Affinity IL-2 Receptor Research Articles

Supplementation with selenium augments the functions of natural killer and lymphokine-activated killer cells.

This study examined the effects of dietary (2.0 ppm for 8 wk) and in vitro (1 x 10(-7)M) supplementation with selenium (Se, as sodium selenite) on the activity of spleen natural killer (NK) cells and plastic-adherent lymphokine-activated killer (A-LAK) cells from C57B1/6J male mice. Dietary supplementation with Se resulted in a significant increase in the lytic activity of activated NK cells, and cells from these highly lytic effector cell populations expressed significantly higher numbers of intermediate affinity interleukin-2 receptors (II-2R)/cell. In the presence of high concentrations of II-2 and 1 x 10(-7)M Se, resting populations of spleen NK cells developed into A-LAK cells that had a significantly enhanced ability to proliferate, as indicated by the significantly higher amounts of nuclear 3H-thymidine incorporation, and a significantly augmented cytolytic activity against both NK-sensitive and NK-resistant target cells. Se appears to enhance the lytic activity of activated NK cells and to augment the proliferation, expansion, and lytic activity of A-LAK cells in the presence of high concentrations of II-2 through its ability to enhance the expression of intermediate affinity II-2R on these cells.

Effects of Viscum album L. on cyclophosphamide-treated peripheral blood mononuclear cells in vitro: sister chromatid exchanges and activation/proliferation marker expression.

Based on recently published data, Viscum album L. (VAL) extracts have been shown to provide a DNA stabilizing effect which seems to be restricted to the peripheral blood mononuclear cells (PBMC). We have now investigated whether VAL exerts effects of cellular protection for phytohemagglutinin-activated PBMC treated with cyclophosphamdie (CP) in vitro. The addition of VAL resulted in a slight reduction of CP-induced sister chromatid exchanges of cultured PBMC from healthy individuals. The incubation with CP significantly reduced the expression of the low affinity interleukin-2 receptor (IL-2R alpha chain) and of transferrin receptor (TfR) on PHA-stimulated T lymphocytes. The addition of 10 micrograms/ml VAL was protective against the CP-induced depression of IL-2R alpha chain and TfR expression on these cells. The simultaneous addition of CP and purified VAL components, such as ML I, ML II/III, and viscotoxins did not significantly change expression of IL-2R alpha chain and TfR on T cells. Thus, so far undefined VAL components might be responsible for the observed protection effects of the whole plant extract. The results presented here should encourage investigation of this drug, which might become an interesting adjuvant in cancer therapy.

Expression of interleukin-2 receptor on enterocytes in conventional and germ-free rats after stimulation with gliadin.

The high affinity interleukin-2 receptor (IL-2R) is a membrane-bound heterodimer resulting from the noncovalent association of at least 2 subunits. Both a (p55, Tac protein) and s (p75) chains bind interleukin-2 (IL-2) with low and intermediate affinity, respectively.

Effects of Route and Formulation on Clinical Pharmacokinetics of Interleukin-2

Interleukin-2 (IL-2) is a hormone of the immune system responsible for control of the proliferation and cytotoxicity of T lymphocytes and natural killer cells as well as the proliferation of B lymphocytes. Recombinant IL-2 has been only minimally to modestly successfully to date in the treatment of cancer and infectious diseases, largely because the drug is associated with toxicity and a narrow therapeutic index. Quantitative measurement of IL-2 can be quickly done by enzyme immunoassay. IL-2 bioassay provides an index of biologically active cytokine. IL-2 action and pharmacokinetics can be understood in the context of the effect IL-2 on high (alpha, beta, gamma trimer) vs intermediate (alpha, beta) vs low (beta only or alpha only) affinity IL-2 receptors on various cells of the immune system. IL-2 undergoes rapid renal elimination. The route of administration is important to determine the provision of sustained drug concentrations adequate to support the proliferation and cytotoxicity of immune cells. When IL-2 is given intravenously it has rapid elimination pharmacokinetics with an initial elimination half-life and terminal elimination half-life (t1/2 beta) of 6 to 12 minutes and 40 to 80 minutes, respectively. Subcutaneous or intramuscular administration of IL-2 results in sustained systemic absorption and approximately 30% of the injected dose is absorbed. Because IL-2 is less rapidly cleared from the site of intracavitary injection, when the drug is given by these less traditional routes (e.g. intraperitoneal, intrapleural, intrathecal, intraventricular, intravesicular, and inhalational administration) sustained local IL-2 activity can result. In some cases this has resulted in an improved therapeutic index compared with that resulting after administration of the drug by high dose intravenous bolus or continuous infusion. Depot IL-2 preparations may offer more convenient administration (e.g. t1/2 beta of polyethylene glycolated IL-2 is approximately 10-fold higher than that of recombinant IL-2) or more favourable biodistribution (e.g. IL-2 liposomes are more potent against lung metastases) compared with IL-2 administered by more conventional routes. An understanding of IL-2 clinical pharmacokinetics in relation to immunobiology of this central cytokine should lead to less toxicity and more effective clinical use.

Suppressin: an endogenous negative regulator of immune cell activation.

We have recently identified a new suppressor molecule we named suppressin (SPN) that has all the characteristics of a global negative regulator of the immune system. SPN is a unique 63-kD monomeric polypeptide with a pI of 8.1 that is produced and secreted under basal conditions by murine splenocytes, human peripheral mononuclear cells, and hormone-secreting pituitary cells. The biological actions of SPN in vitro include the inhibition of mitogen-induced proliferation and immunoglobulin synthesis of lymphocytes and the suppression of interleukin-2-dependent CTLL-2 cell proliferation. In addition, SPN enhances natural killer cell activity by eliciting interferon-alpha and -beta synthesis and secretion. SPN effects are reversible, nontoxic, and require the continuous presence of exogenous SPN. T lymphocytes stimulated with concanavalin A or phytohemagglutinin are more sensitive to SPN (90% inhibition) than are lipopolysaccharide-stimulated B cells (60% inhibition). SPN arrests lymphocytes in the G0/G1 phase of the cell cycle after reduction of their RNA, protein and DNA synthesis, suggesting that SPN inhibits the processes required for G0 transition to G1. SPN is found intracellularly in all unstimulated lymphocyte subsets, monocytes, and in phytohemagglutinin-activated T lymphocytes immunopositive for the low affinity interleukin-2 receptor. These results suggest that SPN may be a major negative regulator of cell proliferation in the immune system. All SPN-producing cell types are also sensitive to SPN. Collectively, the results of these experiments provide the foundations for a model in which SPN regulates lymphocyte proliferation in an autocrine and/or paracrine manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Coiled-coil molecular recognition: directed solution assembly of receptor ectodomains.

The high affinity interleukin-2 (IL-2) receptor is composed of at least three cell surface proteins (alpha, beta and gamma subunits), each of which is independently capable of ligand binding. Physiologically, these subunits cooperate to form dimeric and trimeric complexes that efficiently capture IL-2 and transmit the signal across the membrane. The knowledge of how each subunit functions with respect to ligand capture, signal transmission and internalization is essential for the development of ligand-based IL-2 agonists and antagonists, as well as receptor-related therapeutic and diagnostic reagents. Only one of the subunits (p55 or alpha chain) is capable of interacting with ligand in solution in a manner that resembles cell surface binding. To generate soluble multimeric complexes of the IL-2 receptor subunits that may bind ligand in solution in a fashion that mimics the same receptor complexes on the cell surface, we have added recognition sequences (coiled-coil heptad repeats) to the ectodomains of the individual receptors. Here we describe the expression and characterization of a prototype IL-2 beta receptor ectodomain-coiled-coil fusion protein and demonstrate that this is a feasible approach to the preparation of cytokine receptor solution complexes.

Increased expression of high affinity IL-2 receptors and beta-adrenoceptors on peripheral blood mononuclear cells is associated with clinical and MRI activity in multiple sclerosis.

Enhanced expression of beta-adrenoceptor densities on peripheral blood mononuclear cells (PBMCs) in progressive multiple sclerosis patients has been observed in a number of independent studies. A link between increased number of beta-adrenoceptors and inflammatory disease has been further indicated by studies in rheumatoid arthritis and relapsing-remitting multiple sclerosis patients. In a serial monthly assessment of relapsing-remitting multiple sclerosis patients, we have demonstrated that increased beta-adrenoceptors on PBMCs correlate with the expression of high affinity interleukin-2 receptors (IL-2Rs) and disease activity as determined by clinical and MRI findings. Magnetic resonance imaging activity was strongly correlated with IL-2R expression and it appears to be a sensitive marker of PBMC immunoactivation in multiple sclerosis. In vitro studies showed that beta-agonist stimulation of PBMCs reduces the IL-2R expression and suppresses cell proliferation following mitogenic stimulation. This observation may indicate a recovery role for the enhanced beta-adrenoceptor expression in multiple sclerosis. However, its therapeutic importance remains to be tested by appropriate trials using either beta-agonists or agents activating the second messenger system, c-AMP, in lymphocytes.

Interleukin-2 receptor-directed therapies: antibody-or cytokine-based targeting molecules.

With the exception of certain hematologic malignancies, the high affinity interleukin-2 (IL-2) receptor is only transiently expressed during the brief antigen-triggered proliferative burst of lymphocytes. Hence, we wondered whether administration of anti-IL-2 receptor (IL-2R) monoclonal antibody (mAb) or chimeric IL-2 toxins would provide a utilitarian way to achieve immunosuppression aimed directly at activated lymphocytes, or whether this approach could be used to treat IL-2R+ leukemia/lymphoma. Studies in preclinical autoimmune and transplant models indicate that this approach can be effective. The results of open, uncontrolled studies provide preliminary evidence that a chimeric IL-2 toxin is well tolerated at doses that may induce improvement in patients with IL-2R+ leukemia/lymphoma, as well as in patients with refractory rheumatoid arthritis or new-onset diabetes mellitus.

Tyrosine phosphorylation of receptor beta subunits and common substrates in response to interleukin-3 and granulocyte-macrophage colony-stimulating factor.

The receptors for interleukin-3 (IL-3) and granulocyte-macrophage colony-stimulating factor (GM-CSF) consist of two polypeptides each belonging to a new class of molecules referred to as the hemopoietin receptor family. When expressed alone, receptor polypeptides of this family often bind their respective factors with lower affinity than the receptors identified in whole cells. Despite the lack of structural evidence for any enzymatic activity of the receptor polypeptides, both IL-3 and GM-CSF stimulate tyrosine phosphorylation of multiple intracellular substrates. We investigated IL-3 and GM-CSF receptor structure and signaling in a myeloid cell line, FDC-P1, which is dependent on either IL-3 or GM-CSF for growth. Antiphosphotyrosine antibodies were used to immunoprecipitate tyrosine-phosphorylated proteins from 32P-labeled cells or to probe immunoblots. Both IL-3 and GM-CSF stimulated the phosphorylation of a similar pattern of polypeptides on tyrosine. One tyrosine phosphorylated polypeptide migrated with M(r) = 135,000 and increased to 150,000 over a period of 10 min following stimulation of cells with IL-3 or GM-CSF. The M(r) = 135,000-150,000 polypeptide phosphorylated in response to IL-3 was shown to be primarily the Aic-2A polypeptide, the low affinity IL-3 receptor. Phosphatase treatment showed that the dramatic IL-3-induced shift in apparent molecular weight from M(r) = 125,000 in unstimulated cells was entirely due to phosphorylation. The closely related receptor, Aic-2B, was also tyrosine phosphorylated in response to IL-3, although to a lesser extent than Aic-2A. Treatment with GM-CSF resulted in tyrosine phosphorylation of the Aic-2B polypeptide exclusively. It was intriguing that GM-CSF treatment did affect the mobility of the Aic-2A polypeptide on polyacrylamide gels. Together, these results suggest that the Aic-2A polypeptide is part of the IL-3 receptor complex, but not the GM-CSF receptor. In contrast, the Aic-2B polypeptide is a component of the GM-CSF receptor, but it can also be utilized in an IL-3 receptor.

Human immunodeficiency virus-infected adherent cell-derived inhibitory factor (p29) inhibits normal T cell proliferation through decreased expression of high affinity interleukin-2 receptors and production of interleukin-2.

Adherent cells from HIV-infected subjects as well as in vitro HIV-infected normal adherent cells produce spontaneously a 29-kD (p29) factor that inhibits mitogen-induced proliferation of normal T cells. p29 mediates a partial dose-dependent inhibition of total protein synthesis in both nonstimulated and PHA-activated cells that is associated with impaired PHA-induced expression of IL-2 receptor (IL-2R)alpha chain, HLA-class II molecules, and production of IL-2 by these cells; conversely, p29 does not modify the expression of IL-2R beta chain, 4F2, CD9, or transferrin receptor, or the production of IL-1 and TNF alpha by the cells. 1 h preincubation of the cells with p29 is sufficient to detect its biologic activity and added rIL-2 abrogates p29-induced inhibition of IL-2R alpha chain expression; however, p29 does not display any biologic effect on already expressed IL-2R alpha chains. The impaired expression of IL-2R alpha chain mediated by p29 is not due to a decreased accumulation of the corresponding mRNA transcripts, but is associated with a two-fold increase of intracellular cAMP. Binding experiments with 125I-rIL-2 reveals that p29 induces a 50% decrease in the number of both high and low affinity IL-2R per cell. p29 also inhibits alloantigen-induced proliferation of PBMC, whereas it does not modify IL-2-dependent proliferation of 48-h PHA-blasts that already express high affinity IL-2R. These findings indicate that p29 mediates its biologic activity during early stages of T cell activation affecting the expression of high affinity IL-2R and production of IL-2, through a nontranscriptional mechanism involving an increase of intracellular cAMP.

Monoclonal antibodies specific for low-affinity interleukin-3 (IL-3) binding protein AIC2A: evidence that AIC2A is a component of a high- affinity IL-3 receptor

Mouse interleukin-3 (IL-3) binds to its receptor with high and low affinities. Using anti-Aic2 antibody, two distinct cDNAs (AIC2A and AIC2B) were isolated. The AIC2A gene encodes a protein of 120 Kd that binds IL-3 with low affinity, whereas the AIC2B gene encodes a protein that is 91% identical to AIC2A at the amino acid level, but which does not bind IL-3. To study the structure of the functional high-affinity IL-3 receptor (IL-3R), we generated specific monoclonal antibodies against the AIC2A protein. We produced a soluble AIC2A protein by inserting a termination codon at the beginning of the transmembrane domain of the AIC2A cDNA. Soluble AIC2A protein expressed in COS7 cells was purified to homogeneity and three anti-AIC2A monoclonal antibody- producing hybridomas (3D1, 3D4, and 9D3) were obtained from a rat immunized with the purified soluble AIC2A protein. The antibodies were specific for the AIC2A protein and did not bind to the AIC2B protein. Using chimeric receptors between AIC2A and AIC2B, recognition sites of the antibodies were mapped. The antibodies immunoprecipitated a 120-Kd protein from IL-3-dependent PT18 cells. The N-terminal sequence of the 120-Kd protein was consistent with the predicted processing site of the signal sequence of the AIC2A protein. Staining of IL-3-dependent and IL- 3-independent cell lines with the 9D3 antibody were consistent with the IL-3 binding. The 9D3 antibody inhibited both the high-affinity IL-3 binding and the low-affinity binding, as well as IL-3-dependent proliferation. These results indicate that the AIC2A protein is a binding component of a high-affinity IL-3R.

The antiglucocoticoid RU486 downregulates the expression of interleukin-2 receptors in normal human lymphocytes

The effects of the antiglucocorticoid RU486 on the expression of low and high affinity interleukin-2 receptors (IL-2R) in phytohaemagglutinin (PHA)-activated human peripheral blood lymphocytes were investigated. We demonstrated that RU486 inhibits in a dose-dependent way the expression of both classes of IL-2R, thereby mimicking the effects of the glucocorticoid agonist dexamethasone. The maximal effect on the low affinity binding sites was observed at 10 μM (28 ± 2% of control, P < 0.001) and on the high affinity IL-2R at 1 μM (from2938 ± 74 to 437 ± 108 binding sites per cell, P < 0.001). This inhibition of IL-2R expression occurs at a pretranslational level since RU486 decreased the accumulation of β-chain IL-2R mRNA transcripts. Our data support the concept that the antiglucocorticoid RU486 at pharmacological concentrations can exert agonistic-immunosuppresive effects.

Thymopentin and interleukin-2 in combination with 5-fluorouracil and leucovorin in metastatic colorectal adenocarcinoma: Preliminary results

Thymic hormones have been shown to exhibit immunorestorative effects in vivo and in vitro, and to enhance the expression of high affinity interleukin-2 (IL-2) receptors on normal human lymphocytes stimulated with phytohemagglutinin. Based on these data, a clinical trial was initiated to determine the effects of the combination of 5-fluorouracil (5-FU) and folinic acid (FA) with thymopentin (TP-5) and interleukin-2 (IL-2) in patients with advanced colorectal carcinoma. Fifteen patients were treated with FA, 200 mg/m2/day by IV bolus, and 5-FU, 400 mg/m2/day as a 30-minute infusion, both given for 5 consecutive days every 28 days. TP-5, 50 mg/day, was administered subcutaneously on days 8-11, and IL-2, 9 million IU/m2 twice daily, was given subcutaneously on days 12-16. Of 8 patients evaluable for response, 4 achieved a response. Two patients had stable disease, and two progressed during treatment. There were no instances of life-threatening toxicity. Two patients developed grade III stomatitis and diarrhea followed by leukopenia and fever, requiring hospitalization. Other toxicities were moderate. These results are only preliminary, and a larger number of patients and longer follow-up are needed to draw meaningful conclusions about the merits of this new approach in cancer treatment.

Cloning of an Interleukin-3 Receptor Gene: a Member of a Distinct Receptor Gene Family

Interleukin-3 (IL-3) binds to its receptor with high and low affinities, induces tyrosine phosphorylation, and promotes the proliferation and differentiation of hematopoietic cells. A binding component of the IL-3 receptor was cloned. Fibroblasts transfected with the complementary DNA bound IL-3 with a low affinity [dissociation constant (Kd) of 17.9 +/- 3.6 nM]. No consensus sequence for a tyrosine kinase was present in the cytoplasmic domain. Thus, additional components are required for a functional high affinity IL-3 receptor. A sequence comparison of the IL-3 receptor with other cytokine receptors (erythropoietin, IL-4, IL-6, and the beta chain IL-2 receptor) revealed a common motif of a distinct receptor gene family.

Evidence for a Low-Affinity Interleukin-3 Receptor

Interleukin-3 (IL-3) regulates the proliferation of myeloid, erythroid, and lymphoid cells. Previous reports showed IL-3 binding restricted to a single high-affinity (Kd = 50-200 pM) site. Here, we demonstrate by equilibrium studies an additional binding site for IL-3 with lower apparent affinity (Kd = 5-20 nM). Furthermore, kinetic analysis shows that two binding sites for IL-3 exist: IL-3 dissociates slowly from the first site (T1/2 = 4 hr; k-1 = 2.7 x 10(-3) min-1), whereas it dissociates rapidly (T1/2 = 4.0 min; k-1 = 0.116 min-1) from the second site. Cross-linking showed that [125I]IL-3 binding to the 115- and 140-kD proteins was not saturable at concentrations commensurate with high-affinity binding and IL-3 dissociated rapidly from these same molecules. Thus, the low affinity IL-3 receptor is a molecule(s) of 115- to 140-kD.

Phorbol myristate acetate induces both high affinity and low affinity interleukin 2-receptors on a pre-B leukemic cell line

The human pre-B leukemic cell line Reh6 does not constitutively express the Tac molecule (p55), which is one of the two polypeptide chains of the interleukin 2-receptor (IL2-R). Cell incubation with Phorbol Myristate Acetate (PMA) induces expression of the Tac molecule in a dose and time-dependent manner. Binding experiments with radiolabelled recombinant IL2 (rIL2) revealed both high and low affinity IL2-R (225 ± 105 sites/cell with a K d of 130 ± 51 pM and 24060 sites/cell with a K d of 17.3 nM respectively), whereas unstimulated Reh6 cells only expressed intermediate affinity IL2-R (58 sites per cell with a K d of 460 pM). Cross-linking of radiolabelled rIL2 to PMA-incubated Reh6 cells revealed the presence of two polypeptide chains of mol. wts 55,000 (Tac molecule) and 70,000, as in normal activated T cells, while the 70,000 mol. wt chain alone was observed in unstimulated Reh6 cells. IL2-R-bearing Reh6 cells could absorb rIL2 in a dose-dependent manner and this absorption was inhibited by a monoclonal antibody against the Tac molecule (anti-Tac). Moreover, partial internalization of IL2 bound under high affinity conditions occurred at 37°C. IL2-R expressed on PMA-induced cells were functional since rIL2 specifically enhanced the proliferation in vitro of PMA-treated cells in semi-solid but not liquid cultures. These findings thus demonstrate an IL2-dependent mechanism of proliferation in vitro of pre-B leukemic cells induced by PMA, which can express high affinity, functional IL2-R.

Thymosin alpha 1 modulates the expression of high affinity interleukin-2 receptors on normal human lymphocytes

In this report we demonstrate that thymosin alpha 1 (T α 1), a synthetic peptide composed of 28 amino acid residues, and thymosin fraction 5 (TF5) enhance the number of high affinity interleukin 2 receptors (IL-2R) expressed by human peripheral blood lymphocytes in response to in vitro stimulation with phytohemagglutinin (PHA). Thymosins did not, however, alter the affinity of the IL-2R for its ligand. Dose-response studies using a wide range of concentrations indicated a bimodal distribution of responsiveness to T α 1. In most experiments the high and low concentration peaks of activity were observed at 10 −8M and 10 −12M, respectively, although peak responses were observed at different T α 1 concentrations in different donors. No effects were elicited by thymosins in the absence of mitogenic stimulation. Thymosin enhancement of PHA-induced high affinity IL-2R expression directly correlated with increased levels of Tac antigen expression, as determined by flow cytometry, and enhanced interleukin 2 (IL-2) production. Since the biological effects of IL-2 are associated with the occupancy of high affinity IL-2R, the findings presented in this report strongly suggest that thymosins play a significant role in the regulation of immune responses through the modulation of high affinity IL-2R expression.

Enhanced expression of high affinity interleukin-2 (HIL-2) receptors in scleroderma: Possible role for interleukin-6 (IL-6): [formula omitted]. Medical University of South Carolina, Charleston, SC 29425

Enhanced expression of high affinity interleukin-2 (HIL-2) receptors in scleroderma: Possible role for interleukin-6 (IL-6): [formula omitted]. Medical University of South Carolina, Charleston, SC 29425

Are the IL‐2 Receptors Expressed in the Murine Fetal Thymus Functional?

It is well established that the majority of murine fetal thymocytes (day 15 of gestation) express receptors for interleukin 2 (IL-2), but the functional significance of these IL-2 receptors (IL-2Rs) is not clear. In situ hybridization data show a developmentally regulated expression of IL-2 and IL-2R mRNA. IL-2 binding studies were performed on fetal thymocytes and the results show the presence of both high (kD ≅ 20 pM) and low (kD ≅ 10 nM) affinity IL-2Rs. These IL-2Rs are indeed functional: intact fetal thymic lobes (but not cell suspensions) cultured in IL-2 exhibited an in vitro proliferative response at 20 pM of IL-2, corresponding with the presence of a functional high-affinity IL-2R on fetal thymocytes. The IL-2-dependent growth was primarily observed in the IL-2R + thymic subset, which contains the CD3-/CD4-/CD8- precursor thymocytes. Furthermore, in vitro blocking of IL-2 in intact fetal thymic lobes resulted in a reduction in the cell yield, which predominantly affected the expansion of the immature CD3-/CD4-/CD8-thymocytes. Our findings strongly support the concept that the IL-2/IL-2R pathway is responsible for the proliferation of precursor cells within the fetal thymus.

Crystallization and Preliminary X-ray Diffraction Studies of a Complex between Interleukin-2 and a Soluble Form of the p55 Component of the High Affinity Interleukin-2 Receptor

Human recombinant interleukin-2 (IL-2) and a soluble recombinant form of the human p55 (Tac antigen) component of the IL-2 receptor (IL-2R) have been cocrystallized in 1.7-1.8 M ammonium sulfate, in the pH range 7.0-8.2. Variously glycosylated forms of both receptor and ligand can be cocrystallized under those conditions. The best crystals of the putative receptor-ligand complex involve the enzymatically desialylated receptor and unglycosylated IL-2. These crystals belong to the trigonal space group P3(1)2(1) or its enantiomorph, with unit cell dimensions a = b = 91 A and c = 119 A, and diffract to 3.5 A resolution. There is one receptor-ligand complex asymmetric unit, with a Matthews coefficient of 2.7, assuming the presence of one IL-2 molecule-receptor molecule. Interestingly, in addition to IL-2 (Mr = 14,000), the p55 IL-2 receptor (Mr = 44,000) and two fragments of the receptor, of apparent Mr = 35,000 and 25,000, respectively, in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the crystals are enriched in a reducible dimeric form of the desialylated receptor (apparent Mr = 90,000), as compared with protein solution from which the crystals grow. The overall amino acid content in the crystals is consistent with a 1:1 ratio of receptor to ligand. A native data set has been collected on a multiwire area detector and the search for suitable heavy atom derivatives is in progress.