RESUMEN
Common γ chain (γc) cytokine receptors, including interleukin-2 (IL-2), IL-4, IL-7, IL-9, IL-15, and IL-21 receptors, are activated upon engagement with a common γc receptor (CD132) by concomitant binding of their ectodomains to an interleukin. In this work, we find that direct interactions between the transmembrane domains (TMDs) of both the γc and the interleukin receptors (ILRs) are also required for receptor activation. Moreover, the same γc TMD can specifically recognize multiple ILR TMDs of diverse sequences within the family. Heterodimer structures of γc TMD bound to IL-7 and IL-9 receptor TMDs-determined in a lipid bilayer-like environment by nuclear magnetic resonance spectroscopy-reveal a conserved knob-into-hole mechanism of recognition that mediates receptor sharing within the membrane. Thus, signaling in the γc receptor family requires specific heterotypic interactions of the TMDs.
Asunto(s)
Subunidad gamma Común de Receptores de Interleucina , Subunidad alfa del Receptor de Interleucina-7 , Dominios y Motivos de Interacción de Proteínas , Subunidad gamma Común de Receptores de Interleucina/química , Subunidad gamma Común de Receptores de Interleucina/genética , Unión Proteica , Transducción de Señal , Resonancia Magnética Nuclear Biomolecular , Subunidad alfa del Receptor de Interleucina-7/química , Subunidad alfa del Receptor de Interleucina-7/genéticaRESUMEN
A continuing quest for specific inhibitors of proinflammatory cytokines brings promise for effective therapies designed for inflammatory and autoimmune disorders. Cefazolin, a safe, first-generation cephalosporin antibiotic, has been recently shown to specifically interact with interleukin 15 (IL-15) receptor subunit α (IL-15Rα) and to inhibit IL-15-dependent TNF-α and IL-17 synthesis. The aim of this study was to elucidate cefazolin activity against IL-2, IL-4, IL-15 and IL-21, i.e. four cytokines sharing the common cytokine receptor γ chain (γc). In silico, molecular docking unveiled two potential cefazolin binding sites within the IL-2/IL-15Rß subunit and two within the γc subunit. In vitro, cefazolin decreased proliferation of PBMC (peripheral blood mononuclear cells) following IL-2, IL-4 and IL-15 stimulation, reduced production of IFN-γ, IL-17 and TNF-α in IL-2- and IL-15-treated PBMC and in IL-15 stimulated natural killer (NK) cells, attenuated IL-4-dependent expression of CD11c in monocyte-derived dendritic cells and suppressed phosphorylation of JAK3 in response to IL-2 and IL-15 in PBMC, to IL-4 in TF-1 (erythroleukemic cell line) and to IL-21 in NK-92 (NK cell line). The results of the study suggest that cefazolin may exert inhibitory activity against all of the γc receptor-dependent cytokines, i.e. IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21.
Asunto(s)
Antiinflamatorios/farmacología , Cefazolina/farmacología , Subunidad gamma Común de Receptores de Interleucina/antagonistas & inhibidores , Adulto , Antiinflamatorios/química , Sitios de Unión , Antígeno CD11c/metabolismo , Cefazolina/química , Proliferación Celular/efectos de los fármacos , Células Dendríticas/efectos de los fármacos , Células Dendríticas/metabolismo , Humanos , Interferón gamma/metabolismo , Subunidad gamma Común de Receptores de Interleucina/química , Subunidad gamma Común de Receptores de Interleucina/metabolismo , Interleucina-15/metabolismo , Interleucina-2/metabolismo , Janus Quinasa 3/metabolismo , Masculino , Monocitos/patología , Fosforilación/efectos de los fármacos , Factor de Necrosis Tumoral alfa/biosíntesisRESUMEN
The common γ chain (γc) family of hematopoietic cytokines consists of six distinct four α-helix bundle soluble ligands that signal through receptors which include the shared γc subunit to coordinate a wide range of physiological processes, in particular, those related to innate and adaptive immune function. Since the first crystallographic structure of a γc family cytokine/receptor signaling complex (the active Interleukin-2 [IL-2] quaternary complex) was determined in 2005 [1], tremendous progress has been made in the structural characterization of this protein family, transforming our understanding of the molecular mechanisms underlying immune activity. Although many conserved features of γc family cytokine complex architecture have emerged, distinguishing details have been observed for individual cytokine complexes that rationalize their unique functional properties. Much work remains to be done in the molecular characterization of γc family signaling, particularly with regard to intracellular activation events, and looking forward, new technologies in structural biophysics will offer further insight into the biology of cytokine signaling to inform the design of targeted therapeutics for treatment of immune-linked diseases such as cancer, infection, and autoimmune disorders.
Asunto(s)
Subunidad gamma Común de Receptores de Interleucina , Transducción de Señal , Animales , Humanos , Subunidad gamma Común de Receptores de Interleucina/química , Subunidad gamma Común de Receptores de Interleucina/fisiología , Interleucina-2/química , Interleucina-2/fisiología , Receptores de Citocinas/metabolismo , Transducción de Señal/fisiologíaRESUMEN
IL-17 produced by Th17 cells has been implicated in the pathogenesis of rheumatoid arthritis (RA). It is important to prevent the differentiation of Th17 cells in RA. Homodimeric soluble γc (sγc) impairs IL-2 signaling and enhances Th17 differentiation. Thus, we aimed to block the functions of sγc by inhibiting the formation of homodimeric sγc. The homodimeric form of sγc was strikingly disturbed by sγc-binding DNA aptamer. Moreover, the aptamer effectively inhibited Th17 cell differentiation and restored IL-2 and IL-15 signaling impaired by sγc with evidences of increased survival of T cells. sγc was highly expressed in SF of RA patients and increased in established CIA mice. The therapeutic effect of PEG-aptamer was tested in CIA model and its treatment alleviated arthritis pathogenesis with impaired differentiation of pathogenic Th17, NKT1, and NKT17 cells in inflamed joint. Homodimeric sγc has pathogenic roles to exacerbate RA progression with differentiation of local Th17, NKT1, and NKT17 cells. Therefore, sγc is suggested as target of a therapeutic strategy for RA.
Asunto(s)
Aptámeros de Nucleótidos/farmacología , Artritis Experimental/inducido químicamente , Artritis Reumatoide/inmunología , Proteína 1 Similar a Quitinasa-3/farmacología , Subunidad gamma Común de Receptores de Interleucina/antagonistas & inhibidores , Células Th17/inmunología , Anciano , Animales , Aptámeros de Nucleótidos/química , Diferenciación Celular/efectos de los fármacos , Dimerización , Femenino , Células HEK293 , Humanos , Inflamación/inmunología , Subunidad gamma Común de Receptores de Interleucina/química , Interleucina-15/metabolismo , Interleucina-2/metabolismo , Ratones , Ratones Endogámicos DBA , Persona de Mediana Edad , Células T Asesinas Naturales/inmunología , Técnica SELEX de Producción de Aptámeros/métodos , TransfecciónRESUMEN
BACKGROUND: Primary immunodeficiency diseases are a group of genetic disorders that lead to increased propensity to a variety of infections, sometimes with fatal outcomes. METHOD: In this study, whole-exome sequencing (WES) was used to identify mutations in two patients suspected of having primary immunodeficiency. Sanger sequencing was used to confirm the results in the patients and their family. RESULT: One patient was diagnosed as X-linked severe combined immunodeficiency (X-SCID) and another patient as X-linked chronic granulomatous disease (X-CGD) by WES. Sequencing analysis of IL2RG gene revealed a novel mutation (c.794T>A, p.I265N) and CYBB gene revealed a missense mutation (c.935T>A, p.M312K). DISCUSSION AND CONCLUSION: This study identifies one novel mutation in the IL2RG gene and another, previously described mutation in the CYBB genes. It is the first report establishing a diagnosis of X-SCID and X-CGD using WES in Chinese patients.
Asunto(s)
Estudios de Asociación Genética , Predisposición Genética a la Enfermedad , Síndromes de Inmunodeficiencia/diagnóstico , Síndromes de Inmunodeficiencia/genética , Subunidad gamma Común de Receptores de Interleucina/genética , Mutación , NADPH Oxidasa 2/genética , Alelos , Sustitución de Aminoácidos , Pueblo Asiatico/genética , Biomarcadores , China , Análisis Mutacional de ADN , Genotipo , Enfermedad Granulomatosa Crónica/diagnóstico , Enfermedad Granulomatosa Crónica/genética , Humanos , Inmunofenotipificación , Lactante , Subunidad gamma Común de Receptores de Interleucina/química , Masculino , Modelos Moleculares , NADPH Oxidasa 2/química , Linaje , Fenotipo , Relación Estructura-Actividad , Secuenciación Completa del Genoma , Enfermedades por Inmunodeficiencia Combinada Ligada al Cromosoma X/diagnóstico , Enfermedades por Inmunodeficiencia Combinada Ligada al Cromosoma X/genéticaRESUMEN
IL-7 signaling via IL-7Rα and common γ-chain (γc) is necessary for the development and homeostasis of T cells. Although the delicate mechanism in which IL-7Rα downregulation allows the homeostasis of T cell with limited IL-7 has been well known, the exact mechanism behind the interaction between IL-7Rα and γc in the absence or presence of IL-7 remains unclear. Additionally, we are still uncertain as to how only IL-7Rα is separately downregulated by the binding of IL-7 from the IL-7Rα/γc complex. We demonstrate here that 4G3, TUGm2, and 3E12 epitope masking of γc protein are induced in the presence of IL-7, indicating that the epitope alteration is induced by IL-7 binding to the preassembled receptor core. Moreover, the epitope masking of γc protein is inversely correlated with the expression of IL-7Rα upon IL-7 binding, implying that the structural alteration of γc might be involved in the regulation of IL-7Rα expression. The conformational change in γc upon IL-7 binding may contribute not only to forming the functional IL-7 signaling complex but also to optimally regulating the expression of IL-7Rα.
Asunto(s)
Epítopos/química , Subunidad gamma Común de Receptores de Interleucina/química , Interleucina-7/metabolismo , Receptores de Interleucina-7/química , Animales , Anticuerpos Monoclonales/química , Citocinas/metabolismo , Humanos , Cinética , Ratones , Ratones Endogámicos C57BL , Fosforilación , Unión Proteica , Dominios Proteicos , Transducción de SeñalRESUMEN
Originally identified as the third subunit of the high-affinity IL-2 receptor complex, the common γ-chain (γc) also acts as a non-redundant receptor subunit for a series of other cytokines, collectively known as γc family cytokines. γc plays essential roles in T cell development and differentiation, so that understanding the molecular basis of its signaling and regulation is a critical issue in T cell immunology. Unlike most other cytokine receptors, γc is thought to be constitutively expressed and limited in its function to the assembly of high-affinity cytokine receptors. Surprisingly, recent studies reported a series of findings that unseat γc as a simple housekeeping gene, and unveiled γc as a new regulatory molecule in T cell activation and differentiation. Cytokine-independent binding of γc to other cytokine receptor subunits suggested a pre-association model of γc with proprietary cytokine receptors. Also, identification of a γc splice isoform revealed expression of soluble γc proteins (sγc). sγc directly interacted with surface IL-2Rß to suppress IL-2 signaling and to promote pro-inflammatory Th17 cell differentiation. As a result, endogenously produced sγc exacerbated autoimmune inflammatory disease, while the removal of endogenous sγc significantly ameliorated disease outcome. These data provide new insights into the role of both membrane and soluble γc in cytokine signaling, and open new venues to interfere and modulate γc signaling during immune activation. These unexpected discoveries further underscore the perspective that γc biology remains largely uncharted territory that invites further exploration.
Asunto(s)
Citocinas/inmunología , Inflamación/inmunología , Subunidad gamma Común de Receptores de Interleucina/inmunología , Linfocitos T/inmunología , Empalme Alternativo , Animales , Humanos , Inflamación/genética , Subunidad gamma Común de Receptores de Interleucina/química , Subunidad gamma Común de Receptores de Interleucina/genética , Janus Quinasa 3/inmunología , Activación de Linfocitos , Isoformas de Proteínas/química , Isoformas de Proteínas/genética , Isoformas de Proteínas/inmunología , Receptores de Citocinas/inmunología , Transducción de Señal , Linfocitos T/citología , Linfocitos T/metabolismo , Células Th17/citología , Células Th17/inmunología , Células Th17/metabolismoRESUMEN
X-linked severe combined immunodeficiency (XSCID) is caused by a genetic mutation within the common gamma chain (γc), an essential component of the cytokine receptors for interleukin (IL)-2, IL-4, IL-7, IL-9, IL-15, and IL-21. XSCID patients are most commonly treated with bone marrow transplants (BMT) to restore systemic immune function. However, BMT-XSCID humans and dogs remain at an increased risk for development of cutaneous papillomavirus (PV) infections and their associated neoplasms, most typically cutaneous papillomas. Since basal keratinocytes are the target cell for the initial PV infection, we wanted to determine if canine XSCID keratinocytes have a diminished antiviral cytokine response to poly(dA:dT) and canine papillomavirus-2 (CPV-2) upon initial infection. We performed quantitative RT-PCR for antiviral cytokines and downstream interferon stimulated genes (ISG) on poly(dA:dT) stimulated and CPV-2 infected monolayer keratinocyte cultures derived from XSCID and normal control dogs. We found that XSCID keratinocytes responded similarly to poly(dA:dT) as normal keratinocytes by upregulating antiviral cytokines and ISGs. CPV-2 infection of both XSCID and normal keratinocytes did not result in upregulation of antiviral cytokines or ISGs at 2, 4, or 6 days post infection. These data suggest that the antiviral response to initial PV infection of basal keratinocytes is similar between XSCID and normal patients, and is not the likely source for the remaining immunodeficiency in XSCID patients.
Asunto(s)
Queratinocitos/efectos de los fármacos , Queratinocitos/metabolismo , Infecciones por Papillomavirus/etiología , Poli dA-dT/farmacología , Enfermedades por Inmunodeficiencia Combinada Ligada al Cromosoma X/inmunología , Animales , Secuencia de Bases , Trasplante de Médula Ósea , Citocinas/genética , Citocinas/metabolismo , Modelos Animales de Enfermedad , Perros , Femenino , Expresión Génica , Regulación de la Expresión Génica/efectos de los fármacos , Factores Reguladores del Interferón/genética , Factores Reguladores del Interferón/metabolismo , Subunidad gamma Común de Receptores de Interleucina/química , Subunidad gamma Común de Receptores de Interleucina/genética , Queratinocitos/virología , Datos de Secuencia Molecular , Mutación , Papillomaviridae , Infecciones por Papillomavirus/tratamiento farmacológico , Poli dA-dT/administración & dosificación , Cultivo Primario de Células , ARN Mensajero/genética , Enfermedades por Inmunodeficiencia Combinada Ligada al Cromosoma X/complicaciones , Enfermedades por Inmunodeficiencia Combinada Ligada al Cromosoma X/genética , Enfermedades por Inmunodeficiencia Combinada Ligada al Cromosoma X/terapiaRESUMEN
Most proteins in nature are chemically modified after they are made to control how, when, and where they function. The 3 core features of proteins are posttranslationally modified: amino acid side chains can be modified, peptide bonds can be cleaved or isomerized, and disulfide bonds can be cleaved. Cleavage of peptide bonds is a major mechanism of protein control in the circulation, as exemplified by activation of the blood coagulation and complement zymogens. Cleavage of disulfide bonds is emerging as another important mechanism of protein control in the circulation. Recent advances in our understanding of control of soluble blood proteins and blood cell receptors by functional disulfide bonds is discussed as is how these bonds are being identified and studied.
Asunto(s)
Regulación Alostérica/fisiología , Proteínas Sanguíneas/química , Proteínas Sanguíneas/metabolismo , Disulfuros/química , Angiotensinógeno/química , Angiotensinógeno/metabolismo , Animales , Disulfuros/metabolismo , Humanos , Enlace de Hidrógeno , Subunidad gamma Común de Receptores de Interleucina/química , Subunidad gamma Común de Receptores de Interleucina/metabolismo , Plasminógeno/química , Plasminógeno/metabolismo , beta 2 Glicoproteína I/química , beta 2 Glicoproteína I/metabolismoRESUMEN
Previously, we demonstrated that the γC subunit of type I IL-4 receptor was required for robust tyrosine phosphorylation of the downstream adapter protein, IRS-2, correlating with the expression of genes (ArgI, Retnla, and Chi3l3) characteristic of alternatively activated macrophages. We located an I4R-like motif (IRS-2 docking sequence) in the γC cytoplasmic domain but not in the IL-13Rα1. Thus, we predicted that the γC tail directed enhanced IRS-2 phosphorylation. To test this, IL-4 signaling responses were examined in a mutant of the key I4R motif tyrosine residue (Y325F) and different γC truncation mutants (γ285, γ308, γ318, γ323, and γFULL LENGTH (FL)) co-expressed in L-cells or CHO cells with wild-type (WT) IL-4Rα. Surprisingly, IRS-1 phosphorylation was not diminished in Y325F L-cell mutants suggesting Tyr-325 was not required for the robust insulin receptor substrate response. IRS-2, STAT6, and JAK3 phosphorylation was observed in CHO cells expressing γ323 and γFL but not in γ318 and γ285 mutants. In addition, when CHO cells expressed γ318, γ323, or γFL with IL-2Rß, IL-2 induced phospho-STAT5 only in the γ323 and γFL clones. Our data suggest that a smaller (5 amino acid) interval than previously determined is necessary for JAK3 activation/γC-mediated signaling in response to IL-4 and IL-2. Chimeric receptor chains of the γC tail fused to the IL-13Rα1 extracellular and transmembrane domain did not elicit robust IRS-2 phosphorylation in response to IL-13 suggesting that the extracellular/transmembrane domains of the IL-4/IL-13 receptor, not the cytoplasmic domains, control signaling efficiency. Understanding this pathway fully will lead to rational drug design for allergic disease.
Asunto(s)
Subunidad gamma Común de Receptores de Interleucina/química , Subunidad alfa1 del Receptor de Interleucina-13/química , Interleucina-4/química , Animales , Células CHO , Línea Celular , Cricetinae , Cricetulus , Citocinas/metabolismo , Citoplasma/metabolismo , Citometría de Flujo , Interleucina-13/química , Ratones , Modelos Biológicos , Mutación , Fosforilación , Estructura Terciaria de Proteína , Receptores de IgE/metabolismo , Receptores de Interleucina-4/metabolismo , Factor de Transcripción STAT6/metabolismo , Transducción de SeñalRESUMEN
BACKGROUND: Modeling of transmembrane domains (TMDs) requires correct prediction of interfacial residues for in-silico modeling and membrane insertion studies. This implies the defining of a target sequence long enough to contain interfacial residues. However, too long sequences induce artifactual polymorphism: within tested modeling methods, the longer the target sequence, the more variable the secondary structure, as though the procedure were stopped before the end of the calculation (which may in fact be unreachable). Moreover, delimitation of these TMDs can produce variable results with sequence based two-dimensional prediction methods, especially for sequences showing polymorphism. To solve this problem, we developed a new modeling procedure using the PepLook method. We scanned the sequences by modeling peptides from the target sequence with a window of 19 residues. RESULTS: Using sequences whose NMR-structures are already known (GpA, EphA1 and Erb2-HER2), we first determined that the hydrophobic to hydrophilic accessible surface area ratio (ASAr) was the best criterion for delimiting the TMD sequence. The length of the helical structure and the Impala method further supported the determination of the TMD limits. This method was applied to the IL-2Rß and IL-2Rγ TMD sequences of Homo sapiens, Rattus norvegicus, Mus musculus and Bos taurus. CONCLUSIONS: We succeeded in reducing the variation in the TMD limits to only 2 residues and in gaining structural information.
Asunto(s)
Subunidad gamma Común de Receptores de Interleucina/química , Subunidad beta del Receptor de Interleucina-2/química , Programas Informáticos , Secuencia de Aminoácidos , Animales , Bovinos , Perros , Humanos , Interacciones Hidrofóbicas e Hidrofílicas , Macaca mulatta , Ratones , Datos de Secuencia Molecular , Pan troglodytes , Estructura Terciaria de Proteína , Ratas , Alineación de Secuencia , Homología de Secuencia de AminoácidoRESUMEN
CD132 is the common gamma chain to a number of cytokine receptor complexes including that for IL-2. To identify the functional domain of chicken CD132 (chCD132), the cDNA of chCD132 was cloned, and a neutralizing monoclonal antibody, raised against a recombinant chCD132 protein, was identified by inhibition of IL-2-dependent proliferation of T cells. Flow cytometry analysis revealed that chCD132 molecules are expressed on the surface of splenic mononuclear cells. The functional domain of chCD132 that binds to chicken interleukin 2, Q(84)E(94)L(95)Q(96)N(97)L(98), was found through phage display and peptide-competitive ELISA, and its critical residue Q(96) was further identified. A tertiary structure model shows that the functional domain is positioned at the elbow-like junction of N and C terminal fibronectin-III domains of chCD132. These data provide experimental evidence for elucidating the interaction between chCD132 and chIL-2.
Asunto(s)
Pollos/genética , Pollos/inmunología , Subunidad gamma Común de Receptores de Interleucina/genética , Interleucina-2/genética , Leucocitos Mononucleares/inmunología , Secuencia de Aminoácidos , Animales , Separación Celular , Ensayo de Inmunoadsorción Enzimática , Mapeo Epitopo , Citometría de Flujo , Expresión Génica , Perfilación de la Expresión Génica , Subunidad gamma Común de Receptores de Interleucina/química , Subunidad gamma Común de Receptores de Interleucina/inmunología , Interleucina-2/inmunología , Datos de Secuencia Molecular , Biblioteca de Péptidos , Estructura Terciaria de Proteína , Proteínas Recombinantes/química , Proteínas Recombinantes/genética , Proteínas Recombinantes/inmunología , Reacción en Cadena de la Polimerasa de Transcriptasa InversaRESUMEN
Recent structural information for complexes of cytokine receptor ectodomains bound to their ligands has significantly expanded our understanding of the macromolecular topology and ligand recognition mechanisms used by our three principal shared cytokine signaling receptors-gp130, gamma(c), and beta(c). The gp130 family receptors intricately coordinate three structurally unique cytokine-binding sites on their four-helix bundle cytokine ligands to assemble multimeric signaling complexes. These organizing principles serve as topological blueprints for the entire gp130 family of cytokines. Novel structures of gamma(c) and beta(c) complexes show us new twists, such as the use of a nonstandard sushi-type alpha receptors for IL-2 and IL-15 in assembling quaternary gamma(c) signaling complexes and an antiparallel interlocked dimer in the GM-CSF signaling complex with beta(c). Unlike gp130, which appears to recognize vastly different cytokine surfaces in chemically unique fashions for each ligand, the gamma(c)-dependent cytokines appear to seek out some semblance of a knobs-in-holes shape recognition code in order to engage gamma(c) in related fashions. We discuss the structural similarities and differences between these three shared cytokine receptors, as well as the implications for transmembrane signaling.
Asunto(s)
Receptor gp130 de Citocinas/química , Receptor gp130 de Citocinas/metabolismo , Citocinas/metabolismo , Animales , Sitios de Unión , Subunidad beta Común de los Receptores de Citocinas/química , Subunidad beta Común de los Receptores de Citocinas/metabolismo , Citocinas/química , Humanos , Subunidad gamma Común de Receptores de Interleucina/química , Subunidad gamma Común de Receptores de Interleucina/metabolismo , Ligandos , Unión Proteica , Conformación Proteica , Transducción de SeñalRESUMEN
Interleukin (IL)-15 is a pleiotropic cytokine that plays a pivotal role in both innate and adaptive immunity. IL-15 is unique among cytokines due to its participation in a trans signaling mechanism in which IL-15 receptor alpha (IL-15Ralpha) from one subset of cells presents IL-15 to neighboring IL-2Rbeta/gammac-expressing cells. Here we present the crystal structure of IL-15 in complex with the sushi domain of IL-15Ralpha. The structure reveals that the alpha receptor-binding epitope of IL-15 adopts a unique conformation, which, together with amino acid substitutions, permits specific interactions with IL-15Ralpha that account for the exceptionally high affinity of the IL-15.IL-15Ralpha complex. Interestingly, analysis of the topology of IL-15 and IL-15Ralpha at the IL-15.IL-15Ralpha interface suggests that IL-15 should be capable of participating in a cis signaling mechanism similar to that of the related cytokine IL-2. Indeed, we present biochemical data demonstrating that IL-15 is capable of efficiently signaling in cis through IL-15Ralpha and IL-2Rbeta/gammac expressed on the surface of a single cell. Based on our data we propose that cis presentation of IL-15 may be important in certain biological contexts and that flexibility of IL-15Ralpha permits IL-15 and its three receptor components to be assembled identically at the ligand-receptor interface whether IL-15 is presented in cis or trans. Finally, we have gained insights into IL-15.IL-15Ralpha.IL-2Rbeta.gammac quaternary complex assembly through the use of molecular modeling.