Combinatorial functions of two chimeric antibodies directed to human CD4 and one directed to the alpha-chain of the human interleukin-2 receptor.

The general feasibility of chimerization of monoclonal antibodies (mAbs) has already been shown for a large number of them. In order to evaluate in vitro parameters relevant to immunosuppressive therapy, we have chimerized and synthesized two anti-CD4 mAbs recognizing two different epitopes on the human T-lymphocyte antigen, CD4. The chimerized mAbs are produced at levels corresponding to those of the original hybridoma cell lines. With respect to activation of human complement, the individual Abs are negative; however, when used in combination, complement activation was performed. When applied in combination, they were found to modulate the CD4 antigen, whereas the individual mAb do not display this property. Individually they mediate an up to 60% inhibition of the mixed lymphocyte reaction (MLR). However, by combination of an anti-CD4 mAb with one directed against the alpha-chain of the human IL2 receptor, nearly 100% inhibition of the MLR was achieved, even with reduced dosage of the mAbs. Our data suggest that the combination of an anti-CD4 mAb and an anti-IL2R alpha chain mAb is more effective with respect to immunosuppression than each mAb by itself, indicating that this mAb cocktail could be a new strategy for immunosuppressive therapy.     

TCGF (IL 2)-receptor inducing factor(s). I. Regulation of IL 2 receptor on a natural killer-like cell line (YT cells)

A continuous cell line (YT cells) with inducible receptor for T cell growth factor (TCGF)/interleukin 2 (IL 2) was established from a 15-yr-old boy with acute lymphoblastic lymphoma and thymoma. YT cells were tetraploid, having 4q+ chromosomal markers, and proliferated continuously in vitro without conditioned medium (CM) or IL 2. They were weakly positive for OKT9, OKT11, and Tac antigen (Ag), a determinant closely associated with the receptor for IL 2 (IL 2-R), and were negative for OKT1, OKT3, OKT4, and OKT8 Ag. YT cells also expressed HNK-1 Ag and Fc receptors for IgG, which are expressed on natural killer (NK) cells. They retained a killing activity against human cell lines, including K562 (myeloid), T, and B cell lines. Unlike Tac Ag/IL 2-R(+) cell lines derived from adult T cell leukemia (ATL), YT cells were negative for HTLV, as proved by Southern blotting with cDNA for viral DNA. The expression of Tac Ag was markedly enhanced in 18 hr, when YT cells were incubated with CM from PHA-stimulated peripheral blood leukocytes (PBL) or spleen cells, as determined by immunofluorescence by using flow cytometry and binding assay with 125I-anti-Tac antibody (Ab). The binding study with 125I-labeled recombinant IL 2 showed 3.2 X 10(4) IL 2 receptor sites on YT cells precultured with CM. PHA-P and Con A neither agglutinate nor enhance the expression of IL 2-R/Tac antigen on these non-T cell line cells. Furthermore, neither recombinant IL 2 nor gamma-interferon could induce IL 2-R on YT cells, suggesting the presence of a unique IL 2-R inducing factor in PBL or spleen CM. Unlike Tac Ag on HTLV(+), ATL-derived cell lines (Hut-102, MT-1, ATL-2), the expression of Tac Ag on YT cells was down-regulated by anti-Tac Ab. The induction of Tac Ag/IL 2-R on YT cells seemed specific, because the enhancement of Tac Ag expression was not associated with that of Ia Ag and T9/transferrin receptor.     

Spontaneous production of a suppressor factor by a human macrophage-like cell line U937. II. Suppression of antigen- and mitogen-induced blastogenesis, IL 2 production and IL 2 receptor expression in T lymphocytes

U937, a human macrophage-like cell line, spontaneously produces a factor which inhibited blastogenic responses of human blood T lymphocytes stimulated with tuberculin-purified protein derivative (PPD) or phytohemagglutinin (PHA). We investigated the mechanism of suppressor action of the U937 factor. The U937 suppressor factor inhibited interleukin 2 (IL 2) production by human blood T lymphocytes stimulated with PPD or PHA. IL 1 did not overcome the inhibitory action of the U937 factor on PPD-induced IL 2 production by human blood T lymphocytes. The U937 factor also inhibited the production of IL 2 by a human leukemic cell line, JURKAT, stimulated with PHA. The U937 suppressor factor interfered with the expression of Tac antigen (IL 2 receptor) on PPD- or PHA-stimulated blood T lymphocytes. The inhibitory activity of the U937 factor on Tac expression was not affected by the addition of IL 2 or a crude lymphokine-containing T cell supernatant. Tac expression was more sensitive than IL 2 production to inhibition by U937-conditioned medium. The U937 suppressor factor was precipitable by 33 to 67% saturated ammonium sulfate and was inactivated at pH 2 or pH 11. Sephacryl S-200 Gel filtration analysis of U937 culture supernatants revealed that the inhibitory activities for blastogenesis, IL 2 production, and Tac expression co-purified in fractions with an apparent m.w. between 67,000 and 130,000. These data indicate that U937 spontaneously produces a macromolecular suppressive factor with major locus of action on the production of IL 2 and the expression of the IL 2 receptor.     

Interleukin 2 activates a receptor-associated protein kinase

The interleukin 2 (IL 2) receptor complex has been shown to consist of at least two IL 2 binding molecules, one of 55 to 57 kd (gp57Tac) and one of 75 to 78 kd apparent m.w. The data presented here indicate that a protein of m.w. 78,000 (pp78) co-immunoprecipitates with gp57Tac when a monoclonal antibody against gp57Tac is used. The 78 kd molecule is phosphorylated in vitro within the immune complex only in the presence of exogenously added IL 2, whereas the 57 kd molecule is phosphorylated equally in the presence or absence of IL 2. Phosphorylation in vitro of pp78 was demonstrated in extracts of human peripheral blood T cells (PBL-T) and the human T cell line Jurkat, but not in extracts of the human macrophage line U937 or the murine T cell line 2.8.2. Metabolic phosphorylation in intact cells reflects results observed in vitro; both pp78 and gp57Tac are phosphorylated in PBL-T and Jurkat, but not in U937. These data demonstrate that the IL 2 receptor complex contains an IL 2 responsive protein kinase activity and may signal the cell through a phosphorylation event.     

Induction of T cell-dependent B cell differentiation by anti-CD3 monoclonal antibodies

Three monoclonal antibodies (mAb) recognizing the CD3 (T3) surface complex each induced B cell differentiation (as measured by PFC generation) in cultures containing T + non-T cells. Irradiation of the T cells before culture usually augmented the PFC response. An IgG2a mAb (454) induced PFC in all donors tested, whereas two IgG1 mAb (147 and 446) induced PFC in only 80% of the donors tested. This heterogeneity in PFC response to IgG1 anti-CD3 mAb strictly paralleled the heterogeneity in proliferative response to IgG1 anti-CD3 mAb and was governed by cells within the non-T population. In IgG1 anti-CD3 high responders (HR), all anti-CD3 mAb tested induced Tac expression. In IgG1 anti-CD3 low responders (LR), mAb 454 induced Tac expression, but mAb 147 did not. However, when the cultures were supplemented with exogenous interleukin 2, Tac expression and PFC generation in response to mAb 147 was similar to the response to mAb 454 in both HR and LR. The addition of anti-Tac to the cultures partially inhibited anti-CD3-induced PFC generation. These studies indicate that anti-CD3 mAb can lead to B cell differentiation under appropriate experimental conditions and may be valuable in studying polyclonal T cell-dependent B cell differentiation in normal and disease states.     

Association of protein kinase C activation with IL 2 receptor expression

Tac antigen (as a measure of the IL 2 receptor) acquisition and regulation by IL 2, an antigen-receptor agonist (anti-T3), phorbol esters, and phytohemagglutinin (PHA) were studied. Phorbol esters stimulated de novo acquisition of Tac antigen, which was associated with the subcellular redistribution of protein kinase C (PK-C) from cytosol to particulate membranes of human T lymphocytes. PHA and anti-T3 (alpha-T3) antibody also stimulated a transient redistribution and activation of PK-C that reached a maximum within 20 min after stimulation. Both phorbol esters and alpha-T3 could increase Tac expression and stimulate PK-C translocation on 5 and 12 day activated T cells that were at the G0/G1 stage of the cell cycle due to IL 2 deprivation. Tac antigen-specific mRNA was seen in the nucleus within 2 hr after stimulation. In contrast, IL 2 alone could only increase Tac expression and stimulate PK-C translocation on day 5 but not day 12 activated T cells. IL 2 synergizes with alpha-T3 and phorbol ester for the regulation of Tac expression. Although IL 2 increased expression of Tac, the majority if not all of these receptors possessed low affinity for IL 2. These data suggest that the activation of PK-C is a common transmembrane signal shared by IL 2 and antigen stimulation. The results also imply that PK-C activation is necessary for the regulation of Tac antigen expression.     

Stimulation of immunoglobulin secretion in human B lymphocytes as a direct effect of high concentrations of IL 2

In certain human IgM and IgG cell lines, immunoglobulin (Ig) secretion is highly stimulated by a B cell inducing factor (BIF) that is free of interleukin 2 (IL 2). BIF also induces Ig secretion in purified peripheral blood B cell populations that have been mitogenically stimulated by Staphylococcus aureus bacteria. Low concentrations of IL 2 (less than 20 U/ml) are not active in these systems. We now show that IL 2 at concentrations above 100 U/ml can induce Ig secretion in these blood B cells and B cell lines. Both conventional IL 2, purified from the human JURKAT and gibbon MLA-144 cell lines, and recombinant IL 2 are active. Very high concentrations approaching 10(4) U/ml are optimal for Ig secretion. Antibody to the T cell IL 2 receptor, anti-Tac, did not inhibit stimulation of the IgM cell line SKW6.4 by IL 2, and no Tac antigen was detected on the cells. The 9B11 monoclonal anti-IL 2 antibody that neutralizes T cell growth activity also abrogates stimulation of Ig secretion by conventional and recombinant IL 2 in the SKW6.4 cell line. However, the 1H11 monoclonal anti-(conventional thr3-glycosylated IL 2), which does not neutralize T cell growth activity, does inhibit induction of Ig secretion by the corresponding IL 2 in the B cell line. These results suggest that IL 2 stimulates B cells via a low-affinity interaction with a receptor different from the Tac receptor identified on T cells, and that the active site on the IL 2 molecule for B cells differs from that for T cell targets. If IL 2 promotes Ig secretion by binding with a low affinity to the B cell BIF receptor, IL 2 and BIF could be homologous proteins.     

A murine–human chimeric IgG antibody against vascular endothelial growth factor receptor 2 inhibits angiogenesis in vitro

Vascular endothelial growth factor receptor 2 (VEGFR2) has been reported to play an important role in angiogenesis and tumorigenesis. A murine anti-VEGFR2 mAb (A8H1) has been established in a previous study. To reduce the incompatibility of the murine mAb for human use, the chimeric anti-VEGFR2-IgG was developed by genetic recombination of the variable regions of the A8H1 antibody and the constant regions of human IgG, and was expressed in Sp2/0 cells transfected with the two recombinant vectors containing the heavy chain and the light chain regions. After screening, clone 2F12 was selected and was found to stably secrete the murine–human chimeric anti-VEGFR2-IgG (coded 2F12). This chimeric IgG maintained the specificity and the affinity of the parental murine antibody against VEGFR2, and effectively identified VEGFR2 expressed on the surface of HUVECs and BEL-7402 cells. Furthermore, the 2F12 antibody demonstrated inhibition of angiogenesis in vitro, such as proliferation, migration, invasion and tube formation of HUVECs. This murine–human chimeric IgG may be considered for further development as an anti-angiogenesis and anti-tumor agent.     

Direct activation of human resting T cells by IL 2: the role of an IL 2 receptor distinct from the Tac protein

High concentrations of interleukin 2 (IL 2) were shown to produce a delayed but pronounced proliferation of purified resting T cells in the apparent absence of other activation signals. Because these stimulatory effects of IL 2 occurred in the absence of detectable Tac+ cells, the possibility that IL 2 might be initially interacting with an IL 2 binding protein distinct from the Tac protein was studied. Chemical cross-linking studies with 125I-IL 2 revealed the presence of an IL 2 binding protein distinct from the Tac protein on the surface of these unstimulated T cells. This second IL 2 receptor has an estimated molecular size of 70,000 daltons, lacks reactivity with the anti-Tac antibody, and appears to be identical to the p70 protein recently proposed as a component of the high affinity IL 2 receptor. Scatchard analysis of IL 2 binding assays performed with the unactivated T cells revealed approximately 600 to 700 p70 sites per cell and an apparent Kd of 340 pM. These data indicate that the p70 protein present on resting T cells binds IL 2 with an intermediate affinity compared with the previously recognized high and low affinity forms of the receptor and may account for the high concentration of IL 2 needed to induce resting T cell proliferation. To investigate the early biologic consequences of IL 2 binding to the p70 protein, potential changes in the expression of genes involved in T cell activation were examined. Northern blotting revealed the rapid induction of c-myc, c-myb, and Tac mRNA after stimulation of resting T cells with a high concentration of IL 2. The anti-Tac antibody did not inhibit IL 2 induced expression of these genes, suggesting that the p70 protein rather than the Tac antigen or the high affinity IL 2 receptor complex mediated this signal. However, in contrast to these early activation events, the anti-Tac antibody significantly inhibited IL 2 induced T cell proliferation. This finding implicates the high affinity form of the IL 2 receptor in the proliferative response of the IL 2 activated T cells. Thus these data support a two step model for the induction of resting T cell proliferation by high doses of IL 2 involving the initial generation of an activation or "competence" signal through the p70 protein and a subsequent proliferation or "progression" signal through the high affinity form of the receptor.     

Structural and functional analysis of monomorphic determinants recognized by monoclonal antibodies reacting with the HLA class I alpha 3 domain.

To identify mAb reacting with the HLA class I alpha 3 domain, 14 mAb recognizing monomorphic determinants expressed on HLA-A, B, and C Ag or restricted to HLA-B Ag were screened in indirect immunofluorescence with mouse L cells expressing HLA-B7/H-2Kb chimeric Ag. mAb CR1S63, CR10-215, CR11-115, and W6/32 were found to react with the HLA class I alpha 3 domain in addition to the alpha 2 domain. mAb Q1/28 and TP25.99 were found to react only with the HLA class I alpha 3 domain. The determinants recognized by the six mAb were mapped on the HLA class I alpha 3 domain by indirect immunofluorescence staining of L cells expressing H-2Kb Ag containing different segments of the HLA-B7 alpha 3 domain chimerized with the H-2Kb alpha 3 domain. mAb TP25.99 reacts with chimeric Ag containing the HLA-B7 184 to 199 stretch, mAb CR10-215 and CR11-115 react with chimeric Ag containing the HLA-B7 184 to 246 stretch, mAb CR1S63 and Q1/28 react with chimeric Ag containing the HLA-B7 184 to 256 stretch, and mAb W6/32 reacts with chimeric Ag containing the whole HLA-B7 alpha 3 domain. Functional analysis using human CD8 alpha-bearing mouse H-2Kb-specific T cell hybridoma cells (HTB-Leu2) showed that only mAb TP25.99 inhibited IL-2 production by HTB-Leu2 cells stimulated with L cells expressing KbKbB7 Ag. This inhibition may occur because of the spatial proximity of the determinant defined by mAb TP25.99 to the CD8 alpha binding loop and/or because of change(s) in the conformation of the CD8 alpha binding loop induced by the binding of mAb TP25.99 to the HLA class I molecule. Furthermore, mAb TP25.99 inhibited the cytotoxicity of CD8-dependent and CD8-independent CTL clones. These results indicate that mAb TP25.99 has unique specificity and functional characteristics. Therefore it represents a valuable probe to characterize the role of the HLA class I alpha 3 domain in immunologic phenomena.     

Monoclonal antibodies to meningococcal factor H binding protein with overlapping epitopes and discordant functional activity

Background

Meningococcal factor H binding protein (fHbp) is a promising vaccine candidate. Anti-fHbp antibodies can bind to meningococci and elicit complement-mediated bactericidal activity directly. The antibodies also can block binding of the human complement down-regulator, factor H (fH). Without bound fH, the organism would be expected to have increased susceptibility to bacteriolysis. Here we describe bactericidal activity of two anti-fHbp mAbs with overlapping epitopes in relation to their different effects on fH binding and bactericidal activity.

Methods and Principal Findings

Both mAbs recognized prevalent fHbp sequence variants in variant group 1. Using yeast display and site-specific mutagenesis, binding of one of the mAbs (JAR 1, IgG3) to fHbp was eliminated by a single amino acid substitution, R204A, and was decreased by K143A but not by R204H or D142A. The JAR 1 epitope overlapped that of previously described mAb (mAb502, IgG2a) whose binding to fHbp was eliminated by R204A or R204H substitutions, and was decreased by D142A but not by K143A. Although JAR 1 and mAb502 appeared to have overlapping epitopes, only JAR 1 inhibited binding of fH to fHbp and had human complement-mediated bactericidal activity. mAb502 enhanced fH binding and lacked human complement-mediated bactericidal activity. To control for confounding effects of different mouse IgG subclasses on complement activation, we created chimeric mAbs in which the mouse mAb502 or JAR 1 paratopes were paired with human IgG1 constant regions. While both chimeric mAbs showed similar binding to fHbp, only JAR 1, which inhibited fH binding, had human complement-mediated bactericidal activity.

Conclusions

The lack of human complement-mediated bactericidal activity by anti-fHbp mAb502 appeared to result from an inability to inhibit binding of fH. These results underscore the importance of inhibition of fH binding for anti-fHbp mAb bactericidal activity.     

Comparative biodistribution and antibody-dependent cellular cytotoxicity of native and heavy chain chimeric antibody.

We have recently chimerized the heavy chain of the pan-carcinoma monoclonal antibody (mAb) B72.3. Studies were undertaken to compare the IgG1 chimeric antibody, B72.3-1-3 with native murine B72.3 (nB72.3). Using fluorescence-activated cell sorting analysis, B72.3-1-3 demonstrated specific binding to fresh LS174T tumor cells. Biodistribution of 131I B72.3-1-3 was similar to 131I nB72.3 in nude mice bearing LS174T xenografts. Peak radiolocalization indices were noted on day 6 for B72.3-1-3 and day 8 for nB72.3. Both antibodies were capable of imaging LS174T tumors by radioimmunoscintigraphy. Antibody-dependent cellular cytotoxicity of LS174T by human peripheral blood lymphocytes was tested in 8h 51Cr release assays. With either no antibody or nB72.3, lymphocytes were not capable of killing LS174T cells. However, B72.3-1-3 at a concentration of 5 and 50 micrograms/ml mediated significant lysis of tumor cells by human lymphocytes. These results suggest that chimeric antibodies retain their binding properties to tumor cells and display biodistribution patterns similar to their unmodified counterparts. Such modifications may reduce the deleterious human antimouse antibody response to murine mAbs as well as augment antibody-dependent cellular cytotoxicity of tumor cells by human effectors.     

IL-2 regulation of tyrosine kinase activity is mediated through the p70-75 beta-subunit of the IL-2 receptor

The stimulation of activated human T lymphocytes with IL-2 results in increased tyrosine kinase activity. IL-2 treatment of Tac+ T cells stimulates the rapid phosphorylation of multiple protein substrates at M of 116, 100, 92, 70 to 75, 60, 56, 55, 33, and 32 kDa. Phosphorylation on tyrosine residues was detected by immunoaffinity purification of protein substrates with Sepharose linked antiphosphotyrosine mAb, 1G2. Although phorbol ester stimulated serine phosphorylation of the IL-2R alpha (p55) subunit recognized by alpha TAC mAb, IL-2 did not stimulate any detectable phosphorylation of IL-2R alpha or associated coimmune precipitated proteins. In fact, the tyrosine phosphorylated proteins did not coprecipitate with alpha Tac antibody and similar phosphoproteins were stimulated by IL-2 in IL-2R alpha- human large granular lymphocytes which express only the 70 to 75 kDa IL-2R beta subunit of the high affinity IL-2R. Anti-Tac mAb could inhibit IL-2-stimulated tyrosine phosphorylation in activated T cells, which express both IL-2R subunits that together form the high affinity receptor complex, but not in large granular lymphocytes expressing only the IL-2R beta subunit. The data suggest that IL-2 stimulation of tyrosine kinase activities requires only the IL-2R beta subunit.     

Sequential synergistic effect of interleukin 2 and interferon-gamma on the differentiation of a Tac-antigen-positive B cell line

The presence of Tac-antigen (Tac-Ag) on human B lymphocytes and its functional significance with regard to the ability of interleukin 2 (IL 2) to modulate B cell differentiation is currently an area of high interest. An Epstein-Barr virus-transformed B cell line (CB) that secretes IgG was 30 to 40% Tac-Ag+ and was used as a model for examining the role of Tac-Ag and IL 2 in B cell differentiation. Recombinant IL 2 alone was found to have a modest but significant effect on CB in enhancing IgG secretion, increasing the plaque-forming cell response from 637 to 1734 at high concentrations (1000 U/ml IL 2) and to 888 at lower concentrations (100 U/ml). In contrast, recombinant interferon-gamma (IFN-gamma) alone had no effect on the differentiation of CB. However, both factors together showed marked synergy in increasing the number of plaque-forming cells to over 3000 by using only 10 U/ml of IFN-gamma and 100 U/ml of IL 2. These two factors were shown to act sequentially in that IL 2 was needed initially, while IFN-gamma was required for the next differentiation step into IgG-secreting cells. The effect of IL 2 on stimulating differentiation was blocked by anti-Tac, indicating that the action of IL 2 is mediated through its Tac-Ag receptor. CB cells were also sorted into Tac+ and Tac- populations and were cultured separately. In 2 wk, both populations reverted to the pattern of the original cell line. Moreover, cell cycle analysis when using double staining procedures indicated that Tac-Ag on the cell surface of CB appears and disappears according to the stage of the cell cycle, and that Tac is most strongly expressed in the S and G2 + M phases. Thus, the present study suggests that certain B cells are capable of responding to sequential stimulation by IL 2 and IFN-gamma with terminal differentiation into Ig-secreting cells, and that the amount of Tac-Ag expression is cell cycle dependent.     

Inhibition of IL 2-dependent proliferation of rat T lymphoblasts by the monoclonal antibody ART62 which reacts with MHC class 1 antigens

During the course of studies designed to obtain monoclonal antibodies (mAb) that recognize the rat interleukin 2 receptor, a mouse IgG1 mAb (ART62) was identified which inhibits the interleukin 2 (IL 2)-dependent proliferation of rat T lymphoblasts without affecting the binding of IL 2 to such cells. In order to characterize the cell surface components that react with the mAb ART62, T lymphoblasts were surface-labeled with 125I, and the radioactive molecules were immunoprecipitated by the antibody analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The mAb ART62 precipitated two major components of 48,000 m.w. and 12,000 m.w., respectively, which were different from those which react with the anti-IL 2-receptor antibody ART18, a molecule of 50,000 to 55,000 m.w. Sequential immunoprecipitation studies revealed that the mAb ART62 reacts with the MHC class 1 antigen that reacts with the classical anti-rat MHC class 1 mAb OX18, and vice versa. In contrast to the mAb ART62, OX18 that does not affect and several other mAbs known to inhibit the rat MLR failed to inhibit IL 2-dependent proliferation of rat T lymphoblasts. In contrast to the anti-IL 2 receptor antibody ART18, ART62 effectively inhibited IL 2-driven proliferation even when added to cells already committed to proliferate by IL 2-IL 2 receptor interaction. These data raise the possibility that MHC class 1 antigens could be involved in the chain of reactions mediating the signals required for cell proliferation.     

Structure-function relationships for the IL 2-receptor system. IV. Analysis of the sequence and ligand-binding properties of soluble Tac protein

The Tac protein is one of at least two glycoproteins known to bind the growth and differentiation factor interleukin 2 (IL 2). In addition to its location on the cell surface, where it plays a part in high and low affinity IL 2 receptors, Tac is released from activated lymphocytes in a soluble form. We observed this release both for Tac protein labeled biosynthetically and for Tac protein labeled by surface iodination of intact cells. Competitive binding studies indicated that the soluble Tac protein retained an ability to bind IL 2 with a low affinity (Kd of 11.1 nM). In addition, structural analysis revealed that the polypeptide chain began at position 1 and ended at or just before Cys-192 of the full-length molecule. Thus, the protein was missing its normal transmembrane and intracytoplasmic segments, accounting for its solubility and cellular release. The apparent lack of modification in the amino acid sequence and the termination at Cys-192 are inconsistent with a mechanism of cellular release dependent only on alternate mRNA splicing. Instead, the results suggest that proteolysis may accompany the release of soluble Tac protein from cells expressing IL 2 receptors.     

Therapy with monoclonal antibodies. An in vivo model for the assessment of therapeutic potential.

A set of rat-human and rat-rat chimeric mAb has been created, all possessing V regions identical in their specificity for the mouse CD8 Ag. In vitro all antibodies were able to block cell-mediated lysis but varied greatly in their capacity to utilize rabbit complement. We examined the ability of these chimeric antibodies to deplete in vivo and established a clear hierarchy. Of the human IgG subclasses, only IgG1, 2, and 3 could fix complement in vitro, yet all (IgG1-4) were remarkably potent at depleting CD8+ PBL in vivo. In contrast, human IgA2 and IgE were ineffective at clearing CD8+ PBL. The vector system used to create these antibodies together with the small doses of antibodies required to deplete in vivo make this a simple and rapid system for testing the effects of different antibody isotypes and mutants. We have shown that a mutant of human IgG1, which is incapable of fixing complement, depletes perfectly well in vivo, whereas an aglycosyl IgG1 mutant is rendered inactive. Our model provides a unique opportunity to study effector functions and motifs that are used by mAb in vivo and will help in the design of improved antibodies for human therapy.     

TGF alpha-anti-Tac(Fv)-PE40: a bifunctional toxin cytotoxic for cells with EGF or IL2 receptors

Conventional immunotoxins and chimeric toxins made in bacteria are directed to only one receptor or antigen on target cells. In this report we describe the construction of a chimeric molecule TGF alpha-anti Tac(Fv)-PE40 which is composed of human transforming growth factor type alpha attached to anti-Tac(Fv) which is in turn attached to PE40, a form of pseudomonas exotoxin, devoid of its cell recognition domain. TGF alpha-anti-Tac(Fv)-PE40 is a bifunctional toxin that is produced in E. coli and is active on cells bearing either IL2 or EGF receptors.     

Structure-function relationships for the IL 2-receptor system. II. Localization of an IL 2 binding site on high and low affinity receptors

The ligand-binding component of high and low affinity IL 2 receptors is a 55,000 m.w. glycoprotein termed Tac. Correlating the structure and function of this molecule should provide insight into the mechanism of IL 2-initiated signal transduction and the structural basis for high and low affinity receptor forms. As a first step in this process, various approaches were used to localize the IL 2 binding region of the Tac molecule. Antibodies prepared to synthetic fragments of Tac were tested for their ability to interfere with IL 2 binding and bioactivity. The results delineated segments in the C-terminal portion of the molecule which appeared to be distal to the ligand binding site. In a more direct approach, radioiodinated IL 2 was cross-linked to high and low affinity receptors, and the resulting complexes were subjected to mild tryptic digestion. Consistent with the antibody data, the IL 2 remained covalently associated with an N-terminal tryptic fragment which apparently consisted of residues 1-83 of the Tac protein. These results suggest that the N-terminal region of the Tac molecule contains important contact sites for ligand-receptor interaction.     

Rheumatoid factors may bear the internal image of the Fc gamma-binding protein of herpes simplex virus type 1

Affinity-purified rheumatoid factors (RF) from 20 patients with rheumatoid arthritis were tested for their reactivity with the mAb II-481 against glycoprotein E (gE), the Fc gamma-binding protein of HSV-1, as well as with a panel of mAb against human Fc gamma R. All RF bound to mAb II-481 in preference to mAb IV.3 (anti-human Fc gamma RII) or MOPC 141 (control mAb) which belong to the same IgG2b subclass. Five RF showed strong reactivity with II-481. No significant reactivity was observed between RF and mAb against human Fc gamma R. Non-RF human IgM did not react with any of the mAb. Clear-cut binding to II-481 was also seen with monoclonal IgM-RF derived from MRL/1 mice (mRF-2). The reaction between RF and II-481 was completely inhibited by human IgG. It was also inhibited by BHK cell extract infected with HSV-1, and with purified gE. II-481 inhibited the binding of human IgG Fc to the infected cell extract, confirming that II-481 recognizes the Fc-binding site on gE. II-481 did not react directly with human IgG or Fc of IgG. mAb to human IgG2 showed stronger binding to II-481 than to MOPC 141, suggesting II-481 has conformational similarity to human IgG H chain. These results suggest that at least some RF bear the "internal image" of HSV-1 Fc gamma-binding protein and support the hypothesis that some RF may be generated as anti-idiotype antibodies against antiviral antibodies.