We identified a viral IL-10 homolog encoded by an ORF (UL111a) within the human cytomegalovirus (CMV) genome, which we designated cmvIL-10. cmvIL-10 can bind to the human IL-10 receptor and can compete with human IL-10 for binding sites, despite the fact that these two proteins are only 27% identical. cmvIL-10 requires both subunits of the IL-10 receptor complex to induce signal transduction events and biological activities. The structure of the cmvIL-10 gene is unique by itself. The gene retained two of four introns of the IL-10 gene, but the length of the introns was reduced. We demonstrated that cmvIL-10 is expressed in CMVinfected cells. Thus, expression of cmvIL-10 extends the range of counter measures developed by CMV to circumvent detection and destruction by the host immune system. I L-10 is a pleiotropic immunomodulatory cytokine produced by CD4 ϩ and CD8 ϩ T cells, monocytes͞macrophages, keratinocytes, and activated B cells (1). In addition, its expression is elevated in patients with a variety of peripheral blood or bone marrow-derived leukemias, certain B cell and T cell lymphomas and nasal natural killer cell lymphomas and other hematopoietic and solid tumors (1-5). Two mechanisms of IL-10 action can be used by tumors. IL-10 appears to act as an autocrine growth factor for B cell lymphomas. In addition, IL-10 selectively inhibits certain aspects of the cellular immune response. It blocks proinflammatory cytokine synthesis and suppresses the ability of macrophages to serve as antigen-presenting or costimulatory cells (6-8). Thus, IL-10 is a powerful anti-inflammatory agent and a potent immunosuppressor.Many viruses exploit the strategy of using homologs of cellular cytokines or cytokine receptors to shield virus-infected cells from immune defenses and enhance virus survival in the host. The presence of virus-encoded homologs of cellular proteins may be an indicator of the importance of these cellular components in immune mechanisms for combating this virus in vivo. A number of herpes viruses harbor homologs of IL-10. Epstein-Barr virus (EBV)-encoded IL-10 (ebvIL-10), the first viral homolog of IL-10 identified (9, 10), shares many but not all of the biological activities of cellular IL-10 and may play an important role in the host-virus interaction (1,11,12). In addition to EBV, another virus, the Orf poxvirus (OV), which can infect humans, has its own IL-10 homolog, ovIL-10 (13). Whether it is active on human cells remains to be shown. The exact in vivo roles of viral IL-10 homologs in the viral life cycle, in immune evasion, and͞or in helping virus-infected cells to survive immune surveillance remain to be defined.Human cytomegalovirus (CMV) is a widespread herpes virus that is able to persist for decades in its host. CMV is the major cause of a variety of life-threatening diseases in immunocompromised individuals, including transplant and AIDS patients, and is a leading cause of congenital birth defects (14). CMV is also associated with the development of atherosclerosis, restenosis after coronary...

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;

To expand our understanding of the role of Jak2 in cellular signaling, we used the yeast two-hybrid system to identify Jak2-interacting proteins. One of the clones identified represents a human homologue of the Schizosaccaromyces pombe Shk1 kinase-binding protein 1, Skb1, and the protein encoded by the Saccharomyces cerevisiae HSL7 (histone synthetic lethal 7) gene. Since no functional motifs or biochemical activities for this protein or its homologues had been reported, we sought to determine a biochemical function for this human protein. We demonstrate that this protein is a protein methyltransferase. This protein, designated JBP1 (Jak-binding protein 1), and its homologues contain motifs conserved among protein methyltransferases. JBP1 can be cross-linked to radiolabeled S-adenosylmethionine (AdoMet) and methylates histones (H2A and H4) and myelin basic protein. Mutants containing substitutions within a conserved region likely to be involved in AdoMet binding exhibit little or no activity. We mapped the JBP1 gene to chromosome 14q11.2-21. In addition, JBP1 co-immunoprecipitates with several other proteins, which serve as methyl group acceptors and which may represent physiological targets of this methyltransferase. Messenger RNA for JBP1 is widely expressed in human tissues. We have also identified and sequenced a homologue of JBP1 in Drosophila melanogaster. This report provides a clue to the biochemical function for this conserved protein and suggests that protein methyltransferases may have a role in cellular signaling.

With the use of a partial sequence of the human genome, we identified a gene encoding a novel soluble receptor belonging to the class II cytokine receptor family. This gene is positioned on chromosome 6 in the vicinity of the IFNGR1 gene in a head-to-tail orientation. The gene consists of six exons and encodes a 231-aa protein with a 21-aa leader sequence. The secreted mature protein demonstrates 34% amino acid identity to the extracellular domain of the IL-22R1 chain. Cross-linking experiments demonstrate that the protein binds IL-22 and prevents binding of IL-22 to the functional cell surface IL-22R complex, which consists of two subunits, the IL-22R1 and the IL-10R2c chains. Moreover, this soluble receptor, designated IL-22-binding protein (BP), is capable of neutralizing IL-22 activity. In the presence of the IL-22BP, IL-22 is unable to induce Stat activation in IL-22-responsive human lung carcinoma A549 cells. IL-22BP also blocked induction of the suppressors of cytokine signaling-3 (SOCS-3) gene expression by IL-22 in HepG2 cells. To further evaluate IL-22BP action, we used hamster cells expressing a modified IL-22R complex consisting of the intact IL-10R2c and the chimeric IL-22R1/γR1 receptor in which the IL-22R1 intracellular domain was replaced with the IFN-γR1 intracellular domain. In these cells, IL-22 activates biological activities specific for IFN-γ, such as up-regulation of MHC class I Ag expression. The addition of IL-22BP neutralizes the ability of IL-22 to induce Stat activation and MHC class I Ag expression in these cells. Thus, the soluble receptor designated IL-22BP inhibits IL-22 activity by binding IL-22 and blocking its interaction with the cell surface IL-22R complex.

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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;

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