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)