A novel antibody-drug conjugate targeting SAIL for the treatment of hematologic malignancies

Although several new therapeutic approaches have improved outcomes in the treatment of hematologic malignancies, unmet need persists in acute myeloid leukemia (AML), multiple myeloma (MM) and non-Hodgkin's lymphoma. Here we describe the proteomic identification of a novel cancer target, SAIL (Surface Antigen In Leukemia), whose expression is observed in AML, MM, chronic lymphocytic leukemia (CLL), diffuse large B-cell lymphoma (DLBCL) and follicular lymphoma (FL). While SAIL is widely expressed in CLL, AML, MM, DLBCL and FL patient samples, expression in cancer cell lines is mostly limited to cells of AML origin. We evaluated the antitumor activity of anti-SAIL monoclonal antibodies, 7-1C and 67-7A, conjugated to monomethyl auristatin F. Following internalization, anti-SAIL antibody–drug conjugates (ADCs) exhibited subnanomolar IC₅₀ values against AML cell lines in vitro. In pharmacology studies employing AML cell line xenografts, anti-SAIL ADCs resulted in significant tumor growth inhibition. The restricted expression profile of this target in normal tissues, the high prevalence in different types of hematologic cancers and the observed preclinical activity support the clinical development of SAIL-targeted ADCs.

B lymphocyte stimulator expression in patients with rheumatoid arthritis treated with tumour necrosis factor alpha antagonists: differential effects between good and poor clinical responders

OBJECTIVE

To assess the effects of tumour necrosis factor (TNF) antagonist therapy on B lymphocyte stimulator (BLyS) expression in patients with rheumatoid arthritis (RA).

METHODS

Blood from 38 patients with RA from a single centre was collected prior to and following initiation of TNF antagonist therapy. Plasma BLyS protein levels, blood leukocyte BLyS mRNA levels and disease activity were longitudinally monitored. Twelve patients with RA who either refused or were felt not to be candidates for TNF antagonist therapy and five normal healthy volunteers served as TNF antagonist-naïve controls.

RESULTS

Baseline plasma BLyS protein levels, but not blood leukocyte BLyS mRNA levels, were elevated in patients with RA. Plasma BLyS protein levels declined following initiation of TNF antagonist therapy in good responders (GR) to TNF antagonist therapy but not in poor responders (PR). By contrast, the erythrocyte sedimentation rate (ESR) declined in response to TNF antagonist therapy in GR and PR. TNF antagonist therapy did not promote change in blood leukocyte BLyS mRNA levels in either GR or PR, suggesting that the TNF antagonist-associated changes in circulating BLyS protein levels reflected changes in local BLyS production in the affected joints rather than changes in systemic BLyS production. BLyS expression did not change over time in either the normal or RA control groups.

CONCLUSIONS

A good clinical response to TNF antagonist therapy in patients with RA is associated with a decline in plasma BLyS protein levels. Increased BLyS expression in affected joints may contribute to ongoing disease activity, and reduction of such expression may help promote a favourable clinical response to TNF antagonist therapy.

B cell depletion therapy in systemic lupus erythematosus: relationships among serum B lymphocyte stimulator levels, autoantibody profile and clinical response

OBJECTIVE

To assess the relationships between serum B lymphocyte stimulator (BLyS) levels, autoantibody profile and clinical response in patients with systemic lupus erythematosus (SLE) following rituximab-based B cell depletion therapy (BCDT).

METHODS

A total of 25 patients with active refractory SLE were followed for >or=1 year following BCDT. Disease activity was assessed using the British Isles Lupus Assessment Group (BILAG) system, and serum levels of BLyS and autoantibodies to dsDNA and extractable nuclear antigens (ENA) measured by ELISA. Serum immunoglobulins and anti-dsDNA antibodies were assessed for expression of the 9G4 idiotope (indicating VH4-34 germline gene origin).

RESULTS

Following BCDT, all patients depleted in the peripheral blood and improved clinically for >or=3 months. Pre-BCDT BLyS levels were quantifiable (median 1.9 ng/ml) in 18/25 patients and rose in most patients at 3 months post-BCDT (median 4.15 ng/ml). Nine patients, all with quantifiable pre-BCDT serum BLyS, experienced a disease flare within 1 year. This group of patients was more likely to harbour anti-Ro/SSA antibodies (odds ratio 1.76; p = 0.06) with higher serum levels (p = 0.0027; Mann-Whitney U test). Serum levels of anti-ribonucleoprotein (RNP)/Sm were also higher in this group (p<0.05). Expression of VH4-34 by serum immunoglobulins and anti-dsDNA antibodies had no predictive value for the length of clinical response.

CONCLUSIONS

Patients with SLE with an expanded autoantibody profile and raised BLyS levels at baseline had shorter clinical responses to BCDT. This may reflect a greater propensity to, and degree of, epitope spreading in such patients and suggests that treatment regimens beyond BCDT may be necessary to induce long-lasting clinical remissions in these individuals.

B-Lymphocyte stimulator (BLyS) up-regulation in mixed cryoglobulinaemia syndrome and hepatitis-C virus infection

OBJECTIVES

To investigate the role of B-Lymphocyte stimulator (BLyS) in mixed cryoglobulinaemia syndrome (MCsn), a systemic vasculitis associated with a high risk to develop lymphoma, since BLyS up-regulation may favour both autoimmunity and lymphoproliferation.

METHODS

BLyS serum levels were analysed by enzyme-linked immunosorbent assay (positive when >0.85 ng/ml) in 66 patients with MCsn, 54 (81.8%) of whom were positive for hepatitis-C virus (HCV) infection. Thirty-three HCV-positive patients without MCsn were also studied. Patients were compared with 48 healthy blood donors (HBDs). BLyS modifications after antiviral therapy were also studied.

RESULTS

A significantly higher frequency of BLyS serum positivity was detected both in MCsn patients and in HCV-positive patients without MCsn (37.9 and 30.3%, respectively) when compared with HBDs (4.2%) (P < 0.0001 vs MCsn and P = 0.0026 vs HCV-positive patients without MCsn, respectively). BLyS appeared significantly higher in MCsn (3.70 +/- 2.97 ng/ml) than in HCV-positive patients without MCsn (1.56 +/- 0.63 ng/ml; P = 0.0044). BLyS expression did not correlate with rheumatoid factor levels, cryoglobulin levels or definite MCsn-related systemic features. High BLyS levels were significantly associated only with MCsn-related overt lymphoproliferative disorder. Finally, antiviral treatment significantly increased BLyS levels, independently from HCV-RNA negativization. However, BLyS normalization was noticed after both HCV-RNA negativization and suspension of antiviral therapy by preliminary data.

CONCLUSIONS

BLyS is up-regulated and may play a pathogenetic role in a fraction of patients with MCsn, similarly to other autoimmune diseases. HCV infection likely represents the early event leading to BLyS up-regulation in this setting. BLyS is up-regulated during antiviral treatment. Overall, these data provide new insights for BLyS and virus-related autoimmunity, lymphoproliferation and possible treatment strategies.

The deubiquitinating enzyme DUB-2 prolongs cytokine-induced signal transducers and activators of transcription activation and suppresses apoptosis following cytokine withdrawal

Cytokines, such as interleukin-2 (IL-2), activate intracellular signaling pathways via rapid tyrosine phosphorylation of their receptors, resulting in the activation of many genes involved in cell growth and survival. The deubiquitinating enzyme DUB-2 is induced in response to IL-2 but as yet its function has not been determined. The results of this study show that DUB-2 is expressed in human T-cell lymphotropic virus-I (HTLV-1)-transformed T cells that exhibit constitutive activation of the IL-2 JAK/STAT (signal transducers and activators of transcription) pathway, and when expressed in Ba/F3 cells DUB-2 markedly prolonged IL-2-induced STAT5 phosphorylation. Although DUB-2 did not enhance IL-2-mediated proliferation, when withdrawn from growth factor, cells expressing DUB-2 had sustained STAT5 phosphorylation and enhanced expression of IL-2-induced genes cis and c-myc. Moreover, DUB-2 expression markedly inhibited apoptosis induced by cytokine withdrawal allowing cells to survive. Taken together these data suggest that DUB-2 can enhance signaling through the JAK/STAT pathway, prolong lymphocyte survival, and, when constitutively expressed, may contribute to the activation of the JAK/STAT pathway observed in some transformed cells.

BLyS BINDS TO B CELLS WITH HIGH AFFINITY AND INDUCES ACTIVATION OF THE TRANSCRIPTION FACTORS NF-kappaB AND ELF-1

B lymphocyte stimulator (BLyS) is a novel member of the TNF family of proteins expressed by myeloid cells as membrane-bound and soluble forms. BLyS was shown to act specifically on B cells, inducing proliferation and immunoglobulin production both in vitro and in vivo. The present study was undertaken to characterize binding of radiolabeled BLyS to its cognate receptor on human B lymphocytes and examine intracellular events initiated by BLyS binding. Similar to other TNF family members, BLyS is present in solution as a homotrimer as determined by gel filtration chromatography and light scattering analysis. BLyS binding to B cells is specific as other TNF family members tested did not compete for(125)I-BLyS binding. Analysis of equilibrium binding of(125)I-labeled BLyS to purified human tonsillar B cells demonstrated saturable binding. Scatchard analysis of the binding data revealed a single class of high-affinity binding on human B cells with approximately 2600 binding sites per cell and an apparent dissociation constant (K(D)) of about 0.1 nM. In addition we report that BLyS binding to B cells results in the activation of NF-kappaB and the Ets family transcription factor, ELF-1, and in the induction of mRNA for Polo-like kinase (PLK).

Synthesis and release of B-lymphocyte stimulator from myeloid cells

B-lymphocyte stimulator (BLyS) is a recently identified novel member of the tumor necrosis factor ligand superfamily shown to exist in a membrane-bound and soluble form. BLyS was found to be specifically expressed on cells of myeloid lineage and to selectively stimulate B-lymphocyte proliferation and immunoglobulin production. The expression of a cytokine involved in potentiation of humoral immune responses, such as BLyS, is expected to be strictly controlled. The goal of the present study was to examine regulation of BLyS levels in monocytic cells in response to cytokines and during their differentiation to macrophages and dendritic cells. The presence of BLyS on the cell surface and in the culture medium of both normal blood monocytes and on tumor cells of myelomonocytic origin was demonstrated. BLyS gene expression and levels of membrane-associated and soluble BLyS were found to be regulated by cytokines, in particular interferon (IFN)-gamma and to a lesser extent interleukin-10 (IL-10). The expression of BLyS on monocyte membranes was retained following differentiation into macrophages, but detection on the surface of monocyte-derived dendritic cells required stimulation with IFN-gamma. Both IFN-gamma and IL-10 enhanced the release of soluble BLyS that was active in B-cell proliferation assays. Cells transfected with BLyS complementary DNA mutated in a predicted cleavage site failed to release BLyS into the culture medium, thereby suggesting that soluble BLyS was derived from the membrane form. These results provide further support for an important role for BLyS expressed in myeloid cells in B-cell expansion and antibody responses.

Tumor necrosis factor (TNF) receptor superfamily member TACI is a high affinity receptor for TNF family members APRIL and BLyS

An expression cloning approach was employed to identify the receptor for B-lymphocyte stimulator (BLyS) and identified the tumor necrosis factor receptor superfamily member TACI as a BLyS-binding protein. Expression of TACI in HEK293T cells confers on the cells the ability to bind BLyS with subnanomolar affinity. Furthermore, a TACI-Fc fusion protein recognizes both the cleaved, soluble form of BLyS as well as the membrane BLyS present on the cell surface of a recombinant cell line. TACI mRNA is found predominantly in B-cells and correlates with BLyS binding in a panel of B-cell lines. We also demonstrate that TACI interacts with nanomolar affinity with the BLyS-related tumor necrosis factor homologue APRIL for which no clear in vivo role has been described. BLyS and APRIL are capable of signaling through TACI to mediate NF-kappaB responses in HEK293 cells. We conclude that TACI is a receptor for BLyS and APRIL and discuss the implications for B-cell biology.

CIS associates with the interleukin-2 receptor beta chain and inhibits interleukin-2-dependent signaling

CIS is a cytokine-induced SH2-containing protein that was originally cloned as an interleukin (IL)-3-inducible gene. CIS is known to associate with the IL-3 receptor beta chain and erythropoietin receptor and to inhibit signaling mediated by IL-3 and erythropoietin. We now demonstrate that CIS also interacts with the IL-2 receptor beta chain (IL-2Rbeta). This interaction requires the A region of IL-2Rbeta (residues 313-382), which also mediates the association of IL-2Rbeta with Lck and Jak3. Correspondingly, CIS inhibits functions associated with both of these kinases: Lck-mediated phosphorylation of IL-2Rbeta and IL-2-mediated activation of Stat5. Thus, we demonstrate that CIS can negatively control at least two independent IL-2 signaling pathways. Although a functional SH2 binding domain of CIS was not required for its interaction with IL-2Rbeta in vitro, its phosphotyrosine binding capability was essential for the inhibitory action of CIS. On this basis, we have generated a mutant form of CIS protein with an altered SH2 domain that acts as a dominant negative and should prove useful in further understanding CIS action.

SOCS-3 is tyrosine phosphorylated in response to interleukin-2 and suppresses STAT5 phosphorylation and lymphocyte proliferation

Members of the recently discovered SOCS/CIS/SSI family have been proposed as regulators of cytokine signaling, and while targets and mechanisms have been suggested for some family members, the precise role of these proteins remains to be defined. To date no SOCS proteins have been specifically implicated in interleukin-2 (IL-2) signaling in T cells. Here we report SOCS-3 expression in response to IL-2 in both T-cell lines and human peripheral blood lymphocytes. SOCS-3 protein was detectable as early as 30 min following IL-2 stimulation, while CIS was seen only at low levels after 2 h. Unlike CIS, SOCS-3 was rapidly tyrosine phosphorylated in response to IL-2. Tyrosine phosphorylation of SOCS-3 was observed upon coexpression with Jak1 and Jak2 but only weakly with Jak3. In these experiments, SOCS-3 associated with Jak1 and inhibited Jak1 phosphorylation, and this inhibition was markedly enhanced by the presence of IL-2 receptor beta chain (IL-2Rbeta). Moreover, following IL-2 stimulation of T cells, SOCS-3 was able to interact with the IL-2 receptor complex, and in particular tyrosine phosphorylated Jak1 and IL-2Rbeta. Additionally, in lymphocytes expressing SOCS-3 but not CIS, IL-2-induced tyrosine phosphorylation of STAT5b was markedly reduced, while there was only a weak effect on IL-3-mediated STAT5b tyrosine phosphorylation. Finally, proliferation induced by both IL-2- and IL-3 was significantly inhibited in the presence of SOCS-3. The findings suggest that when SOCS-3 is rapidly induced by IL-2 in T cells, it acts to inhibit IL-2 responses in a classical negative feedback loop.

Autosomal SCID caused by a point mutation in the N-terminus of Jak3: mapping of the Jak3-receptor interaction domain

Signaling through the hematopoietic receptors requires activation of receptor-associated Janus (Jak) kinases. For example, Jak1 and Jak3 bind specifically to the IL-2 receptor beta (IL-2Rbeta) and common gamma (gammac) chains, respectively, and initiate biochemical signals critical in controlling immune responses. The region of Jak responsible for receptor interactions, however, is not well characterized. Here we describe a naturally occurring Jak3 mutation from a patient with autosomal severe combined immunodeficiency (SCID), where a single amino acid substitution, Y100C, in Janus homology domain 7 (JH7) prevents kinase-receptor interaction. This mutation also results in a loss of IL-2-induced signaling in a B-cell line derived from this patient. Using mutational analysis we have identified a region of Jak3, including portions of JH6 and JH7, that is sufficient for kinase-receptor contact and show that this segment interacts with the proline-rich Box1 region of the receptor. Furthermore, a Jak3-Jak1 chimera containing only the JH6 and JH7 domains of Jak3 interacts with gammac and can reconstitute IL-2-dependent responses, including receptor phosphorylation and activation of signal transducer and activator of transcription (STAT) 5b. Our results suggest that the N-terminus of Jak kinases is critical for receptor binding, and is therefore likely to determine specificity of Jak kinase-receptor interactions.

Functional cooperation of the interleukin-2 receptor beta chain and Jak1 in phosphatidylinositol 3-kinase recruitment and phosphorylation

Phosphatidylinositol 3-kinase (PI 3-K) plays an important role in signaling via a wide range of receptors such as those for antigen, growth factors, and a number of cytokines, including interleukin-2 (IL-2). PI 3-K has been implicated in both IL-2-induced proliferation and prevention of apoptosis. A number of potential mechanisms for the recruitment of PI 3-K to the IL-2 receptor have been proposed. We now have found that tyrosine residues in the IL-2 receptor beta chain (IL-2Rbeta) are unexpectedly not required for the recruitment of the p85 component of PI 3-K. Instead, we find that Jak1, which associates with membrane-proximal regions of the IL-2Rbeta cytoplasmic domain, is essential for efficient IL-2Rbeta-p85 interaction, although some IL-2Rbeta-p85 association can be seen in the absence of Jak1. We also found that Jak1 interacts with p85 in the absence of IL-2Rbeta and that IL-2Rbeta and Jak1 cooperate for the efficient recruitment and tyrosine phosphorylation of p85. This is the first report of a PI 3-K-Jak1 interaction, and it implicates Jak1 in an essential IL-2 signaling pathway distinct from the activation of STAT proteins.

Human and simian T-cell leukemia viruses type 2 (HTLV-2 and STLV-2(pan-p)) transform T cells independently of Jak/STAT activation

Human T-lymphotropic virus type 1 (HTLV-1) and HTLV-2 differ in pathogenicity in vivo. HTLV-1 causes leukemia and neurologic and inflammatory diseases, whereas HTLV-2 is less clearly associated with human disease. Both retroviruses transform human T cells in vitro, and transformation by HTLV-1 was found to be associated with the constitutive activation of the Jak/STAT pathway. To assess whether HTLV-2 transformation may also result in constitutive activation of the Jak/STAT pathway, six interleukin-2-independent, HTLV-2-transformed T-cell lines were analyzed for the presence of activated Jak and STAT proteins by electrophoretic mobility shift assay. In addition, the phosphorylation status of Jak and STAT proteins was assessed directly by immunoprecipitation and immunoblotting with an antiphosphotyrosine antibody. Jak/STAT proteins were not found to be constitutively activated in any of the T-cell lines infected by the type 2 human and nonhuman primate viruses, suggesting that HTLV-2 and the cognate virus simian T-lymphotropic virus type 2 from Pan paniscus transform T cells in vitro by mechanisms at least partially different from those used by HTLV-1.

Recruitment of SH2-containing protein tyrosine phosphatase SHP-1 to the interleukin 2 receptor; loss of SHP-1 expression in human T-lymphotropic virus type I-transformed T cells

Interleukin 2 (IL-2) rapidly induces tyrosine phosphorylation of intracellular substrates, including the IL-2 receptor beta chain (IL-2Rbeta), Janus kinase 1 (Jak1), Jak3, signal transducer/activator of transcription proteins, and Shc, but the mechanism underlying dephosphorylation of these proteins is not known. The src homology 2 (SH2) containing tyrosine phosphatase 1 (SHP-1) is recruited by several hematopoietic surface receptors indicating that this phosphatase plays an important role as a regulator of signaling. We have found that IL-2 induces association of SHP-1 with the IL-2 receptor complex, and that once SHP-1 is recruited to the activated receptor it is able to decrease tyrosine phosphorylation of IL-2Rbeta and the associated tyrosine kinases Jak1 and Jak3. This dephosphorylation is specific as expression of a catalytically inactive form of SHP-1, or expression of the related phosphatase SHP-2 did not result in dephosphorylation of the IL-2 receptor components. Furthermore, we have found that SHP-1 expression is greatly decreased or undetectable in a number of IL-2 independent HTLV-I transformed T cell lines that exhibit constitutive Jak/signal transducer/activator of transcription activation. In HTLV-I infected T cells, down-regulation of SHP-1 expression was also found to correlate with the acquisition of IL-2 independence. These observations suggest that SHP-1 normally functions to antagonize the IL-2 signal transduction pathway and that HTLV-I infection and oncogenic transformation can lead to loss of SHP-1 expression resulting in constitutive activation of IL-2 regulated T cell responses.

Proliferation of adult T cell leukemia/lymphoma cells is associated with the constitutive activation of JAK/STAT proteins

Human T cell leukemia/lymphotropic virus type I (HTLV-I) induces adult T cell leukemia/lymphoma (ATLL). The mechanism of HTLV-I oncogenesis in T cells remains partly elusive. In vitro, HTLV-I induces ligand-independent transformation of human CD4+ T cells, an event that correlates with acquisition of constitutive phosphorylation of Janus kinases (JAK) and signal transducers and activators of transcription (STAT) proteins. However, it is unclear whether the in vitro model of HTLV-I transformation has relevance to viral leukemogenesis in vivo. Here we tested the status of JAK/STAT phosphorylation and DNA-binding activity of STAT proteins in cell extracts of uncultured leukemic cells from 12 patients with ATLL by either DNA-binding assays, using DNA oligonucleotides specific for STAT-1 and STAT-3, STAT-5 and STAT-6 or, more directly, by immunoprecipitation and immunoblotting with anti-phosphotyrosine antibody for JAK and STAT proteins. Leukemic cells from 8 of 12 patients studied displayed constitutive DNA-binding activity of one or more STAT proteins, and the constitutive activation of the JAK/STAT pathway was found to persist over time in the 2 patients followed longitudinally. Furthermore, an association between JAK3 and STAT-1, STAT-3, and STAT-5 activation and cell-cycle progression was demonstrated by both propidium iodide staining and bromodeoxyuridine incorporation in cells of four patients tested. These results imply that JAK/STAT activation is associated with replication of leukemic cells and that therapeutic approaches aimed at JAK/STAT inhibition may be considered to halt neoplastic growth.

Interleukin-2 (IL-2)-mediated induction of the IL-2 receptor alpha chain gene. Critical role of two functionally redundant tyrosine residues in the IL-2 receptor beta chain cytoplasmic domain and suggestion that these residues mediate more than Stat5 activation

The interleukin-2 receptor alpha chain (IL-2Ralpha) is potently induced by antigens, mitogens, and certain cytokines that include IL-2 itself. This induction leads to the formation of high affinity IL-2 receptors when IL-2Ralpha is co-expressed with the beta (IL-2Rbeta) and gamma (gammac) chains of this receptor. We investigated the signaling pathways mediating IL-2-induced IL-2Ralpha mRNA expression using 32D myeloid progenitor cells stably transfected with either wild type IL-2Rbeta or mutants of IL-2Rbeta containing tyrosine to phenylalanine substitutions. Of the six cytoplasmic tyrosines in IL-2Rbeta, we have found that only the two tyrosines that mediate Stat5 activation (Tyr-392 and Tyr-510) contribute to IL-2-induced IL-2Ralpha gene expression and that either tyrosine alone is sufficient for this process. Interestingly, the IL-7 receptor contains a tyrosine (Tyr-429)-based sequence resembling the motifs encompassing Tyr-392 and Tyr-510 of IL-2Rbeta. Further paralleling the IL-2 system, IL-7 could activate Stat5 and drive expression of IL-2Ralpha mRNA in 32D cells transfected with the human IL-7R. However, IL-3 could not induce IL-2Ralpha mRNA in 32D cells, despite its ability to activate Stat5 via the endogenous IL-3 receptor. Moreover, the combination of IL-3 and IL-2 could not "rescue" IL-2Ralpha mRNA expression in cells containing an IL-2Rbeta mutant with phenylalanine substitutions at Tyr-392 and Tyr-510. These data suggest that Tyr-392 and Tyr-510 couple to an additional signaling pathway beyond STAT protein activation in IL-2-mediated induction of the IL-2Ralpha gene.

Distinct tumorigenic potential of abl and raf in B cell neoplasia: abl activates the IL-6 signaling pathway

The development of murine plasma cell tumors induced by raf/myc containing retroviruses is facilitated by T cells and completely dependent on IL-6. To determine whether kinases with differing specificities reflect alternative biochemical pathways in B cell tumorigenesis, we have employed an abl/myc containing retrovirus to assess neoplastic development. In contrast with raf/myc, abl/myc disease is T cell and IL-6 independent. An examination of the IL-6 signal transduction pathway reveals that this pathway, as defined by activation of Stat3, is inducible by IL-6 in raf/myc tumors but constitutively activated in abl/myc tumors. These findings provide a mechanism for the derivation of cytokine-independent plasma cell tumors and suggest that both IL-6-dependent and independent tumors may arise in vivo depending on the particular mutational events incurred during tumorigenesis.

Different interleukin 2 receptor beta-chain tyrosines couple to at least two signaling pathways and synergistically mediate interleukin 2-induced proliferation

One of the earliest events induced by interleukin 2 (IL-2) is tyrosine phosphorylation of cellular proteins, including the IL-2 receptor beta chain (IL-2Rbeta). Simultaneous mutation of three tyrosines (Y338, Y392, and Y510) in the IL-2Rbeta cytoplasmic domain abrogated IL-2-induced proliferation, whereas mutation of only Y338 or of Y392 and Y510 inhibited proliferation only partially. While Y392 and Y510 were critical for IL-2-induced activation of signal transducers and activators of transcription (STAT proteins), Y338 was required for Shc-IL-2Rbeta association and for IL-2-induced tyrosine phosphorylation of Shc. Thus, activation of both Jak-STAT and Shc-coupled signaling pathways requires specific IL-2Rbeta tyrosines that together act in concert to mediate maximal proliferation. In COS-7 cells, overexpression of Jak1 augmented phosphorylation of Y338 as well as Y392 and Y510, suggesting that the role for this Jak kinase may extend beyond the Jak-STAT pathway.

Mutation of Jak3 in a patient with SCID: essential role of Jak3 in lymphoid development

Males with X-linked severe combined immunodeficiency (XSCID) have defects in the common cytokine receptor gamma chain (gamma c) gene that encodes a shared, essential component of the receptors of interleukin-2 (IL-2), IL-4, IL-7, IL-9, and IL-15. The Janus family tyrosine kinase Jak3 is the only signaling molecule known to be associated with gamma c, so it was hypothesized that defects in Jak3 might cause an XSCID-like phenotype. A girl with immunological features indistinguishable from those of XSCID was therefore selected for analysis. An Epstein-Barr virus (EBV)-transformed cell line derived from her lymphocytes had normal gamma c expression but lacked Jak3 protein and had greatly diminished Jak3 messenger RNA. Sequencing revealed a different mutation on each allele: a single nucleotide insertion resulting in a frame shift and premature termination in the Jak3 JH4 domain and a nonsense mutation in the Jak3 JH2 domain. The lack of Jak3 expression correlated with impaired B cell signaling, as demonstrated by the inability of IL-4 to activate Stat6 in the EBV-transformed cell line from the patient. These observations indicate that the functions of gamma c are dependent on Jak3 and that Jak3 is essential for lymphoid development and signaling.

Constitutively activated Jak-STAT pathway in T cells transformed with HTLV-I

Human T cell lymphotropic virus I (HTLV-I) is the etiological agent for adult T cell leukemia and tropical spastic paraparesis (also termed HTLV-I-associated myelopathy). HTLV-I-infected peripheral blood T cells exhibit an initial phase of interleukin-2 (IL-2)-dependent growth; over time, by an unknown mechanism, the cells become IL-2-independent. Whereas the Jak kinases Jak1 and Jak3 and the signal transducer and activator of transcription proteins Stat3 and Stat5 are activated in normal T cells in response to IL-2, this signaling pathway was constitutively activated in HTLV-I-transformed cells. In HTLV-I-infected cord blood lymphocytes, the transition from IL-2-dependent to IL-2-independent growth correlated with the acquisition of a constitutively activated Jak-STAT pathway, which suggests that this pathway participates in HTLV-I-mediated T cell transformation.

The role of shared receptor motifs and common Stat proteins in the generation of cytokine pleiotropy and redundancy by IL-2, IL-4, IL-7, IL-13, and IL-15

To understand the molecular bases for cytokine redundancy and pleiotropy, we have compared the Stat proteins activated in peripheral blood lymphocytes (PBLs) by cytokines with shared and distinct actions. Interleukin-2 (IL-2) rapidly activated Stat5 in fresh PBL, and Stat3 and Stat5 in preactivated PBL. IL-7 and IL-15 induced the same complexes as IL-2, a feature explained by the existence of similar tyrosine-phosphorylated motifs in the cytoplasmic domains of IL-2R beta and IL-7R that can serve as docking sites for Stat proteins. IL-13 Induced the same complexes as IL-4, a finding explained by our studies implicating IL-4R as a shared component of the receptors. These studies demonstrate that a single cytokine can activate different combinations of Stat proteins under different physiological conditions, and also indicate two mechanisms by which distinct cytokines can activate the same Stat protein.

An integration-defective U5 deletion mutant of human immunodeficiency virus type 1 reverts by eliminating additional long terminal repeat sequences

Nonoverlapping deletions that eliminated the 5' (HIV-1US/603del), middle (HIV-1U5/206del), and 3' (HIV-1U5/604del) thirds of the U5 region of the human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR) were studied for their effects on virus replication (transient transfection of HeLa cells) and infectivity (T-cell lines and peripheral blood mononuclear cells). All three mutants exhibited a wild-type phenotype in directing the production and release of virus particles from transfected HeLa cells. In infectivity assays, HIV-1U5/206del was usually indistinguishable from wild-type virus whereas HIV-1U%/603del was unable to infect human peripheral blood mononuclear cells or MT4 and CEM cells. Investigations of HIV-1U5/603del particles revealed a packaging defect resulting in a 10-fold reduction of encapsidated genomic RNA. The HIV-1U5/604del mutant either was noninfectious or exhibited delayed infection kinetics, depending on the cell type and multiplicity of infection. Quantitative competitive PCR indicated that HIV-1U5/604del synthesized normal amounts of viral DNA in newly infected cells. During the course of a long-term infectivity assay, a revertant of the HIV-1U5/604del mutant that displayed rapid infection kinetics emerged. Nucleotide sequence analysis indicated that the original 26-nucleotide deletion present in HIV-1U5/604del had been extended an additional 19 nucleotides in the revertant virus. Characterization of the HIV-1U5/604del mutant LTR in in vitro integration reactions revealed defective 3' processing and strand transfer activities that were partially restored when the revertant LTR substrate was used, suggesting that the reversion corrected a similar defect in the mutant virus.