Perfusion of human placenta with hemoglobin introduces preeclampsia-like injuries that are prevented by α1-microglobulin

BACKGROUND

Preeclamptic women have increased plasma levels of free fetal hemoglobin (HbF), increased gene expression of placental HbF and accumulation of free HbF in the placental vascular lumen. Free hemoglobin (Hb) is pro-inflammatory, and causes oxidative stress and tissue damage.

METHODOLOGY

To show the impact of free Hb in PE, we used the dual ex vivo placental perfusion model. Placentas were perfused with Hb and investigated for physical parameters, Hb leakage, gene expression and morphology. The protective effects of α(1)-microglobulin (A1M), a heme- and radical-scavenger and antioxidant, was investigated.

RESULTS

Hb-addition into the fetal circulation led to a significant increase of the perfusion pressure and the feto-maternal leakage of free Hb. Morphological damages similar to the PE placentas were observed. Gene array showed up-regulation of genes related to immune response, apoptosis, and oxidative stress. Simultaneous addition of A1M to the maternal circulation inhibited the Hb leakage, morphological damage and gene up-regulation. Furthermore, perfusion with Hb and A1M induced a significant up-regulation of extracellular matrix genes.

SIGNIFICANCE

The ex vivo Hb-perfusion of human placenta resulted in physiological and morphological changes and a gene expression profile similar to what is observed in PE placentas. These results underline the potentially important role of free Hb in PE etiology. The damaging effects were counteracted by A1M, suggesting a role of this protein as a new potential PE therapeutic agent.

Distribution of iodine 125-labeled alpha1-microglobulin in rats after intravenous injection

The 28-kd plasma protein alpha(1)-microglobulin is found in the blood of mammals and fish in a free, monomeric form and as high-molecular-weight complexes with molecular masses above 200 kd. In this study, iodine 125-labeled free and high-molecular weight rat alpha(1)-microglobulin (a mixture of alpha(1)-microglobulin/alpha(1)-inhibitor-3 and alpha(1)-microglobulin/fibronectin complexes) were injected intravenously into rats. The distribution of the proteins was measured by using scintillation camera imaging. Both forms of (125)I-labeled alpha(1)-microglobulin were rapidly cleared from the blood, with a half-life of 2 and 16 minutes for the initial and late phase, respectively, for free alpha(1)-microglobulin; and a half-life of 3 and 130 minutes for the initial and late phase, respectively, for the complexes. After 45 minutes, 6%, 16%, 27%, 13%, and 34% of the free (125)I-labeled alpha(1)-microglobulin and 18%, 21%, 6%, 10%, and 42% of the (125)I-labeled alpha(1)-microglobulin complexes were found in the blood, gastrointestinal tract, kidneys, liver, and the remainder of the body, respectively. The local distribution of injected (125)I-labeled alpha(1)-microglobulin in intestines and kidneys was investigated by microscopy and autoradiography. In the intestine, both forms were distributed in the basal layers, villi, and luminal contents. The results also suggested intracellular labeling of epithelial cells. Well-defined local regions containing higher concentrations of injected protein could be seen in the intestine. In the kidneys, both forms were found mostly in the cortex. Free (125)I-labeled alpha(1)-microglobulin was found predominantly in epithelial cells of a subset of the tubules, whereas the (125)I-labeled complexes were more evenly distributed. Intracellular labeling was indicated for both alpha(1)-microglobulin forms. The results thus indicate a rapid transport of (125)I-labeled alpha(1)-microglobulin from the blood to most tissues.

Expression of a Functional Proteinase Inhibitor Capable of Accepting Xylose: Bikunin

Bikunin is a Kunitz-type proteinase inhibitor, which is cross-linked to heavy chains via a chondroitin sulfate chain, forming inter-alpha-inhibitor and related molecules. Rat bikunin was produced by baculovirus-infected insect cells. The protein could be purified with a total yield of 20 mg/liter medium. Unlike naturally occuring bikunin the recombinant protein had no galactosaminoglycan chain. Endoglycosidase digestion also suggested that the recombinant form lacked N-linked oligosaccharides. Bikunin is translated as a part of a precursor, alpha1-microglobulin/bikunin, but the functional significance of the cotranslation is unknown. Our results indicate that the proteinase inhibitory function of bikunin is not regulated by the alpha1-microglobulin-part of the alpha1-microglobulin/bikunin precursor since recombinant bikunin had the same trypsin inhibitory activity as the recombinant precursor. Both free bikunin and the precursor were also functional as a substrate in an in vitro xylosylation system. This demonstrates that the alpha1-microglobulin-part is not necessary for the first step of galactosaminoglycan assembly.

The Yersinia protein kinase A is a host factor inducible RhoA/Rac-binding virulence factor

The pathogenic yersiniae inject proteins directly into eukaryotic cells that interfere with a number of cellular processes including phagocytosis and inflammatory-associated host responses. One of these injected proteins, the Yersinia protein kinase A (YpkA), has previously been shown to affect the morphology of cultured eukaryotic cells as well as to localize to the plasma membrane following its injection into HeLa cells. Here it is shown that these activities are mediated by separable domains of YpkA. The amino terminus, which contains the kinase domain, is sufficient to localize YpkA to the plasma membrane while the carboxyl terminus of YpkA is required for YpkAs morphological effects. YpkAs carboxyl-terminal region was found to affect the levels of actin-containing stress fibers as well as block the activation of the GTPase RhoA in Yersinia-infected cells. We show that the carboxyl-terminal region of YpkA, which contains sequences that bear similarity to the RhoA-binding domains of several eukaryotic RhoA-binding kinases, directly interacts with RhoA as well as Rac (but not Cdc42) and displays a slight but measurable binding preference for the GDP-bound form of RhoA. Surprisingly, YpkA binding to RhoA(GDP) affected neither the intrinsic nor guanine nucleotide exchange factor-mediated GDP/GTP exchange reaction suggesting that YpkA controls activated RhoA levels by a mechanism other than by simply blocking guanine nucleotide exchange factor activity. We go on to show that YpkAs kinase activity is neither dependent on nor promoted by its interaction with RhoA and Rac but is, however, entirely dependent on heat-sensitive eukaryotic factors present in HeLa cell extracts and fetal calf serum. Collectively, our data show that YpkA possesses both similarities and differences with the eukaryotic RhoA/Rac-binding kinases and suggest that the yersiniae utilize the Rho GTPases for unique activities during their interaction with eukaryotic cells.

Tissue distribution of the lipocalin alpha-1 microglobulin in the developing human fetus

Alpha-1 microglobulin (alpha(1)m), a lipocalin, is an evolutionarily conserved immunomodulatory plasma protein. In all species studied, alpha(1)m is synthesized by hepatocytes and catabolized in the renal proximal tubular cells. alpha(1)m deficiency has not been reported in any species, suggesting that its absence is lethal and indicating an important physiological role for this protein To clarify its functional role, tissue distribution studies are crucial. Such studies in humans have been restricted largely to adult fresh/frozen tissue. Formalin-fixed, paraffin-embedded multi-organ block tissue from aborted fetuses (gestational age range 7-22 weeks) was immunohistochemically examined for alpha(1)m reactivity. Moderate to strong reactivity was seen at all ages in hepatocytes, renal proximal tubule cells, and a subset of pancreatic islet cells. Muscle (cardiac, skeletal, or smooth), adrenal cortex, a scattered subset of intestinal mucosal cells, tips of small intestinal villi, and Leydig cells showed weaker and/or variable levels of reactivity. Connective tissue stained with variable location and intensity. The following cells/sites were consistently negative: thymus, spleen, hematopoietic cells, lung parenchyma, glomeruli, exocrine pancreas, epidermis, cartilage/bone, ovary, seminiferous tubules, epididymis, thyroid, and parathyroid. The results underscore the dominant role of liver and kidney in fetal alpha(1)m metabolism and provide a framework for understanding the functional role of this immunoregulatory protein.

alpha(1)-Microglobulin: a yellow-brown lipocalin

alpha(1)-Microglobulin, also called protein HC, is a lipocalin with immunosuppressive properties. The protein has been found in a number of vertebrate species including frogs and fish. This review summarizes the present knowledge of its structure, biosynthesis, tissue distribution and immunoregulatory properties. alpha(1)-Microglobulin has a yellow-brown color and is size and charge heterogeneous. This is caused by an array of small chromophore prosthetic groups, attached to amino acid residues at the entrance of the lipocalin pocket. A gene in the lipocalin cluster encodes alpha(1)-microglobulin together with a Kunitz-type proteinase inhibitor, bikunin. The gene is translated into the alpha(1)-microglobulin-bikunin precursor, which is subsequently cleaved and the two proteins secreted to the blood separately. alpha(1)-Microglobulin is found in blood and in connective tissue in most organs. It is most abundant at interfaces between the cells of the body and the environment, such as in lungs, intestine, kidneys and placenta. alpha(1)-Microglobulin inhibits immunological functions of white blood cells in vitro, and its distribution is consistent with an anti-inflammatory and protective role in vivo.

Carbohydrate groups of alpha1-microglobulin are important for secretion and tissue localization but not for immunological properties

The role of the carbohydrates for the structure and functions of the plasma and tissue protein alpha1-microglobulin (alpha1m) was investigated by deletion of the sites for N-glycosylation by site-directed mutagenesis (N17,96-->Q). The mutated cDNA was expressed in a baculovirus-insect cell system resulting in a nonglycosylated protein. The biochemical properties of N17,96Q-alpha1m were compared to nonmutated alpha1m, which carries two short non-sialylated N-linked oligosaccharides when expressed in the same system. Both proteins carried a yellow-brown chromophore and were heterogeneous in charge. Circular dichroism spectra and antibody binding indicated a similar overall structure. However, the secretion of N17,96Q-alpha1m was significantly reduced and approximately 75% of the protein were found accumulated intracellularly. The in vitro immunological effects of recombinant nonmutated alpha1m and N17,96Q-alpha1m were compared to the effects of alpha1m isolated from plasma, which is sialylated and carries an additional O-linked oligosaccharide. All three alpha1m variants bound to human peripheral lymphocytes and mouse T cell hybridomas to the same extent. They also inhibited the antigen-stimulated proliferation of peripheral lymphocytes and antigen-stimulated interleukin 2-secretion of T cell hybridomas in a similar manner. After injection of rats intravenously, the blood clearance of recombinant nonmutated and N17,96Q-alpha1m was faster than that of plasma alpha1m. Nonmutated alpha1m was located primarily to the liver, most likely via binding to asialoglycoprotein receptors, and N17,96Q-alpha1m was located mainly to the kidneys. It is concluded that the carbohydrates of alpha1m are important for the secretion and the in vivo turnover of the protein, but not for the structure or immunological properties.

Structural model of human alpha1-microglobulin: proposed scheme for the interaction with the Gla domain of anticoagulant protein C

Alpha1-microglobulin (alpha1m) is a small glycoprotein with immunomodulatory properties. It is a member of the lipocalin family, a group of proteins that exhibit a well-conserved three-dimensional structure despite low sequence identity and that are known to bind small hydrophobic ligands. The types of ligands carried by alpha1m are still unknown, but it is known that this protein has yellow-brown chromophores attached to three lysines at position 92, 118 and 130. Alpha1m has one unpaired cysteine residue (Cys 34) that can form a disulphide bond with other proteins that also possess an exposed free unpaired cysteine. For instance, alpha1m interacts with the protein C (PC) Gla domain containing the Arg9Cys or Ser12Cys substitution. In order to gain insights about the alpha1m molecule and analyze the intriguing alpha1m-Gla domain interaction, it was decided to use bioinformatics. The three-dimensional structures of alpha1m and PC Gla domain were predicted. Alpha1m Cys 34 is solvent exposed and located near the entrance of the ligand-binding pocket. The chromophore-carrying lysines are found buried into the pocket, and the area around the entrance of this cavity displays about 10 positively charged residues. This electropositive region in alpha1m complements the essentially electronegative Gla domain and may play a role during intermolecular interactions. In addition, a few hydrophobic residues surround alpha1m Cys 34 and could be of importance during its interaction with macromolecular ligands.

Alpha1-microglobulin chromophores are located to three lysine residues semiburied in the lipocalin pocket and associated with a novel lipophilic compound

Alpha1-microglobulin (alpha1m) is an electrophoretically heterogeneous plasma protein. It belongs to the lipocalin superfamily, a group of proteins with a three-dimensional (3D) structure that forms an internal hydrophobic ligand-binding pocket. Alpha1m carries a covalently linked unidentified chromophore that gives the protein a characteristic brown color and extremely heterogeneous optical properties. Twenty-one different colored tryptic peptides corresponding to residues 88-94, 118-121, and 122-134 of human alpha1m were purified. In these peptides, the side chains of Lys92, Lys118, and Lys130 carried size heterogeneous, covalently attached, unidentified chromophores with molecular masses between 122 and 282 atomic mass units (amu). In addition, a previously unknown uncolored lipophilic 282 amu compound was found strongly, but noncovalently associated with the colored peptides. Uncolored tryptic peptides containing the same Lys residues were also purified. These peptides did not carry any additional mass (i.e., chromophore) suggesting that only a fraction of the Lys92, Lys118, and Lys130 are modified. The results can explain the size, charge, and optical heterogeneity of alpha1m. A 3D model of alpha1m, based on the structure of rat epididymal retinoic acid-binding protein (ERABP), suggests that Lys92, Lys118, and Lys130 are semiburied near the entrance of the lipocalin pocket. This was supported by the fluorescence spectra of alpha1m under native and denatured conditions, which indicated that the chromophores are buried, or semiburied, in the interior of the protein. In human plasma, approximately 50% of alpha1m is complex bound to IgA. Only the free alpha1m carried colored groups, whereas alpha1m linked to IgA was uncolored.

The alpha1-microglobulin/bikunin gene: characterization in mouse and evolution

The 129Sv mouse gene coding for the alpha1-microglobulin/bikunin precursor has been isolated and characterized. The 11kb long gene contains ten exons, including six 5'-exons coding for alpha1-microglobulin and four 3'-exons encoding bikunin. Exon 7 also codes for the tribasic tetrapeptide RARR which connects the alpha1-microglobulin and bikunin parts. The sixth intron, which separates the alpha1-microglobulin and bikunin encoding parts, was compared in the human, mouse and a fish (plaice) gene. The size of this intron varies considerably, 6.5, 3.3 and 0.1kb in man, mouse and plaice, respectively. In all three genes, this intron contains A/T-rich regions, and retroposon elements are found in the first two genes. This indicates that this sixth intron is an unstable region and a hotspot for recombinational events, supporting the concept that the alpha1-microglobulin and bikunin parts of this gene are assembled from two ancestral genes. Finally, the nonsynonymous nucleotide substitution rate of the gene was determined by comparing coding sequences from ten vertebrate species. The results indicate that the alpha1-microglobulin part of the gene has evolved faster than the bikunin part.

Isolation of plaice (Pleuronectes platessa) alpha1-microglobulin: conservation of structure and chromophore

A cDNA coding for plaice (Pleuronectes platessa) alpha1-microglobulin (Leaver et al., 1994, Comp. Biochem. Physiol. 108B, 275-281) was expressed and purified from baculovirus-infected insect cells. Specific monoclonal antibodies were then prepared and used to isolate the protein from plaice liver and serum. Mature 28.5 kDa alpha1-microglobulin was found in both liver and serum. The protein consisted of an 184 amino acid peptide with a complex N-glycan in position Asn123, one intrachain disulfide bridge and a yellow-brown chromophore. Physicochemical characterization indicated a globular shape with a frictional ratio of 1.37, electrophoretic charge-heterogeneity and antiparallel beta-sheet structure. A smaller, incompletely glycosylated, yellow-brown alpha1-microglobulin as well as a 45 kDa precursor protein were also found in liver. The chromophore was found to be linked to alpha1-microglobulin intracellularly. Recombinant plaice alpha1-microglobulin isolated from insect cells had the same N-terminal sequence, globular shape and yellow-brown color as mature alpha1-microglobulin, but carried a smaller, fucosylated, non-sialylated N-glycan in the Asn123 position. The concentration of alpha1-microglobulin in plaice serum was 20 mg/l and it was found both as a 28.5 kDa component and as high molecular weight components. Thus, the size, shape, charge and color of plaice alpha1-microglobulin were similar to mammalian alpha1-microglobulin, indicating a high degree of structural conservation between fish and human alpha1-microglobulin. The monoclonal antibodies against plaice alpha1-microglobulin cross-reacted with human alpha1-microglobulin, emphasizing the structural similarity.

Histologic distribution and biochemical properties of alpha 1-microglobulin in human placenta

PROBLEM

The embryo is protected from immunologic rejection by the mother, possibly accomplished by immunosuppressive molecules located in the placenta. We investigated the distribution and biochemical properties in placenta of the immunosuppressive plasma protein alpha 1-microglobulin.

METHOD OF STUDY

Placental alpha 1-microglobulin was investigated by immunohistochemistry and, after extraction, by electrophoresis, immunoblotting and radioimmunoassay.

RESULTS

alpha 1-Microglobulin staining was observed in the intervillous fibrin and in syncytiotrophoblasts, especially at sites with syncytial injury. Strongly stained single cells in the intervillous spaces and variably stained intravillous histiocytes were noted. Solubilization of the placenta-matrix fraction and placenta membrane fraction released predominantly the free form of alpha 1-microglobulin, but, additionally, an apparently truncated form from the placenta-membrane fraction. The soluble fraction of placenta contained two novel alpha 1-microglobulin complexes.

CONCLUSIONS

The biochemical analysis indicates the presence in placenta of alpha 1-microglobulin forms not found in blood. The histochemical analysis supports the possibility that alpha 1-microglobulin may function as a local immunoregulator in the placenta.

Alpha1-microglobulin is found both in blood and in most tissues

In this study we demonstrate that, in addition to blood, alpha1-microglobulin (alpha1m) is present in most tissues, including liver, heart, eye, kidney, lung, pancreas, and skeletal muscle. Western blotting of perfused and homogenized rat tissue supernatants revealed alpha1m in its free, monomeric form and in high molecular weight forms, corresponding to the complexes fibronectin-alpha1m and alpha1-inhibitor-3-alpha1m, which have previously been identified in plasma. The liver also contained a series of alpha1m isoforms with apparent molecular masses between 40 and 50 kD. These bands did not react with anti-inter-alpha-inhibitor antibodies, indicating that they do not represent the alpha1m-bikunin precursor protein. Similarly, the heart contained a 45-kD alpha1m band and the kidney a 50-kD alpha1m band. None of these alpha1m isoforms was present in plasma. Immunohistochemical analysis of human tissue demonstrated granular intracellular labeling of alpha1m in hepatocytes and in the proximal epithelial cells of the kidney. In addition, alpha1m immunoreactivity was detected in the interstitial connective tissue of heart and lung and in the adventitia of blood vessels as well as on cell surfaces of cardiocytes. alpha1m mRNA was found in the liver and pancreas by polymerase chain reaction, suggesting that the protein found in other tissues is transported via the bloodstream from the production sites in liver and pancreas. The results of this study indicate that in addition to its role in plasma, alpha1m may have important functions in the interstitium of several tissues. (J Histochem Cytochem 46:887-893, 1998)

Receptor for alpha1-microglobulin on T lymphocytes: inhibition of antigen-induced interleukin-2 production

The human plasma protein alpha1-microglobulin (alpha1m) was found to inhibit the antigen-induced interleukin-2 (IL-2) production of two different mouse T-helper cell hybridomas. Alpha1m isolated from human plasma and recombinant alpha1m isolated from baculovirus-infected insect cell cultures had similar inhibitory effects. Flow cytometric analysis showed a binding of plasma and recombinant alpha1m to the T-cell hybridomas as well as to a human T-cell line. Radiolabelled plasma and recombinant alpha1m bound to the T-cell hybridomas in a saturable manner and the binding could be eliminated by trypsination of the cells. The affinity constants for the cell binding were calculated to be 0.4-1 x 10(5) M(-1) using Scatchard plotting, and the number of binding sites per cell was estimated to be 5 x 10(5)-1 x 10(6). The cell-surface proteins of one of the T-cell hybridomas were radiolabelled, the cells lysed and alpha1m-binding proteins isolated by affinity chromatography. SDS-PAGE and autoradiography analysis of the eluate revealed major bands with Mr-values around 70, 35 and 15 kDa. The results thus suggest that alpha1m binds to a specific receptor on T cells and that the binding leads to inhibition of antigen-stimulated IL-2 production by T-helper cells.

Association of the haptoglobin phenotype (1-1) with falciparum malaria in Sudan

The haptoglobin phenotypes of Sudanese patients with complicated and uncomplicated falciparum malaria, and those of uninfected randomly selected individuals, were determined by electrophoresis of sera on polyacrylamide gels followed by benzidine staining of the gels. Among 273 malaria patients, the proportions with haptoglobin phenotypes (1-1), (2-1) and (2-2) were 60.8%, 29.7% and 9.5%, respectively, and in 72 cerebral malaria patients the proportions were 63.9%, 29.2%, and 6.9%. The distribution among 208 control individuals was 26.0%, 55.8% and 18.3%, respectively. The difference between patients and controls was highly significant (P < 0.001). The distribution of the different haptoglobin phenotypes among the randomly selected group of 208 Sudanese individuals was comparable to that in many other populations. The results suggests that the haptoglobin phenotype (1-1) is associated with susceptibility to falciparum malaria and the development of severe complications; alternatively, the other phenotypes may confer resistance.

Physicochemical and biochemical characterization of human alpha 1-microglobulin expressed in baculovirus-infected insect cells

DNA encoding the signal peptide and the alpha 1-microglobulin part of the human alpha 1-microglobulin-bikunin gene was expressed in baculovirus-infected insect cells. Recombinant alpha 1-microglobulin was secreted and could be purified from the medium with a yield of 20-30 mg/ L. Biochemical and physicochemical characterization showed that the recombinant protein was very similar to alpha 1-microglobulin isolated from human urine and plasma, except that the recombinant protein had smaller N-linked oligosaccharides, lacked the O-linked oligosaccharide, and was devoid of sialic acid. Recombinant alpha 1-microglobulin migrated upon SDS-PAGE as two bands, 27 and 29 kDa, representing alpha 1-microglobulin with one and two N-linked carbohydrates, respectively. An overall structural similarity was indicated as antibodies raised against human urinary alpha 1-microglobulin were found to recognize recombinant, plasma, and urinary alpha 1-microglobulin in a similar manner. CD studies suggested an almost identical secondary structure for recombinant and urinary alpha 1-microglobulin but a slightly different structure for plasma alpha 1-microglobulin. The absorbance spectrum as well as visual examination demonstrated that recombinant, urinary, and plasma alpha 1-microglobulin carried a yellow-brown chromophore, but that plasma alpha 1-microglobulin was slightly less intensely colored. Although it is still a puzzle why the immunosuppressive plasma protein alpha 1-microglobulin and the protease inhibitor bikunin, which have no known function in common, are cotranslated from the same mRNA, it can be concluded that bikunin is not necessary for an adequate translation, folding, and secretion of alpha 1-microglobulin. Furthermore, since recombinant alpha 1-microglobulin was produced in large amounts and found to be very similar to plasma and urinary alpha 1-microglobulin, it may prove to be useful in structural and functional studies of the protein.

Increase of bikunin and alpha1-microglobulin concentrations in urine of rats during pregnancy is due to decreased tubular reabsorption

Bikunin and alpha1-microglobulin are two plasma proteins of about 25 kDa which are made in the liver from a common precursor. The concentration of bikunin in human urine has been shown to increase several fold during various conditions of stress. The mechanism behind this increase is unknown. We have studied pregnant rats and found that the bikunin and alpha1-microglobulin levels in their urine increased 3-fold towards the end of the pregnancy, whereas those of albumin and orosomucoid did not. There were no significant changes in either the bikunin/alpha1-microglobulin mRNA level or the concentrations of the two proteins in serum. These findings imply that the synthesis and the clearance rates of bikunin and alpha1-microglobulin are normal during pregnancy but that the tubular reabsorption of these proteins is decreased.

Alpha1-microglobulin and bikunin in rats with collagen II-induced arthritis: plasma levels and liver mRNA content

The plasma proteins alpha1-microglobulin (alpha1-m) and bikunin are synthesized in the liver as a common precursor which is cleaved just before secretion. Half of plasma alpha1-m is covalently linked to fibronectin and alpha1-inhibitor-3, and more than 95% of bikunin is part of pre-alpha-inhibitor, inter-alpha-inhibitor and related large molecules. Both alpha1-m and bikunin have been shown to be involved in inflammation, but the regulation of their synthesis is not clear. The authors have measured the plasma and urinary concentrations of alpha1-m and bikunin as well as their hepatic mRNA levels in rats during the development of collagen-induced arthritis. Also, the plasma concentrations of acknowledged acute-phase proteins were measured. The results suggested a biphasic inflammatory reaction: an early response after 1 week, represented by an elevated fibronectin level; and a late response after 3 weeks, represented by elevated alpha1-acid glycoprotein and decreased albumin and alpha1-inhibitor-3 levels. The alpha1-m-bikunin mRNA content in liver was slightly reduced after 1 week and elevated after 3 weeks, but the total concentrations of free and bound alpha1-m and bikunin in plasma were unchanged. The free bikunin fraction as well as the fibronectin/alpha1-m complex in plasma, however, were elevated after 1 week. Urinary bikunin levels were also elevated after 1 week, whereas urinary alpha1-m levels remained unchanged. The results thus suggest that free bikunin in plasma is increased and excreted in the urine at an early stage during the development of collagen-induced arthritis. Later, when the synthesis rate of alpha1-m-bikunin is elevated, both proteins are most likely directed to other locations in the body.

Prothrombin, albumin and immunoglobulin A form covalent complexes with alpha1-microglobulin in human plasma

Molecules containing the 33-kDa plasma protein alpha1-microglobulin were isolated from human plasma by anti-(alpha1-microglobulin) affinity chromatography. Five major bands could be seen after electrophoretic separation of the alpha1-microglobulin-containing proteins under native conditions. Immunoblotting demonstrated alpha1-microglobulin in all five bands. Two of these have been described previously: free alpha1-microglobulin and alpha1-microglobulin complexed with IgA (IgA x alpha1-microglobulin). The other three bands were identified as prothrombin alpha1-microglobulin, albumin x alpha1-microglobulin and dimeric alpha1-microglobulin. Prothrombin x alpha1-microglobulin were 1:2 and 1:1 complexes which carried approximately 1% of total alpha1-microglobulin, had molecular masses of about 145 kDa and 110 kDa upon SDS/PAGE and dissociated completely to free alpha1-microglobulin and prothrombin (72 kDa) when reducing agents were added, suggesting that the complexes were stabilized by disulfide bonds. The alpha1-microglobulin molecules did not inhibit cleavage of prothrombin by factor Xa and were bound to the peptides which were released upon activation of prothrombin. Albumin x alpha1-microglobulin, corresponding to 7% of total plasma alpha1-microglobulin, was a mixture between 1:1 and 1:2 complexes, with masses upon SDS/PAGE of approximately 100 kDa and 135 kDa, respectively. Both these complexes dissociated only partially to free alpha1-microglobulin and albumin when reducing agents were added. The albumin x alpha1-microglobulin complexes carried a yellow-brown chromophore similar to free alpha1-microglobulin. The complex-binding to alpha1-microglobulin did not block the fatty-acid-binding ability of albumin. The plasma concentrations of albumin x alpha1-microglobulin and prothrombin x alpha1-microglobulin were estimated to 5.2 mg/l and 1.1 mg/l, respectively.

Coiled-coil structure of group A streptococcal M proteins. Different temperature stability of class A and C proteins by hydrophobic-nonhydrophobic amino acid substitutions at heptad positions a and d

M proteins and M-like proteins, expressed on the surface of group A streptococci and binding to human plasma proteins, can be divided into two classes, A and C, depending on the structure of the central repeated regions. The class C proteins have been shown to be dimers with a coiled-coil structure. In this work, we have compared the structure and binding of a class A protein, Mrp4, and a class C protein, Arp4, expressed by the same bacterial strain. Circular dichroism spectra, gel filtration, and binding assays showed that both proteins had a coiled-coil dimer configuration and a high-affinity binding at 20 degrees C. However, striking differences were seen at 37 degrees C. The class A protein, Mrp4, was still a coiled-coil dimer with high affinity binding activity, whereas the class C protein, Arp4, had lost both the coiled-coil structure and binding activity. Raising the temperature even higher, Mrp4 retained the coiled-coil structure up to 70-90 degrees C. Furthermore, a recombinant protein, Mrp(C), in which the A-repeats of Mrp4 were replaced by the C-repeats of Arp4, lost its coiled-coil structure and fibrinogen-binding around 40-45 degrees C. These results suggest a high thermal stability of class A proteins and a low stability of class C proteins and that the structural basis for this can be found partly in the A- and C-repeats. Analysis of the amino acid sequences of the A- and C-repeats, revealed a large difference, 87% and 45%, respectively, in the content of hydrophobic amino acid residues in the positions regarded as important for the formation of the coiled-coil structure. In particular, several alanine residues in the A-repeats were replaced by serine residues in the C-repeats. Our results suggest that important structural and functional changes within the M protein family have evolved by specific hydrophobic-nonhydrophobic amino acid replacements.

Bovine alpha 1-microglobulin/bikunin. Isolation and characterization of liver cDNA and urinary alpha 1-microglobulin

cDNA coding for alpha 1-microglobulin, an immunoregulatory plasmaprotein, was isolated from bovine liver. The sequence of a total of 1258 nucleotides revealed an open reading frame of 352 amino acids. This included alpha 1-microglobulin, 182 amino acids, and bikunin, the light chain of the plasmaprotein inter-alpha-inhibitor, 147 amino acids. The two proteins were connected by a basic tetrapeptide, R-A-R-R, which conforms to the consensus sequence recognized by endoproteolytic cleavage enzymes. The deduced amino acid sequence showed a high degree of identity with alpha 1-microglobulin and bikunin sequences from other species, and the alpha 1-microglobulin part displayed sequence motifs typical for members of the lipocalin protein superfamily. A single alpha 1-microglobulin/bikunin mRNA with a size of around 1300 nt was found in bovine liver. The mature alpha 1-microglobulin protein was isolated from bovine urine, and partly characterized. It was found to be a globular molecule with an apparent molecular weight of 23,300, containing one N-linked and at least on O-linked oligosaccharide, one intra-chain disulfide bridge and an electrophoretic heterogeniety with a pI-value of 4.1-5.2.

Structure and stability of protein H and the M1 protein from Streptococcus pyogenes. Implications for other surface proteins of gram-positive bacteria

M proteins and other members of the M protein family, expressed on the surface of Streptococcus pyogenes, bind host proteins such as immunoglobulins, albumin, and fibrinogen. Protein H and the M1 protein are expressed by adjacent genes and both belong to the M protein family. In this work, the structure and stability of these two proteins have been investigated. As judged from sequence analysis and circular dichroism spectroscopy, the proteins are almost entirely in an alpha-helix conformation. The amino acids are arranged in a seven-residue (heptad) repeat pattern along the greater part of the proteins. These observations support the previously accepted model of M proteins as coiled-coil dimers. However, it was also found that the structures of both proteins were thermally unstable; i.e., the content of helix conformation was greatly reduced at 37 degrees C as compared to 25 degrees C or below. Together with previous findings that these proteins appear as monomers at 37 degrees C and dimers at low temperatures, the results suggest that the coiled-coil dimers are unfolded at 37 degrees C. The heptad patterns of protein H and the M1 protein showed a nonoptimal distribution of residues expected for a coiled-coil conformation. This is a possible explanation for the low thermal stability of the proteins. It was also demonstrated that the proteins were stabilized in the presence of the ligands IgG and/or albumin. Protein H and M1 protein show a high degree of sequence similarity in their C-terminal regions, and a fragment from this region displayed a high content of helix conformation, whereas fragments from the nonsimilar N-terminal parts did not adopt any stable folded structure. Thus, the C-terminal parts, which are conserved within the M protein family, may constitute a framework for the formation of the parallel helical coiled-coil structure, and we propose that the less stable N-terminal part may also participate in antiparallel interaction with M proteins on adjacent bacteria. The results suggest that temperature fluctuations in the environment could change the properties of bacterial surface proteins, thereby affecting the molecular interactions between the bacterium and its host.

Allosteric and temperature effects on the plasma protein binding by streptococcal M protein family members

Most group A streptococcal strains bind immunoglobulins (Ig) and fibrinogen to their cell walls. It is shown in this paper that the Ig-binding of three different strains was much weaker at 37 degrees C than at room temperature (20 degrees C), whereas the fibrinogen binding was unaffected by temperature. The binding properties and molecular sizes of two purified group A streptococcal cell surface proteins from the M protein family were studied at various temperatures, M1 protein with affinity for IgG, fibrinogen and albumin, and protein Sir22 with affinity for IgA and IgG. Both proteins appeared as monomers which bound all their ligands, including fibrinogen, very weakly at 37 degrees C, and as strongly binding dimers at 10 and 20 degrees C. Furthermore, the results demonstrated that the plasma protein binding of the bacterial proteins was allosterically regulated, i.e. the binding of a ligand to one site modulated the binding of a ligand to a second site. For example, the binding of albumin or IgG to purified M1 protein at 10 and 20 degrees C strongly enhanced the binding of fibrinogen at 37 degrees C. This indicates that the high affinity dimer form of the bacterial proteins can be stabilized at 37 degrees C, a possible explanation for the strong fibrinogen binding of whole bacteria. Finally, the sizes and binding properties of three M1 protein fragments were studied and the results indicated that the centrally located C-repeats, which are conserved among the members of the M protein family, are important for the formation of the high-affinity dimers of the bacterial proteins.

Expression of rat alpha 1-microglobulin-bikunin in baculovirus-transformed insect cells

cDNA encoding rat alpha 1-microglobulin-bikunin was ligated into the transfer vector pVL 1392 and recombined with a wild-type baculovirus. The resulting alpha 1-microglobulin-bikunin-encoding baculovirus was used to infect Trichoplusia ni (Hi-5) insect cells. The infected cells secreted alpha 1-microglobulin with maximal concentrations of 15 mg/liter 5 days after infection. The secreted proteins migrated upon SDS-PAGE as two major protein bands, 40 and 26 kDa, corresponding to alpha 1-microglobulin-bikunin and free alpha 1-microglobulin. The results suggested that the cells secreted mostly alpha 1-microglobulin-bikunin, which subsequently was cleaved in the medium, yielding free alpha 1-microglobulin. Both forms were isolated by monoclonal anti-alpha 1-microglobulin affinity chromatography, and alpha 1-microglobulin-bikunin separated from free alpha 1-microglobulin by gel chromatography. The yields of purified alpha 1-microglobulin-bikunin and free alpha 1-microglobulin were approximately 1 and 5 mg, respectively, per liter medium. Insect cell alpha 1-microglobulin displayed a size, shape, and charge heterogeneity similar to alpha 1-microglobulin isolated from rat urine. A panel of monoclonal antibodies raised against urinary alpha 1-microglobulin from several different species bound to rat urinary alpha 1-microglobulin and insect cell secreted alpha 1-microglobulin-bikunin and free alpha 1-microglobulin with approximately the same strength, indicating that the three proteins are folded in similar ways. The results of glycosidase treatments and lectin blotting indicate the absence of neuraminic acid but the presence of one N-linked oligosaccharide and an unspecified number of O-linked oligosaccharides in alpha 1-microglobulin-bikunin and free alpha 1-microglobulin.

Formation of the alpha 1-microglobulin chromophore in mammalian and insect cells: a novel post-translational mechanism?

alpha 1-Microglobulin is an immunosuppressive plasma protein synthesized by the liver. The isolated protein is yellow-brown, but the hypothetical chromophore has not yet been identified. In this work, it is shown that a human liver cell line, HepG2, grown in a completely synthetic and serum-free medium, secretes alpha 1-microglobulin which is also yellow-brown, suggesting a de novo synthesis of the chromophore by the cells. alpha 1-Microglobulin isolated from the culture medium of insect cells transfected with the gene for rat alpha 1-microglobulin is also yellow-brown, suggesting that the gene carries information about the chromophore. Reduction and alkylation or removal of N- or O-linked carbohydrates by glycosidase treatment did not reduce the colour intensity of the protein. An internal dodecapeptide (amino acid positions 70-81 in human alpha 1-microglobulin) was also yellow-brown. The latter results indicate that the chromophore is linked to the polypeptide. In conclusion, the results suggest that the alpha 1-microglobulin gene carries information activating a post-translational protein modification mechanism which is present in mammalian and insect cells.

alpha 1-Microglobulin destroys the proteinase inhibitory activity of alpha 1-inhibitor-3 by complex formation

The immunoregulatory plasma protein alpha 1-microglobulin (alpha 1-m) and the proteinase inhibitor alpha 1-inhibitor-3 (alpha 1I3) form a complex in rat plasma. In the present work, it was demonstrated that the alpha 1I3.alpha 1-m complex has no inhibitory activity, the bait region was not cleaved by low amounts of proteinases, and it was unable to covalently incorporate proteinases. The results also indicated that the thiolester bond of the alpha 1I3.alpha 1-m complex was broken. The alpha 1I3.alpha 1-m complex was cleared from the circulation much faster than native alpha 1I3, with a half-life of approximately 7 min. Structurally, however, the alpha 1I3.alpha 1-m complex was similar to native alpha 1I3 rather than alpha 1I3 cleaved by proteinases. It is speculated that the role of alpha 1-m is to destroy the function of alpha 1I3 by blocking the bait region and breaking the thiolester and causing its physical elimination by rapid clearing from the blood circulation. It is also possible that the formation of complexes between alpha 1-m and alpha 1I3 may serve as a mean to regulate the function of alpha 1-m since its complex with alpha 1I3 is taken up rapidly by cellular receptors for alpha-macroglobulins.

On the interaction between single chain Fv antibodies and bacterial immunoglobulin-binding proteins

Using four bacterial immunoglobulin-binding proteins, we have analyzed the binding characteristics of a panel of 34 human single chain Fv antibodies, expressed in E. coli and with known specificity and sequence. Several of the single chain Fv antibodies showed affinity for staphylococcal protein A and peptostreptococcal protein L, but not for the streptococcal proteins G or H. The affinity of the binding was higher for protein L (4.5 and 1.4 x 10(9) M-1) than for protein A (7.7 and 6.7 x 10(8) M-1), using the two single chain Fv antibodies displaying the strongest binding activity to these ligands. The binding was shown to be specific by Western blotting, and the single chain Fv antibodies could be purified from crude bacterial culture media by affinity chromatography on protein L- or A-Sepharose. Protein A, which has affinity for the VH domain of the scFv antibodies, was tested against scFv antibodies containing VH1, VH3, VH4 and VH5 domains, and its binding was restricted to approximately half of the scFv antibodies with a VH3 domain. Protein L, which has affinity for the VL domain, was tested against kappa 1, kappa 4, lambda 1, lambda 2 and lambda 3 domains, and it bound all kappa 1 domains, one lambda 2 and one lambda 3 domain. Comparison of the amino acid sequences of binding and non-binding VL domains demonstrated that amino acid residues crucial to the binding of protein L were distributed over a large area outside the hypervariable antigen-binding regions.

Processing and secretion of rat alpha 1-microglobulin-bikunin expressed in eukaryotic cell lines

The precursor protein alpha 1-microglobulin-bikunin was cleaved to the same degree whether expressed in CHO cells or in mutated CHO cells, RPE.40 cells, suggested to lack a functional form of the intracellular protease furin. Thus, alpha 1-microglobulin-bikunin probably is not cleaved in vivo by furin. However, simultaneous overexpression of the precursor and furin in COS, CHO and RPE.40 cells increased the cleavage, suggesting that compartmentalisation and concentrations of protease and precursor are important for the cleavage, besides the in vitro specificity. Expression of alpha 1-microglobulin and bikunin alone gave different protein patterns of SDS-PAGE as compared to expression of the precursor and subsequent cleavage, suggesting that the precursor protein is important for the post-translational handling of alpha 1-microglobulin and bikunin.

Isolation and characterization of fibronectin-alpha 1-microglobulin complex in rat plasma

Molecules containing the 28 kDa immunoregulatory protein alpha 1-microglobulin (alpha 1-m), also known as protein HC, were isolated from rat plasma or serum by immunoaffinity chromatography. Three molecular species were distinguished on the basis of nondenaturing PAGE. Two of these have been described previously: uncomplexed alpha 1-m, and the complex of alpha 1-m with alpha 1-inhibitor-3. The third species was analysed by denaturing PAGE, immunoblotting, proteinase digestion and N-terminal-sequence analyses, and shown to consist of a complex between alpha 1-m and fibronectin. This complex, with a mass of about 560 kDa, was resistant to dissociation in the presence of denaturants, but not in the presence of reducing agents in combination with denaturants, and we conclude that the two components are linked by disulphide bonds. About 60% of the total detectable plasma alpha 1-m exists as high-molecular-mass complexes distributed approximately evenly between fibronectin and alpha 1-inhibitor-3. Immunochemical analyses were used to determine the proportion of the total plasma pools of fibronectin and alpha 1-inhibitor-3 that circulate in complex with alpha 1-m. About 3-7% of the total plasma fibronectin from three different rat strains contained alpha 1-m, whereas 0.3-0.8% of the total plasma alpha 1-inhibitor-3 contained alpha 1-m. Complexes were found at similar levels in plasma and serum, indicating that coagulation is not responsible for complex formation. Moreover, immunochemical analyses of human plasma revealed small amounts of alpha 1-m in complex with fibronectin and alpha 2-macroglobulin (an alpha 1-inhibitor-3 homologue). The existence of a complex between alpha 1-m and fibronectin in rats and humans suggests a mechanism for the incorporation of the immunoregulatory molecule alpha 1-m into the extracellular matrix.

Autism in thalidomide embryopathy: a population study

Of a population of 100 Swedish thalidomide embryopathy cases, at least four met full criteria for DSM-III-R autistic disorder and ICD-10 childhood autism. Thalidomide embryopathy of the kind encountered in these cases affects fetal development early in pregnancy, probably on days 20 to 24 after conception. It is argued that the possible association of thalidomide embryopathy with autism may shed some light on the issue of which neural circuitries may be involved in autism pathogenesis.

Interaction between streptococcal protein Arp and different molecular forms of human immunoglobulin A

Protein Arp, the IgA-binding protein of the group A Streptococcus, has affinity for the Fc-part of IgA. The binding between protein Arp and several different molecular forms of human IgA was characterized. It was found that protein Arp bound with higher affinity to uncomplexed forms of IgA than to complexed forms (secretory IgA, alpha 1-antitrypsin-IgA and alpha 1-microglobulin-IgA). Thus, the affinity constant was 2.0-5.9 x 10(8) M-1 for the binding to monomeric, dimeric, trimeric, and quadrimeric IgA, and 4.5-5.0 x 10(7) M-1 for binding to the complexed forms. Among the uncomplexed IgA-molecules, the affinity constant was in the same range for J chain-containing forms (dimeric, trimeric and quadrimeric IgA) as for forms without J chain (monomeric and a particular quadrimeric IgA devoid of J chain). Western blotting demonstrated that protein Arp bound exclusively to the alpha-chain of all IgA-forms. Several lines of evidence pointed to a localization of the binding site to the C alpha 3-domain. First, protein Arp did not bind to three N-terminal alpha-chain fragments which lacked a region corresponding to the C alpha 3-domain, including that form a four-chain myeloma IgA, naturally occurring in plasma. Second, the binding to dimeric and tri/quadrimeric IgA was partially blocked by an added secretory component, which has been suggested to bind to the C alpha 2- and C alpha 3-domains of the alpha-chain. Finally, alpha 1-antitrypsin and alpha 1-microglobulin, in the weakly binding IgA-complexes, have been shown to be linked to the C alpha 3-domain via the penultimate amino acid residue of the alpha-chain peptide, supporting the hypothesis of a localization of the binding site of protein Arp to the C alpha 3-domain.

Purification of antibodies using protein L-binding framework structures in the light chain variable domain

Protein L from the bacterial species Peptostreptococcus magnus binds specifically to the variable domain of Ig light chains, without interfering with the antigen-binding site. In this work a genetically engineered fragment of protein L, including four of the repeated Ig-binding repeat units, was employed for the purification of Ig from various sources. Thus, IgG, IgM, and IgA were purified from human and mouse serum in a single step using protein L-Sepharose affinity chromatography. Moreover, human and mouse monoclonal IgG, IgM, and IgA, and human IgG Fab fragments, as well as a mouse/human chimeric recombinant antibody, could be purified from cultures of hybridoma cells or antibody-producing bacterial cells, with protein L-Sepharose. This was also the case with a humanized mouse antibody, in which mouse hypervariable antigen-binding regions had been introduced into a protein L-binding kappa subtype III human IgG. These experiments demonstrate that it is possible to engineer antibodies and antibody fragments (Fab, Fv) with protein L-binding framework regions, which can then be utilized in a protein L-based purification protocol.

Cleavage of the alpha 1-microglobulin-bikunin precursor is localized to the Golgi apparatus of rat liver cells

alpha 1-Microglobulin, a plasma protein with immunoregulatory properties, and bikunin, the light chain of the proteinase inhibitors inter-alpha-inhibitor and pre-alpha-inhibitor, are translated as a precursor protein from the same mRNA. The cosynthesis of alpha 1-microglobulin and bikunin is unique compared to other proproteins such as procomplement components and prohormones, since alpha 1-microglobulin and bikunin have no known functional connection. Different forms of intracellular rat liver alpha 1-microglobulin were isolated and characterized by amino acid sequence analysis, lectin binding and glycosidase treatment. Their subcellular distribution was studied by Nycodenz and sucrose gradient centrifugation, pulse-chase experiments, and electrophoresis with subsequent immunoblotting, using pro-C3 and prohaptoglobin as reference proteins. Two alpha 1-microglobulin-bikunin precursors (40 and 42 kDa), containing one and two N-linked oligosaccharides, respectively, were detected in the endoplasmic reticulum. After transport to the Golgi apparatus, the precursors were cleaved, probably C-terminal to the sequence Arg-Ala-Arg-Arg immediately preceding the bikunin part, yielding free sialylated 28 kDa alpha 1-microglobulin, representing the mature protein. The cleavage was almost complete in phosphatidylinositol 4-kinase-enriched membranes, previously identified as a post-Golgi compartment. A fourth intracellular form of alpha 1-microglobulin, 26 kDa, lacked sialic acid. None of the intracellular forms carried the yellow-brown chromophore associated with alpha 1-microglobulin when purified from serum and urine, suggesting that this chromophore becomes linked to the protein after its secretion from the liver cells.

On the interaction between protein L and immunoglobulins of various mammalian species

Protein L, a cell wall molecule of certain strains of the anaerobic bacterial species Peptostreptococcus magnus, shows high affinity for human immunoglobulin (Ig) light chains. In the present study protein L was tested against a panel of human myeloma proteins of the IgG, IgM, IgA and IgE classes, and strong binding was seen with antibodies carrying kappa light chains. A high degree of specificity for Ig was demonstrated in binding experiments with human plasma proteins. Apart from human Ig, strong protein L-binding activity was also detected in the serum of 12 out of 23 tested additional mammalian species, including other primates and rodents. Subsequent analysis with purified Ig samples demonstrated the binding of protein L to Ig of important laboratory animal species such as the mouse, the rat and the rabbit. The affinity constants for the interactions between protein L and polyclonal IgG of these species were 2.6 x 10(9), 3.9 x 10(8) and 7.4 x 10(7), respectively. In non-human species, the binding of protein L was also found to be mediated through Ig light chains, and the results demonstrate the potential value of protein L as an immunochemical tool.

Protein Arp and protein H from group A streptococci. Ig binding and dimerization are regulated by temperature

Cell surface proteins that bind to the Fc part of Ig are expressed by many strains of group A streptococci, an important human pathogen. Two such bacterial strains, AP4 and AP1, were shown to bind IgA and IgG, respectively, in a temperature-dependent manner. The binding of radiolabeled Ig to the bacterial cells was lower at 37 degrees C than at 22 and 4 degrees C. Similarly, protein Arp, the IgA-binding protein isolated from strain AP4, and protein H, the IgG-binding protein isolated from strain AP1, displayed a strong Ig-binding at 22 degrees C and lower temperatures, and virtually no binding at all at 37 degrees C. The effect was reversible: lowering of the temperature restored the binding and vice versa. A gradual shift between binding and nonbinding took place between 27 and 37 degrees C. Gel chromatography and velocity sedimentation centrifugation showed that protein Arp and protein H appeared as noncovalently associated dimers at 10 and 22 degrees C, and as monomers at 37 degrees C. These results strongly suggest that the dimerization of protein Arp and protein H, rather than the low temperature itself, yielded the strong Ig-binding of the proteins at 10 and 22 degrees C. Indeed, after covalent cross-linking of the dimers at 10 degrees C by incubation with low concentrations of glutaraldehyde, full Ig-binding was achieved even at 37 degrees C. A carboxyl-terminal proteolytic fragment of protein Arp, which completely lacked the IgA-binding capacity at any temperature, showed the same temperature-dependent dimerization as intact protein Arp, suggesting that the Ig-binding part of the protein is not required for dimerization. The implications of these results for the function of Ig-binding group A streptococcal proteins, and their role in the host-parasite relationship are discussed.

Protein Arp and protein H from group A streptococci. Ig binding and dimerization are regulated by temperature

Cell surface proteins that bind to the Fc part of Ig are expressed by many strains of group A streptococci, an important human pathogen. Two such bacterial strains, AP4 and AP1, were shown to bind IgA and IgG, respectively, in a temperature-dependent manner. The binding of radiolabeled Ig to the bacterial cells was lower at 37 degrees C than at 22 and 4 degrees C. Similarly, protein Arp, the IgA-binding protein isolated from strain AP4, and protein H, the IgG-binding protein isolated from strain AP1, displayed a strong Ig-binding at 22 degrees C and lower temperatures, and virtually no binding at all at 37 degrees C. The effect was reversible: lowering of the temperature restored the binding and vice versa. A gradual shift between binding and nonbinding took place between 27 and 37 degrees C. Gel chromatography and velocity sedimentation centrifugation showed that protein Arp and protein H appeared as noncovalently associated dimers at 10 and 22 degrees C, and as monomers at 37 degrees C. These results strongly suggest that the dimerization of protein Arp and protein H, rather than the low temperature itself, yielded the strong Ig-binding of the proteins at 10 and 22 degrees C. Indeed, after covalent cross-linking of the dimers at 10 degrees C by incubation with low concentrations of glutaraldehyde, full Ig-binding was achieved even at 37 degrees C. A carboxyl-terminal proteolytic fragment of protein Arp, which completely lacked the IgA-binding capacity at any temperature, showed the same temperature-dependent dimerization as intact protein Arp, suggesting that the Ig-binding part of the protein is not required for dimerization. The implications of these results for the function of Ig-binding group A streptococcal proteins, and their role in the host-parasite relationship are discussed.

Synthesis of alpha 1-microglobulin in cultured rat hepatocytes is stimulated by interleukin-6, leukemia inhibitory factor, dexamethasone and retinoic acid

The secretion of alpha 1-microglobulin by primary cultures of rat hepatocytes was found to increase upon the addition of interleukin-6 or leukemia inhibitory factor, two mediators of acute phase response. This stimulatory effect was further enhanced by dexamethasone. alpha 1-Microglobulin is synthesized as a precursor also containing bikunin, and the precursor protein is cleaved shortly before secretion. Our results therefore suggest that both alpha 1-microglobulin and bikunin are acute phase reactants in rat hepatocytes. Furthermore, we found that retinoic acid, previously shown to be involved in the regulation of cell differentiation and development, also stimulated alpha 1-microglobulin synthesis. Only free, uncomplexed alpha 1-microglobulin (28,000 Da) was detected in the hepatocyte media, suggesting that the complex between alpha 1-microglobulin and alpha 1-inhibitor 3, found in rat serum, is formed outside the hepatocyte.

Localization of the binding site for streptococcal protein G on human serum albumin. Identification of a 5.5-kilodalton protein G binding albumin fragment

Protein G is a streptococcal cell wall protein with separate and repetitively arranged binding domains for immunoglobulin G (IgG) and human serum albumin (HSA). In this work, the binding of protein G to HSA was studied. The results suggest that a single binding site is present on HSA: the apparent size of the HSA-protein G complex (230 kDa) corresponded to two or three HSA molecules bound to one protein G molecule, and Ouchterlony immunodiffusion did not yield any precipitate between protein G and HSA. HSA was cleaved by pepsin and CNBr into several fragments which were identified by SDS-PAGE and N-terminal amino acid sequencing, and the binding of protein G to the fragments was studied in Western blot experiments. The results indicated that the binding area was located in disulfide loops 6-8, involving both the second (loop 6) and the third (loops 7 and 8) domain of HSA. One of the protein G binding pepsin fragments, with an apparent molecular mass of 5.5 kDa, located in loops 7 and 8, was isolated and found to completely inhibit the binding between protein G and the intact HSA, again suggesting a single protein G binding site on serum albumin. Reducing the disulfide bonds of HSA, and subsequent alkylation of the half-cystine residues, significantly decreased the affinity for protein G. Protein G bound to albumin from baboon, cat, guinea pig, hamster, hen, horse, man, mouse, and rat, but not to albumin from cow, dog, goat, pig, rabbit, sheep, snake, or turkey.

Protein L from Peptostreptococcus magnus binds to the kappa light chain variable domain

Protein L is an immunoglobulin light chain-binding protein expressed by some strains of the anaerobic bacterial species Peptostreptococcus magnus. The major variable region subgroups of human kappa and lambda light chains were tested for protein L binding; V kappa I, V kappa III, and V kappa IV bound protein L, whereas no binding occurred with proteins of the V kappa II subgroup or with any lambda light chain subgroups. Studies of the protein L binding capacity of naturally occurring VL fragments, and VL- and CL-related trypsin- and pepsin-derived peptides prepared from a kappa I light chain, localized the site of interaction to the VL domain. The affinity constant for the binding to an isolated V kappa I fragment was comparable to that for the native protein (Ka 0.9 x 10(9) M-1 and Ka 1.5 x 10(9) M-1, respectively). No binding occurred with CL-related fragments. Extensive reduction and alkylation of the V kappa fragment or the native kappa chain resulted in complete loss of protein L binding. Although it is possible, from comparative amino acid sequence data, to identify certain VL-framework region residues that account for the selective binding of protein L by kappa I, kappa III, and kappa IV proteins, our studies indicate that this interaction is essentially dependent upon the tertiary structural integrity of the kappa chain VL domain.

Rat α1-microglobulin: co-expression in liver with the light chain of inter-α-trypsin inhibitor

A 1162 bp rat liver cDNA clone encoding the immunoregulatory plasma protein alpha 1-microglobulin was isolated and sequenced. The open reading frame encoded a 349 amino acid polyprotein, including alpha 1-microglobulin, 182 amino acids, and bikunin, the light chain of the plasma protein inter-alpha-trypsin inhibitor, 145 amino acids. The alpha 1-microglobulin/bikunin mRNA was found only in the liver when different tissues were examined. Free alpha 1-microglobulin and a polyprotein, containing both alpha 1-microglobulin and inter-alpha-trypsin inhibitor epitopes, were found in the microsomal fraction from rat liver homogenates.

Characterization of monoclonal anti-alpha 1-microglobulin antibodies: binding strength, binding sites, and inhibition of lymphocyte stimulation

Eleven monoclonal antibodies (MoAb) directed against the immunoregulatory plasma glycoprotein alpha 1-microglobulin were characterized. The MoAb were produced in mice immunized with a mixture of alpha 1-microglobulin homologues from man, guinea pig, rat and rabbit. Using radioimmunoassay, western blotting, affinity chromatography, and Scatchard analysis, the affinities and binding sites of the MoAb were analysed. All antibodies were more or less cross-reactive, but most showed a major specificity for one or two of the alpha 1-microglobulin homologues. None of the antibodies was directed against the carbohydrate moiety of alpha 1-microglobulin. Six of the MoAb had high affinity for the antigen and four of these were directed towards the same part of the molecule though differing in their species specificity. Five showed lower affinity for the antigen and were mainly directed towards epitopes on other parts of the molecule. Only some of the antibodies could block the proliferation of lymphocytes induced by human alpha 1-microglobulin. The blocking efficiency of the different antibodies was similar when tested on the stimulation of human or mouse lymphocytes, suggesting that the same part of the alpha 1-microglobulin molecule is responsible in both species. The magnitude of blocking by the different MoAb was not related to their affinities, emphasizing the importance of where on the alpha 1-microglobulin molecule, rather than how strongly, they bind. The binding of the strongest blocking antibody was shown to be directed to a C-terminal peptide of rat alpha 1-microglobulin, indicating that this part of alpha 1-microglobulin is important for the mitogenic effects. Thus the panel of anti-alpha 1-microglobulin MoAb should be a valuable tool for structural and functional studies of alpha 1-microglobulin.

Binding properties of protein Arp, a bacterial IgA-receptor

A cell surface receptor that binds to the Fc region of IgA is expressed by certain strains of group A streptococci. The physico-chemical properties and binding characteristics of this receptor, called protein Arp, were studied. Like bacterial receptors that bind IgG, protein Arp has an elongated shape and no disulfide bonds. The affinity constant of protein Arp for three different molecular forms of IgA was determined, and was found to be more than ten-fold higher for serum IgA than for two complexed forms of IgA: secretory IgA and IgA bound to alpha 1-microglobulin. Cleavage of protein Arp with CNBr resulted in a peptide corresponding to the region located outside the cell wall, except for the N-terminal 52 amino acids. This CNBr-fragment did not bind IgA, which strongly suggests that the IgA-binding region of protein Arp is located in the N-terminal part of the molecule. In addition to the binding of IgA, protein Arp also binds to IgG weakly. The pH-dependence of these two types of binding is different, with maximal binding of IgA at neutral pH (5-7) and maximal binding of IgG at acidic pH (3-5). Both for IgA and IgG, protein Arp shows strong specificity for immunoglobulins of human origin.

Antibody response in immunized rabbits measured with bacterial immunoglobulin-binding proteins

Protein G, an immunoglobulin (Ig)-binding protein isolated from group C or G streptococci, binds to the Fc portion of IgG. Protein L, from the anaerobic bacterium Peptostreptococcus magnus, specifically binds light chains of Ig. In this study, protein G and L were used to measure the production of antibodies in immunized rabbits. Two rabbits were immunized with a mixture of human urinary proteins from a patient with tubular proteinuria, and blood samples were collected regularly from the animals for 6 weeks after the immunization. The antibody levels of the blood samples against six of the proteins in the antigen mixture were then measured by ELISA. Microtiter plates were coated with each of the antigens, incubated with the rabbit serum samples, and the specific antibodies of the IgG class measured by incubation with biotinylated protein G, and antibodies of all Ig classes with biotinylated protein L. Alternatively, Western blotting was employed, where the antibodies which bound to each antigen after separation by SDS-PAGE and transfer to nitrocellulose membranes, were detected by protein G or L. The results showed that antibody production against five of the antigens, albumin, alpha 1 gamma-acid glycoprotein, alpha 1 gamma-microglobulin, Ig light chains, and retinol-binding protein, showed a similar pattern, although the magnitude of the initial IgM response differed somewhat. After 6 weeks, the levels of the protein G-binding antibodies had reached a plateau, while those of protein L-binding antibodies were still increasing. The response to the sixth antigen, beta 2 microglobulin, was considerably different. A dramatic increase of anti-beta 2 gamma-microglobulin antibodies was seen during the 4th week after immunization when protein L was used.

Characterization of an IgA receptor from group B streptococci: specificity for serum IgA

Some strains of group B streptococci express a cell surface protein which binds IgA. This report describes some properties of such an IgA receptor and compares it with a previously described IgA receptor from group A streptococci. The group B receptor was released in an almost pure form from bacteria incubated at elevated pH, and could be isolated by IgA-Sepharose affinity chromatography. The sequence of the N-terminal 19 amino acid residues was unique. The receptor preferentially binds IgA of human origin, as shown in immunoblotting experiments with purified IgA from nine different species. The affinity constant of the purified receptor for serum IgA was determined to be 3.5 x 10(8) M-1, but for secretory IgA it was too low to allow determination. This result indicates that secretory component and/or J chain interferes with the binding of IgA to this type of bacterial receptor, which may be one of the physiological functions of these polypeptides. A reduction in affinity was also observed for another complexed form of IgA, alpha 1-microglobulin-IgA. The group B receptor is antigenically unrelated to the IgA receptor from group A streptococci (protein Arp), but competitive inhibition experiments indicate that they bind to the same region in IgA. The implications of these findings, and the biological role of bacterial IgA receptors, are discussed.

Isolation of rat serum alpha 1-microglobulin. Identification of a complex with alpha 1-inhibitor-3, a rat alpha 2-macroglobulin homologue

Alpha 1-Microglobulin (alpha 1-m), or protein HC, a low molecular weight plasma protein with immunoregulatory properties, was isolated from rat serum by affinity chromatography using Sepharose-coupled monoclonal anti-alpha 1-m antibodies. High molecular weight forms of alpha 1-m were then separated from the low molecular weight alpha 1-m by gel chromatography of the eluted proteins. The apparent Mr (28,000), the charge heterogeneity, the N-linked carbohydrate, and yellow-brown chromophore suggest that the low molecular weight alpha 1-m is the serum counterpart to urinary alpha 1-m, which was purified previously. A high molecular weight complex of alpha 1-m was also isolated by the gel chromatography. It was homogeneous as judged by nondenaturing polyacrylamide gel electrophoresis. The molecule was bound by antibodies against human alpha 2-macroglobulin, and experiments with antisera against the three alpha-macroglobulin variants in rat serum, alpha 1-macroglobulin, alpha 2-macroglobulin, and alpha 1-inhibitor-3 (alpha 1I3) suggested that alpha 1I3 was the complex-partner of alpha 1-m. An antiserum raised against high molecular weight alpha 1-m was then used to isolate the complex-partner of alpha 1-m from rat serum with affinity chromatography, and this molecule was positively identified as alpha 1I3 by its physicochemical properties. Gel chromatography of the alpha 1I3.alpha 1-m complex suggested a molecule with an Mr of 266,000. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, however, it migrated as three major molecular species with apparent molecular weights of 224,000, 205,000, and 194,000 and several minor species of both higher and lower molecular weights, suggesting a complex subunit structure. alpha 1-m and alpha 1I3 could be detected in all three major species by Western blotting, and NH2-terminal amino acid sequencing suggested a molar ratio of 1:1 of alpha 1-m and alpha 1I3 in all three species. alpha 1I3.alpha 1-m was colorless, did not show light absorbance beyond 300 nm which is typical of low molecular weight alpha 1-m and was electrophoretically homogeneous, suggesting that it lacks the chromophore. Finally, the serum concentrations of the alpha 1I3.alpha 1-m complex and free alpha 1-m were determined as 0.16 and 0.010 g/liter, respectively. Thus, alpha 1I3.alpha 1-m constitutes 1-3% of the total alpha 1I3 in rat serum (w/w) and approximately 60% of the total alpha 1-m.

Mitogenic effect of alpha 1-microglobulin on mouse lymphocytes. Evidence of T- and B-cell cooperation, B-cell proliferation, and a low-affinity receptor on mononuclear cells

Human alpha 1-m microglobulin (alpha 1-m), a low molecular weight plasma protein, was found to exert mitogenic effects on mouse lymphocytes from lymph nodes and spleen. The stimulatory effects appeared to be strain-restricted: alpha 1-m induced a varying degree of proliferation of lymphocytes from three strains, whereas one strain responded poorly. Experiments with lymphocyte subpopulations showed only weak stimulatory effects of alpha 1-m on purified T and B lymphocytes cultivated alone. The addition of mitomycin-treated cells of the other subpopulation could not restore the proliferative responses in either T or B lymphocytes. Strong stimulations were recorded only when both T and B lymphocytes were present, indicating that the T and B lymphocytes cooperate to achieve the proliferation. However, FACS studies on cultured splenocytes indicated that the proliferating cells are predominantly B lymphocytes. These data extend our earlier findings of a mitogenic effect of alpha 1-m on guinea pig lymphocytes. Furthermore, results were obtained indicating the presence of a receptor on mononuclear cells. Iodine-labelled alpha 1-m was bound to mononuclear cells prepared from spleens, and the binding could be blocked by an excess of non-labelled alpha 1-m. Scatchard plotting of the data gave an equilibrium constant of 0.7 x 10(5)/M for the binding between alpha 1-m and the receptor. Together with the documented inhibitory activity of alpha 1-m on antigen-driven proliferation of lymphocytes, these results suggest an immunoregulatory role for alpha 1-m.

An intriguing member of the lipocalin protein family: alpha 1-microglobulin

The plasma protein alpha 1-microglobulin is a member of the lipocalin protein superfamily. In the last few years, the work on alpha 1-microglobulin has given unexpected and promising new results. Of particular interest are its molecular association with immunoglobulin A and with proteinase inhibitors, and its interactions with the immune system.

Monoclonal antibodies to the pituitary growth-hormone receptor by the anti-idiotypic approach. Production and initial characterization

We obtained 10/192 and 3/384 antibody-secreting hybrids after immunization of Balb/c mice with either human growth hormone or affinity-purified rabbit anti-(human growth hormone) respectively. Radiolabelled rabbit anti-(human growth hormone) antibodies, but not human growth hormone, were specifically bound by supernatants from the 13 hybrids. The binding was completely inhibited by human-growth-hormone serum binding protein. However, anti-(human growth hormone antibodies) were detected in the sera of all the mice immunized with human growth hormone. In an independent fusion, which was carried out after immunization with fewer doses of human growth hormone, anti-(human growth hormone) antibodies were also obtained. Five hybrids, where the starting antigen was human growth hormone, were selected for ascites production, and the corresponding monoclonal antibodies were partially purified and characterized with respect to their immunoglobulin isotype and their interaction with human-growth-hormone receptors. These antibodies were found to enhance the binding of radioiodinated human growth hormone to human-growth-hormone serum binding protein from human and rabbit plasma by 40%. Scatchard analysis of the effect of one of the monoclonal antibodies showed that this enhancement was due to an increased number of binding sites. All of the partially purified antibodies but one (F12) inhibited the binding of human growth hormone to rat but not rabbit, liver microsomes to various extents, as well as to H-4-II-E rat hepatoma cells. Monoclonal antibody F12 enhanced the binding of radiolabelled human growth hormone to rat liver microsomes and H-4-II-E hepatoma cells. This enhancement was found to be due to an increase in the number of binding sites.