Peptide binding and antigen presentation by class II histocompatibility glycoproteins

The immune system has evolved complex mechanisms for the recognition and elimination of pathogens. CD4+ helper T lymphocytes play a central role in orchestrating immune responses and their activation is carefully regulated. These cells selectively recognize short peptide antigens stably associated with membrane-bound class II histocompatibility glycoproteins that are selectively expressed in specialized antigen presenting cells. The class II-peptide complexes are generated through a series of events that occur in membrane-bound compartments within antigen presenting cells that, collectively, have become known as the class II antigen processing pathway. In the present paper, our current understanding of this pathway is reviewed with emphasis on mechanisms that regulate peptide binding by class II histocompatibility molecules.

CD4+ T cell responses to self- and mutated p53 determinants during tumorigenesis in mice

We analyzed CD4+ T helper responses to wild-type (wt) and mutated (mut) p53 protein in normal and tumor-bearing mice. In normal mice, we observed that although some self-p53 determinants induced negative selection of p53-reactive CD4+ T cells, other p53 determinants (cryptic) were immunogenic. Next, BALB/c mice were inoculated with J774 syngeneic tumor cell line expressing mut p53. BALB/c tumor-bearing mice mounted potent CD4+ T cell responses to two formerly cryptic peptides on self-p53. This response was characterized by massive production of IL-5, a Th2-type lymphokine. Interestingly, we found that T cell response was induced by different p53 peptides depending upon the stage of cancer. Mut p53 gene was shown to contain a single mutation resulting in the substitution of a tyrosine by a histidine at position 231 of the protein. Two peptides corresponding to wt and mutated sequences of this region were synthesized. Both peptides bound to the MHC class II-presenting molecule (Ed) with similar affinities. However, only mut p53.225-239 induced T cell responses in normal BALB/c mice, a result strongly suggesting that high-affinity wt p53.225-239 autoreactive T cells had been eliminated in these mice. Surprisingly, CD4+ T cell responses to both mut and wt p53.225-239 peptides were recorded in J774 tumor-bearing mice, a phenomenon attributed to the recruitment of low-avidity p53.225-239 self-reactive T cells.

DO11.10 and OT-II T cells recognize a C-terminal ovalbumin 323-339 epitope

The OVA323-339 epitope recognized by DO11.10 (H-2d) and OT-II (H-2b) T cells was investigated using amino- and carboxy-terminal truncations to locate the approximate ends of the epitopes and single amino acid substitutions of OVA323-339 to identify critical TCR contact residues of the OVA323-339 peptide. DO11.10 and OT-II T cells are both specific for a C-terminal epitope whose core encompasses amino acids 329-337. Amino acid 333 was identified as the primary TCR contact residue for both cells, and amino acid 331 was found to be an important secondary TCR contact residue; however, the importance of other secondary TCR contact residues and peptide flanking residues differ between the cells. Additional OVA323-339-specific clones were generated that recognized epitopes found in the N-terminal end or in the center of the peptide. These findings indicate that OVA323-339 can be presented by I-Ad in at least three binding registers. This study highlights some of the complexities of peptide Ags such as OVA323-339, which contain a nested set of overlapping T cell epitopes and MHC binding registers.

Aberrant forms of alpha(2)-macroglobulin purified from patients with multiple sclerosis

The biochemical properties of alpha(2)-macroglobulin were investigated in four patients with multiple sclerosis and compared to alpha(2)-macroglobulin from healthy controls. An impaired stability of alpha(2)-macroglobulin from the multiple sclerosis patients was demonstrated as a spontaneous conversion to an electrophoretic"fast" form of alpha(2)-macroglobulin upon purification and storage, with a concomitant decrease in functional capacity to inhibit proteinases. The ability to form complexes with proteinases was significantly reduced in alpha(2)-macroglobulin purified from the multiple sclerosis patients. The aberrant molecular arrangements of the protein were not due to proteinase cleavages in the bait regions of alpha(2)-macroglobulin, as demonstrated by gel electrophoresis and protein sequencing. The number of functional thiol esters, however, was reduced in alpha(2)-macroglobulin purified from the multiple sclerosis patients, an observation compatible with the impaired proteinase binding property. Furthermore, differences in isoelectric points were observed between alpha(2)-macroglobulin from the multiple sclerosis patients and alpha(2)-macroglobulin from healthy controls. The results suggest that aberrant forms of alpha(2)-macroglobulin may be present in patients with multiple sclerosis.

Proprioceptive sensitivity and performance in anterior cruciate ligament-deficient knee joints

We studied the performance and proprioception of the knee joint in a group of non-reconstructed anterior cruciate ligament (ACL)-deficient (n=20) patients and compared them with a group of ACL-reconstructed patients (n=18) and a group of healthy controls (n=20). Each patient was scored according to Lysholm and Tegner and was then asked to subjectively evaluate the performance of the injured knee and the degree of retropatellar discomfort. The knee joint laxity was measured. The performance was assessed based on the performance in a triple jump test and a one-leg one-step leap test. The proprioception in the knee was measured as the threshold when passive movement was detected and as the ability to reproduce a flexion angle from a start position of 60 degrees of flexion or from full extension of the knee. All tests were performed on both legs. The scoring systems and the subjective evaluation showed significant differences between the reconstructed and the non-reconstructed patients. No significant difference in knee joint laxity was found between the two groups. In the triple jump test and the one-step leap test, both groups performed significantly worse on the leg with the injured knee joint than on the non-injured leg. The proprioceptive tests showed decreased ability to recognize and reproduce a prior angle from a start position of 60 degrees. The threshold to detection of passive movement with the injured knee was significantly increased in both groups of patients. No difference was found between the dominant and non-dominant knee in the control group. When reproduction of the same angles started from full extension, the groups did not differ. These data show that decreased performance and changes in the proprioception of the knee joint accompany ACL rupture.

Decreased stability of alpha2-macroglobulin purified from patients with multiple sclerosis

OBJECTIVES

To investigate conformational properties of alpha2-macroglobulin from multiple sclerosis patients.

MATERIALS AND METHODS

alpha2-macroglobulin was purified to homogeneity from plasma of 4 multiple sclerosis patients and 5 healthy controls. The plasma and the purified alpha2-macroglobulin from each individual were investigated using polyacrylamide gel electrophoresis.

RESULTS

Impaired stability of purified alpha2-macroglobulins from multiple sclerosis patients was demonstrated with spontaneous conversion to an electrophoretic "fast" form upon purification and following storage not ascribable to bait region cleavage.

CONCLUSION

alpha2-macroglobulin from multiple sclerosis patients displays altered stability. Possible functional impairments of proteinase inhibition mechanisms are discussed.

Protein transfer of the costimulatory molecule, B7-2 (CD86), into tumor membrane liposomes as a novel cell-free vaccine

Several approaches have been taken to enhance the immunogenicity of tumors. Genetically-modified tumors expressing various cytokines, major histocompatibility complex (MHC) molecules, or costimulatory molecules such as B7-1 (CD80) or B7-2 (CD86) can induce tumor-specific immune responses. In the present study, an alternative approach was explored based on direct protein transfer of purified recombinant B7-2 into tumor cell membranes. B7-2 was purified from recombinant baculovirus infected insect cells. Although differentially glycosolyated, the recombinant B7-2 retained the function to costimulate T-cell proliferation. Purified B7-2 was readily incorporated into tumor membranes using a detergent dialysis technique to form unilameller liposomes. The immunogenicity of tumor membrane proteoliposomes was significantly increased by incorporation of B7-2. These findings suggest an alternative method for the introduction of immunostimulatory molecules into tumor membranes to create novel tumor vaccines.

Processing and reactivity of T cell epitopes containing two cysteine residues from hen egg-white lysozyme (HEL74-90)

The Ag processing and structural requirements involved in the generation of a major T cell epitope from the hen egg-white lysozyme protein (HEL74-88), containing two cysteine residues at positions 76 and 80, were investigated. Several T cell hybridomas derived from both low responder (I-Ab) and high responder (I-Ak) mice recognize this region. These hybridomas are strongly responsive to native HEL, but unresponsive to the reduced and carboxymethylated protein. Air-oxidized HEL74-88 peptide was unable to bind I-Ak molecules and failed to stimulate T cells in the absence of intracellular Ag processing. Further functional competition assays showed that alkylation of cysteine residues with bulky methyl groups interferes with the contacts for the MHC class II molecules (I-Ak) of high responder mice and the I-Ab-restricted TCR of low responder mice. Serine substitutions of the cysteine residues of HEL74-88 either enhanced or abrogated T cell stimulation by the peptides without significant alterations in the class II binding. These results suggest that the cysteine residues of peptides must be free from disulfide bonding for efficient stimulation of T cells and yet frequently used modifications of cysteine residues may not be suitable for peptide-based vaccine development.

Peptide exchange in MHC molecules

Major histocompatibility complex (MHC)-encoded glycoproteins bind peptide antigens through non-covalent interactions to generate complexes that are displayed on the surface of antigen-presenting cells (APC) for recognition by T cells. Peptide-binding site occupancy is necessary for stable assembly of newly synthesized MHC proteins and export from the endoplasmic reticulum (ER). The MHC class II antigen-processing pathway provides a mechanism for presentation of peptides generated in the endosomal pathway of APC. The chaperone protein, invariant chain, includes a surrogate peptide that stabilizes newly synthesized class II molecules during transport to endosomal compartments. The invariant chain-derived peptide must be replaced through a peptide exchange reaction that is promoted by acidic pH and the MHC-encoded co-factor HLA-DM. Peptide exchange reactions are not required for presentation of antigens by MHC class I molecules because they bind antigens during initial assembly in the ER. However, exchange reactions may play an important role in editing the repertoire of peptides presented by both class II and class I molecules, thus influencing the specificity of immunity and tolerance.

Lebetase, an alpha(beta)-fibrin(ogen)olytic metalloproteinase of Vipera lebetina snake venom, is inhibited by human alpha-macroglobulins

The effects of the plasma proteinase inhibitors alpha(2)-macroglobulin (alpha(2)M) and the alpha(2)M-related pregnancy zone protein (PZP) were evaluated towards the metalloproteinase lebetase, isolated from Vipera lebetina venom. We demonstrate that lebetase interacts with both inhibitors. Cleavage of alpha(2)M by lebetase resulted in the formation of 90-kDa fragments, and covalent complexes of alpha(2)M with lebetase were observed. The proteolytic activity of lebetase against fibrinogen and azocasein could be inhibited by alpha(2)M. Cleavage of PZP also resulted in the formation of 90-kDa fragments, and complexes of both dimer and tetramer forms of PZP with lebetase were detected. The amino acid sequence identification of the sites of specific proteolysis of alpha(2)M and PZP demonstrate that the cleavage sites are within the bait regions of both proteins. Lebetase I cleaves between Arg(696)-Leu(697), which is one of the most common cleavage sites in alpha(2)M by proteinases. The other two cleavage sites in alpha(2)M by lebetase are Gly(679)-Leu(680) and His(694)-Ala(695). The cleavage between Pro(689)-Gln(690) is the only cleavage site identified in PZP. In that lebetase is an anticoagulation agent in vivo, we propose that the interaction of lebetase with alpha(2)M may suggest a reduced fibrin(ogen)olytic activity of lebetase in human.

Assessment of the role of determinant selection in genetic control of the immune response to insulin in H-2b mice

The immune response to insulin is regulated by MHC class II genes. Immune response (Ir) gene-linked low responsiveness to protein Ags can be mediated by the low affinity of potential antigenic determinants for MHC molecules (determinant selection) or by the influence of MHC on the functional T cell repertoire. Strong evidence exists that determinant selection plays a key role in epitope immunodominance and Ir gene-linked unresponsiveness. However, the actual measurement of relative MHC-binding affinities of all potential peptides derived from well-characterized model Ags under Ir gene regulation has been very limited. We chose to take advantage of the simplicity of the structure of insulin to study the mechanism of Ir gene control in H-2b mice, which respond to beef insulin (BINS) but not pork insulin (PINS). Peptides from these proteins, including the immunodominant A(1-14) determinant, were observed to have similar affinities for purified IAb in binding experiments. Functional and biochemical experiments suggested that PINS and BINS are processed with similar efficiency. The T cell response to synthetic pork A(1-14) was considerably weaker than the response to the BINS peptide. We conclude that the poor immunogenicity of PINS in H-2b mice is a consequence of the T cell repertoire rather than differences in processing and presentation.

Introduction of a new branchpoint in tetrapyrrole biosynthesis in Escherichia coli by co-expression of genes encoding the chlorophyll-specific enzymes magnesium chelatase and magnesium protoporphyrin methyltransferase

The genes encoding the three Mg chelatase subunits, ChlH, ChlI and ChlD, from the cyanobacterium Synechocystis PCC6803 were all cloned in the same pET9a-based Escherichia coli expression plasmid, forming an artificial chlH-I-D operon under the control of the strong T7 promoter. When a soluble extract from IPTG-induced E. coli cells containing the pET9a-ChlHID plasmid was assayed for Mg chelatase activity in vitro, a high activity was obtained, suggesting that all three subunits are present in a soluble and active form. The chlM gene of Synechocystis PCC6803 was also cloned in a pET-based E. coli expression vector. Soluble extract from an E. coli strain expressing chlM converted Mg-protoporphyrin IX to Mg-protoporphyrin monomethyl ester, demonstrating that chlM encodes the Mg-protoporphyrin methyltransferase of Synechocystis. Co-expression of the chlM gene together with the chlH-I-D construct yielded soluble protein extracts which converted protoporphyrin IX to Mg-protoporphyrin IX monomethyl ester without detectable accumulation of the Mg-protoporphyrin IX intermediate. Thus, active Mg chelatase and Mg-protoporphyrin IX methyltransferase can be coupled in E. coli extracts. Purified ChlI, -D and -H subunits in combination with purified ChlM protein were subsequently used to demonstrate in vitro that a molar ratio of ChlM to ChlH of 1 to 1 results in conversion of protoporphyrin IX to Mg-protoporphyrin monomethyl ester without significant accumulation of Mg-protoporphyrin.

Early motion of the ankle after operative treatment of a rupture of the Achilles tendon. A prospective, randomized clinical and radiographic study

BACKGROUND

Different regimens of early motion of the ankle after operative treatment of a ruptured Achilles tendon have been suggested since the late 1980s. However, as far as we know, no controlled studies comparing these regimens with conventional immobilization in a cast have been reported.

METHODS

In a prospective study, seventy-one patients who had an acute rupture of the Achilles tendon were randomized to either conventional postoperative management with a cast for eight weeks or early restricted motion of the ankle in a below-the-knee brace for six weeks. The brace was modified with an elastic band on the posterior surface, in a manner similar to the principle of Kleinert traction. Metal markers were placed in the tendon, and the separation between them was measured on serial radiographs during the first twelve weeks postoperatively. The patients were assessed clinically when the cast or brace was removed, at twelve weeks postoperatively, and at a median of sixteen months postoperatively.

RESULTS

The separation between the markers at twelve weeks postoperatively was nearly identical in the two groups, with a median separation of 11.5 millimeters (range, zero to thirty-three millimeters) in the patients managed with early motion of the ankle and nine millimeters (range, one to forty-one millimeters) in the patients managed with a cast. The separation was primarily correlated with the initial tautness of the repair (r[S] = 0.45). No patient had excessive lengthening of the tendon. The patients managed with early motion had a smaller initial loss in the range of motion, and they returned to work and sports activities sooner than those managed with a cast. Furthermore, there were fewer visible adhesions between the repaired tendon and the skin in the patients managed with early motion, and these patients were subjectively more satisfied with the overall result. The patients in both groups recovered a median of 89 percent of strength of plantar flexion compared with that of the noninjured limb, as measured with an isometric strain-gauge at 15 degrees of dorsiflexion. The heel-rise index was similar for both groups: 0.88 for the patients managed with early motion and 0.89 for those managed with a cast.

CONCLUSIONS

Early restricted motion appears to shorten the time needed for rehabilitation. There were no complications related to early motion in these patients. However, early unloaded exercises did not prevent muscle atrophy.

Cross-recognition of two middle T protein epitopes by immunodominant polyoma virus-specific CTL

We recently identified the immunodominant epitope for polyoma virus-specific CTL as the Dk-associated peptide MT389-397 derived from the middle T (MT) viral oncoprotein. Another Dk-restricted peptide corresponding to residues 236-244 of MT was recognized by nearly all MT389-397-reactive CTL clones, but required concentrations at least 2 logs higher to sensitize syngeneic target cells for lysis. Except for identity at the three putative Dk-peptide anchor residues, MT236-244 shares no homology with MT389-397. Using a novel europium-based class I MHC-peptide binding immunoassay, we determined that MT236-244 bound Dk 2-3 logs less well than MT389-397. Infection with a mutant polyoma virus whose MT is truncated just before the MT389-397 epitope or immunization with MT389-397 or MT236-244 peptides elicited CTL that recognized both MT389-397 and MT236-244. Importantly, infection with a polyoma virus lacking MT389-397 and mutated in an MT236-244 Dk anchor position induced polyoma virus-specific CTL recognizing neither MT389-397 nor MT236-244 epitopes. Despite predominant usage of the Vbeta6 gene segment, MT389-397/MT236-244 cross-reactive CTL clones possess diverse complementarity-determining region 3beta domains; this is functionally reflected in their heterogeneous recognition patterns of alanine-monosubstituted MT389-397 peptides. Using Dk/MT389-397 tetramers, we directly visualized MT236-244 peptide-induced TCR down-modulation of virtually all MT389-397-specific CD8+ T cells freshly explanted from polyoma-infected mice, suggesting that a single TCR recognizes both Dk-restricted epitopes. The availability of immunodominant epitope-specific CTL capable of recognizing a second epitope in MT, a viral protein essential for tumorigenesis, may serve to amplify the CTL response to the immunodominant epitope and prevent the emergence of immunodominant epitope-loss viruses and virus-induced tumors.

ATPase activity associated with the magnesium-protoporphyrin IX chelatase enzyme of Synechocystis PCC6803: evidence for ATP hydrolysis during Mg2+ insertion, and the MgATP-dependent interaction of the ChlI and ChlD subunits

Insertion of Mg2+ into protoporphyrin IX catalysed by the three-subunit enzyme magnesium-protoporphyrin IX chelatase (Mg chelatase) is thought to be a two-step reaction, consisting of activation followed by Mg2+ chelation. The activation step requires ATP and two of the subunits, ChlI and ChlD (I and D respectively), and it has been speculated that this step results in the formation of an I-D-ATP complex. The subsequent step, in which Mg2+ is inserted into protoporphyrin, also requires ATP and the third subunit, H, in addition to ATP-activated I-D complex. In the present study, we examine the interaction of the I and D subunits of the Mg chelatase from the cyanobacterium Synechocystis PCC 6803. We demonstrate the purification of an I-D complex, and show that ATP and Mg2+ are absolute requirements for the formation of this complex, probably as MgATP. However, ATP may be replaced by the slowly hydrolysable analogue, adenosine 5'-[gamma-thio]triphosphate, and, to a minor extent, by ADP and the non-hydrolysable ATP analogue, adenosine 5'-[beta,gamma-imido]triphosphate, all of which suggests that ATP hydrolysis is not necessary for the formation of the ChlI-ChlD complex. A sensitive continuous assay was used to detect ATPase activity during Mg2+ chelation, and it was found that the maximum rate of ATP hydrolysis coincided with the maximum rate of Mg2+ insertion. The rate of ATP hydrolysis depended on factors that determined the rate of Mg2+ chelation, such as increasing the concentration of the H subunit and the concentration of protoporphyrin. Thus ATP hydrolysis has been identified as an absolute requirement for the chelation step. The I subunit possessed strong ATPase activity when assayed on its own, whereas the D subunit had no detectable activity, and when the I and D subunits were assayed in combination, the ATPase activity of the I subunit was repressed.

Mechanisms of antigen presentation

T-lymphocytes recognize short peptide antigens bound stably to polymorphic major histocompatibility complex (MHC)-encoded glycoproteins expressed on the surface of antigen-presenting cells (APC). Two general pathways have evolved to generate peptide-MHC complexes. The MHC class II antigen processing pathway provides a mechanism for sampling proteins present in endosomal compartments. CD4+ regulatory T-cells recognize peptides bound to MHC class II molecules, which are selectively expressed in specialized APC that have efficient mechanisms for uptake of microbial antigens, and express costimulatory molecules required for activating naive T-cells. CD8+ T-cells recognize peptides bound to MHC class I molecules. Class I molecules are widely expressed and bind peptides derived from the normal turnover of cellular proteins, providing a mechanism to display a sampling of cellular components to be monitored for abnormalities by cytotoxic T-cells. Specialized accessory proteins influence the efficiency of antigen presentation and the specificity of immune responses through their roles in generating peptides, targeting antigen and MHC glycoproteins to selected intracellular compartments, and by direct participation in the peptide-loading mechanism. It has recently been discovered that some viruses have evolved ways to inhibit or subvert discrete steps in antigen processing, providing a mechanism to evade immune recognition.

Class II-associated invariant chain peptide-independent binding of invariant chain to class II MHC molecules

The class II-associated invariant chain peptide (CLIP) region of invariant chain (Ii) is believed to play a critical role in the assembly and transport of MHC class II alphabetaIi complexes through its interaction with the class II peptide-binding site. The role of the CLIP sequence was investigated by using mutant Ii molecules with altered affinity for the DR1 peptide-binding site. Both high- and low-affinity mutants were observed to efficiently assemble with DR1 and mediate transport to endosomal compartments in COS cell transfectants. Using N- and C-terminal truncations, a region adjacent to CLIP within Ii(103-118) was identified that can complement loss of affinity for the peptide-binding site in mediating efficient assembly of alphabetaIi. A C-terminal fragment completely lacking the CLIP region, Ii(103-216), was observed binding stably to class II molecules in immunoprecipitation studies and experiments with purified proteins. The Ii(103-118) region was required for this binding, which occurs through interactions outside of the alphabeta peptide-binding groove. We conclude that strong interactions involving Ii(103-118) and other regions of Ii cooperate in the assembly of functional alphabetaIi under conditions where CLIP has little or no affinity for the class II peptide-binding site. Our results support the hypothesis that the CLIP sequence has evolved to avoid high-stability interactions with the peptide-binding sites of MHC class II molecules rather than as a promiscuous binder with moderate affinity for all class II molecules.

Magnesium chelatase from Rhodobacter sphaeroides: initial characterization of the enzyme using purified subunits and evidence for a BchI-BchD complex

The enzyme magnesium-protoporphyrin IX chelatase (Mg chelatase) catalyses the insertion of Mg into protoporphyrin IX, the first committed step in (bacterio)chlorophyll biosynthesis. In the photosynthetic bacterium Rhodobacter sphaeroides, this reaction is catalysed by the products of the bchI, bchD and bchH genes. These genes have been expressed in Escherichia coli so that the BchI, BchD and BchH proteins are produced with N-terminal His6 affinity tags, which has led to the production of large amounts of highly purified, highly active Mg chelatase subunits from a single chromatography step. Furthermore, BchD has been purifed free of contamination with the chaperone GroEL, which had proven to be a problem in the past. BchD, present largely as an insoluble protein in E. coli, was purified in 6 M urea and refolded by addition of BchI, MgCl2 and ATP, yielding highly active protein. BchI/BchD mixtures prepared in this way were used in conjunction with BchH to determine the kinetic parameters of R. sphaeroides Mg chelatase for its natural substrates. We have been able to demonstrate for the first time that BchI and BchD form a complex, and that Mg2+ and ATP are required to establish and maintain this complex. Gel filtration data suggest that BchI and BchD form a complex of molecular mass 200 kDa in the presence of Mg2+ and ATP. Our data suggest that, in vivo, BchD is only folded correctly and maintained in its correct conformation in the presence of BchI, Mg2+ and ATP.

HLA-DM and the MHC class II antigen presentation pathway

The MHC class II antigen processing pathway provides a mechanism to selectively present peptides generated in the endosomal compartments of antigen presenting cells to CD4+ T cells. Transport of newly synthesized class II molecules to the endosomal pathway requires the function of an accessory protein, invariant chain, which contains a region that interacts directly with the class II peptide binding site. Release of invariant chain and peptide loading by class II molecules are facilitated by a second accessory protein, HLA-DM. This MHC-encoded membrane protein catalyzes peptide exchange reactions, influencing the repertoire of peptides that are available for recognition by T cells.

Mouse CD94/NKG2A is a natural killer cell receptor for the nonclassical major histocompatibility complex (MHC) class I molecule Qa-1(b)

Natural killer (NK) cells preferentially lyse targets that express reduced levels of major histocompatibility complex (MHC) class I proteins. To date, the only known mouse NK receptors for MHC class I belong to the Ly49 family of C-type lectin homodimers. Here, we report the cloning of mouse NKG2A, and demonstrate it forms an additional and distinct class I receptor, a CD94/NKG2A heterodimer. Using soluble tetramers of the nonclassical class I molecule Qa-1(b), we provide direct evidence that CD94/NKG2A recognizes Qa-1(b). We further demonstrate that NK recognition of Qa-1(b) results in the inhibition of target cell lysis. Inhibition appears to depend on the presence of Qdm, a Qa-1(b)-binding peptide derived from the signal sequences of some classical class I molecules. Mouse NKG2A maps adjacent to CD94 in the heart of the NK complex on mouse chromosome six, one of a small cluster of NKG2-like genes. Our findings suggest that mouse NK cells, like their human counterparts, use multiple mechanisms to survey class I expression on target cells.

Binding of soluble myelin basic protein to various conformational forms of alpha2-macroglobulin

Myelin basic protein is known to be released into the circulation following traumatic injuries or demyelination within the central nervous system, resulting in the generation of potentially immunogenic myelin basic protein material. In this investigation we have studied the binding of bovine and human myelin basic protein to human alpha2-macroglobulin, which was found to be the only major myelin basic protein-binding protein in human plasma. Myelin basic protein bound to all three conformational forms of alpha2-macroglobulin studied, i.e., native alpha2-macroglobulin, methylamine-treated alpha2-macroglobulin, and chymotrypsin-treated alpha2-macroglobulin. Zinc chloride (1 mM) or 1 mM iodoacetamide partly blocked the complex formation between myelin basic protein and alpha2-macroglobulin, while 1 mM magnesium chloride, 1 mM calcium chloride, or 1 mM EDTA had no effect on binding. Chymotrypsin and trypsin can degrade myelin basic protein to fragments which do not bind to alpha2-macroglobulin. However, when myelin basic protein was complexed with any of the conformational forms of alpha2-macroglobulin, no significant release of Na[125I]-labeled myelin basic protein occurred after proteinase treatment. The results suggest that binding of myelin basic protein to alpha2-macroglobulin may protect extracellular compartments in vivo from immunogenic myelin basic protein fragments and alpha2-macroglobulin may participate in the specific clearance of myelin basic protein from the circulation.

The role of [Ca2+]i, membrane potential and pHi in the relaxation of rat mesenteric arteries to hyperosmolar acetate

In vitro both acetate and hyperosmolarity cause vasodilation, which could be physiologically important during food ingestion and during peritoneal dialysis. The purpose of this study was to investigate the role of the intracellular calcium concentration ([Ca2+]i, measured with fura-2), membrane potential (measured with glass microelectrodes) and intracellular pH [pHi, measured with bis-carboxyethylcarboxyfluorescein (BCECF)] in the vasodilation. Hyperosmolar sodium acetate (30 mM) concentration dependently relaxed noradrenaline-precontracted arteries. This response was associated with hyperpolarization and a fall in [Ca2+]i. In arteries precontracted with 50 mM K+ the relaxation was associated with a decrease of [Ca2+]i but no change in membrane potential. Isoosmolar sodium acetate neither relaxed or affect [Ca2+]i of K+-precontracted arteries, but induced a small relaxation with no reduction in [Ca2+]i in noradrenaline-precontracted arteries. Hyperosmolar acetate caused a transient reduction of pHi that was unrelated to relaxation. It is concluded that the mechanisms responsible for the relaxation to hyperosmolar acetate involve a decrease of [Ca2+]i, which is only partly explained by hyperpolarization and probably a decrease in the sensitivity of the contractile proteins to [Ca2+]i. pHi seems not to play a role in these effects.

Transporters associated with antigen processing (TAP)-independent presentation of soluble insulin to alpha/beta T cells by the class Ib gene product, Qa-1(b)

T cell hybridomas isolated from nonresponder H-2(b) mice immunized with pork insulin were stimulated by insulin in the presence of major histocompatibility complex (MHC)-unmatched antigen presenting cells. The restriction element used by these CD4(-) T cells was mapped to an oligomorphic MHC class Ib protein encoded in the T region and identified as Qa-1(b) using transfectants. The antigenic determinant was localized to the insulin B chain, and experiments with truncated peptides suggested that it is unexpectedly long, comprising most or all of the 30 amino acid B chain. The antigen processing pathway used to present insulin to the Qa-1(b)- restricted T cells does not require transporters associated with antigen processing (TAP), and it is inhibited by chloroquine. A wide variety of cell lines from different tissues efficiently present soluble insulin to Qa-1(b)-restricted T cells, and insulin presentation is not enhanced by phagocytic stimuli. Our results demonstrate that Qa-1(b) can function to present exogenous protein to T cells in a manner similar to MHC class II molecules. Therefore, this class Ib protein may have access to a novel antigen processing pathway that is not available to class Ia molecules.

Determinants of catalytic activity with the use of purified I, D and H subunits of the magnesium protoporphyrin IX chelatase from Synechocystis PCC6803

The I, D and H subunits (ChlI, ChlD and ChlH respectively) of the magnesium protoporphyrin IX chelatase from Synechocystis have been purified to homogeneity as a result of the overexpression of the encoding genes in Escherichia coli and the production of large quantities of histidine-tagged proteins. These subunits have been used in an initial investigation of the biochemical and kinetic properties of the enzyme. The availability of pure ChlI, ChlD and ChlH has allowed us to estimate the relative concentrations of the three protein components required for optimal activity, and to investigate the dependence of chelatase activity on the concentrations of MgCl2, ATP and protoporphyrin IX. It was found that, whereas ChlD and ChlH are likely to be monomeric, ChlI can aggregate in an ATP-dependent manner, changing from a dimeric to an octameric structure. Subunit titration assays suggest an optimal ratio of ChlI, ChlD and ChlH of 2:1:4 respectively. However, the dependence of chelatase activity on increasing concentrations of ChlI and ChlH with respect to ChlD suggests that these two subunits, at least in vitro, behave as substrates in their interaction with ChlD. Mg chelation could not be detected unless the Mg2+ concentration exceeded the ATP concentration, suggesting at least two requirements for Mg2+, one as a component of MgATP2-, the other as the chelated metal. The steady-state kinetic parameters were determined from continuous assays; the Km values for protoporphyrin, MgCl2 and ATP were 1.25 microM, 4.9 mM and 0.49 mM respectively. The rate dependence of Mg2+ was clearly sigmoidal with a Hill coefficient of 3, suggesting positive co-operativity. Initiating the reaction by the addition of one of the substrates in these continuous assays resulted in a significant lag period of at least 10 min before the linear production of Mg protoporphyrin. This lag was significantly decreased by preincubating ChlI and ChlD with ATP and MgCl2, and by mixing it with ChlH that had been preincubated with protoporphyrin IX, ATP and MgCl2. This suggests not only a close MgATP2--dependent interaction between ChlI and ChlD but also an interaction between ChlH and the protoporphyrin substrate that also is stimulated by ATP and MgCl2.

The unimportance of being (protein kinase C) epsilon

The purpose of our study was to determine the mechanism through which phorbol esters and smooth muscle myosin phosphatase inhibitors can induce contraction of smooth muscle in the absence of Ca2+. Protein kinase C-epsilon (PKC-epsilon) was previously implicated in this process based largely on its supposed absence in the ferret portal vein, and a correlation was drawn between the presence of this isoform and the ability of smooth muscle to contract independently of Ca2+ and phosphorylation of the 20 kDa regulatory light chains of myosin (MLC20). We demonstrate here, with two antibodies, one to the NH2 terminus and the other to the COOH terminus of PKC-epsilon, that epsilon is present in both ferret portal vein and rabbit portal vein smooth muscle, neither of which exhibits phorbol ester-induced contraction in the absence of Ca2+. However, in the presence of clamped submaximal Ca2+, phorbol es ter increased MLC20 phosphorylation from 17.7+/-1.7% to 46.4+/-3.6% in ferret portal vein smooth muscle and evoked an increase in force. Prolonged (48 h) incubation of ferret portal vein with phorbol esters completely down-regulated PKC-epsilon, as shown by Western blots, and abolished the phorbol ester-evoked contraction at submaximal Ca2+, but not Ca2+-independent, contractions induced by the phosphatase inhibitor microcystin. Contractions induced by microcystin in Ca2+-free solution were associated with increased phosphorylation of myosin light chain kinase (MLCK). Activation of MLCK by autophosphorylation in the absence of Ca2+ occurs in vitro (1). We conclude that PKC-epsilon is neither necessary nor sufficient for Ca2+-independent regulation of myosin II in smooth muscle, but contractions induced by agents that inhibit smooth muscle myosin phosphatase in the absence of Ca2+ may be mediated by MLCK autophosphorylated or activated by another Ca2+-independent kinase.