We have identified a cDNA from a human phytohemagglutinin-activated lymphoblast library encoding a protein that binds 125

We previously described the cloning of a cDNA encoding an interleukin-12 receptor (IL-12R) subunit, designated beta, that bound IL-12 with low affinity when expressed in COS cells. We now report that a pair of monoclonal antibodies (mAb), 2B10 and 2.4E6, directed against different epitopes on the IL-12R beta chain, when used in combination, strongly inhibited IL-12-induced proliferation of activated T cells, IL-12-induced secretion of interferon-gamma by resting peripheral blood mononuclear cells (PBMC), and IL-12-mediated lymphokine-activated killer cell activation. The mAb had no effect on lymphoblast proliferation induced by IL-2, -4, or -7. Thus, the IL-12R beta chain appears to be an essential component of the functional IL-12R on both T and natural killer (NK) cells. We previously observed that high affinity IL-12R were expressed on activated T and NK cells, but not B cells. Studies using flow cytometry and reverse transcription-polymerase chain reaction analysis showed that IL-12R beta chain was expressed on several human T, NK, and (surprisingly) B cell lines, but not on non-lymphohematopoietic cell lines. The Kit225/K6 (T cell) and SKW6.4 (B cell) lines were found to express the greatest amounts of IL-12R beta chain (800-2500 sites/cell); however, Kit225/K6 but not SKW6.4 cells bound IL-12. Similar to SKW6.4 B cells, activated tonsillar B lymphocytes expressed IL-12R beta chain but, consistent with previous results, did not display detectable IL-12 binding. Likewise, up to 72% of resting PBMC from normal volunteer donors expressed IL-12R beta, but did not bind measurable amounts of IL-12. These results indicate that expression of IL-12R beta is essential, but not sufficient, for expression of functional IL-12R. We speculate that expression of functional, high-affinity IL-12R may require the presence of a second subunit that is more restricted in its expression than IL-12R beta.

We have previously described the identification of a protein, now designated IL-12R beta 1, that binds 125I-huIL-12 with a Kd of about 10 nM, corresponding to the low affinity 125I-huIL-12 binding sites seen on PHA-activated human lymphoblasts. Using expression cloning techniques, we have recently identified an additional IL-12-binding protein subunit, IL-12R beta 2, which binds 125I-huIL-12 with a Kd of about 5 nM when expressed alone in COS-7 cells. Coexpression of IL-12R beta 1 and IL-12R beta 2 in COS-7 cells results in formation of two classes of 125 I-huIL-12-binding sites with Kds of about 50 pM and 5 nM. Mouse IL-12 p40 subunit homodimer (mo(p40)2) blocked 125I-huIL-12 binding to human IL-12R beta 1, but did not inhibit binding to human IL-12R beta 2. In contrast, anti-huIL-12 monoclonal antibody 20C2, which does not block 125I-huIL-12 binding to human IL-12R beta 1, completely blocked binding to human IL-12R beta 2. These results demonstrate that two classes of IL-12 inhibitors, one that primarily blocks IL-12/IL-12R beta 1 interaction (e.g., mo(p40)2), and one that primarily blocks IL-12/IL-12R beta 2 interaction (e.g., 20C2), can be identified.

A cDNA encoding a human IL-12R subunit was isolated by expression cloning. This subunit is a 662 amino acid type I transmembrane protein with an extracellular domain of 516 amino acids and a cytoplasmic domain of 91 amino acids. It is a member of the hemopoietin receptor superfamily and is most closely related over its entire length to gp130 and the receptors for granulocyte-CSF (G-CSF) and leukemia-inhibitory factor. When expressed in COS cells, this IL-12R subunit binds both human and murine IL-12 with an apparent affinity of 2 to 5 nM. The transfected COS cells express both monomers and disulfide-linked dimers or oligomers of the IL-12R subunit on their surface. However, unlike the IL-6-induced dimerization of gp130, the oligomerization of the IL-12R subunit is not dependent on binding of IL-12. Only the IL-12R subunit dimers/oligomers but not the monomers bind IL-12 with an affinity of 2 to 5 nM. A polyclonal antiserum raised against this receptor subunit specifically inhibits IL-12-induced proliferation of PHA-activated PBMC. The data are consistent with the hypotheses that 1) a dimer/oligomer of the cloned IL-12R subunit (IL-12R-beta) represents the low affinity IL-12 binding site identified on human lymphoblasts, 2) the cloned receptor subunit is involved in IL-12 signal transduction, and 3) an additional, as of yet unidentified subunit is required to generate a high affinity IL-12R complex.

IL-12 is a heterodimeric cytokine, composed of a p40 and a p35 subunit, that exerts its biological effects by binding to specific cell surface receptors. Two IL-12R proteins, designated human IL-12 (huIL-12) receptor β1 (huIL-12Rβ1) and huIL-12Rβ2, have been previously identified. These IL-12R individually bind huIL-12 with low affinity and in combination bind huIL-12 with high affinity and confer IL-12 responsiveness. In this study the interactions of huIL-12 with these two identified human IL-12R protein subunits are examined. The heterodimer-specific anti-huIL-12 mAb 20C2, which neutralizes huIL-12 bioactivity but does not block 125I-huIL-12 binding to huIL-12Rβ1, blocked binding of huIL-12 to huIL-12Rβ2. In contrast, anti-huIL-12Rβ1 mAb 2B10 and mouse IL-12 p40 subunit homodimer (mo(p40)2) blocked 125I-huIL-12 binding to huIL-12Rβ1, but not to huIL-12Rβ2. Therefore, two classes of IL-12 inhibitors can be identified that differ in their ability to block huIL-12 interaction with either huIL-12Rβ1 or huIL-12Rβ2. Both mo(p40)2 and 20C2 blocked high affinity binding to huIL-12Rβ1/β2-cotransfected COS-7 cells, although, as previously reported, mo(p40)2 does not block high affinity binding to IL-12R on PHA-activated human lymphoblasts. Furthermore, these two classes of IL-12 inhibitors synergistically decreased huIL-12-stimulated proliferation and IFN-γ production. Therefore, IL-12, in binding to the high affinity IL-12R, interacts with IL-12Rβ1 primarily via regions on the IL-12 p40 subunit and with IL-12Rβ2 via 20C2-reactive, heterodimer-specific regions of IL-12 to which the p35 and p40 subunits both contribute.

Type I IFNs (IFN-α/β), in addition to IL-12, have been shown to play an important role in the differentiation of human, but not mouse, Th cells. We show here that IFN-α/β act directly on human T cells to drive Th1 development, bypassing the need for IL-12-induced signaling, whereas IFN-α cannot substitute IL-12 for mouse Th1 development. The molecular basis for this species specificity is that IFN-α/β activate Stat4 in differentiating human, but not mouse, Th cells. Unlike IL-12, which acts only on Th1 cells, IFN-α/β can activate Stat4 not only in human Th1, but also in Th2 cells. However, restimulation of human Th2 lines and clones in the presence of IFN-α does not induce the production of IFN-γ. These results suggest that activation of Stat4, which is necessary for the differentiation of naive T cells into polarized Th1 cells, is not sufficient to induce phenotype reversal of human Th2 cells.

Great attention has been placed on the possibility of distinguishing Th1 from Th2 cells on the basis of differential expression of surface receptors. We have recently shown that the differential expression of the IL-12Rβ2 chain in Th1 and Th2 cells, as measured at the mRNA level, accounts for an important regulatory mechanism in the differentiation of the two cell subsets. In this study, we identify IL-12R expression at the protein level. We have generated an anti-IL-12Rβ2-specific mAb and analyzed IL-12Rβ2 expression on polarized Th cell populations generated in vitro and on T cells derived from patients with Th1- or Th2-mediated inflammatory conditions. Although IL-12Rβ2 was absent in freshly isolated PBMC and in cord blood cells, we were able to detect IL-12Rβ2 expression selectively in differentiated Th1 and T cytotoxic 1, but not Th2 or T cytotoxic 2 cells. In the presence of IL-12, cell surface expression of the IL-12Rβ2 subunit was readily detected on T cells after 24 h, reached the maximum at day 5, and declined thereafter. Most importantly, the anti-IL-12Rβ2 mAb recognizes lung T cells from patients with sarcoidosis, a disease characterized by a typical cell-mediated, Th1-type inflammatory response. In contrast, IL-12Rβ2 was absent in lung T cells from patients with allergic asthma, a disease characterized by a Th2-type inflammatory response. The mAb reported in this study should represent a powerful tool to investigate the role of Th1 and Th2 cells in inflammatory conditions and to monitor therapies aimed at altering the balance of Th cell subsets.