Abstract:
Methods and compositions for using the MHC class II invariant chain polypeptide, Ii (also known as CD74), as a receptor for macrophage migration inhibitory factor (MIF), are disclosed. These include methods and compositions for using this receptor, as well as agonists and antagonists of MIF which bind to this receptor, or which otherwise modulate the interaction of MIF with CD74 or the consequences of such interaction, in treatment of conditions characterized by locally or systemically altered MIF levels, particularly inflammatory conditions and cancer.

Description:
[0001]    This application claims priority from U.S. Provisional Application Ser. No. 60/279,435 filed Mar. 29, 2001. The entirety of that provisional application is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to methods and compositions for using the MHC class II invariant chain polypeptide, Ii (also known as CD74), as a receptor for macrophage migration inhibitory factor (MIF), including methods and compositions for using this receptor, as well as agonists and antagonists of MIF which bind to this receptor or which otherwise modulate the interaction of MIF with CD74 or the consequeces of such interaction, in methods for treatment of conditions characterized by locally or systemically altered MIF levels, particularly inflammatory conditions and cancer. 
         [0004]    2. Background of the Technology 
         [0005]    Macrophage migration inhibitory factor (MIF), the first cytokine activity to be described, has emerged to be seen as a critical regulator of the innate and adaptive immune response 1-3 . MIF is encoded by a unique gene, and crystallization studies have shown MIF to define a new protein fold and structural superfamily 4 . Despite the fact that the biological activity attributed to MIF first was described almost 30 years ago, information regarding MIF&#39;s precise role in cell physiology and immunity has emerged only recently 1-9,18 . MIF is centrally involved in macrophage and T cell activation and in the development of septic shock, arthritis, and other inflammatory conditions 2 . Also, MIF has been linked to cancer 32 . 
         [0006]    MIF is critically involved in the expression of innate and acquired immunity. MIF is released by a variety of cell types and is a necessary factor for the activation or proliferative responses of macrophages 18 , T cells 6 , and fibroblasts 7 . MIF&#39;s mitogenic effects proceed via an autocrine/paracrine activation pathway involving the p44/p42 (ERK-1/2) mitogen-activated protein kinase cascade 7 . MIF −/− mice are highly resistant to endotoxic shock 3 , and immunoneutralization of MIF confers protection against septic shock 25  and a variety of immuno-inflammatory pathologies such as delayed-type hypersensitivity 26 , arthritis 27 , and glomerulonephritis 28 . MIF&#39;s actions on cells also show a number of unique features. These include a global, counter-regulatory action on glucocorticoid-induced immunosuppression 5,6 , the induction of a sustained pattern of ERK-1/2 activation 7 , and functional antagonism of p53-dependent apoptosis 6 . 
         [0007]    MIF&#39;s pro-inflammatory properties have been linked to its capacity to counter-regulate the immunosuppressive effects of glucocorticoids 5,6 , and its interactions with cells have been presumed to require a receptor-based mechanism of action 7,8  or to reflect a specialized, intracellular mode of action 9 . Numerous in vitro and in vivo studies have been consistent with MIF acting by engagement of a cell surface receptor, however lack of progress toward the identification of candidate receptors has prompted interest in either specialized, intracellular modes of action 9  or the potential biological role of MIPs tautomerase activity 2,21 . There also is evidence that MIF may function as an isomerase 4 . 
         [0008]    The MHC class II-associated invariant chain, Ii (CD74) 10 , has been established to play an important role in the processing and transport of MHC class II proteins from the endoplasmic reticulum to the Golgi 10 . Most Ii dissociates from the class II complex as antigenic peptides load onto their class II binding sites. Approximately 2-5% of total cellular Ii also is expressed on the cell surface 17 , where it has been shown to function as an accessory molecule for T cell activation 11 . Ii has been previously implicated in signaling and accessory functions for immune cell activation 11-13 . 
         [0009]    U.S. Pat. No. 5,559,028 to Humphreys et al. discloses gene constructs for expression of wild type and mutant Ii chains in recombinant cells. U.S. Pat. No. 5,726,020 to Humphreys , et al. discloses and claims expressible reverse gene constructs and oligonucleotides that hybridize with an Ii mRNA molecule, thereby inhibiting translation of the Ii mRNA molecule. 
       SUMMARY OF THE INVENTION 
       [0010]    The invention is based in part upon the identification, utilizing expression cloning and functional analyses, that the Class II-associated invariant chain polypeptide, Ii (or CD74) 10 , is a cellular receptor for MIF. Thus, MIF binds to the extracellular domain of Ii, a Type II membrane protein, and Ii is required for MIF-induced cell activation and/or phenotypic changes including, for instance, signaling via the extracellular signal-related kinase (ERK)-1/2MAP kinase cascade and cell proliferation. The inventive relationship provides a mechanism for MIF&#39;s activity as a cytokine and identify it as a natural ligand for Ii, which has been previously implicated in signaling and accessory functions for immune cell activation. 
         [0011]    Accordingly, one aspect of the present invention relates to methods for screening compounds to identify positive or negative modulators of MIF binding to, or activity in connection with binding to, CD74. In a first instance, such a method comprises a biochemical (i.e., acellular) binding assay, comprising: contacting an MHC class II invariant chain (Ii) polypeptide with MIF in the presence and absence of a test compound, and comparing the binding interaction of the MIF and Ii polypeptides in the presence of the test compound with their interaction in the absence of the test compound, whereby a compound that positively modulates the interaction of MIF with the Ii polypeptide is identified as an enhancer of MIF binding activity and a compound that negatively modulates the interaction of MIF with the Ii polypeptide is identified as an inhibitor of MIF binding activity. Enhancers so identified are candidate therapeutic agonists or enhancers of MIF, whereas inhibitors so identified are candidate therapeutic antagonists of MIF. For instance, a test compound may reinforce the binding of MIF to the Ii polypeptide (i.e., increase the affinity of the interaction) and thereby enhance the interaction of MIF and the Ii polypeptide. Such an enhancer is thereby identified as an agonist or enhancer of MIF, and is identified as a candidate therapeutic agent to enhance, independently or in connection with endogenous or exogenous MIF, MIF effects in subjects requiring such augmentation. Alternatively, a test compound that competes with MIF for binding to the Ii polypeptide or otherwise inhibits the interaction of the MIF with the Ii polypeptide is identified as an antagonist of MIF, and is identified as a candidate therapeutic agent to antagonize MIF effects in subjects requiring such antagonism. In this biochemical binding assay, the Ia polypeptide comprises the complete Ia sequence or an MIF-binding fragment thereof, and the assay is conveniently conducted with recombinantly prepared MIF and Ii peptides, one of which is optionally immobilized to a solid support, and one of which (or a binding partner thereto, such as an antibody) is labeled to facilitate detection and measurement of the MIF:Ii binding interaction. 
         [0012]    In a second aspect, the binding assay may be a cellular binding assay, comprising CD74 expressed (either normally or as a consequence of genetic engineering for Ii expression) by a cell (prokaryotic or eukaryotic), typically on the cell surface, and MIF binding thereto is detected and measured in the presence or absence of a test compound. As in the above described biochemical or acellular assay, a comparison is made of the binding interaction of the MIF and the cell-displayed Ii polypeptide in the presence of the test compound with their interaction in the absence of the test compound, whereby a compound that positively modulates the interaction of MIF with the Ii polypeptide (i.e., increases their affinity) is identified as an enhancer of MIF binding activity and a compound that negatively modulates the interaction of MIF with the Ii polypeptide (i.e., decreases their affinity) is identified as an inhibitor of MIF binding activity. Enhancers so identified are candidate therapeutic agonists or enhancers of MIF, whereas inhibitors so identified are candidate therapeutic antagonists of MIF. 
         [0013]    In a third aspect, the cellular assay is a signaling assay, in which the activity of an intracellular signaling cascade is measured before and after MIF is contacted to cell-displayed CD74 polypeptide, either in the presence or the absence of a test compound. Preferably, the signaling assay is an ERK-1/2 activation assay. A test compound that positively modulates the signaling activity of MIF via interaction with the Ii polypeptide is identified as an enhancer of MIF signaling activity and a compound that negatively modulates the signaling of MIF via interaction of MIF with the Ii polypeptide is identified as an inhibitor of MIF signaling activity. Enhancers so identified are candidate therapeutic agonists or enhancers of MIF, whereas inhibitors so identified are candidate therapeutic antagonists of MIF. 
         [0014]    In a fourth aspect, the cellular assay is a cellular activity or cell phenotype assay, in which the activity or phenotype of a target cell is measured before and after MIF is contacted to cell-displayed CD74 polypeptide, either in the presence or the absence of a test compound. Preferably, the activity or phenotype assay is a proliferation assay or an assay for functional antagonism of p53-dependent apoptosis. A test compound that positively modulates the chosen cellular activity or phenotypic change mediated by MIF via interaction with the Ii polypeptide is identified as an enhancer of MIF cellular activity and a compound that negatively modulates the chosen cellular activity or phenotypic change mediated by MIF via interaction with the Ii polypeptide is identified as an inhibitor of MIF cellular activity. Enhancers so identified are candidate therapeutic agonists or enhancers of MIF, whereas inhibitors so identified are candidate therapeutic antagonists of MIF. 
         [0015]    The invention also provides an enhancer of MIF, including an agonist, or an inhibitor, including an antagonist of MIF, identified by any of the methods above. One form of such an agonist or antagonist would be an antibody or antigen-binding fragment thereof, such as an anti-CD74 antibody. Anti-CD74 antibodies and CD74-binding fragments thereof are known in the art. For instance, the anti-CD74 antibody may be a monoclonal antibody and also may be a human, humanized or chimeric antibody, made by any conventional method. 
         [0016]    Another aspect of the invention relates to a method of inhibiting an effect of MIF on a cell comprising on its surface an MHC class II invariant chain (Ii) polypeptide which binds MIF and thereby mediates the effect of MIF. This method comprises: contacting the cell with an antagonist or other inhibitor of MIF, where the antagonist or inhibitor inhibits, in a first instance, binding of MIF to the Ii polypeptide; in a second instance, signaling initiated by MIF:Ii interaction; and in a third instance, a change in cellular activity, metabolism or phenotype effected by MIF:Ii interaction. In any of these methods the antagonist or inhibitor may be an antibody or fragment thereof which binds to the Ii polypeptide. Alternatively, the inhibitor may be soluble Ii polypeptide or a soluble MIF-binding fragment thereof which inhibits the interaction of MIF and Ia polypeptide (or the cellular consequences of such interaction) by binding to MIF or by interacting with Ii polypeptide on the surface of a cell. In some cases, the cell comprising Ii polypeptide is present in a mammal and the antagonist or other inhibitor is administered to the mammal in a pharmaceutical composition. A mammal that would benefit from this method is a mammal suffering from a condition or disorder characterized by MIF levels locally or systemically elevated above the normal range found in mammals not suffering from such a condition. In such a case, the antagonist or inhibitor is administered in an amount effective to treat the condition or disorder. For instance, the mammal may be suffering from cancer or an inflammatory disorder, and the antagonist or inhibitor is administered in an amount effective to treat the cancer or inflammatory disorder. The inflammatory disorder may be, for instance, septic shock or arthritis. 
         [0017]    More particularly, one aspect of the invention is a method of inhibiting an activity of MIF, which method comprises: contacting MIF with an MHC class II invariant chain (Ii) polypeptide or a fragment thereof which binds to MIF. The fragment of the MHC class II invariant chain (Ii) polypeptide which binds to MIF may be a soluble form of the polypeptide, particularly a soluble form that comprises the extracellular binding domain of this type II transmembrane polypeptide. In some cases, the MIF to be inhibited is in a mammal and the Ii polypeptide or a fragment thereof is administered to the mammal in a pharmaceutical composition. Where the mammal suffers from cancer or an inflammatory disorder, such as septic shock or arthritis, the Ii polypeptide or fragment thereof is administered in an amount effective to treat the disorder. In a further instance, the MIF antagonist or inhibitor is administered in an amount effective to treat an infectious disease, in which disease MIF or a polypeptide evolutionarily related to MIF (as evidenced by sequence homology) deriving from the infecting pathogen (whether a virus, bacterial, fungus, or especially, a parasite) is present locally, systemically, or at the host:pathogen interface. 
         [0018]    Yet another aspect of the invention relates to a method of purifying MIF comprising: contacting a sample comprising MIF with an MHC class II invariant chain (Ii) polypeptide or a fragment thereof which binds to MIF, under conditions that promote the specific binding of MIF to the Ii polypeptide or fragment thereof, and separating the MIF:Ii polypeptide complex thereby formed from materials which do not bind to the Ii polypeptide or fragment thereof. In this method, the Ii polypeptide may be immobilized on a solid support matrix. The invention also provides a method of assaying for the presence of MIF comprising: contacting a sample with an MHC class II invariant chain (Ii) polypeptide or a fragment thereof which binds to MIF under conditions that promote the specific binding of MIF to the Ii polypeptide or fragment thereof, and detecting any MIF:Ii polypeptide or MIF:Ii polypeptide fragment complex thereby formed. 
         [0019]    Still another method provided by the invention is a method for reducing an effect of MIF on a cell comprising on its surface an MHC class II invariant chain (Ii) polypeptide or fragment thereof which binds MIF and thereby mediates the effect of MIF. This method comprises: providing to the cell an antisense nucleic acid molecule in an amount effective to reduce the amount of Ii polypeptide produced by the cell. The antisense nucleic acid molecule specifically binds to a portion of mRNA expressed from a gene encoding the MHC class II invariant chain (Ii) polypeptide and thereby decreases translation of the mRNA in the cell and, ultimately, the level of Ii polypeptide on the surface of the cell. In this method the cell comprising the Ii polypeptide may be in a mammal, for instance, a mammal suffering from a condition or disorder characterized by MIF levels locally or systemically elevated above the normal range in mammals not suffering from such a condition or disorder. For instance, the mammal may be suffering from a cancer or an inflammatory disorder, such as septic shock or arthritis. In such a case, the antisense nucleic acid is administered in a pharmaceutical composition, in an amount effective to treat the condition or disorder. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0020]      FIG. 1  illustrates high affinity binding of MIF to THP-1 monocytes, a, Alexa-MIF shows full retention of dose-dependent MIF biological activity as assessed by activation of the p44/p42 (ERK-1/2) MAP kinase cascade, visualized by western blotting of cell lysates using antibodies specific for phospho-p44/p42 or total p44/p42; and b, suppression of p53-dependent apoptosis induced by serum starvatuin (CM: complete medium, SFM: serum-free medium). MIF or Alexa-MIF were added at 50 ng/ml. Data shown are Mean±SD of triplicate wells and are representative of 3 independent experiments. Further evidence for the retention of native structure by Alexa-conjugation was provided by the measurement of MIF tautomerase activity using L-dopachrome methyl ester as substrate 25 . No difference in the tautomerase activity of Alexa-MIF versus unconjugated MIF was observed (Alexa-MIF: ΔOD 475 =0.275 sec −1  μg −1  protein, versus rMIF: ΔOD 475 =0.290 sec −1  μg −1 ; P=NS) c, Flow cytometric analysis shows the binding of Alexa-MIF to THP-1 monocytes is markedly enhanced by IFN-δ treatment. Competition for Alexa-MIF binding was performed in the presence of 1 μg/ml unlabeled, rMIF d, Direct visualization of Alexa-MIF binding to THP-1 monocytes by confocal microscopy THP-1 cells were grown on cover slips, incubated with INFγ (1 ng/ml) for 72 hrs and stained with Alexa-MIF (left panel) or Alexa-MIF plus excess, unlabeled rMIF (right panel). Cell bound Alexa-MIF was rapidly internalized upon shifting cells from 4° to 37° for 15 mins (right panel). Magnification: 630× e, Binding characteristics of Alexa-MIF to IFNγ-activated, THP-1 monocytes. The inset shows the binding data transformed by Scatchard analysis, indicating two distinct binding activities; one with K d =3.7×10 −8  m the other with K d =3.5×10 5 , data are representative of 3 independent experiments. 
           [0021]      FIG. 2  show that Ii is a cell surface binding protein for MIF a, Sequential cycles of fluorescence-activated cell-sorting of COS-7 cell transfectants shows enrichment for MIF binding activity b, Diagrams indicating structure of Ii (35 kDa isoform), and three of ten representative cDNA clones with MIF binding activity. IC, TM, and EC are the intracellular, transmembrane, and extracellular domains. M1 and M17 refer to two sites of alternative translation initiation. c, Flow cytometry analysis of MIF binding to Ii-expressing cells. Enhanced binding of Alexa-MIF to Ii-transfected versus control vector-transfected COS-7 cells (left panel), inhibited binding of Alexa-MIF to Ii-transfected COS-7 cells incubated with anti-Ii mAb (clone LN2) versus an isotypic mAb control (con mAb) (middle panel), and enhanced binding of Alexa-MIF to IFNγ-stimulated, THP-1 monocytes incubated with anti-Ii mAb (clone LN2) versus an isotypic mAb control (right panel). The data shown are representative of at least three independent experiments. The anti-Ii mAb, LN2 (PharMingen), is reactive with an epitope residing within 60 amino acids of the extracytoplasmic, C-terminus of the protein d, MIF binds to the extracellular domain of Ii in vitro. [ 35 S]-Ii protein was prepared in a coupled transcription and translation reaction utilizing plasmids encoding Ii fragments of different lengths. Protein-protein interaction was assessed by measuring bound radioactivity in 96-well plates that were pre-coated with MIF (n=6 wells per experiment). The data shown are representative of three experiments, showing that MIF binding is severely compromised with vectors expressing Ii fragments of amino acids 1-72 or 1-109 versus robust binding with vectors expressing Ii fragments of amino acids 1-149 or 1-232 (full-length). 
           [0022]      FIG. 3  illustraes Ii mediation of MIF stimulation of ERK-1/2 (p44/p42) phosphorylation in COS-7 cells a, ERK-1/2 phosphorylation is induced by MIF in COS-7 cells transfected with Ii vector (COS-7/Ii) or control vector (COS-7/V). Cells were treated without or with various doses of rMIF for 2.5 hrs and analyzed for phospho-p44/p42 and total p44/p42 by western blotting b, There is dose-dependent inhibition of MIF-induced ERK-1/2 phosphorylation by anti-Ii mAb. COS-7 cells were transfected with an Ii vector and stimulated with 50 ng/ml MIF for 2.5 hrs in the presence of an isotypic control mAb or an anti-Ii mAb (clone LN2) at different doses. In control experiments, anti-Ii showed no effect on ERK-1/2 phosphorylation in the absence of added MIF (data not shown). 
           [0023]      FIG. 4  illustrates western blots of MIF-induced phosphorylation a, MIF dose dependently stimulates ERK-1/2 (p44/p42) phosphorylation in human Raji B cells, as visualized by western blotting for phospho-p44/p42 b, There is inhibition of MIF-induced ERK-1/2 phosphorylation in Raji cells by anti-Ii mAb also. Raji cells were stimulated with 50 ng/ml of MIF for 2.5 hrs in the presence of an isotype control antibody (Con Ab) or the two anti-Ii mAbs,—B741 or LN2, each added at 50 μg/ml. c, Anti-Ii inhibits MIF-induced Raji cell proliferation quantified by  3 H-thymidine incorporation d, Anti-Ii inhibits MIF-induced proliferation of human fibroblasts also. Antibodies were added to a final concentration of 50 μg/ml. The results shown are the Mean±SD of triplicate assays and are representative of at least three separate experiments. Anti-Ii antibodies showed no effect on cell proliferation in the absence of added MIF (data not shown). 
           [0024]      FIG. 5  shows the complete nucleotide sequence (SEQ ID NO: 1) and longest translated amino acid sequence (beginning at nt 8; SEQ ID NO: 2) of the human mRNA for the Ii polypeptide (HLA-DR antigens associated invariant chain p33 [GenBank Accession Nr. X00497 M14765]), as reported in Strubin, M. et al., The complete sequence of the mRNA for the HLA-DR-associated invariant chain reveals a polypeptide with an unusual transmembrane polarity. EMBO J., 3, 869-872 (1984). 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    The following abbreviations are used herein: Alexa-MIF: Alexa 488-MIF conjugate, ERK: extracellular-signal-regulated kinase, Ii: MHC class II-associated invariant chain (CD74), INFγ: interferon-γ, mAb: monoclonal antibody, MIF: macrophage migration inhibitory factor. 
         [0026]    Utilizing expression cloning and functional analyses, we have identified as a cellular receptor for MIF the Class II-associated invariant chain, Ii (CD74) 10 . MIF binds to the extracellular domain of Ii, a Type II membrane protein, and Ii is required for MIF-induced cellular effects, including for instance, activation of the ERK-1/2 MAP kinase cascade and cell proliferation. These data provide a mechanism for MIF&#39;s activity as cytokine and identify it as a natural ligand for Ii, which has been previously implicated in signaling and accessory functions for immune cell activation 11-13 . We linked the fluorescent dye Alexa 488 14  to recombinant MIF by standard techniques, verified the retention of biological activity of the conjugate ( FIG. 1A , B), and conducted binding experiments with a panel of cell types known to respond to MIF. By way of illustration, using flow cytometry, we observed high-affinity binding of Alexa-MIF to the surface of the human monocytic cell line, THP-1. This binding activity was induced by activation of monocytes with interferon-γ (IFNγ), and was competed by the addition of excess, unlabeled MIF ( FIG. 1C ). Confocal microscopy and direct visualization of IFNγ-treated monocytes at 4° C. showed surface binding of Alexa-MIF, and cell-bound Alexa-MIF was internalized upon shifting temperature to 37° C. ( FIG. 1D ). Quantitative binding studies performed with increasing concentrations of Alexa-MIF revealed two apparent classes of cell surface receptors ( FIG. 1E ). The higher affinity binding activity showed a K d  of 3.7×10 −8  M and 3.1×10 4  binding sites per cell, and the lower affinity binding showed a K d  of 3.5×10 −7  M and 4.9×10 4  sites per cell. 
         [0027]    To identify the MIF receptor, we prepared cDNA from IFNγ-activated THP-1 monocytes and constructed a mammalian expression library in the lambdaZAP-CMV vector 15 . Library aliquots representing a total of 1.5×10 7  recombinants were transfected into COS-7 cells, which we had established previously to exhibit little detectable binding activity for MIF, and the transfectants were analyzed by flow cytometry for Alexa-MIF binding. Positively-staining cells were isolated by cell sorting, and the cDNA clones collected, amplified, and re-transfected into COS-7 cells for additional rounds of cell sorting ( FIG. 2A ). After four rounds of selection, single colonies were prepared in  E. coli  and 250 colonies were randomly picked for analysis. We sequenced 50 clones bearing cDNA inserts of ≧1.6 kB and observed that 10 encoded the Class II-associated invariant chain, Ii (CD74), a 31-41 kD Type II transmembrane protein 16 . While the isolated clones differed with respect to their total length, each was in the sense orientation and encoded a complete extracellular and transmembrane domain ( FIG. 2B ). 
         [0028]    To confirm that Ii is a cell surface binding protein for MIF, we analyzed the binding of Alexa-MIF to COS-7 cells transfected with an Ii expression plasmid ( FIG. 2C ). Binding was inhibited by excess, unlabeled MIF (data not shown), and by an anti-Ii mAb directed against the extracellular portion of the protein. Anti-Ii mAb also inhibited the binding of Alexa-MIF to IFN-γ stimulated THP-1 monocytes. The inhibition by anti-Ii mAb of Alexa-MIF binding to THP-1 monocytes was significant, but partial, consistent with the interpretation that Ii represents one of the two classes of cell surface receptors for MIF revealed by Scatchard analysis ( FIG. 1E ). [ 35 S]-Ii protein prepared by a coupled transcription and translation reticulocyte lysate system bound to MIF in vitro, and the principal binding epitope was localized to a 40 amino acid region contained within the Ii extracellular domain ( FIG. 2D ). 
         [0029]    To verify the functional significance of MIF binding to Ii in an exemplary system, we examined the activity of MIF to stimulate ERK-1/2 activation and cellular proliferation in different Ii-expressing cells. We observed an MIF-mediated increase, and a dose-dependent, anti-Ii mAb-mediated decrease, in ERK-1/2 phosphorylation in Ii-transfected COS-7 cells ( FIG. 3 ). Irrespective of Ii gene transfection however, we could not detect any proliferative effect of MIF on this monkey epithelial cell line (data not shown). We then examined the activity of MIF to induce ERK-1/2 activation and downstream proliferative responses in the human Raji B cell line, which expresses a high level of Ii 19 . MIF stimulated the phosphorylation of ERK-1/2 in quiescent Raji cells, and each of two anti-Ii mAbs blocked this stimulatory effect of MIF (FIG.  4 A,B). Of note, the inhibitory effect of anti-Ii on ERK-1/2 phosphorylation was associated with a significant decrease in the MIF-stimulated proliferation of these cells ( FIG. 4C ). Additionally, we confirmed the role of the MIF-Ii stimulation pathway in cells outside the immune system. MIF extends the lifespan of primary murine fibroblasts 8 , and both MIF&#39;s mitogenic effects and its induction of the ERK-1/2 signal transduction cascade have been best characterized in this cell type 7 . Fibroblasts express low levels of Ii 20 , and we observed that anti-Ii significantly inhibited both ERK-1/2 phosphorylation and the mitogenic effect of MIF on cultured fibroblasts ( FIG. 4D  and data not shown). 
         [0030]    In prior experiments, we have experienced considerable difficulty in preparing a bioactive,  125 I-radiolabelled MIF, and have observed the protein to be unstable to the pH conditions employed for biotin conjugation. By contrast, modification of MIF by Alexa 488 at a low molar density produced a fully bioactive protein which enabled identification of MIF receptors on human monocytes, and the expression cloning of Ii as a cell surface MIF receptor. These data significantly expand our understanding of Ii outside of its role in the transport of class II proteins, and support recent studies which have described an accessory signaling function for Ii in B and T cell physiology 10-13 . 
         [0031]    These findings provide a first insight into the long sought-after MIF receptor, although additional proteins are likely involved in some MIF-mediated activities. For instance, like MIF, Ii is a homotrimer 23 , and the Ii intracellular domain consists of 30-46 amino acids, depending on which of two in-phase initiation codons are utilized 16 . Monocyte-encoded Ii has been shown to enhance T cell proliferative responses, and this accessory function of Ii has been linked to a specific, chondroitin-sulphate-dependent interaction between Ii and CD44 11 . We have observed an inhibitory effect of anti-CD44 on ERK-1/2 phosphorylation, but not MIF binding, in Ii-expressing cells. This is consistent with the inference that MIF-bound Ii is a stimulating ligand for CD44-mediated MAP kinase activation. CD44 is a highly polymorphic Type I transmembrane glycoprotein 24 , and CD44 likely mediates some of the downstream consequences of MIF binding to Ii. 
         [0032]    Interference in the signal transduction pathways induced by MIF-Ii interaction, for instance by providing antagonists or inhibitors of MIF-Ii interaction, offers new approaches to the modulation of cellular immune and activation responses to MIF. Agents active in this regard (agonists and antagonists and other inhibitors) have predicted therapeutic utility in diseases and conditions typified by local or systemic changes in MIF levels. 
         [0033]    The specific binding interaction between MIF and the class II invariant chain polypeptide, Ii, also makes convenient the use of labeled MIF reagents as “Trojan horse-type” vehicles by which to concentrate a desired label or toxin in cells displaying cell surface Ii. Briefly, a desired label or toxic entity is associated with an MIF ligand (for instance, by covalent attachment), and the modified MIF ligand then is presented to cells displaying cell surface-localized Ii, which class II invariant chain polypeptide binds to and causes the internalization of the modified MIF ligand, thus causing the operative cell to become specifically labeled or toxicated. The Ii-displaying cells may be exposed to the modified MIF ligand in vitro or in vivo, in which latter case Ii-displaying cells may be specifically identified or toxicated in a patient. A wide variety of diagnostic and therapeutic reagents can be advantageously conjugated to an MIF ligand (which may be biologically active, full length MIF or an Ii-binding fragment thereof, or a mutein of either of the preceding and particularly such a mutein adapted to be biologically inactive and/or to be more conveniently coupled to a labeling or toxicating entity), providing a modified MIF ligand of the invention. Typically desirable reagents coupled to an MIF ligand include: chemotherapeutic drugs such as doxorubicin, methotrexate, taxol, and the like; chelators, such as DTPA, to which detectable labels such as fluorescent molecules or cytotoxic agents such as heavy metals or radionuclides can be complexed; and toxins such as Pseudomonas exotoxin, and the like. 
       Methods 
       [0034]    MIF and Antibodies 
         [0035]    Human recombinant MIF was purified from an  E. coli  expression system as described previously 22  and conjugated to Alexa 488 14  by the manufacturer&#39;s protocol (Molecular Probes, Eugene Oreg.). The average ratio of dye ligand to MIF homotrimer was 1:3, as determined by matrix-assisted laser-desorption ionization mass spectrometry (Kompact probe/SEQ, Kratos Analytical Ltd, Manchester, UK). Anti-human Ii monoclonal antibodies (clones LN2 and M-B741) were obtained from PharMingen (San Jose Calif.). 
         [0036]    Flow Cytometry, Scatchard Analysis, and Confocal Microscopy. THP-1 cells (2.5×10 5  cells/ml) were cultured in DMEM/10% FBS with or without IFNγ (1 ng/ml, R&amp;D Systems, Minneapolis, Minn.) for 72 hrs. After washing, 5×10 5  cells were resuspended in 0.1 ml of medium and incubated with 200 ng of Alexa-MIF at 4° C. for 45 mins. The cells then were washed with ice-cold PBS (pH 7.4) and subjected to flow cytometry analysis (FACSCalibur, Becton Dickinson, San Jose, Calif.). In selected experiments, THP-1 monocytes or COS-7 transfectants were incubated with Alexa-MIF together with 50 μg/ml of an anti-Ii mAb or an isotypic control mAb. For Scatchard analysis, triplicate samples of IFNγ-treated, THP-1 cells (1×10 6 ) were incubated for 45 mins at 4° C. in PBS/1% FBS together with Alexa-MIF (0-1.5 μM, calculated as MIF trimer), washed 3× with cold PBS/1% FBS, and analyzed by flow cytometry using CellQuest Software (Becton Dickinson, San Jose, Calif.) 29 . The specific binding curve was calculated by subtracting non-specific binding (measured in the presence of excess unlabeled MIF) from total binding. Confocal fluorescence microscopy of Alexa-MIF binding to cells was performed with an LSM 510 laser scanning instrument (Carl Zeiss, Jena Germany). THP-1 cells were incubated with INFγ for 72 hrs and washed 3× with PBS/1% FBS prior to staining for 30 mins (4° C.) with 2 ng/μl of Alexa-MIF, or Alexa-MIF plus 50 ng/μl unlabeled, rMIF. 
         [0037]    cDNA Library Construction, Expression, and Cell Sorting 
         [0038]    cDNA was prepared from the poly(A) +  RNA of IFNγ-activated, THP-1 monocytes, cloned into the lambdaZAP-CMV vector (Stratagene, La Jolla, Calif.), and DNA aliquots (2.5 μg/ml) transfected into 15×10 6  COS-7 cells by the DEAE-dextran method 30 . The transfected cells were incubated with Alexa-MIF for 45 min at 4° C., washed, and the positively-staining cells isolated 31  with a Moflo cell sorter (Cytomation, Fort Collins, Colo.). In a typical run,1.5×10 7  cells/ml were injected and analyzed at a flow rate of 1×10 4  cells/sec. Recovery was generally ≧90%. Plasmid DNA was extracted from sorted cells using the Easy DNA kit (Invitrogen, Carlsbad, Calif.) and transformed into  E.coli  XL-10 gold (Stratagene, La Jolla, Calif.) for further amplification. Purified plasmid DNA then was re-transfected into COS-7 cells for further rounds of sorting. After 4 rounds of cell sorting, 250 single colonies were picked at random and the insert size analyzed by PCR. Clones with inserts &gt;1.6 Kb were individually transfected into COS-7 cells and the MIF binding activity re-analyzed by flow cytometry. 
         [0039]    In vitro Transcription and Translation 
         [0040]    Using a full-length Ii cDNA clone as template, three truncated (1-72aa, 1-109aa, 1-149aa) and one full-length (1-232 aa) Ii product were generated by PCR and subcloned into the pcDNA 3.1/V5-HisTOPO expression vector (Invitrogen). The complete nucleotide sequence of an exemplary Ii cDNA clone and the putative Ii polypeptide forms that it encodes are presented in  FIG. 5 . The fidelity of vector construction was confirmed by automated DNA sequencing and the constructs then used as template for coupled transcription and translation using the T N T Reticulocyte Lysate system (Promega, Madison Wis.). The binding of [ 35 S]-labeled Ii to immobilized MIF was assessed by a 3 hr incubation at room temperature, as recommended by the T N T protocol. 
         [0041]    Activity Assays 
         [0042]    The dose-dependent phosphorylation of ERK-1/2 was measured by western blotting of cell lysates using specific antibodies directed against phospho-p44/p42 or total p44/p42 following methods described previously 7 . MIF-mediated suppression of apoptosis was assessed in serum-deprived, murine embryonic fibroblasts by immunoassay of cytoplasmic histone-associated DNA fragments (Roche Biochemicals, Indianapolis, Ind.) 8 . Proliferation studies were performed by a modification of previously published procedures 7,8 . Human Raji B cells (American Type Tissue Culture, Rockville, Md.) were cultured in RPMI/10% FBS, plated into 96 well plates (500-1000 cells/well), and rendered quiescent by overnight incubation in RPMI/0.5% serum. The cells were washed, the RPMI/0.5% serum replaced, and the MIF and antibodies added as indicated. After an additional overnight incubation, 1 μCi of [ 3 H]-thymidine was added and the cells harvested 12 hrs later. Fibroblast mitogenesis was examined in normal human lung fibroblasts (CCL210, American Type Tissue Culture) cultured in DMEM/10% FBS, resuspended in DMEM/2% serum, and seeded into 96 well plates (150 cells/well) together with rMIF and antibodies as shown. Isotype control or anti-Ii mAbs were added at a final concentration of 50 μg/ml. Proliferation was assessed on Day 5 after overnight incorporation of [ 3 H]-thymidine into DNA. 
         [0043]    As will be apparent to a skilled worker in the field of the invention, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described herein. 
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         [0076]    All publications and patent applications mentioned in the specification are herein incorporated by reference to the same extent as if each individual publication or patent application had been specifically and individually indicated to be incorporated by reference. The discussion of the background to the invention herein is included to explain the context of the invention. Such explanation is not an admission that any of the material referred to was published, known, or part of the prior art or common general knowledge anywhere in the world as of the priority date of any of the aspects listed above. 
         [0077]    While the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modifications and that this application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth.