113). In another group of studies, the response to the pathogen in mutant mice was normal, without a concomitant Th2-mediated disorder, although abnormally high levels of Th2 cytokines and/or Ig isotypes were detected (see Table 1 for a summary of all disease models in which IFN-γR1 (-/-) mice were used). For instance, the outcome of mouse mammary tumor virus (MMTV) infection is similar in mutant and control mice, but is characterized by increased IL-4 levels in the former ( 114). In other models, IFN-γR1 (-/-) mice elaborated defective Th1 responses with no associated Th2 response. For example, while wild-type mice are resistant to infection by either coronavirus or Leishmania major, IFN-γR1 mutant mice are susceptible to these pathogens ( 115, 116). An analysis of the pathologic changes in these mice revealed no evidence of a Th2-type immune response. In summary, some of these studies uncovered global defects in T-cell responses, others demonstrated a specific Th1 dysfunction, and yet others revealed an altered Th1/Th2 balance in IFN-γR1 (-/-) mice. However, the extent of the difference in the clinical course of disease between wild-type and mutant mice was found to be highly variable. Nevertheless, these data suggest an interesting trend in T-cell-mediated immunity in IFN-γR1 (-/-) mice: a Th2-bias with impaired Th1 responses. The observed set of T-cell phenotypes may be the consequence of a host of factors. For instance, IFN-γ-insensitive macrophages and dendritic cells are impaired in their ability to upregulate expression of MHC class II molecules on their cell surface ( 81). This defect in antigen presentation may alter CD4+ T-cell activation in these mice. IFN-γR1 (-/-) antigen-presenting cells (APCs) are also unable to secrete normal amounts of IL-12, an important Th1-polarizing stimulus ( 120). This may lead to a bias toward Th2-type responses. Moreover, it is believed that Th1-derived IFN-γ can cross-regulate Th2 cells ( 99). Unresponsiveness of Th2 cells to IFN-γ would therefore lead to a bias toward type 2 immune responses. It has also been suggested that in the absence of IFN-γ responses, monocytes can become alternatively activated, and immature dendritic cells can develop into alternatively activated dendritic cells (Dc2) ( 121–123). Alternatively activated macrophages and Dc2 cells have been shown to support differentiation of Thp cells to the Th2 phenotype. The role of these unusual APCs in promoting the Th2 pathway may, in part, account for the inability of IFN-γ-unresponsive mice to mount type 1 inflammatory processes. Of course, it is also likely that IFN-γR1 (-/-) T cells have intrinsic differences in their response to antigen. Robust type 2 responses seen in some of these immune models suggest that Th2 cells are less dependent than Th1 cells on antigen presentation by IFN-γ-primed APCs ( 96, 109, 121, 122). Finally, T cells must be able to migrate to their target sites in order to be effective regulators and mediators of immunity. The expression of adhesion molecules on vascular endothelial cells and on T cells, as well as the production of a number of chemokines, is regulated by IFN-γ ( 117–119). Therefore, part of the observed T-cell defect may be secondary to impaired migration to target sites. Like primary T-cell-mediated immunity, immunologic memory, which relies on T helper cell function, may be affected by the absence of a functional IFN-γ system ( 124–126). To assay the role of IFN-γ responsiveness in secondary T-cell responses, IFN-γR1 (-/-) mice were used in a number of immunization models. In every instance, mutant mice were unable to respond normally to antigenic rechallenge. Antigenic recall was impaired in these mice in response to rechallenge with Chlamydia trachomatis, pseudorabies virus, Schistosoma mansoni, and Plasmodium yoeli ( 107, 127–129). In fact, previously sensitized IFN-γR1 (-/-) mice elaborate inappropriate, and therefore unproductive, T helper cell-mediated responses upon re-exposure to S. mansoni ( 129). While the normal response to rechallenge with this pathogen is characterized by a strong Th1 component, helping contain the infection, the response in mutant mice is Th2 in nature, and it is ineffective in controlling parasitemia. Interestingly, repeated exposure of IFN-γR1 (-/-) mice to P. yoeli led to lasting immunity against this pathogen, thereby somehow bypassing the need for the IFN-γ system ( 129). It therefore appears that T-cell memory and, perhaps more specifically, Th1-mediated secondary responses are usually dependent on IFN-γ signaling. Like CD4+ T helper cells, CD8+ cytotoxic T lymphocytes (CTLs) are activated by an antigen presented in the context of an MHC molecule. However, unlike MHC class II, MHC class I molecules (which present intracellularly derived peptides to CD8 cells) are ubiquitously expressed ( 130, 131). Interestingly, recent evidence suggests that CD8+ T cells must first be activated by dendritic cells (that, in this case, encounter and then present extracellularly derived antigen in the context of MHC class I molecules) before leaving lymphoid areas and surveying the periphery ( 132). Moreover, whereas antigen presentation through the MHC class II pathway is enhanced mainly by IFN-γ, antigen presentation through the MHC class I pathway can be stimulated by either IFN-γ or IFN-α ( 2, 130, 131, 133, 134). Tc1 cells, which constitute the predominant CD8+ T-cell subset, secrete substantial amounts of the IFN-γ that is thought to participate in some aspects of their effector function ( 135). It was therefore of interest to examine CTL function in IFN-γR1 (-/-) mice in order to clarify the relevance of IFN-γ sensitivity to the function of these cells. CTLs derived from IFN-γR1 mutant mice infected with vaccinia virus lysed target cells infected with this virus normally, suggesting that IFN-γ-insensitive CTLs are capable of elaborating normal cytotoxic effector and memory functions ( 26). CD8+ cells in LCMV-infected IFN-γR1 (-/-) mice exhibited delayed kinetics of clonal exhaustion as compared to wild-type mice, indicating that they are less susceptible to activation-induced cell death (AICD), perhaps owing to inefficient activation (which may be secondary to either an antigen presentation or an intrinsic T-cell defect) ( 136). A number of studies suggest that CD8+ T cells are important mediators of the effector phase of contact hypersensitivity (CHS) responses to haptens ( 137–140). Although tissue swelling in response to oxazolone and TNCB was comparable in IFN-γR1 (-/-) mice and wild-type mice, mutant mice had reduced dermal mononuclear infiltrates ( 141). Because of the complexity of this model of immunity, impaired CHS may be caused by defects in either the sensitization phase (dendritic cell function, homing, or antigen presentation) or elicitation phase (chemokine secretion; CD8 T-cell activation or migration) of this response ( 142–144). Therefore, reduced dermal mononuclear infiltrates in IFN-γR1-deficient mice may not be caused by an intrinsic CD8+ T-cell defect. Although CD8+ CTL also participate in graft rejection ( 145–147), models of allo- and xeno-transplantation failed to uncover differences in the ability of wild-type and IFN-γR1 mutant mice to reject transplanted grafts ( 148, 149). These data suggest that, for the most part, intrinsic CTL function appears to be intact; CD8+ T cells can elaborate normal effector and memory functions despite their inability to transduce an IFN-γ signal. Both IFN-γR1- and the IFN-αR-deficient mice proved to be valuable model systems in elucidating the specific roles of types I and II interferons (IFN-α/β and IFN-γ, respectively) in fighting viral infection ( 114, 115, 128, 150–153). In vitro studies have revealed that interferons must initiate signaling via their respective cell-surface receptors in order to protect cells against the cytopathic effects of viruses ( 26, 76). In vivo studies, on the other hand, demonstrated that while type I interferons are essential for protection against viral infection, the relative importance of IFN-γ is pathogen-dependent, suggesting that the antiviral actions of type II interferon are partially redundant, as they duplicate those of type I interferons ( 114, 115, 128, 150–153). IFN-γR1-deficient mice were found to be resistant to some viruses (such as vesicular stomatitis virus, LCMV, and MMTV) while susceptible to others (such as murine γ-herpesvirus, vaccinia virus, Theiler's virus, murine cytomegalovirus, and coronavirus) ( 114, 115, 128, 150–153). Even though IFN-γR1-deficient mice were able to resolve some viral infections, they developed exacerbated disease in response to these pathogens as compared to control mice. For example, mutant mice could clear an infection with LCMV but exhibited delayed kinetics of viral clearance as compared to wild-type mice ( 151). LCMV-infected IFN-γR1 (-/-) mice developed greater organ abnormality and had an increased frequency of latently infected cells relative to control mice. These studies suggest that the immune system elaborates a number of simultaneous and/or sequential, functionally complementary, antiviral responses. Some of these may be dependent on IFN-γ, although not all may be essential for a healing or normal response. Survival and clearance of infection with intracellular bacteria are dependent upon innate and cell-mediated immunity ( 154, 155). IFN-γR1-deficient mice uniformly exhibit greater susceptibility to, or increased severity of bacterial infection relative to wild-type mice. Infection of mutant mice with Chlamydia trachomatis is characterized by a longer resolution time than in normal mice ( 156, 157). While mice normally contain and resolve infections with intracellular bacteria such as bacillus Calmette-Guérin (an attenuated form of Mycobacterium bovis), Bordetella pertussis, and Listeria monocytogenes, these pathogens disseminate, often fatally, in mutant mice ( 82, 156, 157). As discussed earlier, IFN-γ is an important activating stimulus for macrophages, which are involved in granuloma formation, a mechanism essential for containing and eliminating intracellular bacteria. Activated macrophages also undergo a respiratory burst, producing reactive oxygen species and NO, both of which are important for their bactericidal function ( 2). Therefore, it is likely that an underlying macrophage defect is the cause of the exquisite sensitivity of IFN-γR1 (-/-) mice to infection with intracellular bacteria ( 81). Control of parasitic infection is dependent upon the development of an appropriate immune response. In general, Th1 responses are associated with healing responses to intracellular parasites, such as Leishmania major, whereas Th2 responses are associated with protection from extracellular pathogens, such as Nippostrongylus brasiliensis ( 158, 159). The innate immune system is also important in protection against parasites. The role of IFN-γ in protection against parasitic infection and in the development of Th1- and Th2-type immune responses was explored with IFN-γR1 (-/-) mice. Protection against infection with L. major depends on normal macrophage and Th1 function ( 160, 161). Infection of IFN-γR1 (-/-) mice with L. major or trypanosomes is associated with significantly greater mortality than that seen in control mice in the absence of any detectable parameters of Th2-type immunity ( 80, 83, 116). Interestingly, iNOS (inducible nitric oxide synthase)-deficient mice display a similar phenotype in response to Trypanosoma cruzi infection to the IFN-γR1 (-/-) ( 80). These studies show that the IFN-γ system is critical for host defense against these intracellular pathogens, and indicate a defect in macrophage function as a key contributor to the observed phenotype. Studies of infection with Toxoplasma gondii suggest that the primary defect in antiprotozoan immunity in IFN-γR1-deficient mice may not be due to a T-cell dysfunction ( 79). When infected with a low-virulence form of this protozoan, normal mice are able to control infection, but develop chronic toxoplasmosis. On the other hand, IFN-γR1 (-/-) mice are unable to control infection, developing a necrotizing hepatitis, and ultimately succumbing to the pathogen. T. gondii-specific memory T cells derived from wild-type mice are unable to confer immunity when adoptively transferred to mutant mice. Furthermore, hepatic macrophages in infected IFN-γR1 (-/-) mice produced lower levels of TNF-α, iNOS, and IL-1-β ( 79). These observations suggest that the specific impaired response to T. gondii infection in IFN-γR1 (-/-) mice may be secondary to alteration of macrophage function. Infection of IFN-γR1 (-/-) mice with Plasmodium species recapitulates some of the immune system dysfunction seen in other infections studies ( 129, 163). Relative to normal mice, the symptoms of infections with a number of Plasmodium species of mutant mice range from prolonged convalescence to increased incidence of death. The immune response in IFN-γR1-deficient mice to some of these pathogens was Th2-biased relative to the anti-Plasmodium immunity seen in normal mice. This, again, suggests that in the absence of IFN-γ signaling, the elaboration of Th1 responses is impaired. In sum, mutant mice infected with parasites elaborate impaired or inappropriate immune responses that alter the clinical course of the disease, and at times lead to increased mortality as compared to normal mice. Organ-specific autoimmune diseases are believed to be mediated and regulated by Th1 cells and IFN-γ ( 102, 113, 164). This paradigm, however,