Synthesis and Characterization of p-Borono-Phenylalanine-Branched Polypeptide-Monoclonal Antibody Ternary Systems for Potential Use in Boron Neutron Capture Therapy (BNCT)

The application of the 10B ( n,α) 7Li capture reaction to cancer radiotherapy (Boron Neutron Capture Therapy) was studied to avoid the inherent disadvantages of conventional radiation therapy. p-Borono-phenylalanine (Bph) was used as the 10B source and mAb produced against HCMB melanoma cells was applied as targeting device. Since extensive direct boronation of mAb led diminished recognition of antigens, an intermediate carrier was used. Nontoxic, biocompatible, biodegradable and weakly immunogenic branched polypeptides with a polylysine backbone was used to carry a high number of 10B. Protected 10B-Bph was coupled by four different methods to polycationic branched polypeptides. The coupling efficiency varied according to the experimental conditions, with a maximum of 90%. The chiroptical properties of the conjugates indicated an ordered conformation which increased with the number of coupled Bph. The whole body survival (WBS) and tissue distribution profile of mAb (8/6 IgG2a) were markedly altered after conjugation with Bph-branched polypeptide. Decreased WBS and intermediate-carrier-dependent accumulation in the spleen, liver and kidney was observed 24 h after iv. administration. After joining only a few chains of the highly loaded Bph-AK conjugate to mAb, the binding activity of the mAb in the ternary system was preserved compared to control.

Systemic inflammatory response syndrome (SIRS) induced by carbon tetrachloride in rats

We have observed the symptoms of systemic inflammatory response syndrome (SIRS) in male rats intoxicated by carbon tetrachloride (CCl(4)). Severe hypothermia, tachypnoea and increase in the heart beat min were diagnosed. These symptoms developed in the first hour of intoxication. The hepatic dysfunction was characterized by elevated bilirubin levels. In the sera we have measured increases in the activity of secretable (group II) phospholipase A(2) sPLA(2) (2,8x) and 6-ketoprostaglandin F(1alpha) (KPGF) (1,44x). Supposedly the free radicals derived from CCl(4)-mainly trichloromethyl-could induce the prompt reaction of SIRS and the release of sPLA(2) as well as the formation of KPGF. Our findings show that in the early phase of CCl(4) intoxication the symptoms of SIRS can be related to elevation of sPLA(2) and the products of cyclooxygenase II.

Physiological up-regulation of inhibitory receptors Fc gamma RII and CR1 on memory B cells is lacking in SLE patients

Under physiological conditions immune complexes (IC) are efficiently cleared from the circulation and meanwhile provide important feedback signals for the immune system via Fc gamma Rs and complement receptors. Dysregulation of these mechanisms have been implicated in conditions where IC concentrations reach pathological levels and inflict diseases, like systemic lupus erythematosus (SLE). Our aim was to compare distinct sub-populations of CD19(+) B cells of healthy individuals and SLE patients with regard to their expression of Fc gamma R type II (Fc gamma RII, CD32), complement receptor type 1 (CR1, CD35) and complement receptor type 2 (CR2, CD21) and sIgG/IgM. The following four groups of peripheral CD19(+) B cells were investigated: IgM(+)/CD27(-) naive, IgM(+)/CD27(+) and IgM(-)/CD27(+) memory cells and CD27(high) plasmablasts. We demonstrate that the expression of the inhibitory receptors Fc gamma RII and CR1 is up-regulated on peripheral memory B cells of healthy controls, whereas this up-regulation is considerably impaired on the memory B cells of SLE patients. This reduction affects both the IgM(+) and switched memory B cells. We found a striking difference between the expression of complement receptors CD21 and CD35; namely, no up-regulation of CD21 occurred on the memory B cells of healthy donors, and its decreased expression in SLE patients was characteristic for both the CD27(-) naive and the CD27(+) memory B-cell populations. Our results clearly demonstrate that the previously reported reduced expression of IC-binding receptors is mainly due to the disturbed memory compartment; however, the higher frequency of CD19(+)/CD27(high)/sIg(low) plasmablasts expressing minimal levels of these receptors also contributes to this diminution.

Altered Expression of Fc and Complement Receptors on B Cells in Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease characterized by B cell hyper-reactivity, autoantibody production, immune complex (IC) deposition, and multiple organ damage. The contribution of IC and B cell-mediated changes in the pathogenesis of SLE is well established, however, the exact role of IC-binding receptors expressed on B cells, Fcgamma receptors, and complement receptors CR1 and CR2 in these pathological processes is unclear. Development of lupus-like symptoms in mice defective for the inhibitory Fc-gammaRIIb and genetic association of certain FcgammaR genes with SLE demonstrate a significant role for these receptors but reports indicating alterations of Fcgamma or complement receptor-mediated B cell functions in human SLE are relatively few. The present review highlights a selected set of data including our own discussing the significance of animal models, genetics, and functional alterations of these IC-binding receptors in the etiopathogenesis of SLE.

[On the way to understand the mechanism of immune tolerance]

This introductory paper deals with mechanisms playing important role in the development of immunological tolerance. Discusses the problems of "immunological self" and "non-self", the importance of the roles of AIRE gene products, as well as the role of dendritic cells and regulatory T-cells (T-reg cells) in immune tolerance and autoimmunity.

The role of CR2 in autoimmunity

Complement activation is one of the most powerful mechanisms taking place during inflammation and immune responses. Over the last 30 years increasing evidence has proven the role of C3 and receptors for its activation fragments in the initiation and regulation of immune responses. Since complement also has a basic importance in the maintenance of immune homeostasis, abnormalities affecting complement proteins and their receptors may lead to pathological conditions. Autoimmune conditions develop as a result of a range of genetic and environmental factors. Findings obtained from animal models support the notion that malfunctioning of complement receptors, particularly CR2, might be involved in the breakdown of tolerance and excessive antibody production by auto reactive B-cell clones. In addition to B cells, activated, CR2-bearing T cells may also contribute to the pathogenesis of autoimmunity as they can receive activating/survival signals in the inflamed tissue. Results obtained from mouse experiments however, should be extended to the human system with great care, since there are basic differences between the structure and function of human and murine CR1 and CR2.

Synthesis and receptor binding of IgG1 peptides derived from the IgG Fc region

The IgG binding Fcgamma receptors (FcgammaRs) play a key role in defence against pathogens by linking humoral and cell-mediated immune responses. Impaired expression and/or function of FcgammaR may result in the development of pathological autoimmunity. Considering the functions of FcgammaRs, they are potential target molecules for drug design to aim at developing novel anti-inflammatory and immunomodulatory therapies. Previous data mostly obtained by X-ray analysis of ligand-receptor complexes indicate the profound role of the CH2 domain in binding to various FcgammaRs. Our aim was to localize linear segments, which are able to bind and also to modulate the function of the low affinity FcgammaRs, like FcgammaRIIb and FcgammaRIIIa. To this end a set of overlapping octapeptides was prepared corresponding to the 231-298 sequence of IgG1 CH2 domain and tested for binding to human recombinant soluble FcgammaRIIb. Based on these results, a second group of peptides was synthesized and their binding properties to recombinant soluble FcgammaRIIb, as well as to FcgammaRs expressed on the cell surface, was investigated. Here we report that peptide representing the Arg(255)-Ser(267) sequence of IgG1 is implicated in the binding to FcgammaRIIb. In addition we found that peptides corresponding to the Arg(255)-Ser(267), Lys(288)-Ser(298) or Pro(230)-Val(240) when presented in a multimeric form conjugated to branched chain polypeptide in uniformly oriented copies induced the release of TNFalpha, a pro-inflammatory cytokine from MonoMac monocyte cell line. These findings indicate that these conjugated peptides are able to cluster the activating FcgammaRs, and mediate FcgammaR dependent function. Peptide Arg(255)-Ser(267) can also be considered as a lead for further functional studies.

Functional mapping of the Fc gamma RII binding site on human IgG1 by synthetic peptides

Receptors specific for the Fc part of IgG (Fc gamma R) are expressed by several cell types and play diverse roles in immune responses. Impaired function of the activating and inhibitory Fc gamma R may result in autoimmunity. Thus, the modulation of IgG-Fc gamma R interaction can be a target for the development of treatments for some autoimmune and inflammatory diseases. This study addresses the localization and functional characterization of linear sequences in human IgG1 which bind to Fc gamma RII. Peptides with overlapping sequences derived from the CH2 domain of human IgG1 between P(234) and S(298) were synthesized and used in binding and functional experiments. Binding of the peptides to Fc gamma R was assayed in vitro and ex vivo, and peptides found to interact were functionally tested. The shortest effective peptide was T(256)-P(271), which bound to soluble recombinant Fc gamma RIIb with K(d)=6 x 10(6) M(-1). The biotinylated peptides R(255)-P(271) and T(256)-P(271) complexed by avidin exhibited functional activity; they induced Fc gamma RIIb-mediated inhibition of the BCR-triggered Ca(2+) response of human Burkitt lymphoma cells, and inflammatory cytokine production (TNF-alpha and IL-6) by the human monocyte cell line MonoMac. In conclusion, our results suggest that the selected peptides functionally represent the Fc gamma RII-binding part of IgG1.

Characterization of a trypsin-like serine protease of activated B cells mediating the cleavage of surface proteins

Activated B cells may cleave their surface receptors due to the proteolytic activity on the cell membrane or in its vicinity. We attempted to isolate and characterize the protease(s) responsible for this cleavage. Zymograms prepared from the supernatant and the plasma membrane fraction of activated human B cells and BL41/95 cell line exhibited a 85-90 kDa doublet band with protease activity, while that of resting B cells did not. Soybean trypsin inhibitor (STI), Nalpha-p-tosyl-L-lysine chloromethyl ketone (TLCK) and EDTA treatment abolished the activity of this protease. The excess of Zn(2+) ions in EDTA did not restore the enzymatic activity, while it was completely recovered in the presence of Ca(2+). We affinity-purified a 85-90 kDa protease from the supernatant of BL41/95 cells using STI coupled to Sepharose 4B beads, and measured its kinetic parameters. For the arginyl substrate K(M) was 358+/-59 microM and for the lysyl substrate 582+/-103 microM. TLCK and benzamidine inhibited the protease at micromolar, while STI at nanomolar concentrations. Both the inhibition profile and the substrate specificity suggest that it is a trypsin-like serine protease. We assume that the 85-90 kDa serine protease expressed on and secreted by activated B cells and BL41/95 cell line is responsible for the cleavage of various membrane proteins, including Fcgamma receptors; thus it may play a crucial role in regulating B cell's function.

Signaling pathways leading to apoptosis or survival in immature and mature B cells

In the absence of phosphorylated Gab2, immature B cells are unable to maintain a prolonged signal transduction, which is necessary for the transcriptional regulation and activation of the cells. Thus, the lack of Gab2 phosphorylation may drive immature B cells to apoptosis.

Developmental differences in B cell receptor-induced signal transduction

We have compared early signaling events at various stages of B cell differentiation using established mouse cell lines. Clustering of pre-B cell antigen receptor (BCR) or BCR induced the tyrosine phosphorylation of various proteins in all cells, although the phosphorylation pattern differed. In spite of the pre-BCR-induced tyrosine phosphorylation, we could not detect an intracellular Ca(2+) signal in pre-B cells. However, co-clustering of the pre-BCR with CD19 did induce Ca(2+) mobilization. In contrast to the immature and mature B cells, where the B cell linker protein (BLNK) went through inducible tyrosine phosphorylation upon BCR clustering, we observed a constitutive tyrosine phosphorylation of BLNK in pre-B cell lines. Both BLNK and phospholipase C (PLC)gamma were raft associated in unstimulated pre-B cells, and this could not be enhanced by pre-BCR engagement, suggesting a ligand-independent PLC gamma-mediated signaling. Further results indicate that the cell lines representing the immature stage are more sensitive to BCR-, CD19- and type II receptors binding the Fc part of IgG (Fc gamma RIIb)-mediated signals than mature B cells.

Co-clustering of Fcgamma and B cell receptors induces dephosphorylation of the Grb2-associated binder 1 docking protein

The immunoreceptor tyrosine-based inhibitory motif (ITIM) of human type IIb Fcgamma receptor (FcgammaRIIb) is phosphorylated on its tyrosine upon co-clustering with the B cell receptor (BCR). The phosphorylated ITIM (p-ITIM) binds to the SH2 domains of polyphosphoinositol 5-phosphatase (SHIP) and the tyrosine phosphatase, SHP-2. We investigated the involvement of the molecular complex composed of the phosphorylated SHIP and FcgammaRIIb in the activation of SHP-2. As a model compound, we synthesized a bisphosphopeptide, combining the sequences of p-ITIM and the N-terminal tyrosine phosphorylated motif of SHIP with a flexible spacer. This compound bound to the recombinant SH2 domains of SHP-2 with high affinity and activated the phosphatase in an in vitro assay. These data suggest that the phosphorylated FcgammaRII-SHIP complexes formed in the intact cells may also activate SHP-2. Grb2-associated binder 1 (Gab1) is a multisite docking protein, which becomes tyrosine-phosphorylated in response to various types of signaling, including BCR. In turn it binds to the SH2 domains of SHP-2, SHIP and the p85 subunit of phosphatidyl inositol 3-kinase (PtdIns3-K) and may regulate their activity. Gab1 is a potential substrate of SHP-2, thus its binding to FcgammaRIIb may modify the Gab1-bound signaling complex. We show here that Gab1 is part of the multiprotein complex assembled by FcgammaRIIb upon its co-clustering with BCR. Gab1 may recruit SH2 domain-containing molecules to the phosphorylated FcgammaRIIb. SHP-2, activated upon the binding to FcgammaRIIb-SHIP complex, partially dephosphorylates Gab1, resulting in the release of PtdIns3-K and ultimately in the inhibition of downstream activation pathways in BCR/FcgammaRIIb co-aggregated cells.

Proximity relationships between residue 117 of rabbit skeletal troponin-I and residues in troponin-C and actin

We used resonance energy transfer and site-directed photo-cross-linking to probe the Ca(2+)-dependent proximity relationships between residue 117 next to the C-terminus of the inhibitory region in rabbit skeletal troponin-I (TnI) and residues in troponin-C (TnC) and in actin. A mutant TnI that contains a single cysteine at position 117 (I117) was constructed, and the distance between TnI residue 117 and TnC residue 98 was measured with the following results: for both the binary TnC-TnI complex and the ternary troponin complex, this distance was 30 and 41 A in the presence and absence of Ca(2+), respectively. The distance between TnI residue 117 and Cys374 of actin was 48 and 41 A in the presence and absence of Ca(2+), respectively. Six additional distances from this TnI residue to cysteines in TnC mutants were measured and used to localize this residue with respect to the crystal structure of TnC. The results show that in the presence of Ca(2+) it is localized near the B and C helices of TnC's N-terminal domain. In the absence of Ca(2+) this residue moves away from this location by approximately 8 A. Photo-cross-linking experiments show that I117 labeled with 4-maleimidobenzophenone photo-cross-linked to TnC but not to actin in both the presence and absence of Ca(2+). Taken together these results provide independent experimental support for the proposal (Y. Luo, J. L. Wu, B. Li, K. Langsetmo, J. Gergely, and T. Tao, 2000, J. Mol. Biol. 296:899-910) that upon Ca(2+) removal the region comprising TnI residues 114-125 triggers the movements of residues 89-113 and 130-150 toward actin, but does not itself interact with actin.

Proximity relationships between residue 6 of troponin I and residues in troponin C: further evidence for extended conformation of troponin C in the troponin complex

Skeletal muscle troponin C (TnC) adopts an extended conformation when crystallized alone and a compact one when crystallized with an N-terminal troponin I (TnI) peptide, TnI(1-47) [Vassylyev et al. (1998) Proc. Natl. Acad. Sci. U.S.A. 95, 4847-4852]. The N-terminal region of TnI (residues 1-40) was suggested to play a functional role of facilitating the movement of TnI's inhibitory region between TnC and actin [Tripet et al. (1997) J. Mol. Biol. 271, 728-750]. To test this hypothesis and to investigate the conformation of TnC in the intact troponin complex and in solution, we attached fluorescence and photo-cross-linking probes to a mutant TnI with a single cysteine at residue 6. Distances from this residue to residues of TnC were measured by the fluorescence resonance energy transfer technique, and the sites of photo-cross-linking in TnC were determined by microsequencing and mass spectrometry following enzymatic digestions. Our results show that in the troponin complex neither the distance between TnI residue 6 and TnC residue 89 nor the photo-cross-linking site in TnC, Ser133, changes with Ca(2+), in support of the notion that this region plays mainly a structural rather than a regulatory role. The distances to residues 12 and 41 in TnC's N-domain are both considerably longer than those predicted by the crystal structure of TnC.TnI(1-47), supporting an extended rather than a compact conformation of TnC. In the binary TnC.TnI complex and the presence of Ca(2+), Met43 in TnC's N-domain was identified as the photo-cross-linking site, and multiple distances between TnI residue 6 and TnC residue 41 were detected. This was taken to indicate increased flexibility in TnC's central helix and that TnC dynamically changes between a compact and an extended conformation when troponin T (TnT) is absent. Our results further emphasize the difference between the binary TnC.TnI and the ternary troponin complexes and the importance of using intact proteins in the study of structure-function relationships of troponin.

Bacterially expressed human Fc gamma RIIb is soluble and functionally active after in vitro refolding

A recombinant soluble form of the human Fc gamma receptor was produced by engineering a cDNA construct containing the extracellular part of the mature protein. After expression in bacteria as inclusion body, the polypeptide was highly purified and was refolded in vitro with a method that was developed for the renaturation of immunoglobulin fragments. With this method oxidation of the disulfide bridges within the domains of the protein is done in the presence of an artificial 'chaperone' which protects the polypeptide molecules from unwanted protein protein interactions thereby inhibiting the incorrect oxidation of the SH-groups. and misfolding of the protein. The refolded recombinant soluble Fc gamma RIIb showed several characteristics of the native receptor: (i) it was recognized by a series of monoclonal antibodies specific for, and in most cases produced against the native cell-surface receptor: (ii) it is bound to its ligand (the Fc-region of different immunoglobulins) under very diverse conditions: and (iii) it is competed strongly and specifically with the native cell surface receptor for both ligand and antibody binding in experiments with distinct read-outs; (iv) monoclonal antibodies produced against the recombinant protein specifically recognized Fc gamma RIIb on different cells. From these data it was concluded that the recombinant soluble Fc-receptor was in a native, functionally active form, and its function was not affected by the lack of glycosylation.

Photocrosslinking of benzophenone-labeled single cysteine troponin I mutants to other thin filament proteins

The interaction sites of rabbit skeletal troponin I (TnI) with troponin C (TnC), troponin T (TnT), tropomyosin (Tm) and actin were mapped systematically using nine single cysteine residue TnI mutants with mutation sites at positions 6, 48, 64, 89, 104, 121, 133, 155 or 179 (TnI6, TnI48 etc.). Each mutant was labeled with the heterobifunctional photocrosslinker 4-maleimidobenzophenone (BP-Mal), and incorporated into the TnI.TnC binary complex, the TnI.TnC.TnT ternary troponin (Tn) complex, and the Tn.Tm.F-actin synthetic thin filament. Photocrosslinking reactions carried out in the presence and absence of Ca(2+) yielded the following results: (1) BP-TnI6 photocrosslinked primarily to TnC with a small degree of Ca(2+)-dependence in all the complex forms. (2) BP-TnI48, TnI64 and TnI89 photocrosslinked to TnT with no Ca(2+)-dependence. Photocrosslinking to TnC was reduced in the ternary versus the binary complex. BP-TnI89 also photocrosslinked to actin with higher yields in the absence of Ca(2+) than in its presence. (3) BP-TnI104 and TnI133 photocrosslinked to actin with much higher yields in the absence than in the presence of Ca(2+). (4) BP-TnI121 photocrosslinked to TnC with a small degree of Ca(2+)-dependence, and did not photocrosslink to actin. (5) BP-TnI155 and TnI179 photocrosslinked to TnC, TnT and actin, but all with low yields. All the labeled mutants photocrosslinked to TnC with varying degrees of Ca(2+)-dependence, and none to Tm. These results, along with those published allowed us to construct a structural and functional model of TnI in the Tn complex: in the presence of Ca(2+), residues 1-33 of TnI interact with the C-terminal domain hydrophobic cleft of TnC, approximately 48-89 with TnT, approximately 90-113 with TnC's central helix, approximately 114-125 with TnC's N-terminal domain hydrophobic cleft, and approximately 130-150 with TnC's A-helix. In the absence of Ca(2+), residues approximately 114-125 move out of TnC's N-terminal domain hydrophobic cleft and trigger the movements of residues approximately 89-113 and approximately 130-150 away from TnC and towards actin.

Development of a speech-based dialogue system for report dictation and machine control in the endoscopic laboratory

BACKGROUND AND STUDY AIMS

Reporting and machine control based on speech technology can enhance work efficiency in the gastrointestinal endoscopy laboratory.

MATERIALS AND METHODS

The status and activation of endoscopy laboratory equipment were described as a multivariate parameter and function system. Speech recognition, text evaluation and action definition engines were installed. Special programs were developed for the grammatical analysis of command sentences, and a rule-based expert system for the definition of machine answers. A speech backup engine provides feedback to the user. Techniques were applied based on the "Hidden Markov" model of discrete word, user-independent speech recognition and on phoneme-based speech synthesis. Speech samples were collected from three male low-tone investigators.

RESULTS

The dictation module and machine control modules were incorporated in a personal computer (PC) simulation program. Altogether 100 unidentified patient records were analyzed. The sentences were grouped according to keywords, which indicate the main topics of a gastrointestinal endoscopy report. They were: "endoscope", "esophagus", "cardia", "fundus", "corpus", "antrum", "pylorus", "bulbus", and "postbulbar section", in addition to the major pathological findings: "erosion", "ulceration", and "malignancy". "Biopsy" and "diagnosis" were also included. We implemented wireless speech communication control commands for equipment including an endoscopy unit, video, monitor, printer, and PC. The recognition rate was 95%.

CONCLUSIONS

Speech technology may soon become an integrated part of our daily routine in the endoscopy laboratory. A central speech and laboratory computer could be the most efficient alternative to having separate speech recognition units in all items of equipment.

Conformational changes induced in troponin I by interaction with troponin T and actin/tropomyosin

Troponin I (TnI) is the inhibitory component of the striated muscle Ca2+ regulatory protein troponin (Tn). The other two components of Tn are troponin C (TnC), the Ca2+-binding component, and troponin T (TnT), the tropomyosin-binding component. We have used limited chymotryptic digestion to probe the local conformation of TnI in the free state, the binary TnC*TnI complex, the ternary TnC*. TnI*TnT (Tn) complex, and in the reconstituted Tn*tropomyosin*F-actin filament. The digestion of TnI alone or in the TnC*TnI complex produced initially two major fragments via a cleavage of the peptide bond between Phe100 and Asp101 in the so-called inhibitory region. In the ternary Tn complex cleavage occurred at a new site between Leu140 and Lys141. In the absence of Ca2+ this was followed by digestion of the 1-140 fragment at Leu122 and Met116. In the reconstituted thin filament the same fragments as in the case of the ternary complex were produced, but the rate of digestion was slower in the absence than in the presence of Ca2+. These results indicate firstly that in both free TnI and TnI complexed with TnC there is an exposed and flexible site in the inhibitory region. Secondly, TnT affects the conformation of TnI in the inhibitory region and also in the region that contains the 140-141 bond. Thirdly, the 140-141 region of TnI is likely to interact with actin in the reconstituted thin filament when Ca2+ is absent. These findings are discussed in terms of the role of TnI in the mechanism of thin filament regulation, and in light of our previous results [Y. Luo, J.-L. Wu, J. Gergely, T. Tao, Biochemistry 36 (1997) 13449-13454] on the global conformation of TnI.

Fcgamma receptor-mediated inhibition of human B cell activation: the role of SHP-2 phosphatase

Co-clustering of the type II receptors binding the Fc part of IgG (FcgammaRIIb) and B cell receptors results in the translocation of cytosolic, negative regulatory molecules to the phosphorylated immunoreceptor tyrosine-based inhibitory motif (P-ITIM) of the FcgammaRIIb. SH2 domain-containing protein tyrosine phosphatases (SHP-1 and SHP-2), and the polyphosphoinositol 5'-phosphatase (SHIP) have been reported earlier to bind to murine FcgammaRIIb P-ITIM. However, neither the functional substrates of these enzymes, nor the mechanism of the inhibition are fully resolved. We show here that the human FcgammaRIIb binds SHP-2 when co-clustered with the B cell receptors, whereas its synthetic P-ITIM peptide bindes SHP-2 and SHIP in lysates of the Burkitt's lymphoma cell line BL41. The P-ITIM peptide binding enhances SHP-2 activity, resulting in dephosphorylation and release of P-ITIM-bound SHIP and Shc. Moreover, P-ITIM-bound SHP-2 dephosphorylates synthetic peptides corresponding to the sites of tyrosine phosphorylation on SHIP and Shc, indicating that these proteins are its potential substrates. Thus SHP-2-induced dephosphorylation may modulate the intracellular localization and/or activity of SHIP and Shc, thereby inhibiting further activation pathways which they mediate.

Residues 48 and 82 at the N-terminal hydrophobic pocket of rabbit skeletal muscle troponin-C photo-cross-link to Met121 of troponin-I

It has been proposed [Herzberg et al. (1986) J. Biol. Chem. 261, 2638-2644], and confirmed by structural studies [Gagne et al. (1995) Nat. Struct. Biol. 2, 784-789], that the binding of Ca2+ to the triggering sites in troponin-C (TnC) causes the opening of the N-terminal hydrophobic pocket bound by the B, C, and D helices. This conformational change is believed to provide an additional binding site for troponin-I (TnI) and to lead to further events in the Ca2+ regulation process. To answer the question of which part of TnI interacts with this hydrophobic patch of TnC, we constructed two TnC mutants, each with a single cysteine, one at residue 48 between helices B and C and the other at residue 82 on the D helix. Each mutant was labeled with the photoactivatable cross-linker benzophenone-4-iodoacetamide, followed by reconstitution and UV irradiation. Studies were made in the binary complex composed of TnC and TnI, the ternary complex composed of TnC, TnI, and troponin-T (TnT), and the synthetic thin filament composed of troponin, tropomyosin, and F-actin. TnC-TnI photo-cross-linking was observed for both mutants and for all three types of complexes. Although no Ca2+ dependence in the photo-cross-linking was observed on the binary and ternary complexes, the extent of cross-linking was reduced in the absence vs the presence of Ca2+ in the thin filament. TnI Met121, five residues from the C-terminus of the inhibitory region, was identified as the cross-linking site for both TnC mutants using microsequencing and mass spectrometry following proteolysis. These results, obtained with intact TnC.TnI complexes, indicate that the TnI segment containing Met121 is in close contact with the N-terminal hydrophobic patch of TnC, and that in the thin filament the segment containing this residue moves away slightly from the hydrophobic patch in the absence of Ca2+, possibly triggering the translocation of the actin-binding region(s) of TnI toward actin.

Cooperation between SHP-2, phosphatidyl inositol 3-kinase and phosphoinositol 5-phosphatase in the Fc gamma RIIb mediated B cell regulation

Co-clustering B cell receptors (BCR) and type II receptors binding the Fc part of IgG (Fc gamma RIIb) inhibits B cell activation and antibody production. Tyrosine phosphorylation of an intracellular motif of Fc gamma RIIb has been shown to be a prerequisite of the inhibition. After being phosphorylated by BCR-activated tyrosine kinases, the immunoreceptor tyrosine-based inhibitory motif (P-ITIM) of Fc gamma RIIb recruits SH2 domain containing protein tyrosine phosphatase(s) (PTPs) and polyphosphoinositol 5-phosphatase (SHIP) to the vicinity of BCR, which in turn dephosphorylate their specific substrates. This leads to the interruption of signal transduction, consequently to the anergy and/or apoptosis of the cell. The downstream signaling pathways affected by Fc gamma RIIb-BCR co-clustering are not clarified yet, neither the substrates of PTPs are known. We have studied the Fc gamma RIIb mediated B cell inhibition on human Burkitt lymphoma cell line (BL41). From the lysates of BL41 cells SHP-2 and phosphatidylinositol 3-kinase (PI3-K), as well as the protein tyrosine kinase (PTK) Lyn bind both to the BCR-co-clustered Fc gamma RIIb and to its P-ITIM peptide. Lyn hyperphosphorylates the P-ITIM associated molecules, including SHIP in the in vitro protein tyrosine kinase activity assay. The P-ITIM-compelled multi-phosphoprotein complex binds to and activates SHP-2, which in turn dephosphorylates SHIP and Shc and probably other substrates. Subcellular localisation of these signaling molecules is regulated by the phosphotyrosine-SH2 domain interactions, thus dephosphorylation may result in the re-direction of Shc and SHIP within the cell, consequently, in the modulation of their activity. Finally, co-clustering Fc gamma RIIb and BCR or Fc gamma RIIb and CD19 on the intact cells inhibited PI3-K activity as detected in the anti-phosphotyrosine (anti-PY) precipitates. The results indicate that SHP-2 bound to and activated by the BCR co-clustered Fc gamma RIIb, may down-regulate PI3-K activity by dephosphorylating a yet unidentified regulatory molecule, which recruits PI3-K to the cell membrane.

Immunoreceptor tyrosine-based inhibition motif-bearing receptors regulate the immunoreceptor tyrosine-based activation motif-induced activation of immune competent cells

ITIM-bearing receptors, a family which only recently has been recognized, play a key role in the regulation of the ITAM-induced activation of immune competent cells. The mechanism of ITM-mediated regulation in various cells was recently clarified. The present review focuses on ITIM bearing membrane proteins that negatively regulate the activation of cells when co-crosslinked with ITAM containing receptors, illustrates the inhibitory processes by the negative regulation of B-, NK-, T-cells and mast cells and summarizes current views on the mechanism of ITIM-mediated inhibition.

Molecular switches in troponin

The contraction of vertebrate striated muscle contraction, and hence its work output, is controlled by Ca2+, which binds to troponin (Tn) associated with tropomyosin (TM) and actin in the thin filaments. Tn consists of three subunits: TnC, the Ca(2+)-receptor; TnI, an inhibitor of actomyosin activity; and TnT, anchoring Tn to TM. Of the four Ca(2+)-binding sites, I and II in the N-terminal domain are Ca-specific sites, while sites III and IV, the high affinity Ca-Mg sites, are in the C-domain. The former are recognized as the functionally important triggering sites. TnC, whose structure has been solved by X-ray crystallography and recently by high-resolution NMR, contains two homologous globular domains connected by an unusual single alpha-helix. The C-terminal domain exhibits an open hydrophobic area regardless of whether Ca2+ or Mg2+ is bound to sites III and IV. In contrast, the N-terminal domain is a closed structure that opens a hydrophobic patch upon Ca(2+)-binding to its two "triggering" sites producing a TnI binding area. Crosslinking and fragment binding studies indicate that, in the main, the two polypeptide chains run in opposite directions in the complex of TnC with Tn. A model of TnC-TnI interactions based on low angle X-ray and neutron scattering is discussed in light of biochemical and other physico-chemical studies. The opening of the structure in the N-terminal domain of TnC may be regarded as a molecular switch. It activates a molecular switch in TnI, reflected in the movement of portions of its C-terminal half, including Cys 133, away from actin and closer to TnC, as well as other structural changes in TnI. Finally the role of TnT in switching and transmitting the Ca(2+)-signal is discussed.

Professor Ebashi's impact on the study of the regulation of striated muscle contraction

The field of striated muscle regulation has changed tremendously over the last forty years. Many of the problems solved by Dr. Ebashi and by those stimulated by him offer new challenges for future generations of scientists. Many questions remain to be solved, and it should give particular pleasure to Dr. Ebashi to see how the seeds sown by him and his colleagues have now grown into a beautiful tree that bears rich fruit at present and will continue to do so for a long time in the future.

[Experimental study of metoprolol-induced side effects]

To study side effects of drugs in preclinical as well as in postmarketing surveillance phase is very important. In our experiments the influence of metoprolol on carbohydrate- and lipid metabolism was investigated in male. Wistar rats. Metoprolol is a liposoluble beta1-selective adrenoceptor antagonists. We have calculated therapeutic dose which reduced heart frequency/min of the animals by 25%. This was 10 mg/kg. The blood glucose and triglyceride values of healthy rats are in the normal human domain. Blood glucose was high after the first metoprolol dose and increased further with continued treatment. Drug administration period comprised 16 days. At finishing experiments diminished glycogen content was measured which may be related to higher glucose output. In blood samples obtained one hour after last 16. metoprolol dose administration triglyceride values were high and HDL-C decreased. These data pertain to the development of a secondary hypertriglyceridaemia. Hyperglycemic and hypertriglyceridaemic responses were established with therapeutic doses regimen so they may be considered as unwanted effects.

Identification of the photocrosslinking sites in troponin-I with 4-maleimidobenzophenone labelled mutant troponin-Cs having single cysteines at positions 158 and 21

Our previous studies have shown that 4-maleimidobenzophenone (BP-Mal) attached to troponin-C (TnC) mutants with single cysteines at positions 12, 57, 89 and 98 forms crosslinks to troponin-I (TnI), and the identified crosslinking regions indicate an antiparallel course of the two interacting polypeptide chains, in agreement with other studies using fragments of TnC and TnI. In this work we extended the mapping of the TnC-TnI interface by analysing photocrosslinking between TnI and BP-Mal labelled TnC mutants with single Cys residues at positions 21 (TnC21) and 158 (TnC158). We determined the sites of these photocrosslinks in TnI by progressive proteolysis of the crosslinked product, followed by N-terminal sequencing and mass spectrophotometric analyses. The results show that whereas TnC158 forms a specific crosslink with Met-21, TnC21 forms multiple crosslinks in the range of residues 96 to 134 of TnI. The results are discussed in light of the antiparallel model of the TnI-TnC complex and a structural model derived from low-angle X-ray and neutron scattering studies.

Localization of Cys133 of rabbit skeletal troponin-I with respect to troponin-C by resonance energy transfer

We have used the technique of resonance energy transfer in conjunction with distance geometry analysis to localize Cys133 of troponin-I (TnI) with respect to troponin-C (TnC) in the ternary troponin complex and the binary TnC.TnI complex in the presence and absence of Ca2+. Cys133 of TnI was chosen because our previous work has shown that the region of TnI containing this residue undergoes Ca2+-dependent movements between actin and TnC, and may play an important role in the regulatory function of troponin. For this purpose, a TnI mutant with a single Cys at position 133, and TnC mutants, each with a single Cys at positions 5, 12, 21, 41, 49, 89, 98, 133, and 158, were constructed by site-directed mutagenesis. The distances between TnI Cys133 and each of the nine residues in TnC were then measured. Using a least-squares minimization procedure, we determined the position of TnI Cys133 in the coordinate system of the crystal structure of TnC. Our results show that in the presence of Ca2+, TnI Cys133 is located near residue 12 beneath the N-terminal lobe of TnC, and moves away by 12.6 A upon the removal of Ca2+. TnI Cys133 and the region of TnC that undergoes major change in conformation in response to Ca2+ are located roughly on opposite sides of TnC's central helix. This suggests that the region in TnI that undergoes Ca2+-dependent interaction with TnC is distinct from that interacting with actin.

[Computerized speech recognition-based endoscopic findings]

Discrete, Hidden Markov model based speech recognition and phoneme based speech synthesis techniques were applied for gastroscopy reporting and machine control. The authors developed a special program for grammatical analysis of the sentences. Altogether 100 patient findings were grammatically analysed. The sentences were grouped according to the topographical order of the investigation: oesophagus, cardia, fundus, corpus, antrum, pylorus, bulbus, postbulbar section, and the pathological findings: erosion, ulceration, malignancy. Speech samples from 3 deep voiced male investigators were collected. The recognition rate was above 95%. A simulation program was also developed for dictation and controlling of the different equipment (monitor, printer, video, endoscope) in the gastroscopy laboratory by speech recognition. Speech synthesis was applied for the evaluation of understanding. This module artificially synthesizes the answer of the system giving backup for the understood information. With additional developments the discrete word speech 'recognition' achieved the level of routine application in medical reporting. However, ready-to-use developments need the joint activity of speech technology and endoscopy industry with end-user teams.

Fc gammaRIIb inhibits both B cell receptor- and CD19-induced Ca2+ mobilization in Fc gammaR-transfected human B cells

Fc gammaRIIb (CD32) controls antibody production by down-regulating cell activation, when co-clustered with B cell antigen receptors (BCR) in vivo, via immune complexes consisting of secreted IgG and antigen. Fc gammaRIIb-BCR co-ligation in vitro was shown to inhibit the Ca2+ influx from the extracellular space, the mechanism of which is not fully understood. Human B cells express Fc gammaRIIb1 and Fc gammaRIIb2, differing only in a 19 amino acid long insert in the cytoplasmic tail of the former. To elucidate whether Fc gammaRIIb1 and Fc gammaRIIb2 isoforms show any difference in the down-regulation of B cells, we have studied the effect of co-clustering of BCR and Fc gammaRIIb1 or Fc gammaRIIb2 on the Ca2+ signaling in a Burkitt's lymphoma cell line, ST486, transfected with the two isoforms respectively. We have shown here, for the first time, that co-aggregation of BCR and Fc gammaRIIb may also inhibit Ca2+ release from the endoplasmic reticulum pool of human B cells. Both isoforms mediated this inhibition and the inhibitory effect depended on the ratio of BCR to Fc gammaRIIb cross-linking. In contrast to Fc gammaRIIb, the CD21/CD19 complex was shown to up-regulate B cell response by lowering the activation threshold. We have shown here that co-clustering of Fc gammaRIIb with CD19 inhibited the CD19-induced Ca2+ influx. Furthermore, the three party co-aggregation of Fc gammaRIIb with BCR and CD19 resulted in a decreased Ca2+ response, as compared to the BCR- plus CD19-induced one, indicating that Fc gammaRIIb may inhibit CD19-induced enhancement of B cell activation. On the basis of these data we suggest that IgG-containing and C3d-fixing immune complexes may down-regulate the B cell response by interfering with both BCR- and CD19-mediated Ca2+ mobilization.

Troponin T and Ca2+ dependence of the distance between Cys48 and Cys133 of troponin I in the ternary troponin complex and reconstituted thin filaments

Contraction of vertebrate striated muscle is regulated by the interaction of Ca2+ with the heterotrimeric protein troponin (Tn), composed of troponin-C (TnC), troponin-I (TnI), and troponin-T (TnT). Although much is known about the Ca2+-induced conformational changes in TnC, the Ca2+-binding subunit of Tn, little is known about how TnI, the inhibitory subunit, responds to the binding of Ca2+ to TnC. In this work, we used resonance energy transfer to measure the distance between probes attached at Cys48 and Cys133 in the N- and C-terminal domains, respectively, of TnI. A mutant rabbit skeletal TnI, TnI48/133 (C64S), was constructed by converting Cys64 into Ser. The remaining two thiols at Cys48 and Cys133 were labeled with the fluorescent donor 1,5-IAEDANS, and the nonfluorescent acceptor, DAB-Mal. We found an interprobe distance of approximately 41 A for both uncomplexed TnI and TnI in the binary complex with TnC. This distance increased to 51 A in the ternary Tn complex with TnT. These distances did not change significantly on binding of Ca2+ to TnC. In the reconstituted thin filament, this distance remained to be 50 A in the presence of saturating Ca2+, but increased to approximately 66 A on removing Ca2+ with EGTA in the presence of Mg2+. Our results indicate firstly that while TnC has only small effects on the global conformation of TnI, the presence of TnT in the ternary Tn complex gives rise to an apparent elongation of TnI. Secondly, whereas there is no detectable Ca2+-dependent change in the global conformation of TnI in the Tn complex free in solution, the removal of Ca2+ caused a substantial separation of the N- and C-terminal TnI regions in the reconstituted thin filament, owing to the interaction between the C-terminal region of TnI and actin in the relaxed state.

Fc gamma receptor type IIb induced recruitment of inositol and protein phosphatases to the signal transductory complex of human B-cell

Co-clustering of Fc gamma RIIb and B-cell receptor (BCR) inhibits cell activation by interrupting BCR stimulated signal transduction. The immunoreceptor tyrosine-based inhibitory motif (ITIM) of Fc gamma RIIb becomes tyrosyl phosphorylated (P-ITIM) upon co-clustering with BCR then P-ITIM interacts with several signalling molecules, some of which negatively regulate the cell activation process. The molecules recruited by the P-ITIM of human Fc gamma RIIb have not been characterised yet. In order to affinity isolate the potential functional partner molecules of human Fc gamma RIIb, synthetic peptides were designed to cover almost the entire intracellular Fc gamma RIIb domain, including Fc gamma RIIb2 specific sequences and stretches containing the phosphorylated and non-phosphorylated ITIM. We report here that several tyrosyl phosphorylated proteins bind to the P-ITIM peptide from both resting and activated B-cell lysates, the 53-56 kDa being the most prominent one. A fraction of the 53-56 kDa bands were identified as the protein tyrosine kinase (PTK), Lyn which also bound to ITIM peptide, pointing to its role in initiating Fc gamma RIIb-mediated negative regulation. Among the P-ITIM associated tyr phosphorylated components, the 145 kDa one was identified as the inositol polyphosphate 5-phosphatase, SHIP and the 72 kDa protein as the protein tyrosine phosphatase (PTP) SHP2, whereas SHP1 was not detected. Phosphatase activity assays showed that P-ITIM bound about five times higher SHIP and four times higher PTP activity than the ITIM containing peptide. Furthermore, we detected PKC and MAPK in both ITIM and P-ITIM peptides precipitated samples. Since human B-cells express both Fc gamma RIIb1 and Fc gamma RIIb2, differing in a 19 amino acid insert in the cytoplasmic tail of the former, we investigated the components binding to Fc gamma RIIb1 and Fc gamma RIIb2 specific sequences. Synthetic peptide representing Fc gamma RIIb1 and Fc gamma RIIb2 specific sequences weakly bound unidentified tyr phosphorylated proteins at 50-56 kDa, while the insert itself did not bind a detectable amount of protein. Neither of the ITIM or P-ITIM bound molecules were observed in samples precipitated with peptides corresponding to Fc gamma RIIb1 or Fc gamma RIIb2 specific sequences. These observations suggest that protein kinases associate with both ITIM and P-ITIM of human Fc gamma RIIb, Lyn being responsible for the tyrosyl phosphorylation of ITIM. SHIP and SHP2 phosphatases selectively bind to the phosphorylated ITIM. Based on these data we assume that SHIP and SHP2 recruited in vivo to the Fc gamma RIIb co-clustered BCR are responsible for the Fc gamma RIIb mediated negative regulation of human B-cell activation.

Integration of activatory and inhibitory signals in human B-cells

Fc gamma receptors type IIb1 (Fc gammaRIIb1) inhibit B-cell activation when co-ligated with B-cell antigen receptors (BCR) by immune complexes. In murine B-cells the inhibition is mediated by the interaction of the phosphorylated immunoreceptor tyrosine-based inhibitory motif (P-ITIM) of Fc gammaRIIb1 with the SH2 domain containing protein tyrosine phosphatase. SHP1. To clarify the mechanism of Fc gammaRIIb mediated inhibition of human B-cells we have studied the association of signaling molecules with human Fc gammaRIIb1 after co-ligating with BCR. Fc gammaRIIb1 were affinity purified from the Burkitt lymphoma cell line, BL41. Several tyrosine phosphorylated proteins were co-isolated with Fc gammaRIIb1 at 145, 110, and 50 60 kDa, which were not present in Fc gammaRIIb1 free immune complexes. Among these molecules we have identified the p52 Shc adaptor protein. Furthermore, we have shown that the insolubilised synthetic peptide corresponding P-ITIM bound Shc, Lyn and the p75 and p 10 unidentified tyrosine phosphorylated proteins. Here we describe that the cell membrane associated Shc is partially dephosphorylated in BCR-Fc gammaRIIb1 co-ligated samples, suggesting that its function in regulating p21ras monomeric G protein is impaired. Indeed, we have detected a lower p21ras activity in BCR-Fc gammaRIIb1 co-crosslinked samples. These data indicate that co-ligation of BCR and Fc gammaRIIb1 interrupts signal transduction between protein tyrosine kinase activation and p21ras mediated activation pathway. Since in contrast to the mouse B-cells both Fc gammaRIIb1 and Fc gammaRIIb2 are expressed in human B-cells, we have investigated the inhibitory function of the two receptors in Fc gammaRIIb negative Burkitt lymphoma cell line ST486 transfected with Fc gammaRIIb1 and Fc gammaRIIb2, respectively. Both Fc gammaRIIb1 and Fc gammaRIIb2 inhibited the rise of intracellular Ca2+ induced by the crosslinking of BCR. The rate of the inhibition depended on the ratio of the co-crosslinked receptors (BCR-Fc gammaRIIb1) to the crosslinked BCR (BCR-BCR). Co-crosslinking of the two receptors inhibited not only the capacitive Ca2+ entry but rather the total Ca2+ response in both Fc gammaRIIb1 and Fc gammaRIIb2 transfected human B-cells. CD19 represents the signal transduction unit of complement receptor, CR2 (CD21), and is responsible for the complement activating IgM-immune complex induced enhancement of B-cell activation. Co-crosslinking of CD19 and BCR was shown to enhance B-cell activation due to the recruitment of further signaling molecules to the activator complex by the phosphorylated tyrosine residues of CD19. Here we show a novel finding that co-ligation of CD19 with Fc gammaRIIb1 inhibits the CD19-induced upregulation of Ca2+ response. The results indicate that IgG plus complement containing immune complexes may inhibit B-cell activation in vivo, due to the Fc gammaRIIb1-mediated interruption of signal transduction via both BCR and CD19.

Membrane-bound ezrin is involved in B-cell receptor-mediated signaling: potential role of an ITAM-like ezrin motif

Ezrin is a cytoskeleton-plasma membrane linker molecule which is implicated in the T-cell antigen receptor signaling as one of the major tyrosine phosphorylated components. Its function in B-lymphocyte activation has not yet been clarified. Here we studied the potential involvement of ezrin in the B-cell receptor (BCR) signaling in BL41 Burkitt lymphoma cells. Our data demonstrate that ezrin, which shows predominantly cytosolic distribution in unstimulated cells, undergoes only a moderate tyrosine phosphorylation in response to BCR triggering, with no concomitant translocation of the protein from the cytosol to the plasma membrane. Instead, BCR-independent stimulants like oxidant stress induced by phenylarsine oxide, resulted in rapid redistribution of ezrin to the plasma membrane. When BCR triggering was preceded by membrane recruitment of ezrin, it became one of the main and earliest substrates of tyrosine kinases activated by BCR. No detectable influence on distribution or phosphorylation of ezrin was triggered by the tyrosine phosphatase inhibition by orthovanadate, suggesting that these effects of phenylarsine oxide are not attributable to its tyrosine phosphatase inhibitory capacity. The notion that BCR-mediated phosphorylation of ezrin negatively correlates with activation events such as phosphorylation of tyrosine kinase, syk and induction of calcium mobilization response, suggests that ezrin might be implicated in the regulation of transmembrane signaling and cellular responsiveness. As will be discussed, the regulatory function of ezrin may be due to an immunoreceptor tyrosine-based activation motif (ITAM)-like sequence.

Human type II Fcgamma receptors inhibit B cell activation by interacting with the p21(ras)-dependent pathway

Co-ligation of antigen receptors and type II Fcgamma receptors (FcgammaRIIb) on B cells interrupts signal transduction and ultimately inhibits antibody production. We have identified p52 Shc in the FcgammaRIIb1-specific immunoprecipitates isolated from the membrane fraction of BL41 Burkitt lymphoma cells following B cell receptor-FcgammaRIIb1 co-ligation. The insolubilized synthetic peptide representing the phosphorylated form of the tyrosine-based inhibitory motif of FcgammaRIIb also binds Shc from the lysates of activated but not from resting BL41 cells. This suggests that the binding does not depend on the interaction of FcgammaRIIb1-phosphotyrosine with the SH2 domain of Shc. Tyr phosphorylation of FcgammaRIIb1-associated Shc is low, indicating an impaired function. Shc is implicated in regulating p21(ras) activation; thus, we have compared p21(ras) activities in BL41 cells treated in different ways. p21(ras) activity is reduced when B cell receptor and FcgammaRIIb1 are co-ligated. p21(ras) couples protein-tyrosine kinase-dependent events to the Ser/Thr kinase-mediated signaling pathway leading to the activation of mitogen-activated protein kinases (MAPK). Our results show that B cell receptor-FcgammaRIIb1 co-cross-linking partially inhibits mitogen-activated protein kinase activity. We conclude that FcgammaRIIb1-dependent inhibition of human B cell activation may be based on interrupting signal transduction between protein-tyrosine kinases and the p21(ras)/mitogen-activated protein kinase-dependent activation pathway.

Kinetics and isotype profile of rheumatoid factor production during viral infection: organ distribution of antibody secreting cells

The kinetics and isotype profile of influenza virus-specific IgG antibodies were studied in correlation with the serum titre of IgG-reactive autoantibodies. An increased level of IgG isotype-specific, rheumatoid factor-type autoantibody secretion was observed in the late phase of the virus-specific memory response. These rheumatoid factors were specific for the IgG2a and IgG1 subclasses which dominated the anti-viral antibody response. As revealed by a preparative immunosorbent technique combined with isotype quantitation the majority of IgG2a- or IgG1-bound immunoglobulins isolated from the serum of virus-infected mice belonged to the same subclass as the target antibody. Comparison of the kinetics of appearance and the number of IgM-, IgG- and IgA-type IgG2a-reactive autoantibody secreting cells during the primary and memory anti-viral antibody responses showed isotype switch of IgM rheumatoid factor secreting cells predominantly to IgA. Localization of IgM and IgA antibody secreting cells demonstrated the wide organ distribution of IgM-type rheumatoid factor secreting cells. On the contrary, IgA rheumatoid factor production was observed only in Peyer's patches and at the site of the local virus-specific immune response, i.e. in mediastinal lymph nodes and in the lung. These results demonstrate that B cells specific for self IgG are activated and differentiated in concert with the viruspecific antibody response in similar microenvironments. The predominant involvement of the mediastinal lymph nodes and the spleen in the production of IgG2a-specific IgM-type autoantibodies suggest a regulatory function of this type of autoantibodies in modulating IgG2a production in both systemic and local anti-viral immune responses. The results also suggest a strictly regulated rheumatoid factor production which, however, can be unbalanced by repeated viral infections resulting in the escape of high affinity, isotype-switched autoantibodies.

Fc gamma RII-mediated regulation of human B cells

Recent studies have provided considerable insight into the mechanism of BCR-mediated B-cell activation, but the inhibitory signals transferred by Fc gamma Rs leading to down-regulation of BCR-activated B lymphocytes are not clarified yet. In the present paper the authors give an overview on new findings regarding BCR structure and signal transduction mechanisms induced by the B-cell antigen receptor complex and outline, partly based on their own observations, the possible mechanisms resulting in Fc gamma R-mediated inhibition of B cells.

Properties of troponin C acetylated at lysine residues

We have studied the properties of rabbit skeletal troponin C (TnC) fully acetylated at its lysine residues (AcTnC). Acetylation causes a decrease in thermal stability of both domains of TnC in the absence of Ca2+. At 25 degrees C, the acetylated C-terminal domain of TnC is almost completely unfolded and the melting temperature of the N-terminal domain monitored by far-UV circular dichroism is decreased by 16.3 degrees C. In the presence of 1 mM CaCl2, no cooperative unfolding can be detected up to 90 degrees C for either TnC or AcTnC. At 25 degrees C, CD spectra show that AcTnC has a slightly lower alpha-helix content in the absence of Ca2+, but higher in the presence of Ca2+ as compared to unmodified TnC. Acetylation causes a 3.5-fold increase in affinity for Ca2+ at the low-affinity sites and a 2-fold decrease at the high-affinity sites. Polyacrylamide gel electrophoresis under nondissociating conditions (no SDS, no urea, pH 8.6) indicates that acetylation has little effect on the apparent affinity of TnC for troponin I; however, the binding of the acetylated peptides corresponding to the N-terminal domain of TnC to troponin I is significantly stronger than that of the unmodified peptides. Troponin T binding to AcTnC is significantly enhanced, the altered properties of the N-terminal domain being predominantly responsible for the increase. Titration of the ATPase activity of TnC-depleted myofibrils with AcTnC or native TnC indicates that acetylation increases TnC's affinity for myofibrils in the presence of Ca2+ approximately 6 times; at saturation the ATPase activity is the same for the two forms of TnC. The Ca2+ dependence of the ATPase activity of myofibrils containing AcTnC is shifted to lower Ca2+ concentrations, consistent with the higher Ca2+ affinity of AcTnC at the low-affinity sites. These data indicate that positively charged lysine side chains, especially those located in the N-terminal domain, modulate TnC's structural stability and interactions with Ca2+ and other troponin components.

Extensive interactions between troponins C and I. Zero-length cross-linking of troponin I and acetylated troponin C

Interactions between troponin C (TnC) and troponin I (TnI) play an important role in the Ca(2+)-dependent regulation of vertebrate striated muscle contraction. Earlier studies have led to the proposal that the "inhibitory region" (residues 96-116) of TnI binds to an alpha-helical segment of TnC comprising residues 89-100 in the nonregulatory, C-terminal domain. Subsequently, on the basis of the results of zero-length cross-linking, we suggested that the inhibitory region of TnI also interacts with the N-terminal, regulatory domain of TnC [Leszyk, J., Grabarek, Z., Gergely, J., & Collins, J. H. (1990) Biochemistry 29, 299-304]. In the present study, we acetylated the epsilon-NH2 groups of the nine lysines of TnC in order to avoid complications which may arise from intramolecular cross-linking between NH2 and COOH groups of TnC. We then activated the COOH groups of acetylated TnC (AcTnC) with 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide and N-hydroxysuccinimide. The activated AcTnC was combined with TnI, and zero-length cross-links were formed between COOH groups in AcTnC and lysine epsilon-NH2 groups in TnI. The cross-linked heterodimer (AcCxI) was cleaved with CNBr and proteases, and the resulting cross-linked peptides were separated by HPLC and then sequenced. Our results show extensive cross-linking between AcTnC and TnI, involving both the N-terminal and C-terminal domains of TnC, as well as the N-terminal, C-terminal, and inhibitory regions of TnI.

IgG isotype-specific auto-antibodies bind preferentially to cross-linked membrane Ig

Under equilibrium conditions, the affinities of five anti-IgG2a mAb isolated from virus-infected mice were comparable to other high-affinity auto-antibodies. Similar to rheumatoid factors, these anti-IgG2a auto-antibodies bound to aggregated or complexed IgG2a with 50 to 1500-fold higher avidity than their monomeric counterparts. Despite their high functional affinity to IgG2a, flow cytometric analysis revealed no binding or marginal mAb binding to four distinct lines of B cells expressing different densities of membrane-anchored IgG2a. If, however, surface IgG2a was cross-linked by polyclonal light chain-specific antibodies, IgM and IgA mAb binding resulted, and was detected as an increase in mean fluorescence intensity compared with isotype-matched control antibodies. The binding of one IgM mAb to cross-linked IgG2a patches of the cell surface was also visualized by confocal microscopy. Pretreatment of cells with aggregated IgG2a caused increased fluorescence intensity, demonstrating that the IgM and IgA mAb were also able to interact with IgG2a aggregates bound on the B cell surface via Fc gamma RIIB. It also permitted efficient co-ligation of the aggregated B cell receptors (BCR) with Fc gamma RIIB-fixed immune complexes known to deliver a negative signal in B cell activation. Cross-linking of IgG2a complexes bound to Fc gamma RI on macrophages or dendritic cells with antigen-specific BCR and/or T cells via their Fc gamma RIIB may accelerate the physical contact of cells involved in the antigen-specific response.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of effector functions by immune complexes of mouse IgG2a with isotype-specific autoantibodies

Analysis of five monoclonal autoantibodies, rheumatoid factors produced by hybridomas generated from spleen cells of BALB/c mice repeatedly infected with A/PR/8/34 human influenza A virus, revealed that they recognized distinct but spatially related epitopes. The differing isoallotypic specificity of the IgM and IgA monoclonal antibodies correlated with the presence of Ile258 and Ala305, respectively. Although these data suggest that the epitopes recognized are within the CH2 domain, all antibodies failed to inhibit IgG antigen reactivity with Staphylococcus aureus protein A (SpA), C1q, mouse C3, human Fc gamma RI or mouse Fc gamma RII, activities known to be predominantly determined by CH2 domain structures. Reactivity of the IgA antibody, Z34, with IgG2b allowed further specificity studies using a panel of 26 mutant IgG2b proteins, each having single amino acid replacements over the surface of the CH2 domain. The only substitution that affected Z34 reactivity was Asn/Ala297, which destroyed the glycosylation sequon, resulting in secretion of an aglycosylated IgG molecule. The epitope recognized by Z34 therefore seems to be located outside of the Fc gamma R and C1q binding sites, but to be dependent on the presence of carbohydrate for expression. In contrast to the binding studies, complement activation by aggregated IgG2a, through classical or alternative pathways, was inhibited by the presence of autoantibodies. The functional significance of isotype-specific autoantibody in immune regulation is discussed.

Interaction of signaling molecules with human Fc gamma RIIb1 and the role of various Fc gamma RIIb isoforms in B-cell regulation

The low-affinity type-IIb IgG Fc-binding receptors (Fc gamma RIIb) are expressed on B cells. When cross-linked with mIgM Fc gamma RIIb are known to down-regulate B-cell activation by interrupting signal transduction upstream from G-protein-activated events. We have studied Fc gamma RII isoforms expressed on resting and activated B cells and the interaction of Fc gamma RIIb1 with molecules transducing the antigen receptor-mediated signals. Expression of Fc gamma RII isoforms was studied by reverse transcription and polymerase chain reaction. Resting B cells express both Fc gamma RIIb2 and Fc gamma RIIb1 isoforms. Activation with anti-IgM or IL-4 induces the splicing of Fc gamma RIIb1 mRNA, while the alternative splicing of Fc gamma RIIb2 mRNA is down-regulated, resulting in the surface expression of Fc gamma RIIb1. Functional differences were found between the two isoforms in inhibiting B-cell activation, suggesting that Fc gamma RIIb2 might influence the threshold of signals necessary for activation of resting B cells, while Fc gamma RIIb1 may regulate in later phases of antibody response. To explore the mechanism by which Fc gamma RII may uncouple antigen receptor-mediated signal transduction, we have investigated the association of signaling molecules with Fc gamma RII. Beside the protein tyrosine kinase (PTK) fyn, protein kinase C (PKC) was found to be co-isolated with Fc gamma RIIb1, suggesting a tight connection between these kinases and Fc gamma RII. We suggest that PKC might be responsible for the activation-induced phosphorylation of Fc gamma RII on serine residues.(ABSTRACT TRUNCATED AT 250 WORDS)