Identification of a monoclonal antibody specific for a neoantigenic determinant on alpha 2-macroglobulin: use for the purification and characterization of binary proteinase-inhibitor complexes

A monoclonal antibody was obtained from the fusion of spleen cells of mice, immunized with methylamine-treated alpha 2-macroglobulin (alpha 2M), with the myeloma cell line P3-X63-Ag8.653. A competitive binding assay demonstrated that the antibody was specific for a neoantigen expressed on alpha 2M when the inhibitor reacts with proteinases or with methylamine. When immobilized, the monoclonal antibody retained its ability to specifically bind alpha 2M-proteinase complexes or methylamine-treated alpha 2M, both of which could be quantitatively recovered from the immunoaffinity column by lowering the pH to 5.0. Binary alpha 2M-proteinase complexes of trypsin, plasmin, and thrombin, prepared by incubating large amounts of alpha 2M with a small amount of enzyme, were isolated by immunoaffinity chromatography. Each purified complex was characterized with regard to proteinase content, extent of alpha 2M subunit cleavage, extent of thiol ester hydrolysis, and extent of conformational change. Each complex contained 0.8-0.9 mol of proteinase/mol of inhibitor. In the alpha 2M-thrombin, alpha 2M-plasmin, and alpha 2M-trypsin complexes, approximately 50%, 60%, and 75% of the subunits are cleaved, respectively. Titration of sulfhydryl groups revealed that all purified binary complexes contained 2 +/- 0.5 mol of thiol/mol of complex, suggesting that each complex retains two intact thiol ester bonds. When the purified complexes were incubated with excess trypsin or with methylamine, an additional 1-2 mol of sulfhydryl/mol of complex could be titrated.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evolution of human alpha 2-macroglobulin

A papain-binding protein (PBP) resembling human alpha 2-macroglobulin (alpha 2M) but of Mr half that of alpha 2M was purified from plaice (Pleuronectes platessa L.) plasma. The plaice protein displayed most of the distinctive inhibitory properties of the human macroglobulin, and was therefore considered, despite its smaller molecular size, to be homologous with alpha 2M. Plaice PBP was shown to consist of four dissimilar subunits; two I chains (Mr 105 000) and two II chains (Mr 90 000). Each of the larger I chains contained a "bait region" sensitive to proteolytic attack by a variety of proteinases, and an autolytic site analogous to the autolytic site of alpha 2M. Subunit I, almost certainly at the autolytic site, formed SDS-stable, covalent links with methylamine or a proportion of the trapped proteinase molecules. A scheme is proposed for the evolution of human alpha 2M from the smaller fish protein, and the possibility of a shared evolutionary origin for alpha 2M and the complement components C3 and C4 is discussed.     

Expression and characterization of pro alpha 2-plasmin inhibitor

alpha s-Plasmin inhibitor (alpha 2PI), one of the serine protease inhibitors in plasma, was expressed in baby hamster kidney (BHK) cell line. The expression vector was constructed with its genomic DNA and cDNA, and was transfected into BHK cells by the calcium phosphate method. The recombinant alpha 2PI which was secreted from the cells was estimated by SDS-PAGE to have a molecular mass of 67 kDa, which is indistinguishable from that of normal plasma alpha 2PI. The leader peptide of 12 amino acids was retained at the amino terminus of the recombinant alpha 2PI. This finding suggests that alpha 2PI has pre-pro type processing and the propeptide of 12 amino acids is not removed in BHK cells. This pro-alpha 2PI shows essentially the same inhibitory activity on plasmin and the same affinity for plasmin(ogen) as those of normal alpha 2PI. However, the cross-linking ability to fibrin is reduced to less than one-third of that of normal alpha 2PI. The cross-linking site is the glutamine residue located at the second position from the amino terminus of normal alpha 2PI. The conformational change of this region caused by the addition of the propeptide may have affected the cross-linking capacity of the inhibitor.     

Characterization of thrombin binding to alpha 2-macroglobulin

The formation and structural characteristics of the human alpha 2-macroglobulin (alpha 2M)-thrombin complex were studied by intrinsic protein fluorescence, sulfhydryl group titration, electrophoresis in denaturing and nondenaturing polyacrylamide gel systems, and in macromolecular inhibitor assays. The interaction between alpha 2M and thrombin was also assessed by comparison of sodium dodecyl sulfate-gel electrophoretic patterns of peptides produced by Staphylococcus aureus V-8 proteinase digests of denatured alpha 2M-125I-thrombin and alpha 2M-125I-trypsin complexes. In experiments measuring fluorescence changes and sulfhydryl group exposure caused by methylamine, we found that thrombin produced its maximum effects at a mole ratio of approximately 1.3:1 (thrombin:alpha 2M). Measurements of the ability of alpha 2M to bind trypsin after prior reaction with thrombin indicated that thrombin binds rapidly at one site on alpha 2M, but occupies the second site with some difficulty. Intrinsic fluorescence studies of trypsin binding to alpha 2M at pH 5.0, 6.5, and 8.0 not only revealed striking differences in trypsin's behavior over this pH range, but also some similarities between the behavior of thrombin and trypsin not heretofore recognized. Structural studies, using sodium dodecyl sulfate-polyacrylamide gel electrophoresis to measure alpha 2M-125I-thrombin covalent complex formation, indicated that covalency reached a maximum at a mole ratio of approximately 1.5:1. At this ratio, only 1 mol of thrombin is bound covalently per mol of alpha 2M. These gel studies and those of proteolytic digests of denatured alpha 2M-125I-trypsin and alpha 2M-125I-thrombin complexes suggest that proteinases form covalent bonds with uncleaved alpha 2M subunits. The sum of our results is consistent with a mechanism of proteinase binding to alpha 2M in which the affinity of the proteinase for alpha 2M during an initial reversible interaction determines its binding ratio to the inhibitor.     

Proteolytic activities of cobra venoms based on inactivation of alpha 2-macroglobulin

The venoms of various cobra species showed a wide range of abilities to cleave hide powder azure, with Naja naja kaouthia and Ophiophagus hannah venoms showing the lowest activities and Naja nivea venom showing the greatest activity on this dye-linked substrate. The activities of the venoms on hide powder did not completely correlate with their ability to inactivate the alpha 2-macroglobulin of human serum. Incubation of 4-5 micrograms of Naja nigricollis venom per microliter of serum for 30 min caused loss of 95% of the alpha 2-macroglobulin activity of the serum. The inactivation was rapid, reaching 80% inactivation 5 min after mixing. This loss of alpha 2-macroglobulin activity was used to quantitate the weak proteolytic activity of N. nigricollis venom and a partially purified sample of the major fibrinogenolytic proteinase of the venom. The inactivation of alpha 2-macroglobulin was also used to compare the proteinase activities of venoms from seven species or subspecies of cobra. Based on alpha 2-macroglobulin inactivation, N. nigricollis had the highest proteinase activity among the tested venoms. The measurement of alpha 2-macroglobulin inactivation should provide a useful alternative to hide powder digestion for demonstration of weak proteolytic activities in venoms.     

Isolation and characterization of horse alpha 2-macroglobulin protease inhibitor

Several publications have described in the past properties of partly purified horse alpha 2-macroglobulin (alpha 2M) which are strikingly different from the human alpha 2M. Horse alpha 2M was therefore isolated to purity by classical procedures, i.e. affinity chromatography, ion exchange chromatography and gel filtration, and its properties are compared with those of its human counterpart. The molecular weight of the native protein and its subunits, the isoelectrofocusing pattern and the change in electrophoretic mobility caused by interaction with protease were similar to those of human alpha 2M. Horse alpha 2M had a broad enzyme specificity and inhibited enzymatic action on macromolecules but not on small molecular weight synthetic substrates. In addition the horse and human alpha 2M were found to be immunochemically related when examined by specific antisera to human as well as to horse alpha 2-macroglobulin.     

Kinetics of the reaction of streptokinase-plasmin complex with purified human and mouse alpha 2-macroglobulin. Implications for mechanism.

Streptokinase-human plasmin complex (Sk-hPm) reacted rapidly with purified mouse alpha 2-macroglobulin (m alpha 2M) in vitro at 37 degrees C. Approx. 98% of the plasmin in Sk-hPm bound covalently to at least one m alpha 2M subunit. Most of the streptokinase dissociated (95%). The rate of Sk-hPm inactivation clearly depended on the m alpha 2M concentration. With 1.2 microM-m alpha 2M, 50% of the Sk-hPm (0.02 microM) reacted in less than 50 s. A double-reciprocal plot comparing pseudo-first-order rate constants (kapp.) and m alpha 2M concentration yielded a second-order rate constant of 2.3 x 10(4) M-1.s-1 (r = 0.97). This value is an approximation, since Sk-hPm preparations are heterogeneous. Sk-hPm reacted with human alpha 2M (h alpha 2M), forming alpha 2M-plasmin complex (98% covalent). More than 99% of the streptokinase dissociated. The rate of reaction of Sk-hPm with h alpha 2M did not clearly depend on inhibitor concentration. The kapp. values determined with 0.6-1.2 microM-h alpha 2M were decreased 10-20-fold compared with m alpha 2M. In order to study the effect of Sk-hPm heterogeneity on the reaction with alpha 2M, the proteinase was incubated for various amounts of time at 37 degrees C before addition of inhibitor. The enzyme amidase activity was maximal within 5 min; however, reaction of Sk-hPm with m alpha 2M or h alpha 2M was most extensive after 20 min and 2 h respectively. After incubation for more than 1 h, Sk-hPm acquired fibrinogenolytic activity, suggesting plasmin dissociation. Therefore the enhanced reaction of h alpha 2M with 'older' Sk-hPm preparations may have resulted in part from dissociated plasmin or 'plasmin-like' species. By contrast, the reaction of Sk-hPm with m alpha 2M was most rapid when the proteinase preparation was free of plasmin, indicating direct reaction of Sk-hPm with m alpha 2M as the only major mechanism. Finally, streptokinase-cat plasminogen complex reacted more extensively with m alpha 2M than with h alpha 2M, suggesting that m alpha 2M may be a superior inhibitor with this class of plasminogen activators in general.     

Human transforming growth factor beta.alpha 2-macroglobulin complex is a latent form of transforming growth factor beta

125I-Labeled human platelet-derived transforming growth factor beta (125I-TGF-beta) and human alpha 2-macroglobulin (alpha 2M) formed a complex as demonstrated by 5% native polyacrylamide gel electrophoresis. The 125I-TGF-beta.alpha 2M complex migrated at a position identical to that of the fast migrating form of alpha 2M. Most of the 125I-TGF-beta.alpha 2M complex could be dissociated by acid or urea treatment. When 125I-TGF-beta was incubated with serum, the high molecular weight form of 125I-TGF-beta could be immunoprecipitated by anti-human alpha 2M anti-sera as demonstrated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. alpha 2M purified from platelet-rich plasma also showed the latent transforming growth factor activity and immunoreactivity of TGF-beta. These results suggest that TGF-beta.alpha 2M complex is a latent form of TGF-beta.     

Mechanism of hypochlorite-mediated inactivation of proteinase inhibition by alpha 2-macroglobulin.

The proteinase-proteinase inhibitor balance plays an important role in mediating inflammation-associated tissue destruction. alpha 2-Macroglobulin (alpha 2M) is a high-affinity, broad-spectrum proteinase inhibitor found abundantly in plasma and interstitial fluids. Increased levels of alpha 2M and proteinase-alpha 2M complexes can be demonstrated in patients with sepsis, emphysema, peridontitis, rheumatoid arthritis, and other inflammatory diseases. Despite these increased levels, proteolysis remains a significant problem. We hypothesized that a mechanism for inactivating alpha 2M-mediated proteinase inhibition must exist and recently demonstrated that alpha 2M isolated from human rheumatoid arthritis synovial fluid is oxidized and has decreased functional activity. The oxidant responsible for alpha 2M inactivation and the mechanism of such destruction were not studied. We now report that while hypochlorite and hydroxyl radical both modify amino acid residues on alpha 2M, only hypochlorite can abolish the ability of alpha 2M to inhibit proteinases. Hydrogen peroxide, on the other hand, has no effect on alpha 2M structure or function. Protein unfolding with increased susceptibility to proteolytic cleavage appears to be involved in alpha 2M inactivation by oxidation. The in vivo relevance of this mechanism is supported by the presence of multiple cleavage fragments of alpha 2M in synovial fluid from patients with rheumatoid arthritis, where significant tissue destruction occurs, but not in patients with osteoarthritis. These results provide strong evidence that hypochlorite oxidation contributes to enhanced tissue destruction during inflammation by inactivating alpha 2M.     

PDGF-stimulated fibroblast proliferation is enhanced synergistically by receptor-recognized alpha 2-macroglobulin.

alpha-Macroglobulins derived from plasma or secreted by macrophages are platelet-derived growth factor (PDGF) binding proteins that compete with cell-surface receptors on fibroblasts for PDGF binding. alpha 2-Macroglobulin (alpha 2M) derived from bovine plasma was tested for its ability to modulate the PDGF-induced proliferation of primary passage rat lung fibroblasts (RLFs) and a human skin fibroblast cell line (CRL 1508). Fibroblasts were grown in 10% fetal bovine serum (FBS) for 24 hr, then washed with serum-free medium before adding serum-free defined medium (SFDM) containing insulin and transferrin. To this medium were added varying concentrations of human plasma-derived AB-PDGF and alpha 2 M, alone or in combination. Receptor-recognized alpha 2M was prepared by treatment with methylamine. Both native alpha 2M and the alpha 2M-methylamine (alpha 2M-MA) were tested for growth promoting activity in the absence or presence of PDGF. After 3 days, a concentration-dependent growth curve of fibroblast proliferation was demonstrated for PDGF alone, with near maximal stimulation reached at 15-20 ng/ml PDGF. alpha 2M and alpha 2M-MA alone had no effect on cell proliferation. However, alpha 2M-MA concentrations above 32 micrograms/ml synergistically enhanced PDGF-stimulated proliferation greater than 100% in the presence of 15 ng/ml PDGF. Native alpha 2M enhanced PDGF-stimulated growth 80-100% above PDGF controls only at low concentrations (32-64 micrograms/ml alpha 2M). High concentrations of native alpha 2M (128-256 micrograms/ml) either had no effect on growth or were inhibitory to PDGF-stimulated growth, depending on the cell type tested. Rat lung fibroblasts were shown to secrete a factor(s) that inhibited the trypsin-binding capacity of native alpha 2M. We further demonstrated that early passage RLFs possess specific cell-surface receptors for [125I]-PDGF and [125I]-alpha 2M-MA, and preincubation of RLFs with alpha 2M-MA increased the specific binding of [125I]-PDGF to the cell surface of these fibroblasts. Considered together, these data support the view that receptor-recognized alpha 2M synergistically enhances the proliferative capacity of PDGF. We postulate that receptor-recognized alpha Ms enhance PDGF-stimulated growth by increasing the local concentration of PDGF at the cell surface, where the PDGF could be released in close proximity to its own receptors.     

The conformation-dependent interaction of alpha 2-macroglobulin with vascular endothelial growth factor. A novel mechanism of alpha 2-macroglobulin/growth factor binding

alpha(2)-Macroglobulin (alpha(2)M) is a highly conserved proteinase inhibitor present in human plasma at high concentration (2-4 mg/ml). alpha(2)M exists in two conformations, a native form and an activated, receptor-recognized form. While alpha(2)M binds to numerous cytokines and growth factors, in most cases, the nature of the alpha(2)M interaction with these factors is poorly understood. We examined in detail the interaction between alpha(2)M and vascular endothelial growth factor (VEGF) and found a novel and unexpected mechanism of interaction as demonstrated by the following observations: 1) the binding of VEGF to alpha(2)M occurs at a site distinct from the recently characterized growth factor binding site; 2) VEGF binds different forms of alpha(2)M with distinct spatial arrangement, namely to the interior of methylamine or ammonia-treated alpha(2)M and to the exterior of native and proteinase-converted alpha(2)M; and 3) VEGF (molecular mass approximately 40 kDa) can access the interior of receptor-recognized alpha(2)M in the absence of a proteinase trapped within the molecule. VEGF bound to receptor-recognized forms of alpha(2)M is internalized and degraded by macrophages via the alpha(2)M receptor, the low density lipoprotein receptor-related protein. Oxidation of both native and receptor-recognized alpha(2)M results in significant inhibition of VEGF binding. We also examined the biological significance of this interaction by studying the effect of alpha(2)M on VEGF-induced cell proliferation and VEGF-induced up-regulation of intracellular Ca(2+) levels. We demonstrate that under physiological conditions, alpha(2)M does not impact the ability of VEGF to induce cell proliferation or up-regulate Ca(2+).     

Coexpression of tumor-associated alpha 2-macroglobulin and growth factors in human melanoma cell lines.

alpha 2-Macroglobulin (alpha 2M) is known as an inhibitor of various proteinases and to bind several of the growth factors. We previously demonstrated that clonal variation exists in the production of alpha 2M in a human melanoma and that this variation may be associated with growth stimulation. We have now analyzed six human melanoma cell lines for the simultaneous expression of TGF-alpha, TGF-beta, PDGF-A chain, PDGF-B chain, and tumor-associated alpha 2M. In Northern blot analysis TGF-alpha was detected in four of the cell lines, TGF-beta in all, PDGF-A chain in three, and PDGF-B chain in none of the cell lines. alpha 2M, detected by immunoblotting, varied significantly between the different melanoma cell lines and only one cell line was found to be negative. Evaluation of growth-promoting activity in conditioned media suggested that alpha 2-macroglobulin, secreted by these tumor cell lines, is a significant modulator of melanoma cell growth.     

Primary structure of alpha 2-macroglobulin receptor-associated protein. Human homologue of a Heymann nephritis antigen.

The alpha 2-macroglobulin (alpha 2M) receptor complex as purified by affinity chromatography contains three polypeptides: a 515-kDa heavy chain, an 85-kDa light chain, and a 39-kDa associated protein. Previous studies have established that the 515/85-kDa components are derived from a 600-kDa precursor whose complete sequence has been determined by cDNA cloning (Herz, J., Hamann, U., Rogne, S., Myklebost, O., Gassepohl, H., and Stanley, K. (1988) EMBO J. 7,4119-4127). We have now determined the primary structure of the human 39-kDa polypeptide, termed alpha 2M receptor-associated protein, by cDNA cloning. The deduced amino acid sequence contains a putative signal sequence that precedes the 323-residue mature protein. Comparative sequence analysis revealed that alpha 2M receptor-associated protein has 73% identity with a rat protein reported to be a pathogenic domain of Heymann nephritis antigen gp 330 and 77% identity to a mouse heparin-binding protein termed HBP-44. The high overall identity suggests that these molecules are interspecies homologues and indicates that the pathogenic domain, previously thought to be a portion of gp 330, is in fact a distinct protein. Further, the 120-residue carboxyl-terminal region of alpha 2M receptor-associated protein has 26% identity with a region of apolipoprotein E containing the low density lipoprotein receptor binding domain. Pulse-chase experiments revealed that the newly formed alpha 2M receptor-associated protein remains cell-associated, while surface labeling experiments followed by immunoprecipitation suggest that this protein is present on the cell surface forming a complex with the alpha 2M receptor heavy and light chains.     

Mechanism of insulin incorporation into alpha 2-macroglobulin: implications for the study of peptide and growth factor binding.

In recent years, many studies have suggested a direct role for alpha 2-macroglobulin (alpha 2M), a plasma proteinase inhibitor, in growth factor regulation. When coincubated in the presence of either trypsin, pancreatic elastase, human neutrophil elastase, or plasmin, 125I-insulin rapidly formed a complex with alpha 2M which was greater than 80% covalent. The covalent binding was stable to reduction but abolished by competition with beta-aminopropionitrile. Neither native alpha 2M nor alpha 2M pretreated with proteinase or methylamine incorporated 125I-insulin. Experiments utilizing alpha 2M cross-linked with cis-dichlorodiammineplatinum(II) indicated that 125I-insulin must be present during alpha 2M conformational change to covalently bind. A maximum stoichiometry of 4 mol of insulin bound per mole of alpha 2M and the short half-life of the alpha 2M intermediate capable of covalent incorporation were consistent with thiol ester involvement. Protein sequence analysis of unlabeled insulin-alpha 2M complexes, together with results of beta-aminopropionitrile competition, confirmed that insulin incorporation occurs via the same gamma-glutamyl amide linkage responsible for covalent proteinase and methylamine binding to alpha 2M. Although intact insulin apparently incorporated through its sole lysine residue on the B chain, we found that isolated A chain also bound covalently to alpha 2M. Phenyl isothiocyanate derivatization of the N-terminus had no effect on A-chain binding, supporting the possibility of heretofore unreported gamma-glutamyl ester linkages to alpha 2M.     

Alpha 2-macroglobulin restricts plasminogen activation to the surface of RC2A leukemia cells.

Human RC2A myelomonocytic leukemia cells are able to activate the prourokinase (pro-u-PA) they secrete so that active u-PA is present both in serum-free conditioned medium from these cells, as well as on the cell surface. When the cells are grown in serum-containing medium, no u-PA activity can be found in the medium but active u-PA is found bound to the cell surface where it can generate bound plasmin. This distribution of u-PA activity was shown to be, first, the net result of slow inactivation of free active u-PA by serum inhibitor(s) and simultaneous rapid uptake of u-PA onto the cell surface. Binding to cells was at least six times faster than inactivation by 10% serum. The principal serum inhibitor of u-PA was identified as alpha 2-macroglobulin (alpha 2M), and prior inactivation of u-PA by purified human alpha 2M was also shown to prevent uptake of u-PA activity onto cells. Second, although endogenous u-PA could form covalent complexes with purified alpha 2M in the culture medium of RC2A cells, covalent alpha 2M complexes were not formed by u-PA on the cell surface; the u-PA taken up in this compartment was protected against alpha 2M inhibition. u-PA anchored to plastic surfaces via monoclonal antibodies to the amino-terminal region of u-PA was also protected against alpha 2M, suggesting that the protection of cell surface u-PA results from a steric effect. These results provide evidence as to how the active u-PA produced by leukemia cells can contribute to proteolytic activity on their cell surface in the presence of serum inhibitors.     

Binding of alpha 2-macroglobulin-thrombin complexes and methylamine-treated alpha 2-macroglobulin to human blood monocytes

The binding of alpha 2-macroglobulin (alpha 2M) to human peripheral blood monocytes was investigated. Monocytes, the precursors of tissue macrophages, were isolated from fresh blood by centrifugal elutriation or density gradient centrifugation. Binding studies were performed using 125I-labeled alpha 2M. Cells and bound ligand were separated from free ligand by rapid vacuum filtration. Nonlinear least-squares analysis of data obtained in direct binding studies at 0 degrees C showed that monocytes bound the alpha 2M-thrombin complex with a Kd of 3.0 +/- 0.9 nM and the monocyte had 1545 +/- 153 sites/cell. Thrombin alone did not compete for the site. Binding was divalent cation dependent. Direct binding studies also demonstrated that monocytes bound methylamine-treated alpha 2M in a manner similar to alpha 2M-thrombin. Competitive binding studies showed that alpha 2M-thrombin and methylamine-treated alpha 2M bound to the same sites on the monocyte. In contrast, native alpha 2M did not compete with alpha 2M-thrombin for the site. Studies done at 37 degrees C suggested that after binding, the monocyte internalized and degraded alpha 2M-thrombin and excreted the degradation products. Receptor turnover and degradation of alpha 2M-thrombin complexes were blocked in monocytes treated with chloroquine, an inhibitor of lysosomal function. Our results indicate that human monocytes have a divalent cation dependent, high-affinity binding site for alpha 2M-thrombin and methylamine-treated alpha 2M which may function to clear alpha 2M-proteinase complexes from the circulation.     

Immunosuppressive properties of electrophoretically "slow" and "fast" form alpha 2-macroglobulin. Effects on cell-mediated cytotoxicity and (allo-) antigen-induced T cell proliferation

Lymphokine activated killer cell lysis of K562 cells was inhibited by alpha 2-macroglobulin (alpha 2M), soybean trypsin inhibitor, and alpha 1-proteinase inhibitor. In serum free medium 2 mg/ml alpha 2M suppressed target cell lysis in a 4-h cytotoxic assay with about 40%. Suppression was dose and time dependent. Cytotoxicity was unaffected by alpha 2M concentrations less than 0.25 mg/ml, and by alpha 2M added later than 1.5 h from start of assay. Pre-treatment of effector (but not of target) cells with alpha 2M was even more suppressive than the presence of alpha 2M during assay. Cell-mediated cytotoxicity was not inhibited by alpha 2M treated with methylamine or by various alpha 2M-proteinase complexes. In contrast, alpha 2M-proteinase complex as well as native alpha 2M suppressed the proliferation of Ag-activated T cells. However, methylamine-treated alpha 2M did not inhibit T cell proliferation, and suppression by alpha 2M-proteinase complex was significantly reduced after inhibition of the alpha 2M-bound proteinase. On incubation at 4 degrees C with lymphokine-activated killer cells, alpha 2M reacted with cell associated proteinases and changed from electrophoretically "slow" to "fast" form. Cell associated proteinases bound by alpha 2M showed chymotrypsin- and trypsin-like specificities and their activity surpassed activity caused by cellular leakage and secretion. The present results strongly indicate that alpha 2M mediates immunosuppression in its capacity as a proteinase inhibitor and suggest inhibition of (T)cell surface-associated proteinases as a possible mode of suppression.     

Alpha 2-macroglobulin is the primary inhibitor of miniplasmin in vitro and in vivo in the mouse. Comparison with alpha 2-antiplasmin in simultaneous reaction experiments.

Miniplasmin reacted rapidly with purified human alpha 2-macroglobulin (alpha 2M). More than 98% of the complexes were stabilized by at least one covalent bond. The second-order rate constant for the reaction of alpha 2M with miniplasmin at 4 degrees C was 5.1 x 10(5) M-1.s-1. This value was determined by measuring the formation of covalent alpha 2M-125I-miniplasmin complex; however, the rate constant most likely reflects the bait-region cleavage step in the reaction mechanism. Miniplasmin bound primarily to alpha 2M when incubated at 37 degrees C with various mixtures of alpha 2-antiplasmin (alpha 2AP) and alpha 2M. A 2.4-fold molar excess of alpha 2AP was required to yield an equal distribution of proteinase between the two inhibitors. alpha 2M was the primary miniplasmin inhibitor in human and murine plasma (4 degrees C and 37 degrees C). The extent of covalent-bond formation with murine alpha 2M was approx. 96%. Intravenously injected miniplasmin cleared rapidly from the circulation of mice and was recovered principally in the liver. The catabolic pathway was distinctly different from that of miniplasminogen, which was sequestered mainly in the kidneys. The rate of miniplasmin clearance was much faster than that of purified alpha 2AP-miniplasmin complex, suggesting reaction with alpha 2M in vivo. This was confirmed in clearance competition experiments with alpha 2M-methylamine.     

Absence of binding of pancreatic and urinary kallikreins to alpha 2-macroglobulin.

Pancreatic and urinary kallikreins failed to form the typical serine proteinase complex with alpha2M (alpha2-macroglobulin). Studies were performed to compare this with the binding of trypsin to alpha2M at various molar binding ratios, with the use of Sephadex G-200 gel filtration to separate free and alpha2M-bound enzyme fractions. The subunit conversion was totally absent with pancreatic kallikrein from lhich traces of a binding proteinase had been removed. The lack of binding is believed to be the result of the restricted specificity of the kallikreins.     

Binding and degradation of alpha 2-macroglobulin by cultured fibroblasts

We studied the interactions of alpha 2-macroglobulin, a major protease inhibitor of plasma and of serum-containing culture medium, with cultured fibroblasts. Iodinated human alpha 2-macroglobulin bound specifically to washed cell layers of cultured human fibroblasts. At 0--4 degrees C, binding was saturated at a concentration of 10--20 micrograms/ml. At 37 degrees C, radiolabel appeared in the medium in a form soluble in 10% trichloroacetic acid. Sodium dodecyl sulfate polyacrylamide gel electrophoresis indicated that ingested iodinated alpha 2-macroglobulin transiently forms a complex with a trypsin-like protease. Indirect immunofluorescence demonstrated alpha 2-macroglobulin in vacuoles of fibroblasts grown in 10% human serum or incubated with purified alpha 2-macroglobulin. Fibroblasts transformed by SV-40 (VA-13 cells) bound and degraded less 125I-labeled alpha 2-macroglobulin than non-transformed fibroblasts and had fewer vacuoles containing alpha 2-macroglobulin. These observations indicate that cultured fibroblasts bind, take up by endocytosis, and degrade alpha 2-macroglobulin. Binding and endocytosis of alpha 2-macroglobulin by a cell may be a means of modulating proteases in the microenvironment of the cell and during endocytosis.