Identification and functional characterization of a second chain of the interleukin-10 receptor complex cells, blocking their ability to secrete cytokines such as Serguei V.Kotenko, Christopher D.Krause, interferon-γ (IFN-γ) and IL-2 (Fiorentino et al., 1991;

Expression of a dominant negative mutant IFNgammaR1 in murine SCK and K1735 tumor cells rendered them relatively unresponsive to IFNgamma in vitro and more tumorigenic and less responsive to IL-12 therapy in vivo. IL-12 induced histologic evidence of ischemic damage only in IFNgamma-responsive tumors, and in vivo Matrigel vascularization assays revealed that while IFNgamma-responsive and -unresponsive tumor cells induced angiogenesis equally well, IL-12 and its downstream mediator IFNgamma only inhibited angiogenesis induced by the responsive cells. IL-12 induced angiogenesis inhibitory activity in the responsive cells, which may be attributable to production of the chemokine IP-10. Thus, IL-12 and IFNgamma inhibit tumor growth by inducing tumor cells to generate antiangiogenic activity.

Cell lines that are mutated in interferon (IFN) responses have been critical in establishing an essential role for the JAK family of nonreceptor tyrosine kinases in interferon signalling. Mutant ␥1A cells have previously been shown to be complemented by overexpression of JAK2. Here, it is shown that these cells carry a defect in, and can also be complemented by, the ␤-subunit of the IFN-␥ receptor, consistent with the hypothesis that the mutation in these cells affects JAK2-receptor association. In contrast, mutant ␥2A cells lack detectable JAK2 mRNA and protein. By using ␥2A cells, the role of various domains and conserved tyrosine residues of JAK2 in IFN-␥ signalling was examined. Individual mutation of six conserved tyrosine residues, mutation of a potential phosphatase binding site, or mutation of the arginine residue in the proposed SH2-like domain had no apparent effect on signalling in response to IFN-␥. Results with deletion mutants, however, indicated that association of JAK2 with the IFN-␥R2 subunit requires the amino-terminal region but not the pseudokinase domain. Consistent with this, in chimeras with JAK1, the JAK2 amino-terminal region was required for receptor association and STAT1 activation. Conversely, a JAK1-JAK2 chimera with the amino-terminal domains of JAK1 linked to the pseudokinase and kinase domains of JAK2 is capable of reconstituting JAK-STAT signalling in response to IFN-␣ and -␥ in mutant U4C cells lacking JAK1. The specificity of the JAKs may therefore lie mainly in their structural interaction with different receptor and signalling proteins rather than in the substrate specificity of their kinase domains.Janus kinase (JAK) nonreceptor tyrosine kinases and signal transducers and activators of transcription (STATs) are essential in signalling by the interferons (IFNs) and other cytokines (19,27,30,31,34,48,49). JAK1, Tyk2, STAT1, and STAT2 are required for the major IFN-␣/␤ response, and JAK1, JAK2, and STAT1 are required for the IFN-␥ response. The IFN-␥ receptor (IFN-␥R) belongs to the class 2 cytokine receptor family (5) and consists of a 90-kDa ligand-binding ␣ subunit (IFN-␥R1) (1) and a ␤ subunit or accessory factor 1 (AF-1; IFN-␥R2) (16, 44), which is required for receptor function (17). JAK1 is constitutively associated with the IFN-␥R1 subunit (18, 21) and JAK2 with the IFN-␥R2 subunit (4, 24, 39). Work from a number of groups has led to the following model for the initial events at the receptor. On ligand-induced dimerization or oligomerization of the preexisting receptor subunit/JAK complexes, JAK2 phosphorylates itself and JAK1, which further activates both kinases. JAK1 is thought to phosphorylate the IFN-␥R1 subunit of the receptor, creating a docking site for STAT1. JAK2 may again be required for the phosphorylation of STAT1. The activated and phosphorylated STAT1 is released and dimerizes, and the homodimers translocate to the nucleus, where they bind to specific DNA elements to activate transcription (6). Signalling through the IFN-␣␤ receptor(s) is less well unders...

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Cellular interleukin 10s (cIL-10s) of human and murine origin have extensive sequence and structural homology to the Epstein-Barr virus BCRF-I gene product, known as viral IL-10 (vIL-10). Although these cytokines share many immunosuppressive properties, vIL-10 lacks several of the immunostimulatory activities of cIL-10 on certain cell types. The molecular and cellular bases for this dichotomy are not currently defined. Here, we show that the single amino acid isoleucine at position 87 of cIL-10 is required for its immunostimulatory function. Substitution of isoleucine in cIL-10 with alanine, which corresponds to the vIL-10 residue, abrogates immunostimulatory activity for thymocytes, mast cells, and alloantigenic responses while preserving immunosuppressive activity for inhibition of interferon γ production and prolongation of cardiac allograft survival. Conversely, substitution of alanine with isoleucine in vIL-10 converts it to a cIL-10–like molecule with immunostimulatory activity. This single conservative residue alteration significantly affects ligand affinity for receptor; however, affinity changes do not necessarily alter specific activities for biologic responses in a predictable fashion. These results suggest complex regulation of IL-10 receptor–ligand interactions and subsequent biological responses. These results demonstrate that vIL-10 may represent a captured and selectively mutated cIL-10 gene that benefits viral pathogenesis by leading to ineffective host immune responses. The ability to manipulate the activity of IL-10 in either a stimulatory or suppressive direction may be of practical value for regulating immune responses for disease therapy, and of theoretical value for determining what aspects of IL-10 activity are important for normal T cell responses.

Each cytokine which utilizes the Jak-Stat signal transduction pathway activates a distinct combination of members of the Jak and Stat families. Thus, either the Jaks, the Stats, or both could contribute to the specificity of ligand action. With the use of chimeric receptors involving the interferon ␥ receptor (IFN-␥R) complex as a model system, we demonstrate that Jak2 activation is not an absolute requirement for IFN-␥ signaling. Other members of the Jak family can functionally substitute for Jak2. IFN-␥ can signal through the activation of Jak family members other than Jak2 as measured by Statl␣ homodimerization and major histocompatibility complex class I antigen expression. This indicates that Jaks are interchangeable and indiscriminative in the JakStat signal transduction pathway. The necessity for the activation of one particular kinase during signaling can be overcome by recruiting another kinase to the receptor complex. The results may suggest that the Jaks do not contribute to the specificity of signal transduction in the Jak-Stat pathway to the same degree as Stats.The Jak-Stat signal transduction pathway was first discovered for interferon ␣ (IFN-␣) 1 and interferon ␥ (IFN-␥) by the complementation of mutant cell lines defective in response to IFN-␥ and/or IFN-␣ (Velazquez et al., 1992;Watling et al., 1993;Mü ller et al., 1993aMü ller et al., , 1993bDarnell et al., 1994;Leung et al., 1995). It has subsequently been shown that the same general pathway is activated by most cytokines and some growth factors (for review, see Ihle and Kerr (1995) and Taniguchi (1995)). This pathway is activated predominantly through receptors which do not possess intrinsic intracellular kinase domains and belong to the class I or class II cytokine receptor superfamily. The lack of inherent catalytic activity in these receptors is overcome through the use of receptor-associated kinases of the Janus kinase (Jak) family. Upon ligand binding, the receptor chains oligomerize allowing the associated kinases to interact and likely cross-activate each other by tyrosine phosphorylation. Subsequently, the activated Jaks directly phosphorylate the intracellular domains of the receptors on specific tyrosine residues. This phosphorylation allows the selective recruitment of SH2-domain containing proteins, particularly Stats (signal transducers and activators of transcription), through a specific interaction between the Stat SH2 domains and the phosphotyrosines within the Stat recruitment sites of the intracellular domains of the receptor chains. These receptor-associated Stats are then rapidly phosphorylated, likely by the activated Jaks . The phosphorylation of the Stats is followed by Stat dimerization, translocation to the nucleus, and activation of cytokine inducible genes.The Jak and Stat families are growing rapidly. The Jak family consists of four members so far: Jak1, Jak2, Jak3, and Tyk2 (Wilks et al., 1991;