HIV-1 Vpr enhances viral burden by facilitating infection of tissue macrophages but not nondividing CD4+ T cells

Prior experiments in explants of human lymphoid tissue have demonstrated that human immunodeficiency virus type 1 (HIV-1) productively infects diverse cellular targets including T cells and tissue macrophages. We sought to determine the specific contribution of macrophages and T cells to the overall viral burden within lymphoid tissue. To block infection of macrophages selectively while preserving infection of T cells, we used viruses deficient for viral protein R (Vpr) that exhibit profound replication defects in nondividing cells in vitro. We inoculated tonsil histocultures with matched pairs of congenic viruses that differed only by the presence of a wild-type or truncated vpr gene. Although these viruses exhibited no reduction in the infection or depletion of T cells, the ability of the Vpr-deficient R5 virus to infect tissue macrophages was severely impaired compared with matched wild-type R5 virus. Interestingly, the Vpr-deficient R5 virus also exhibited a 50% reduction in overall virus replication compared with its wild-type counterpart despite the fact that macrophages represent a small fraction of the potential targets of HIV-1 infection in these tissues. Collectively, these data highlight the importance of tissue macrophages in local viral burden and further implicate roles for CC chemokine receptor 5, macrophages, and Vpr in the life cycle and pathogenesis of HIV-1.

HIV-1 actively replicates in naive CD4(+) T cells residing within human lymphoid tissues

Although HIV-1 gene expression is detected in naive, resting T cells in vivo, such cells are resistant to productive infection in vitro. However, we found that the endogenous microenvironment of human lymphoid tissues supports de novo infection and depletion of this population. Cell cycle analysis and DNA labeling experiments established that these cells were definitively quiescent and thus infected de novo. Quantitation of the "burst size" within naive cells further demonstrated that these cells were productively infected and contributed to the local viral burden. These findings demonstrate that lymphoid tissues support active HIV-1 replication in resting, naive T cells. Moreover, these cells are not solely reservoirs of latent virus but are permissive hosts for viral replication that likely targets them for elimination.

Cytopathicity of human immunodeficiency virus type 1 primary isolates depends on coreceptor usage and not patient disease status

It has been hypothesized that human immunodeficiency virus type 1 (HIV-1) evolves toward increased cytopathicity in conjunction with disease progression in infected patients. A viral property known to evolve in some but not all patients is coreceptor utilization, and it has been shown that a switch in coreceptor utilization is sufficient for the development of increased cytopathicity. To test the hypothesis that the evolution of other viral properties also contributes to accelerating cytopathicity in vivo, we used human lymphoid tissue explants to assay the cytopathicity of a panel of primary HIV-1 isolates derived from various stages of disease characterized by the presence or absence of changes in coreceptor preference. We found no evidence of coreceptor-independent increases in cytopathicity in isolates obtained either before coreceptor preference changes or from patients who progressed to AIDS despite an absence of coreceptor evolution. Instead, the cytopathicity of all HIV-1 isolates was determined solely by their coreceptor utilization. These results argue that HIV-1 does not evolve toward increased cytopathicity independently of changes in coreceptor utilization.

Susceptibility of rat-derived cells to replication by human immunodeficiency virus type 1

Progress in developing a small animal model of human immunodeficiency virus type 1 (HIV-1) disease would greatly facilitate studies of transmission, pathogenesis, host immune responses, and antiviral strategies. In this study, we have explored the potential of rats as a susceptible host. In a single replication cycle, rat cell lines Rat2 and Nb2 produced infectious virus at levels 10- to 60-fold lower than those produced by human cells. Rat-derived cells supported substantial levels of early HIV-1 gene expression, which was further enhanced by overexpression of human cyclin T1. Rat cells displayed quantitative, qualitative, and cell-type-specific limitations in the late phase of the HIV-1 replication cycle including relative expression levels of HIV-1 Gag proteins, intracellular Gag processing, and viral egress. Nb2 cells were rendered permissive to HIV-1 R5 viruses by coexpression of human CD4 and CCR5, indicating that the major restriction on HIV-1 replication was at the level of cellular entry. We also found that primary rat lymphocytes, macrophages, and microglia expressed considerable levels of early HIV-1 gene products following infection with pseudotyped HIV-1. Importantly, primary rat macrophages and microglia, but not lymphocytes, also expressed substantial levels of HIV-1 p24 CA and produced infectious virions. Collectively, these results identify the rat as a promising candidate for a transgenic small animal model of HIV-1 infection and highlight pertinent cell-type-specific restrictions that are features of this species.

Folate receptor-alpha is a cofactor for cellular entry by Marburg and Ebola viruses

Human infections by Marburg (MBG) and Ebola (EBO) viruses result in lethal hemorrhagic fever. To identify cellular entry factors employed by MBG virus, noninfectible cells transduced with an expression library were challenged with a selectable pseudotype virus packaged by MBG glycoproteins (GP). A cDNA encoding the folate receptor-alpha (FR-alpha) was recovered from cells exhibiting reconstitution of viral entry. A FR-alpha cDNA was recovered in a similar strategy employing EBO pseudotypes. FR-alpha expression in Jurkat cells facilitated MBG or EBO entry, and FR-blocking reagents inhibited infection by MBG or EBO. Finally, FR-alpha bound cells expressing MBG or EBO GP and mediated syncytia formation triggered by MBG GP. Thus, FR-alpha is a significant cofactor for cellular entry for MBG and EBO viruses.

Human immunodeficiency virus type 1 clade A and D neurotropism: molecular evolution, recombination, and coreceptor use

Human immunodeficiency virus type 1 (HIV-1) non-B clade viral infections of the brain have not been studied to date. Among nine AIDS patients from Nairobi, Kenya, infected with HIV-1 A (N = 5) or D (N = 4) clade strains, brain-derived HIV-1 env sequences displayed greater evolutionary distance than B clade brain-derived viruses (P < 0.001). Similarly, molecular diversity between matched brain and spleen env clones was clade-dependent and concentrated in the hypervariable V4 region (P < 0.001), with phylogenetic clustering of sequences derived from the same organ. Brain-derived A and D clade sequences displayed significantly lower ratios of nonsynonymous/synonymous substitution rates (d(N)/d(S)) compared to matched spleen-derived clones and brain-derived B clade viruses. Interclade recombination events were infrequently observed among the present env sequences. A chimeric virus containing the C2V3 region from an A clade brain-derived sequence preferentially used CD4 and CCR5 for infection. These findings demonstrate that differences in molecular diversity in brain-derived sequences were dependent on the individual clade and domain within the env gene, but both B and non-B clade brain-derived viruses exhibit a preference for CCR5 as a coreceptor.

Primary and recombinant HIV type 1 strains resistant to protease inhibitors are pathogenic in mature human lymphoid tissues

Preserved peripheral CD4+ T cell counts despite virologic failure in patients undergoing protease inhibitor (PI)-containing antiviral regimens are a frequent occurrence in human immunodeficiency virus (HIV) disease. One hypothesis to explain the relative sparing of CD4+ T cells is that HIV strains exhibiting PI resistance concomitantly are attenuated in terms of cytopathicity for mature T cells. To test this hypothesis, we used a three-dimensional human tonsil histoculture microenvironment to assess the pathogenic potential of a panel of primary and recombinant HIV-1 strains derived from patients experiencing PI failure. All the viruses tested replicated efficiently in these cultures and, in some cases, better than comparable wild-type viral isolates. Furthermore, the PI-resistant strains depleted CD4+ T cells potently and comparably with wild-type isolates in these ex vivo lymphoid tissues. These results demonstrate that PI-resistant viruses are not inherently less pathogenic for mature T cells. Therefore, the sustained peripheral lymphocyte counts in patients with selective virologic failure may be due to specific defects in viral replication in other cell compartments or to an undefined host adaptation to viral infection during PI therapy.

Molecular function of the CD4 D1 domain in coreceptor-mediated entry by HIV type 1

The surface molecule CD4 plays a key role in initiating cellular entry by the human immunodeficiency virus type 1 (HIV-1), and it is now recognized as acting synergistically with select chemokine receptors (coreceptors) in the infection process. The present study was undertaken to determine whether the extracellular region of CD4 is sufficient to induce fusion of HIV-1 virions with target cells in the absence of its anchoring function. Using pseudotype reporter viruses to quantitate infection, soluble CD4 (sCD4) was tested for its ability to induce fusion by viruses utilizing CCR5 as their coreceptor. We found that sCD4 was competent to replace membrane-bound CD4 to trigger infection mediated by several HIV-1 envelopes. Furthermore, in a comparison of the envelopes of HIV-1 NL4-3 and a chimera containing the gp120 V3 loop of Ba-L, the V3 region was found to be one factor affecting susceptibility to induction by sCD4. In addition, using truncated and mutant derivatives of sCD4, the amino-terminal D1 domain of CD4 was found to be necessary and sufficient for induction of fusion and to require an intact gp120-binding site for this activity. These results delineate determinants on CD4 and gp120 required for fusion induction in collaboration with a coreceptor, and suggest a mechanism whereby CD4 may contribute to viral infection in trans.

Cytopathicity of human immunodeficiency virus type 2 (HIV-2) in human lymphoid tissue is coreceptor dependent and comparable to that of HIV-1

Epidemiological studies have shown that human immunodeficiency virus type 2 (HIV-2) is markedly less pathogenic than HIV-1 in vivo. Individuals infected with HIV-2 exhibit a remarkably slow rate of disease development, and these clinical properties have been attributed presumptively to an "attenuated" phenotype of HIV-2 itself. Here, we investigated the impact of coreceptor usage on the cytopathicity of HIV-2 and compared its pathogenic potential with that of HIV-1 in a unique human lymphoid histoculture model. We found that HIV-2 strains, as well as closely related simian immunodeficiency viruses (SIV), displayed mildly or highly aggressive cytopathic phenotypes depending on their abilities to use the coreceptor CCR5 or CXCR4, respectively. A side-by-side comparison of primary X4 HIV-1 and HIV-2 strains revealed similar, high degrees of cytopathicity induced by both HIV types. Furthermore, we found that HIV-2 coreceptor specificity for CCR5 and CXCR4 determined the target cell population for T-cell depletion in lymphoid tissue. Finally, utilization of the alternate coreceptors BOB and Bonzo did not significantly increase the cytopathic properties of HIV-2. These findings demonstrate that coreceptor preference is a key regulator of target cell specificity and the cytopathic potential of HIV-2, with indistinguishable rules compared with HIV-1. Moreover, HIV-2 strains are not characterized by an intrinsically lower cytopathicity than HIV-1 strains. Therefore, direct cytopathic potential per se does not explain the unique behavior of HIV-2 in people, highlighting that other unknown factors need to be elucidated as the basis for their lesser virulence in vivo.

Human immunodeficiency virus type 1 coreceptor preferences determine target T-cell depletion and cellular tropism in human lymphoid tissue

The present study sought to determine how usage of coreceptors by human immunodeficiency virus type 1 dictates cell tropism and depletion of CD4(+) T cells in human lymphoid tissues cultured ex vivo. We found that coreceptor preferences control the marked, preferential depletion of coreceptor-expressing CD4(+) lymphocytes. In addition, there was a strong, but not absolute, preference shown by CXCR4-using strains for lymphocytes and by CCR5-using strains for macrophages.

Distinct mechanisms of entry by envelope glycoproteins of Marburg and Ebola (Zaire) viruses

Since the Marburg (MBG) and Ebola (EBO) viruses have sequence homology and cause similar diseases, we hypothesized that they associate with target cells by similar mechanisms. Pseudotype viruses prepared with a luciferase-containing human immunodeficiency virus type 1 backbone and packaged by the MBG virus or the Zaire subtype EBO virus glycoproteins (GP) mediated infection of a comparable wide range of mammalian cell types, and both were inhibited by ammonium chloride. In contrast, they exhibited differential sensitivities to treatment of target cells with tunicamycin, endoglycosidase H, or protease (pronase). Therefore, while they exhibit certain functional similarities, the MBG and EBO virus GP interact with target cells by distinct processes.

Viral entry through CXCR4 is a pathogenic factor and therapeutic target in human immunodeficiency virus type 1 disease

The chemokine receptors CCR5 and CXCR4 function as the principal coreceptors for human immunodeficiency virus type 1 (HIV-1). Coreceptor function has also been demonstrated for a variety of related receptors in vitro. The relative contributions of CCR5, CXCR4, and other putative coreceptors to HIV-1 disease in vivo have yet to be defined. In this study, we used sequential primary isolates and recombinant strains of HIV-1 to demonstrate that CXCR4-using (X4) viruses emerging in association with disease progression are highly pathogenic in ex vivo lymphoid tissues compared to CXCR4-independent viruses. Furthermore, synthetic receptor antagonists that specifically block CXCR4-mediated entry dramatically suppressed the depletion of CD4(+) T cells by recombinant and clinically derived X4 HIV-1 isolates. Moreover, in vitro specificity for the additional coreceptors CCR3, CCR8, BOB, and Bonzo did not augment cytopathicity or diminish sensitivity toward CXCR4 antagonists in lymphoid tissues. These data provide strong evidence to support the concept that adaptation to CXCR4 specificity in vivo accelerates HIV-1 disease progression. Thus, therapeutic intervention targeting the interaction of HIV-1 gp120 with CXCR4 may be highly valuable for suppressing the pathogenic effects of late-stage viruses.

Genetic evidence for an additional factor required for erythropoietin-induced signal transduction

Erythropoietin (EPO) and its receptor (EPOR) are required for the development of mature erythrocytes. After binding of ligand, the EPOR activates a variety of signaling pathways that ultimately control cellular proliferation, survival, and specific gene expression. Although erythroid progenitors appear to be the principal EPO-responsive cell type in vivo due to the restricted expression of the EPOR, many growth factor-dependent cell lines expressing the EPOR can respond to EPO by activating many or all of these pathways. In the present study, we have identified a cellular context (the interleukin-2 [IL-2]-dependent HT-2 line) in which the EPO stimulation of the EPOR fails to support cellular proliferation, STAT-5 induction, or MAPK activation, despite efficient phosphorylation of the EPOR and JAK2 and inhibition of apoptosis after withdrawal of IL-2. Interestingly, when we fused HT-2 cells expressing the EPOR with Ba/F3 cells in a complementation assay, the resulting hybridomas proliferated and potently activated STAT-5 and MAPK in response to EPO. These data indicate that an unidentified cellular factor is needed to mediate signaling by the EPOR. Moreover, Ba/F3 cells apparently express this factor(s) and somatic fusions can, therefore, confer EPO-responsiveness to HT-2 cells that lack this factor.

A trans-receptor mechanism for infection of CD4-negative cells by human immunodeficiency virus type 1

Chemokine receptors, particularly CCR5 and CXCR4, act as essential coreceptors in concert with CD4 for cellular entry by human immunodeficiency virus type 1 (HIV-1; reviewed in [1]). But infection of CD4(-) cells has also been encountered in various tissues in vivo, including astrocytes, neurons and microvascular endothelial cells of the brain [2] [3] [4] [5] [6], epithelial cells [5] [7], CD4(-) lymphocytes and thymocytes [8] [9], and cardiomyocytes [10]. Here, we present evidence for the infection of CD4(-) cell lines bearing coreceptors by well-known HIV-1 strains when co-cultured with CD4(+) cells. This process requires contact between the coreceptor-bearing and CD4(+) cells and supports the full viral replication cycle within the coreceptor-bearing target cell. Furthermore, CD4 provided in trans facilitates infection of primary human cells, such as brain-derived astrocytes. Although the pathobiological significance of infection of CD4(-) cells in vivo remains to be elucidated, this trans-receptor mechanism may facilitate generation of hidden reservoirs of latent virus that confound antiviral therapies and that contribute to specific AIDS-associated clinical syndromes.

Shared and unique determinants of the erythropoietin (EPO) receptor are important for binding EPO and EPO mimetic peptide

We have shown previously that Phe93 in the extracellular domain of the erythropoietin (EPO) receptor (EPOR) is crucial for binding EPO. Substitution of Phe93 with alanine resulted in a dramatic decrease in EPO binding to the Escherichia coli-expressed extracellular domain of the EPOR (EPO-binding protein or EBP) and no detectable binding to full-length mutant receptor expressed in COS cells. Remarkably, Phe93 forms extensive contacts with a peptide ligand in the crystal structure of the EBP bound to an EPO-mimetic peptide (EMP1), suggesting that Phe93 is also important for EMP1 binding. We used alanine substitution of EBP residues that contact EMP1 in the crystal structure to investigate the function of these residues in both EMP1 and EPO binding. The three largest hydrophobic contacts at Phe93, Met150, and Phe205 and a hydrogen bonding interaction at Thr151 were examined. Our results indicate that Phe93 and Phe205 are important for both EPO and EMP1 binding, Met150 is not important for EPO binding but is critical for EMP1 binding, and Thr151 is not important for binding either ligand. Thus, Phe93 and Phe205 are important binding determinants for both EPO and EMP1, even though these ligands share no sequence or structural homology, suggesting that these residues may represent a minimum epitope on the EPOR for productive ligand binding.

V3 recombinants indicate a central role for CCR5 as a coreceptor in tissue infection by human immunodeficiency virus type 1

Binding of the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein gp120 to both CD4 and one of several chemokine receptors (coreceptors) permits entry of virus into target cells. Infection of tissues may establish latent viral reservoirs as well as cause direct pathologic effects that manifest as clinical disease such as HIV-associated dementia. We sought to identify the critical coreceptors recognized by HIV-1 tissue-derived strains as well as to correlate these coreceptor preferences with site of infection and dementia diagnosis. To reconstitute coreceptor use, we cloned HIV-1 envelope V3 sequences encoding the primary determinants of coreceptor specificity from 13 brain-derived and 6 colon-derived viruses into an isogenic (NL4-3) viral background. All V3 recombinants utilized the chemokine receptor CCR5 uniformly and efficiently as a coreceptor but not CXCR4, BOB/GPR15, or Bonzo/STRL33. Other receptors such as CCR3, CCR8, and US28 were inefficiently and variably used as coreceptors by various envelopes. CCR5 without CD4 present did not allow for detectable infection by any of the tested recombinants. In contrast to the pathogenic switch in coreceptor specificity frequently observed in comparisons of blood-derived viruses early after HIV-1 seroconversion and after onset of AIDS, the characteristics of these V3 recombinants suggest that CCR5 is a primary coreceptor for brain- and colon-derived viruses regardless of tissue source or diagnosis of dementia. Therefore, tissue infection may not depend significantly on viral envelope quasispeciation to broaden coreceptor range but rather selects for CCR5 use throughout disease progression.

CXCR4 utilization is sufficient to trigger CD4+ T cell depletion in HIV-1-infected human lymphoid tissue

The human chemokine receptors CCR5 and CXCR4 have emerged as the predominant cofactors, along with CD4, for cellular entry of HIV-1 in vivo whereas the contribution of other chemokine receptors to HIV disease has not been yet determined. CCR5-specific (R5) viruses predominate during primary HIV-1 infection whereas viruses with specificity for CXCR4 (R5/X4 or X4 viruses) often emerge in late stages of HIV disease. The evolution of X4 viruses is associated with a rapid decline in CD4+ T cells, although a causative relationship between viral tropism and CD4+ T cell depletion has not yet been proven. To rigorously test this relationship, we assessed CD4+ T cell depletion in suspensions of human peripheral blood mononuclear cells and in explants of human lymphoid tissue on exposure to paired viruses that are genetically identical (isogenic) except for select envelope determinants specifying reciprocal tropism for CXCR4 or CCR5. In both systems, X4 HIV-1 massively depleted CD4+ lymphocytes whereas matched R5 viruses depleted such cells only mildly despite comparable viral replication kinetics. These findings demonstrate that the coreceptor specificities of HIV-1 are a causal factor in CD4+ T cell depletion ex vivo and strongly support the hypothesis that the evolution of viral envelope leading to usage of CXCR4 in vivo accelerates loss of CD4+ T cells, causing immunodeficiency.

An antagonist peptide-EPO receptor complex suggests that receptor dimerization is not sufficient for activation

Dimerization of the erythropoietin (EPO) receptor (EPOR), in the presence of either natural (EPO) or synthetic (EPO-mimetic peptides, EMPs) ligands is the principal extracellular event that leads to receptor activation. The crystal structure of the extracellular domain of EPOR bound to an inactive (antagonist) peptide at 2.7 A resolution has unexpectedly revealed that dimerization still occurs, but the orientation between receptor molecules is altered relative to active (agonist) peptide complexes. Comparison of the biological properties of agonist and antagonist EMPs with EPO suggests that the extracellular domain orientation is tightly coupled to the cytoplasmic signaling events and, hence, provides valuable new insights into the design of synthetic ligands for EPOR and other cytokine receptors.

Parallel evolution of CCR5-null phenotypes in humans and in a natural host of simian immunodeficiency virus

The C-C chemokine receptor CCR5 in humans and rhesus macaques (Macaca mulatta) serves as the primary coreceptor for cellular entry by macrophagetropic strains of human immunodeficiency virus type 1 (HIV-1) and all reported strains of simian immunodeficiency virus (SIV) [1-6]. Humans homozygous for a 32 bp deletion allele of CCR5, resulting in a null phenotype, are highly resistant to infection by HIV-1 [7-9], prompting development of therapies and vaccines targeting CCR5. We now report a novel deletion allele of CCR5, with an allele frequency of 0.04, in sooty mangabey monkeys (Cercocebus torquatus atys), a natural host of SIV (SIVsmm) [10]. The mutant protein was not expressed at the cell surface and accordingly did not function as a viral coreceptor. Primary activated lymphocytes from mangabeys heterozygous for the deletion allele expressed significantly less CCR5 on the cell surface. Moreover, SIV seroprevalence and viremia were comparable among CCR5 heterozygotes and wild-type animals. Parallel evolution of CCR5-null alleles in humans and sooty mangabeys suggests that similar negative selection pressures have acted against CCR5, as would occur during epidemics of infectious agents that require CCR5 for pathogenesis. Sooty mangabeys bred to homozygosity for the deletion allele will be useful for experimental studies on the context-dependent role of CCR5 in host defense and microbial pathogenesis.

Rabbit cells expressing human CD4 and human CCR5 are highly permissive for human immunodeficiency virus type 1 infection

To evaluate the feasibility of using transgenic rabbits expressing CCR5 and CD4 as a small-animal model of human immunodeficiency virus type 1 (HIV) disease, we examined whether the expression of the human chemokine receptor (CCR5) and human CD4 would render a rabbit cell line (SIRC) permissive to HIV replication. Histologically, SIRC cells expressing CD4 and CCR5 formed multinucleated cells (syncytia) upon exposure to BaL, a macrophagetropic strain of HIV that uses CCR5 for cell entry. Intracellular viral capsid p24 staining showed abundant viral gene expression in BaL-infected SIRC cells expressing CD4 and CCR5. In contrast, neither SIRC cells expressing CD4 alone nor murine 3T3 cells expressing CCR5 and CD4 exhibited significant expression of p24. These stably transfected rabbit cells were also highly permissive for the production of virions upon infection by two other CCR5-dependent strains (JR-CSF and YU-2) but not by a CXCR4-dependent strain (NL4-3). The functional integrity of these virions was demonstrated by the successful infection of human peripheral blood mononuclear cells (PBMC) with viral stocks prepared from these transfected rabbit cells. Furthermore, primary rabbit PBMC were found to be permissive for production of infectious virions after circumventing the cellular entry step. These results suggest that a transgenic rabbit model for the study of HIV disease may be feasible.

Absence of cytokine receptor-dependent specificity in red blood cell differentiation in vivo

Erythropoietin (EPO) is required for red blood cell development, but whether EPO-specific signals directly instruct erythroid differentiation is unknown. We used a dominant system in which constitutively active variants of the EPO receptor were introduced into erythroid progenitors in mice. Chimeric receptors were constructed by replacing the cytoplasmic tail of constitutively active variants of the EPO receptor with tails of diverse cytokine receptors. Receptors linked to granulocyte or platelet production supported complete erythroid development in vitro and in vivo, as did the growth hormone receptor, a nonhematopoietic receptor. Therefore, EPOR-specific signals are not required for terminal differentiation of erythrocytes. Furthermore, we found that cellular context can influence cytokine receptor signaling.

JAK/STAT signaling by cytokine receptors

The JAK/STAT pathway is recognized as one of the major mechanisms by which cytokine receptors transduce intracellular signals. This system is regulated at multiple levels, including JAK activation, nuclear trafficking of STAT factors, and negative feedback loops. Gene deletion studies have implicated selected STAT factors as predominant mediators for a limited number of lymphokines. This signaling pathway influences normal cell survival and growth mechanisms and may contribute to oncogenic transformation.

Heteromerization of the gammac chain with the interleukin-9 receptor alpha subunit leads to STAT activation and prevention of apoptosis

Interleukin-9 (IL-9) is a cytokine with pleiotropic effects on mast cell and T cell lines. It exerts its effects through the IL-9R complex consisting of IL-9Ralpha and the common gammac subunit. Here we report functional evidence for receptor heteromerization for efficient signal transduction, and we define minimal requirements in the two receptor subunits for IL-9R function. Tyrosine 336 of the IL-9Ralpha and the membrane-proximal segment of gammac are both crucial for signaling. The activated IL-9R complex employs the Janus kinases JAK1 and JAK3 for subsequent activation of the signal transducer and activator transcription (STAT) factors STAT-1, STAT-3, and STAT-5. This process is independent of Tyk2. We demonstrate further that the activated STAT complexes consist of STAT-1 and STAT-5 homodimers and STAT-1-STAT-3 heterodimers. Finally, we show that IL-9R signaling in a T cell line does not result in detectable mitogen-activated protein kinase activation and leads to unsustained proliferation. Nonetheless, these T cells are efficiently protected from dexamethasone-induced apoptosis. These results further define the molecular architecture of the IL-9R and its specific connections to various biologic responses.

Redundant and selective roles for erythropoietin receptor tyrosines in erythropoiesis in vivo

Cytokine receptors have been shown in cell culture systems to use phosphotyrosine residues as docking sites for certain signal transduction intermediates. Studies using various cellular backgrounds have yielded conflicting information about the importance of such residues. The present studies were undertaken to determine whether or not tyrosine residues within the erythropoietin receptor (EPOR) are essential for biologic activity during hematopoiesis in vivo. A variant of the EPOR was constructed that contains both a substitution (R129C) causing constitutive receptor activation as well as replacement of all eight cytoplasmic tyrosines by phenylalanines (cEPORYF). A comparison between animals exposed to recombinant retroviruses expressing cEPOR and cEPORYF showed that efficient red blood cell (RBC) development in vivo is dependent on the pressence of tyrosine residues in the cytoplasmic domain of the EPOR. In addition, an inefficient EPOR tyrosine independent pathway supporting RBC development was detected. Tyrosine add-back mutants showed that multiple individual tyrosines have the capacity to restore full erythropoietic potential to the EPOR as determined in whole animals. The analysis of primary erythroid progenitors transduced with the various cEPOR tyrosine mutants and tyrosine add-backs showed that only tyrosine 343 (Y1) and tyrosine 479 (Y8) were capable of supporting immature burst-forming unit-erythroid progenitor development. Thus, this receptor is characterized by striking functional redundancy of tyrosines in a biologically relevant context. However, selective tyrosine residues may be uniquely important for early signals supporting erythroid development.

Janus kinases in interleukin-2-mediated signaling: JAK1 and JAK3 are differentially regulated by tyrosine phosphorylation

BACKGROUND

Cytokines mediate a variety of effector cell functions, including cellular proliferation, differentiation, and modulation of the immune response. Many cytokines activate receptor-associated Janus kinases (JAKs) that promote tyrosine phosphorylation of signal transducers and activators of transcription (STAT) factors. Although JAK activation has been correlated with phosphorylation, the role of this tyrosine phosphorylation in the regulation of JAK1 and JAK3 remains unclear. Furthermore, the relative roles of JAK1 and JAK3 in the activation of STAT5 by interleukin-2 (IL-2) remain poorly understood.

RESULTS

We targeted two conserved tyrosine residues within the activation loop of the JAK1 and JAK3 kinase domains for substitution with phenylalanines. In an overexpression system, the catalytic function of JAK1 strictly required the presence of the first of these tyrosines, Y1033. In contrast, JAK3 retained catalytic activity when either or both of these activation-loop tyrosines were mutated. Analysis of JAK1/3 chimeras demonstrated that JAK activity was also controlled by intramolecular interactions involving the amino-terminal domain of the JAK as well as by the inherent signaling properties of the kinase domain. Finally, we have reconstituted IL-2-dependent STAT5 induction in a cell line that lacks detectable expression of JAK1 and JAK3. Catalytically active versions of both JAK1 and JAK3 must be present for effective induction of STAT5.

CONCLUSIONS

JAK1 and JAK3 are differentially regulated by specific tyrosines within their respective activation loops. Additionally, the amino-terminal domain of JAK3 appears to contain regulatory sequences that modify the function of the kinase domain. Finally, both JAK1 and JAK3 must retain catalytic function for IL-2-induced STAT5 activation.

Selective employment of chemokine receptors as human immunodeficiency virus type 1 coreceptors determined by individual amino acids within the envelope V3 loop

The chemokine receptor CCR5 acts as an essential cofactor for cell entry by macrophage-tropic human immunodeficiency virus type 1 (HIV-1) strains, whereas CXCR4 acts as an essential cofactor for T-cell-line-adapted strains. We demonstrated that the specific amino acids in the V3 loop of the HIV-1 envelope protein that determine cellular tropism also regulate chemokine coreceptor preference for cell entry by the virus. Further, a strong correlation was found between HIV-1 strains classified as syncytium inducing in standard assays and those using CXCR4 as a coreceptor. These data support the hypothesis that progressive adaptation to additional coreceptors is a key molecular basis for HIV-1 phenotypic evolution in vivo.

A Kaposi's sarcoma-associated herpesvirus-encoded cytokine homolog (vIL-6) activates signaling through the shared gp130 receptor subunit

The present studies analyzed the biologic activity of a gene product (vIL-6) encoded by the recently discovered Kaposi's sarcoma-associated herpesvirus (KSHV) bearing 24.8% amino acid identity with human interleukin-6 (huIL-6). Based on this similarity, we hypothesized that this viral homolog might trigger the JAK/STAT pathway, which typically is engaged by IL-6 and other cytokines. Activation of receptor-associated Janus tyrosine kinases (JAKs) results in the subsequent phosphorylation of signal transducers and activators of transcription (STATs) leading to nuclear entry and transcriptional regulation of target genes. Treatment of HepG2 cells with culture medium containing recombinant KSHV-encoded vIL-6 led to rapid induction of JAK1 phosphorylation and a nuclear DNA-binding activity found to contain STAT1 and STAT3. An antibody to the IL-6 receptor (IL-6R) alpha subunit effectively neutralized the response to huIL-6 but failed to block STAT activation by vIL-6. In contrast, an antibody reactive with the gp130 subunit of IL-6R abrogated signaling of both responses. Moreover, a transfected cell line expressing human gp130 without IL-6Ralpha exhibited a robust response to vIL-6 but not to huIL-6. These results demonstrate that KSHV encodes a cytokine that activates specific JAK/STAT signaling via interactions with the gp130 signal transducing subunit independently of the IL-6Ralpha chain. This activity may have an impact on gp130-mediated signaling in response to native cytokines and thereby influence disease pathogenesis upon KSHV infection.

Molecular uncoupling of C-C chemokine receptor 5-induced chemotaxis and signal transduction from HIV-1 coreceptor activity

The C-C chemokine receptor 5 (CCR5) plays a crucial role in facilitating the entry of macrophage-tropic strains of the HIV-1 into cells, but the mechanism of this phenomenon is completely unknown. To explore the role of CCR5-derived signal transduction in viral entry, we introduced mutations into two cytoplasmic domains of CCR5 involved in receptor-mediated function. Truncation of the terminal carboxyl-tail to eight amino acids or mutation of the highly conserved aspartate-arginine-tyrosine, or DRY, sequence in the second cytoplasmic loop of CCR5 effectively blocked chemokine-dependent activation of classic second messengers, intracellular calcium fluxes, and the cellular response of chemotaxis. In contrast, none of the mutations altered the ability of CCR5 to act as an HIV-1 coreceptor. We conclude that the initiation of signal transduction, the prototypic function of G protein coupled receptors, is not required for CCR5 to act as a coreceptor for HIV-1 entry into cells.

Shared gamma(c) subunit within the human interleukin-7 receptor complex. A molecular basis for the pathogenesis of X-linked severe combined immunodeficiency

Genetic evidence suggests that mutations in the gamma(c) receptor subunit cause X-linked severe combined immunodeficiency (X-SCID). The gamma(c) subunit can be employed in receptor complexes for IL-2, -4, -7, -9, and -15, and the multiple signaling defects that would result from a defective gamma(c) chain in these receptors are proposed to cause the severe phenotype of X-SCID patients. Interestingly, gene disruption of either IL-7 or the IL-7 receptor (IL-7R) alpha subunit in mice leads to immunological defects that are similar to human X-SCID. These observations suggest the functional importance of gamma(c) in the IL-7R complex. In the present study, structure/function analyses of the IL-7R complex using a chimeric receptor system demonstrated that gamma(c) is indeed critical for IL-7R function. Nonetheless, only a limited portion of the cytoplasmic domain of gamma(c) is necessary for IL-7R signal transduction. Furthermore, replacement of the gamma(c) cytoplasmic domain by a severely truncated erythropoeitin receptor does not affect measured IL-7R signaling events. These findings support a model in which gamma(c) serves primarily to activate signal transduction by the IL-7R complex, while IL-7R alpha determines specific signaling events through its association with cytoplasmic signaling molecules. Finally, these studies are consistent with the hypothesis that the molecular pathogenesis of X-SCID is due primarily to gamma(c)-mediated defects in the IL-7/IL-7R system.

Multiple extracellular elements of CCR5 and HIV-1 entry: dissociation from response to chemokines

The human beta-chemokine receptor CCR5 is an important cofactor for entry of human immunodeficiency virus-type 1 (HIV-1). The murine form of CCR5, despite its 82 percent identity to the human form, was not functional as an HIV-1 coreceptor. HIV-1 entry function could be reconstituted by fusion of various individual elements derived from the extracellular region of human CCR5 onto murine CCR5. Analysis of chimeras containing elements from human CCR5 and human CCR2B suggested that a complex structure rather than single contact sites is responsible for facilitation of viral entry. Further, certain chimeras lacking the domains necessary to signal in response to their natural chemokine ligands retained vigorous HIV-1 coreceptor activity.

Interleukin-4-specific signal transduction events are driven by homotypic interactions of the interleukin-4 receptor alpha subunit

Interleukin-4 (IL-4) exerts its effects through a heterodimeric receptor complex (IL-4R), which contains the IL-4R(alpha) and gamma(c) subunits. IL-4R(alpha) also functions with other partner subunits in several receptor types, including the IL-13 receptor. To examine the roles of the individual subunits within IL-4R complexes, we employed a chimeric system that recapitulates native IL-4R function as verified by the activation of the kinases, JAK1 and JAK3, and induction of STAT-6. When a mutant gamma(c) subunit in which the four cytoplasmic tyrosines were converted to phenylalanine was paired with the cytoplasmic domain of the IL-4R(alpha) chain, specificity within the JAK-STAT pathway was not altered. Signaling events were examined further in cells expressing the IL-4R(alpha) chimera alone without the gamma(c) chimera. Ligand-induced homodimerization of these receptors activated the IL-4 signaling program despite the absence of gamma(c), including induction of JAK1 and STAT-6, phosphorylation of the insulin-related substrate 1 and cellular proliferation. Thus, homotypic interactions of the IL-4R(alpha) subunit are sufficient for the initiation and determination of IL-4-specific signaling events, and such interactions may be integral to signaling through IL-4R complexes.

Distinct tyrosine residues within the interleukin-2 receptor beta chain drive signal transduction specificity, redundancy, and diversity

To explore the basis for interleukin (IL)-2 receptor (IL-2R) signaling specificity, the roles of tyrosine-based sequences located within the cytoplasmic tails of the beta and gammac chains were examined in the murine helper T cell line HT-2. Activation of the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway, cellular proliferation, and the induction of various genes were monitored. All four of the cytoplasmic tyrosine residues as well as the distal portion of the gammac proved dispensable for the entire spectrum of IL-2R signaling responses studied. Conversely, select tyrosine residues within the beta chain were essential and differentially required for various signaling events. Specifically, activation of c-fos gene expression was found to occur exclusively through the most membrane proximal tyrosine, Tyr-338, whereas proliferation and the activation of STAT-5 were induced either through Tyr-338 or through the two C-terminal tyrosine residues, Tyr-392 and Tyr-510. These tyrosine residues mediated the induction of two different STAT-5 isoforms, which were found to form heterodimers upon receptor activation. In contrast to the tyrosine dependence of c-fos and STAT-5 induction, bcl-2 gene induction proceeded independently of all IL-2Rbeta tyrosine residues. Thus, the tyrosine-based modules present within the IL-2Rbeta cytoplasmic tail play a critical role in IL-2R signaling, mediating specificity, redundancy, and multifunctionality.

The alpha chain of the IL-2 receptor determines the species specificity of high-affinity IL-2 binding

Interleukin 2 (IL-2) mediated signalling results from ligand binding and subsequent heterodimerization of IL-2r beta and gamma c. The high-affinity IL-2 receptor (IL-2r) is a heterotrimer comprised of the IL-2r alpha, IL-2r beta and gamma c subunits. Whereas human IL-2 effectively binds to either human or murine lymphocytes, murine IL-2 binds with markedly higher affinity to murine receptor complexes than to human complexes. Using cell lines stably expressing heterotrimeric IL-2r that vary in the species origin of individual subunits, we have demonstrated that IL-2r alpha is primarily responsible for the species specificity of IL-2 binding. Studies of ligand binding to the low affinity receptor demonstrated that IL-2r alpha displays a similar species preference to the heterotrimeric complex. Moreover, differences in ligand binding are reflected in differences in proliferation. A cell line expressing human IL-2r alpha and IL-2r beta along with murine gamma c vigorously proliferated only in response to human IL-2 at low doses, while both human and murine IL-2 stimulated proliferation of a cell line containing murine IL-2r alpha (as well as human IL-2r beta and murine gamma c). Therefore, IL-2r alpha is the chain primarily responsible for the species specificity of ligand binding.

The molecular role of the common gamma c subunit in signal transduction reveals functional asymmetry within multimeric cytokine receptor complexes

The specific signal transduction function of the gamma c subunit in the interleukin (IL) 2, IL-4, IL-7, IL-9, and IL-15 receptor complexes remains undefined. The present structure-function analyses demonstrated that the entire cytoplasmic tail of gamma c could be functionally replaced in the IL-2 receptor (IL-2R) signaling complex by a severely truncated erythropoietin receptor cytoplasmic domain lacking tyrosine residues. Heterodimerization of IL-2R beta with either gamma c or the truncated erythropoietin receptor chain led to an array of specific signals normally derived from the native IL-2R despite the substitution of Janus kinase JAK2 for JAK3 in the receptor complex. These findings thus suggest a model in which the gamma c subunit serves as a common and generic "trigger" chain by providing a nonspecific Janus kinase for signaling program initiation, while signal specificity is determined by the unique "driver" subunit in each of the gamma c- containing receptor complexes. Furthermore, these results may have important functional implications for the asymmetric design of many cytokine receptor complexes and the evolutionary design of receptor subfamilies that share common trigger or driver subunits.

Analysis of interleukin-2-dependent signal transduction through the Shc/Grb2 adapter pathway. Interleukin-2-dependent mitogenesis does not require Shc phosphorylation or receptor association

The interleukin (IL)-2 receptor system has previously been shown to signal through the association and tyrosine phosphorylation of Shc. This study demonstrates that the IL-2 receptor beta (IL-2R beta) chain is the critical receptor component required to mediate this effect. The use of IL-2R beta chain deletion mutants transfected into a Ba/F3 murine cell model describes a requirement for the IL-2R beta "acid-rich" domain between amino acids 315 and 384 for Shc tyrosine phosphorylation and receptor association. COS cell co-transfection studies of IL-2R beta chain constructs containing point mutations of tyrosine to phenylalanine along with the tyrosine kinase Jak-1 and a hemagglutinin-tagged Shc revealed that the motif surrounding phosphorylated tyrosine 338 within the acid-rich domain of the IL-2R beta is a binding site for Shc. Deletion of this domain has previously been shown to abrogate the ability of IL-2 to activate Ras but does not affect IL-2-dependent mitogenesis in the presence of serum. Proliferation assays of Ba/F3 cells containing IL-2R beta chain deletion mutants in serum-free medium with or without insulin shows that deletion of the acid-rich domain does not affect IL-2-driven mitogenesis regardless of the culture conditions. This study thus defines the critical domain within the IL-2R beta chain required to mediate Shc binding and Shc tyrosine phosphorylation and further shows that Shc binding and phosphorylation are not required for IL-2-dependent mitogenesis. Neither serum nor insulin is required to supplement the loss of induction of the Shc adapter or Ras pathways, which therefore suggests a novel mechanism for mitogenic signal transduction mediated by this hematopoietin receptor.

Identification of a variable region within the cytoplasmic tail of the IL-2 receptor beta chain that is required for growth signal transduction

Interleukin-2 (IL-2) regulates numerous biological events, including T lymphocyte proliferation. Interleukin-2 receptor (IL-2R)-mediated signaling is triggered by ligand-induced heterodimerization of the IL-2R beta and gamma c subunits, which results in the activation of signaling intermediates that are associated with either IL-2R beta or gamma c. Previous mutagenesis studies of the IL-2R beta cytoplasmic tail demonstrated that the partially conserved box 1 and box 2 motifs and specific tyrosine residues are critical for growth signaling. By deletion and alanine scanning mutagenesis, another set of residues that are critical for IL-2R-mediated signaling has now been identified. These residues lie within the divergent 35-amino acid "spacer" region separating box 1 and box 2. The role of this receptor subregion in early phases of IL-2R signaling was evaluated using BA/F3 stable cell lines expressing three functionally impaired mutants from this region. All three cell lines displayed substantially diminished growth responsiveness to IL-2. Receptor-mediated STAT factor activation, IL-2R beta phosphorylation, and Janus kinase activation were also markedly impaired. These findings indicate that this variable spacer region, which we have termed the V-box, is essential for the initiation of IL-2R-mediated signal transduction.

Growth signal transduction by the human interleukin-2 receptor requires cytoplasmic tyrosines of the beta chain and non-tyrosine residues of the gamma c chain

To evaluate the possible role for receptor-based tyrosine phosphorylation in growth signaling induced by interleukin-2 (IL-2), a series of substitution tyrosine mutants of the IL-2 receptor beta and gamma c chains was prepared and analyzed. Concurrent mutation of all six of the cytoplasmic tyrosines present in the beta chain markedly inhibited IL-2-induced growth signaling in both pro-B and T cell lines. Growth signaling in a pro-B cell line was substantially reconstituted when either of the two distal tyrosines (Tyr-392, Tyr-510) was selectively restored in the tyrosine-negative beta mutant, whereas reconstitution of the proximal tyrosines (Tyr-338, Tyr-355, Tyr-358, Tyr-361) did not restore this signaling function. Furthermore, at least one of the two cytoplasmic tyrosines that is required for beta chain function was found to serve as a phosphate acceptor site upon induction with IL-2. Studies employing a chimeric receptor system revealed that tyrosine residues of the beta chain likewise were important for growth signaling in T cells. In contrast, although the gamma c subunits is a target for tyrosine phosphorylation in vivo, concurrent substitution of all four cytoplasmic tyrosines of this chain produced no significant effect on growth signaling by chimeric IL-2 receptors. However, deletion of either the Box 1, Box 2, or intervening (V-Box) regions of gamma c abrogated receptor function. Therefore, tyrosine residues of beta but not of gamma c appear to play a pivotal role in regulating growth signal transduction through the IL-2 receptor, either by influencing cytoplasmic domain folding or by serving as sites for phosphorylation and subsequent association with signaling intermediates. These findings thus highlight a fundamental difference in the structural requirements for IL-2R beta and gamma c in receptor-mediated signal transduction.

Signaling through the interleukin 2 receptor beta chain activates a STAT-5-like DNA-binding activity

To explore the possible involvement of STAT factors ("signal transducers and activators of transcription") in the interleukin 2 receptor (IL-2R) signaling cascade, murine HT-2 cells expressing chimeric receptors composed of the extracellular domain of the erythropoietin receptor fused to the cytoplasmic domains of the IL-2R beta or -gamma c chains were prepared. Erythropoietin or IL-2 activation of these cells resulted in rapid nuclear expression of a DNA-binding activity that reacted with select STAT response elements. Based on reactivity with specific anti-STAT antibodies, this DNA-binding activity was identified as a murine homologue of STAT-5. Induction of nuclear expression of this STAT-5-like factor was blocked by the addition of herbimycin A, a tyrosine kinase inhibitor, but not by rapamycin, an immunophilin-binding antagonist of IL-2-induced proliferation. The IL-2R beta chain appeared critical for IL-2-induced activation of STAT-5, since a mutant beta chain lacking all cytoplasmic tyrosine residues was incapable of inducing this DNA binding. In contrast, a gamma c mutant lacking all of its cytoplasmic tyrosine residues proved fully competent for the induction of STAT-5. Physical binding of STAT-5 to functionally important tyrosine residues within IL-2R beta was supported by the finding that phosphorylated, but not nonphosphorylated, peptides corresponding to sequences spanning Y392 and Y510 of the IL-2R beta tail specifically inhibited STAT-5 DNA binding.

Dissociation of the CD4 downregulation and viral infectivity enhancement functions of human immunodeficiency virus type 1 Nef

Recent evidence indicates that the nef gene of human immunodeficiency virus type 1 augments rather than inhibits viral replication in both cell culture and in vivo models. In addition, nef alters various normal cellular processes, including the display of CD4 on the cell surface. However, it remains unknown whether the enhancement of infectivity and the downregulation of CD4 represent linked or independent biologic properties of this single protein. In the present studies, mutational analyses were performed to define structure-function relationships within the Nef protein that mediate these effects. To assess the functional consequences of these mutations, sensitive and reliable assays were developed to quantitate the viral infectivity enhancement and CD4 downregulation functions of Nef. The results indicate that membrane-targeting sequences at the N terminus of Nef are important for both functions of Nef, while certain other conserved regions are dispensable for both functions. A conserved proline-X-X repeat segment in the central core of the protein, which is reminiscent of an SH3-binding domain, is critical for the enhancement of infectivity function but is dispensable for CD4 downregulation. However, the downregulation of CD4 by Nef appears to involve a two-step process requiring the initial dissociation of p56lck from CD4 to permit engagement of the endocytic apparatus by CD4. Together, these findings demonstrate that the infectivity enhancement and CD4 downregulation activities of human immunodeficiency virus type 1 Nef can be dissociated. Thus, these processes may be independent of one another in the viral replication cycle.

Ligand binding by the IL-2 receptor is modulated by intracellular determinants of the IL-2 receptor beta-chain

The biologic actions of IL-2 are mediated by the IL-2R, a multisubunit receptor complex displayed on the surface of lymphocytes and select other hematopoietic lineages. The IL-2R exhibits multiple affinities for IL-2 that result from the monomeric (alpha), heterodimeric (alpha beta and beta gamma), and heterotrimeric (alpha beta gamma) assembly of different receptor subunits. In the present studies, we have used a series of IL-2R mutants in a transient mammalian expression system to investigate the potential role of intracellular receptor regions in the ligand-binding functions of the IL-2R. Analyses of chimeric and deletion mutants of the IL-2R beta subunit have revealed that its intracellular domain critically and selectively influences high affinity ligand binding mediated through the extracellular domains of the alpha beta-heterodimeric receptor. In contrast, intermediate affinity binding of IL-2 by beta gamma-heterodimeric receptors exhibits no dependence on the cytoplasmic domain of IL-2 R beta. Further, co-expression of either a full-length or severely truncated form of IL-2 R gamma to generate an alpha beta gamma-heterotrimeric complex also overcomes the functional dependence upon the cytoplasmic tail of IL-2 R beta. Collectively, our findings suggest that the cytoplasmic domain of IL-2R beta produces intrasubunit transmembrane conformational changes in this receptor subunit that promote extracellular IL-2 binding in combination with IL-2R alpha. These findings have important implications for the receptor dynamics involved in both ligand binding and signal transduction as well as for clinical applications pertaining to altering IL-2R function.

Ligand binding by the IL-2 receptor is modulated by intracellular determinants of the IL-2 receptor beta-chain

The biologic actions of IL-2 are mediated by the IL-2R, a multisubunit receptor complex displayed on the surface of lymphocytes and select other hematopoietic lineages. The IL-2R exhibits multiple affinities for IL-2 that result from the monomeric (alpha), heterodimeric (alpha beta and beta gamma), and heterotrimeric (alpha beta gamma) assembly of different receptor subunits. In the present studies, we have used a series of IL-2R mutants in a transient mammalian expression system to investigate the potential role of intracellular receptor regions in the ligand-binding functions of the IL-2R. Analyses of chimeric and deletion mutants of the IL-2R beta subunit have revealed that its intracellular domain critically and selectively influences high affinity ligand binding mediated through the extracellular domains of the alpha beta-heterodimeric receptor. In contrast, intermediate affinity binding of IL-2 by beta gamma-heterodimeric receptors exhibits no dependence on the cytoplasmic domain of IL-2 R beta. Further, co-expression of either a full-length or severely truncated form of IL-2 R gamma to generate an alpha beta gamma-heterotrimeric complex also overcomes the functional dependence upon the cytoplasmic tail of IL-2 R beta. Collectively, our findings suggest that the cytoplasmic domain of IL-2R beta produces intrasubunit transmembrane conformational changes in this receptor subunit that promote extracellular IL-2 binding in combination with IL-2R alpha. These findings have important implications for the receptor dynamics involved in both ligand binding and signal transduction as well as for clinical applications pertaining to altering IL-2R function.

The cytoplasmic domain of the interleukin-2 receptor beta chain contains both unique and functionally redundant signal transduction elements

Binding of interleukin-2 (IL-2) to the IL-2 receptor (IL-2R) triggers a series of intracellular events culminating in lymphocyte proliferation and differentiation. A novel transient assay of signal transduction leading to proliferation is now described which allows the rapid functional assessment of wild type and mutant receptors, including the IL-2R and other members of the cytokine receptor superfamily. This assay has been used to define domains and specific residues within the IL-2R beta intracellular region that contribute to growth signal transduction. In these studies, internal deletion of either the conserved "Box 1" or "Box 2" proximal cytokine receptor homology segments significantly impaired receptor function. Similarly, mutation of specific key residues within or between Box 1 and Box 2, or deletion of the C-terminal 94 residues of the IL-2R beta chain, impaired growth signaling. In contrast, either replacement of the transmembrane domain with that of the CD4 molecule or internal deletion of the 119 amino acids immediately downstream of Box 2 had no impact on growth signaling competence. These studies thus further define the functional architecture of the intracellular region of IL-2R beta, and reveal specific receptor domains that are dispensable, unique, or functionally redundant.

A transfected human muscarinic receptor fails to substitute for the T cell antigen receptor complex in CD2-initiated signal transduction

Several T cell surface molecules can activate signal transduction pathways that lead to T cell activation. Like the T cell antigen receptor (TCR), several other molecules, including the sheep erythrocyte receptor CD2, are able to activate the phosphatidylinositol (PI) signal transduction pathway upon stimulation with appropriate agonists. However, CD2-initiated activation of this pathway is dependent on the functional expression of the TCR. Since the T cell does not express other known receptors that activate the PI pathway independent of the TCR, the specificity of the CD2 requirement for a functional TCR is not known. To evaluate the specificity of this requirement, we examined the functional capacity of CD2 to activate the PI pathway in a TCR-deficient cell which had been transfected with a heterologous receptor, the human muscarinic subtype 1 receptor (HM1). HM1 is a member of the cholinergic family of receptors and is known to activate the PI pathway. HM1 can function in the absence of the TCR in a Jurkat-derived T cell host. Here we demonstrate through calcium fluorimetry and PI metabolism assays that HM1 is unable to substitute functionally for the TCR in CD2-initiated signal transduction. These results suggest a specific functional interaction between CD2 and the TCR in CD2-mediated activation of the PI pathway in T cells.

Molecular characterization of a Leydig cell tumor presenting as congenital adrenal hyperplasia

We present an unusual patient with a Leydig cell tumor to show that greatly elevated serum concentrations of 17-hydroxyprogesterone (17OHP) may not be diagnostic of congenital adrenal hyperplasia (CAH). A 3.5-yr-old boy had a small testicular mass and plasma 17OHP concentrations of 147-333 nmol/L (4,850-11,000 ng/dL), suggesting CAH with adrenal rests. However, normal plasma cortisol values and the unresponsiveness of the 17OHP concentration to dexamethasone suppression or ACTH stimulation suggested a diagnosis of Leydig cell tumor. A 4-fold elevation in plasma 21-deoxycortisol compared with a 200-fold elevation in 17OHP suggested that the elevated 17OHP derived from the normal pathway of testosterone synthesis in the testis. This was proven by normalization of all hormonal values after tumor resection. Compared to the abundance of mRNA for P450c17, the tumor contained unusually large amounts of mRNA for P450scc, the cholesterol side-chain cleavage enzyme, which is the rate-limiting step in steroid hormone synthesis. Increased P450scc activity, which increased the conversion of cholesterol to pregnenolone, apparently permitted the 17,20-lyase activity of P450c17 to become rate limiting, thus accounting for the increased secretion of 17OHP. Thus, Leydig cell tumors can produce quantities of 17OHP previously reported only in CAH due to 21-hydroxylase deficiency. The molecular characterization of steroidogenic mRNAs in this tumor indicates an unusual ratio in the expression of the genes for the steroidogenic enzymes, probably accounting for the unusual pattern of serum steroids.

Function of a heterologous muscarinic receptor in T cell antigen receptor signal transduction mutants

Previously we have described a system of somatic cell genetics (J.CaM1 and J.CaM2) for analyzing signal transduction via the T cell antigen receptor complex (CD3/Ti). Here we describe a third mutant, J.CaM3, which also expresses high levels of receptors that are functionally impaired. Like J.CaM1, J.CaM3 demonstrates partial signal transduction via CD3/Ti to only certain stimuli. J.CaM1, J.CaM2, and J.CaM3 define three non-Ti complementation groups involved in receptor function. To evaluate the mutations further we have introduced a heterologous receptor, the human muscarinic receptor 1 (HM1), into the parental Jurkat and mutant cell lines. This receptor demonstrates signal transduction competence in all these hosts, indicating that 1) T cells express the necessary apparatus for the coupling of HM1 to second messenger generation and 2) the mutations in the J.CaM family all affect molecules that are specific to CD3/Ti, and not HM1, function. Finally, the HM1 receptor exhibits partial sensitivity to cholera toxin in Jurkat cells, in contrast to the virtually complete sensitivity of CD3/Ti to cholera toxin.

Ligand-induced association between the T-cell antigen receptor and two glycoproteins

We have identified two cell surface glycoproteins of 34 and 38 kDa (gp34 and gp38) that associate with the T-cell antigen receptor (TCR). The coimmunoprecipitation of these proteins with the TCR is increased by treatment with monoclonal antibodies (mAbs) directed against the TCR prior to cell solubilization and immunoprecipitation. Treatment of T cells with mAbs directed against other cell surface molecules, CD2 or HLA, does not induce the association of these proteins with the TCR. The coimmunoprecipitation of gp34 and gp38 with the TCR requires solubilization in the presence of an alkylating agent, suggesting that subunit alkylation stabilizes the interaction. J.CaM1 and J.CaM2 are signal-transduction mutant cell lines derived from Jurkat. These cell lines fail to activate the inositol phospholipid second messenger pathway in response to anti-TCR mAbs. Treatment with mAb C305 (anti-TCR) induces the association of gp34 and gp38 with the TCR in J.CaM2 cells but not in J.CaM1. J.CaM1 modulates the TCR normally in response to anti-TCR antibody treatment. This observation suggests that gp34 and gp38 are involved in the signal-transduction pathway of the TCR complex rather than receptor internalization. Furthermore, since these proteins do associate with the TCR of J.CaM2, the induced association with the TCR is not a consequence of signal transduction.