Mapping of biologically relevant sites on human IL-1 beta using monoclonal antibodies

mAb have been raised that recognize human IL-1 beta. Using overlapping peptide fragments expressed in yeast and bacteria, we have mapped the regions of the protein to which these antibodies bind. To assess the relevance of the different regions of IL-1 beta for the expression of its biologic activity, the ability of the antibodies to block IL-1 activity was assayed. Antibodies recognizing the regions 133-148 and 251-269 of human IL-1 beta could inhibit the activity of IL-1 beta, but not of IL-1 alpha, in two different biologic assays, the murine thymocyte proliferation and PGE2 release from human fibroblasts. Conversely, antibodies that recognize the region 218-243 have only a moderate inhibitory effect on the IL-1 beta biologic activity in both assays. Finally, an antibody mapping to the region 148-192 did not inhibit IL-1 beta activity either on thymocytes or on fibroblasts. It is suggested that IL-1 beta-induced cell activation involves different regions of the protein and that both N-terminal and C-terminal fragments are involved in the correct functioning of the IL-1 beta molecule.     

Blockade of NK cell lysis is a property of monoclonal antibodies that bind to distinct regions of T-200

The previously described NK inhibitory monoclonal antibody 13.1 is shown to immunoprecipitate a series of high m.w. glycoproteins homologous with the murine T-200/Ly-5 molecules. Not all antibodies to the human T-200 molecule, however, have an inhibitory effect on NK cell function. A comparison is made between two noninhibitory anti-T-200 antibodies, 13.5 and 13.6, and two inhibitory anti-T-200 antibodies, 13.1 and 13.3. All antibodies are of the IgG1 subclass. Sequential immunoprecipitation experiments show that these antibodies react with the same set of molecules. The differences in NK-blocking activity could not be explained by the amount of antibody bound per cell in NK-enriched populations, nor by the avidity with which they bound. It is shown by competitive radiobinding assays that the 13.1 and 13.3 antibodies define a region, termed region A, distinct from that defined by the nonblocking antibodies 13.5 and 13.6, termed region B. Region B is shown to reside between the membrane and region A. These findings show that the inhibition of NK lysis by anti-T-200 antibodies is a function of the site on that molecule to which these antibodies bind. This may also explain the ability of antibodies to the A region of T-200 to block selectively the lysis of myeloid and erythroid tumor targets, with no effect on the lysis of T lymphoma targets.     

Mapping of multiple binding domains of the superantigen staphylococcal enterotoxin A for HLA.

Multiple binding sites on the staphylococcal enterotoxin A (SEA) molecule which interact with class II MHC Ag have been suggested by previous studies comparing SEA binding with that of another superantigen, toxic shock syndrome toxin-1. Using the synthetic peptide approach we have identified multiple regions of the SEA molecule which are responsible for binding to HLA Ag on Raji cells. Overlapping peptides were synthesized corresponding to the complete amino acid sequence of SEA: SEA(1-45), SEA(39-66), SEA(62-86), SEA(83-104), SEA(102-124), SEA(121-149), SEA(146-173), SEA(166-193), SEA(187-217), and SEA(211-233). Like the native SEA molecule, all of the peptides exhibited relatively high beta-sheet and low alpha-helical structure as determined by circular dichroism spectroscopy. A direct competition assay was employed with peptide blockage of 125I-SEA binding to MHC Ag. SEA(1-45), SEA(39-66), SEA(62-86), and SEA(121-149) but none of the other peptides blocked binding to Raji cells. The relative potency of the peptides in blocking SEA binding was determined with SEA(39-66) much greater than SEA(1-45) = SEA(62-86) = SEA(121-149). Peptide competition was seen at concentrations as low as 55 microM. Further, antibodies were produced to all of the peptides and tested for their ability to bind to SEA and inhibit SEA binding to HLA. Consistent with the direct inhibition of binding, antisera to SEA(1-45), SEA(39-66), and SEA(62-86) reduced the ability of SEA to bind Raji cells, whereas, antisera to the remaining peptides failed to block binding. The data suggest that the binding of the superantigen SEA to MHC molecules involves several N-terminal regions on SEA as well as an additional internal domain. This allows for the presence of multiple binding sites in an extended N-terminal region of the SEA molecule or a discontinuous binding epitope.     

Genetic control of immune response to staphylococcal nuclease. XII: Analysis of nuclease antigenic determinants using anti-nuclease monoclonal antibodies

Summary SJL mice, which are high responders to Staphylococcal nuclease (nuclease), were immunized and used to produce hybridoma cell lines secreting anti-nuclease monoclonal antibodies (mAb). Ten stable clones were derived from a single fusion. Seven of these produced antibodies of the IgG_1, isotype and were more precisely characterized for antigenic specificity. Only one hybridoma cell line (54-10-4) produced anti-nuclease antibodies capable of inhibiting enzymatic activity of nuclease. Binding inhibition analyses strongly suggest that the other monoclonal antibodies, which failed to inhibit nuclease activity detect two different antigenic regions, or epitopes, of the molecule: epitope cluster 1 domain is defined by hybridomas 54-2-7, 54-5-2, 54-9-8, and 54-10-8; epitope cluster 2 by 54-5-1 and 54-1-9. Because of its capacity to inhibit nuclease enzymatic activity mAb 54-10-4 was considered specific for a third epitope of the nuclease molecule called epitope 3. Binding studies of these monoclonal antibodies were extended to peptide fragments of the nuclease molecule in order to examine possible cross-reactions with such fragments, as has previously been reported for antibodies purified from polyclonal antisera. Monoclonal antibodies specific for epitope cluster 1 on the native molecule also bound to the fragments 1–126 and 49–149 but failed to bind to fragment 99–149, suggesting that the corresponding epitope(s) is determined by amino acids localized between residues 49 and 99. The epitope clusters 2 and 3 appeared to be expressed only on the native molecule. Monoclonal antibodies of different clusters exhibited very different migration patterns on isoelectric focusing while monoclonal antibodies of the same cluster were indistinguishable, which suggests that they may have originated from the same B cell precursor. Taken together these data suggest that this panel of monoclonal antibodies detects at least three distinct epitopes of the nuclease molecule, one of which could be involved in the determination of the enzymatic site.     

Fibronectin-plasma membrane interaction in the adhesion of hemopoietic cells

Many hemopoietic cell lines were examined for their ability to adhere to culture dishes coated with extracellular matrix proteins. Adhesion assay was performed with murine and human leukemic cell lines representative of different stages of differentiation along both erythroid and myeloid lineages. All the hemopoietic cell lines tested adhered to fibronectin but not to laminin, types I, III, and IV collagen, serum-spreading factor, and cartilage proteoglycans. In addition to immortalized cell lines, immature erythroid and myeloid mouse bone marrow cells adhered to fibronectin. To define the fibronectin region involved in hemopoietic cell adhesion, proteolytic fragments, monoclonal antibodies, and synthetic peptides were used. Among different fibronectin fragments tested, only a 110-kD polypeptide, corresponding to the fibroblast attachment domain, was active in promoting adhesion. Moreover, a monoclonal antibody to the cell binding site located within this domain prevented hemopoietic cell adhesion. Finally, the tetrapeptide Arg-Gly-Asp-Ser, which corresponds to the fibronectin sequence recognized by fibroblastic cells, specifically and competitively inhibited attachment of hemopoietic cells to this molecule. The cell surface molecule involved in the interaction of mouse hemopoietic cells with fibronectin was identified as a 145,000-D membrane glycoprotein by adhesion-blocking antibodies. This glycoprotein was found to be antigenically and functionally related to the GP135 membrane glycoprotein involved in the adhesion of fibroblasts to fibronectin (Giancotti, F. G., P. M. Comoglio, and G. Tarone, 1986, Exp. Cell Res., 163:47-62). On the basis of these data, we conclude that interaction of hemopoietic cells with fibronectin involves a specific fibronectin sequence and a 145,000-D cell surface glycoprotein. We speculate that this property might be relevant for the interaction of hemopoietic cells with the bone marrow stroma, which represents the natural site of hemopoiesis.     

Neoantigens in complement component C3 as detected by monoclonal antibodies. Mapping of the recognized epitopes by synthetic peptides.

The different fragments of the third complement component, C3, generated upon complement activation/inactivation have the ability to bind to several other complement components and receptors as well as to proteins of foreign origin. These multiple reactivities of C3 fragments are associated with a series of conformational changes occurring in the C3 molecule during its degradation. The conformations acquired by the different C3 fragments are also associated with the exposure of neoantigenic epitopes that are specific for (a) particular fragment(s). In order to study these epitopes and thus the conformational changes occurring in C3, monoclonal antibodies (mAbs) recognizing such epitopes were produced in Balb/c mice after immunization with denatured human C3. Two of the three antibodies (7D84.1 and 7D264.6) presented in this study recognized predominantly surface-bound iC3b, and one mAb (7D323.1) recognized both surface-bound and fluid-phase iC3b. Although none of the mAbs recognized any other fluid-phase C3 fragment, all three antibodies detected micro-titre-plate-fixed C3b and iC3b, but not C3c or C3d. In addition to the reaction with human C3, mAb 7D323.1 also bound to micro-titre-plate-fixed rabbit C3. The epitopes recognized by the three mAbs were further localized by using synthetic peptides that were designed on the basis of the differential binding of the mAbs to the C3 fragments. All three antibodies reacted with C3-(924-965)-peptide, which represents the region of C3 between the kallikrein-cleavage site (923-924) and the elastase-cleavage site (965-966). On the basis of the binding of the mAbs to five different overlapping peptides spanning the region between residues 924 and 965 of the human C3 sequence, and the sequence similarity between human C3 and rabbit C3 within this area, the epitopes recognized by these antibodies are mapped. The contribution of the individual amino acid residues in the formation of the epitopes is discussed.     

The terminal inverted repeats of IS911: requirements for synaptic complex assembly and activity

The bacterial insertion sequence IS911 transposes via a covalently closed circular intermediate. Circle formation involves transposase-mediated pairing of both insertion sequence ends. While full-length transposase, OrfAB, binds poorly in vitro to IS911 DNA fragments carrying a copy of the IS911 end, truncated protein derivatives carrying the first 135 (OrfAB[1-135]) or 149 (OrfAB[1-149]) amino acid residues bind efficiently. They generate a paired-end complex containing two such fragments which resembles that expected for the first synaptic complex. Shorter protein derivatives lacking a region involved in multimerisation do not form these complexes but modify the binding of OrfAB[1-135] and OrfAB[1-149]. DNaseI footprinting demonstrated that OrfAB[1-149] protects a sub-terminal (internal) region of the inverted repeats which includes two blocks of sequence (beta and gamma) conserved between the left (IRL) and right (IRR) ends. DNA binding assays in vitro and measurement of recombination activity in vivo of sequential deletion derivatives of the two inverted repeats suggested a model in which the N-terminal region of OrfAB binds the conserved boxes beta and gamma in a sequence-specific manner and anchors the two insertion sequence ends into a paired-end complex. The external region of the inverted repeat is proposed to contact the C-terminal transposase domain carrying the catalytic site.     

Binding of antibodies against antigenic domains of murine lactate dehydrogenase-C4 to human and mouse spermatozoa

Peptide fragments of lactate dehydrogenase-C4 (LDH-C4) that contain antigenic sequences of the native protein have been identified. The present study describes the binding to murine and human spermatozoa of antibodies that were produced against synthetic peptides containing two of these sequences. Rabbits were immunized with peptides designated MC5-15 and MC211-220, conjugated to diphtheria toxoid (DT). Antisera from these rabbits were tested for binding to washed mouse epididymal sperm or human ejaculated spermatozoa using a solid-phase radioimmunoassay. Antisera bind to mouse sperm in this system at dilutions of 1:64,000. When these antisera are first absorbed with the native LDH-C4 molecule, significant inhibition of binding to sperm results. Antisera to both DT-MC5-15 and DT-MC211-220 bind to human sperm with similar but weaker patterns than seen with mouse sperm. These data indicate that the immune response to synthetic peptides containing antigenic sequences of LDH-C4 includes antibodies that specifically bind to this enzyme on the surface of sperm. In addition, there are shared antigenic sequences between mouse and human LDH-C4, including the MC5-15 and MC211-220 peptides.     

Structure of fragment E species from human cross-linked fibrin

Fragments E1, E2, and E3 are plasmic derivatives of fibrin encompassing the NH2-terminal region of the molecule. The first two species, but not the third, can bind to fragment DD, forming a (DD)E complex, and therefore probably contain binding sites involved in the polymerization of fibrin. For localization of these sites the structure of the fragments was determined by establishing the NH2- and COOH-terminal boundaries of the molecules and using the published amino acid sequence of fibrinogen. Fragment E1 encompasses Gly-alpha 17 to Lys-alpha 78, Gly-beta 15 to Lys-beta 122, and Tyr-gamma 1 to Lys-gamma 62, this representing the intact NH2-terminal region of fibrin. Fragment E2 is an asymmetric molecule which is lacking the sequence of Gly-beta 15 to Lys-beta 53 in one beta-chain remnant. This fragment E2 also lost Lys-beta 122 from the COOH terminal of the beta chain as compared with fragment E1. These cleavages did not affect the ability of fragment E2 to bind to fragment DD. Fragment E3 was heterogeneous, the main species encompassing Val-alpha 20 to Lys-alpha 78, Lys-beta 54 to Leu-beta 120, and Tyr-gamma 1 to Lys-gamma 53. Thus, the loss of the binding function involved in the formation of fibrin clot was associated with the removal of small fragments from all three polypeptide chains: alpha 17-19 (Gly-Pro-Arg), beta 15-53 from the remaining half of the molecule, beta 121 (Leu), and gamma 54-58 (Thr-Ser-Glu-Val-Lys).     

Xenogeneic beta 2-microglobulin substitution affects functional binding of MHC class I molecules by CD8+ T cells

NK cells and CD8+ T cells bind MHC-I molecules using distinct topological interactions. Specifically, murine NK inhibitory receptors bind MHC-I molecules at both the MHC-I H chain regions and beta2-microglobulin (beta2m) while TCR engages MHC-I molecules at a region defined solely by the class I H chain and bound peptide. As such, alterations in beta2m are not predicted to influence functional recognition of MHC-I by TCR. We have tested this hypothesis by assessing the capability of xenogeneic beta2m to modify the interaction between TCR and MHC-I. Using a human beta2m-transgenic C57BL/6 mouse model, we show that human beta2m supports formation and expression of H-2K(b) and peptide:H-2K(b) complexes at levels nearly equivalent to those in wild-type mice. Despite this finding, the frequencies of CD8+ single-positive thymocytes in the thymus and mature CD8+ T cells in the periphery were significantly reduced and the TCR Vbeta repertoire of peripheral CD8+ T cells was skewed in the human beta2m-transgenic mice. Furthermore, the ability of mouse beta2m-restricted CTL to functionally recognize human beta2m+ target cells was diminished compared with their ability to recognize mouse beta2m+ target cells. Finally, we provide evidence that this effect is achieved through subtle conformational changes occurring in the distal, peptide-binding region of the MHC-I molecule. Our results indicate that alterations in beta2m influence the ability of TCR to engage MHC-I during normal T cell physiology.     

Potentiation of NK cytotoxicity by antibody-C3b/iC3b heteroconjugates

The interaction of two Burkitt lymphoma lines, Raji and Rael, with human C and NK cells was analyzed. Raji cells activate the alternative C pathway (ACP) and then bind C3 fragments. Consequently, the cells become more sensitive to lysis by CR3-bearing NK cells but not to C lysis. In contrast, Rael cells are poor ACP activators, do not bind C3 fragments, and are therefore resistant to C-dependent NK lysis. As suggested earlier, the difference between Raji and Rael could be attributed to the presence or absence of CR2, respectively, on their surface. To potentiate C- and NK-dependent lysis of target cells, we generated heteroconjugates composed of a murine antitransferrin receptor mAb and of human C C3b or iC3b. Antibody-C3b conjugates induced C3 deposition on Rael cells and elevated C3 deposition on Raji cells in human serum. Both Raji and Rael cells coated with antibody-C3b conjugates were efficiently lyzed by the cytolytic ACP in human serum. This conjugate had a small enhancing effect on target cell lysis by NK cells which could be markedly increased by combined treatment of the target cell with antibody-C3b conjugate and C5-depleted human serum. On the other hand, antibody-iC3b conjugates efficiently potentiated lysis of target cells by NK cells in the absence of serum. The iC3b-directed cytotoxicity was mediated by CR3-bearing NK effector cells. Anti-C3 but not anti-mouse Ig antibodies abrogated the activity of the antibody-iC3b conjugate. These results further demonstrate that NK cytotoxicity may be potentiated by opsonizing the target cells with C3 fragments and suggest that antibody-C3b/iC3b conjugates could be potent tools for targeting and potentiation of the lytic action of both C and NK cells against tumor cells.     

Epitope mapping of the human immunodeficiency virus type 1 gp120 with monoclonal antibodies.

A soluble form of recombinant gp120 of human immunodeficiency virus type 1 was used as an immunogen for production of murine monoclonal antibodies. These monoclonal antibodies were characterized for their ability to block the interaction between gp120 and the acquired immunodeficiency syndrome virus receptor, CD4. Three of the monoclonal antibodies were found to inhibit this interaction, whereas the other antibodies were found to be ineffective at blocking binding. The gp120 epitopes which are recognized by these monoclonal antibodies were mapped by using a combination of Western blot (immunoblot) analysis of gp120 proteolytic fragments, immunoaffinity purification of fragments of gp120, and antibody screening of a random gp120 gene fragment expression library produced in the lambda gt11 expression system. Two monoclonal antibodies which blocked gp120-CD4 interaction were found to map to adjacent sites in the carboxy-terminal region of the glycoprotein, suggesting that this area is important in the interaction between gp120 and CD4. One nonblocking antibody was found to map to a position that was C terminal to this CD4 blocking region. Interestingly, the other nonblocking monoclonal antibodies were found to map either to a highly conserved region in the central part of the gp120 polypeptide or to a highly conserved region near the N terminus of the glycoprotein. N-terminal deletion mutants of the soluble envelope glycoprotein which lack these highly conserved domains but maintain the C-terminal CD4 interaction sites were unable to bind tightly to the CD4 receptor. These results suggest that although the N-terminal and central conserved domains of intact gp120 do not appear to be directly required for CD4 binding, they may contain information that allows other parts of the molecule to form the appropriate structure for CD4 interaction.     

Covalent attachment of sulfhydryl-specific, electron spin resonance spin-labels to Fab' fragments of murine monoclonal antibodies that recognize human platelet membrane glycoproteins. Development of membrane protein specific spin probes

A general method for the production of high-affinity, nitroxide-labeled, protein-specific spin probes is described in this paper. Fab' fragments are generated from protein-specific, murine monoclonal antibodies by pepsin digestion and mild reduction with cysteine. The free sulfhydryl group located in the carboxy-terminal region of these molecules and produced de novo by this manipulation is then alkylated by reaction with 4-maleimido-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO-maleimide), thereby generating spin-labeled Fab' fragments of these monoclonal antibodies. Two prototypic monoclonal antibodies were tested, each specific for a different integral membrane glycoprotein of human blood platelets. The results indicate that Fab' spin probes generated by this method retain the ability to bind to these glycoproteins within the membrane of intact platelets. These reagents thus represent probes that can be generally used to monitor integral membrane protein mobility on the surface of the intact cell.     

Identification of a conserved domain of the HIV-1 transmembrane protein gp41 which interacts with cholesteryl groups

A soluble form of the HIV-1 envelope glycoprotein gp160 devoid of the transmembrane anchor domain was found to bind to cholesteryl-hemisuccinate agarose. The external subunit gp120 failed to bind to the resin, suggesting that the site responsible for the binding to cholesterol was located in the transmembrane protein gp41. We constructed a series of maltose binding protein (MBP) fusion proteins representing overlapping fragments of the gp41 molecule and we studied their capacity to bind to cholesteryl beads. The domain responsible for binding to cholesterol was localised within the residues 668 to 684 immediately adjacent to the membrane spanning domain. We identified a short sequence (LWYIK, aa 678-683) comparable to the cholesterol interaction amino acid consensus pattern published by Li and Papadopoulos [Endocrinology 139 (1998) 4991]. We demonstrated that the sequence LWYIK synthesized fused to the MBP was able to bind to cholesteryl groups. A synthetic peptide containing the sequence LWYIK was found to inhibit the interaction between cholesteryl beads and MBP44, an MBP fusion HIV-1 envelope protein that contains the putative cholesterol binding domain. Human sera obtained from HIV-1 seropositive patients did not react in ELISA to the LWYIK sequence, suggesting that this region is not exposed to the immune system. The biological significance of the interaction between gp41 and cholesterol is discussed.     

Mapping of the high affinity Fc epsilon receptor binding site to the third constant region domain of IgE.

Identification of the precise region(s) on the IgE molecule that take part in the binding of IgE to its high affinity receptor (Fc epsilon RI) may lead to the design of IgE analogues able to block the allergic response. To localize the Fc epsilon RI-binding domain of mouse IgE, we attempted to confer on human IgE, which normally does not bind to the rodent receptor, the ability to bind to the rat Fc epsilon RI. Employing exon shuffling, we have expressed chimeric epsilon-heavy chain genes composed of a mouse (4-hydroxy-3-nitrophenyl)acetic acid (NP)-binding VH domain, and human C epsilon in which various domains were replaced by their murine counterparts. This has enabled us to test the Fc epsilon RI-binding of each mouse IgE domain while maintaining the overall conformation of the molecule. All of the chimeric IgE molecules which contain the murine C epsilon 3, bound equally to both the rodent and human receptor, as well as to monoclonal antibodies recognizing a site on IgE which is identical or very close to the Fc epsilon RI binding site. Deletion of the second constant region domain did not impair either the binding capacity of the mutated IgE or its ability to mediate mast cell degradation. These results assign the third epsilon domain of IgE as the principal region involved in the interaction with the Fc epsilon RI.     

Studies on the Fc gamma receptor of the murine macrophage-like cell line P388D1. II. Binding of human IgG subclass proteins and their proteolytic fragments.

Binding studies of human IgG proteins to murine P388D1 cells indicated that they bind to an apparently homogeneous Fc receptor population. The association constant was 0.89 x 10(6)M-1 at 22 degrees C and was comparable to the binding affinities of homologous murine IgG2a and IgG2b. The number of receptor sites was found to be approximately 6 x 10(5)/cell. Fc gamma 1 and Fc gamma 3 fragments bound with an affinity comparable to that of the parent proteins. The P388D1 receptors could discriminate between the human IgG subclasses; the relative cytophilic activity was IgG3 greater than IgG1 greater than IgG4 and IgG2 was devoid of binding activity. Fragments corresponding to the C gamma 2 and C gamma 3 domains of human IgG1 were both unable to bind to the P388D1 receptors either alone or in equimolar combination. This suggests that the cytophilic site may be formed cooperatively by interaction between the two domains. The integrity of the hinge region appeared to be essential for full expression of cytophilic activity since reduction of the hinge-region disulfides in both human IgG1 and its Fc fragment markedly decreased their binding affinity. In addition, a mutant IgG1 molecule lacking the hinge region was significantly less cytophilic than its normal counterpart.     

The sequence and topology of human complement component C9.

A partial nucleotide sequence of human complement component C9 cDNA representing 94% of the coding region of the mature protein is presented. The amino acid sequence predicted from the open reading frame of this cDNA concurs with the amino acid sequence at the amino-terminal end of three proteolytic fragments of purified C9 protein. No long stretches of hydrophobic residues are present, even in the carboxy-terminal half of the molecule which reacts with lipid-soluble photoaffinity probes. Monoclonal antibody epitopes have been mapped by comparing overlapping fragments of C9 molecule to which the antibodies bind on Western blots. Several of these epitopes map to small regions containing other surface features (e.g., proteolytic cleavage sites and N-linked oligosaccharide). The amino-terminal half of C9 is rich in cysteine residues and contains a region with a high level of homology to the LDL receptor cysteine-rich domains. A model for C9 topology based on these findings is proposed.     

Binding sites involved in the interaction of actin with the N-terminal region of dystrophin.

Two actin-binding sites have been identified on human dystrophin by proton NMR spectroscopy of synthetic peptides corresponding to defined regions of the polypeptide sequence. These are Actin-Binding Site 1 (ABS1) located at residues 17-26 and Actin-Binding Site 2 (ABS2) in the region of residues 128-156. Using defined fragments of the actin amino acid sequence, ABS1 has been shown to bind to actin in the region represented by residues 83-117 and ABS2 to the C-terminal region represented by residues 350-375. These dystrophin-binding sites lie on the exposed domain in the actin filament.     

Construction, expression and characterization of humanized antibodies directed against the human alpha/beta T cell receptor.

Completely humanized antibodies with specificity for the human alpha/beta TCR have been produced by genetic engineering. The L and H chain V region exons encoding the murine mAb BMA 031 CD regions and human EU framework regions were synthesized and replaced into previously isolated genomic fragments. These fragments were inserted into mammalian expression vectors containing the human kappa and gamma 1 C region exons. Two variants were constructed each containing selected BMA 031 amino acids within the human frameworks. The humanized genes were transfected into Sp2/0 hybridoma cells by electroporation and transfectomas secreting humanized antibody were isolated. Levels of antibody expression up to 7 pg/cell/24 h were obtained. The humanized antibody, BMA 031-EUCIV2, competed poorly with murine BMA 031 for binding to T cells. BMA 031-EUCIV3, however, bound specifically to T cells and competed effectively with both the murine BMA 031 antibody and a previously constructed chimeric BMA 031 antibody for binding to these cells. The relative affinity of BMA 031-EUCIV3 was about 2.5 times lower than BMA 031. The ability to promote antibody dependent cell-mediated cytolysis was significantly enhanced with the engineered antibodies as compared to murine BMA 031. Humanized BMA 031 is a clinically relevant, genetically engineered antibody with potential uses in transplantation, graft vs host disease, and autoimmunity.