Monoclonal antibodies to human protein C: effects on the biological activity of activated protein C and the thrombin-catalyzed activation of protein C1

Thirteen monoclonal antibodies designated as MFC-1 to MFC-13 were obtained from hybridoma cells cloned after the fusion of mouse myeloma cells with spleen cells of mice immunized with purified human protein C. Studies were made to determine where the antibodies bound to the molecule of protein C and whether they affected the biological actions of protein C. By using the immunoblotting technique, six of these antibodies were shown to bind to the light chain of protein C, and five to the heavy chain of protein C and also activated protein C. The remaining two antibodies bound to neither the light chain nor the heavy chain, though both antibodies bound to the intact protein C. Antibodies specific for the light chain did not bind to the gamma-carboxyglutamic acid-domain. Two of the antibodies specific for the heavy chain (MFC-13 and -1) inhibited the amidolytic activity of activated protein C. The MFC-13 also inhibited the activity of bovine activated protein C, but not that of human Factor IXa, Factor Xa, or thrombin. In addition to these two antibodies, another one for the heavy chain (MFC-10) and two antibodies for the light chain (MFC-9 and -11) inhibited the inactivation of Factor Va by human activated protein C. One of the antibodies which inhibited the enzyme activity (MFC-1) blocked the inhibition of activated protein C by protein C inhibitor. Another one for the heavy chain (MFC-5) inhibited the activation of protein C by thrombin regardless of the presence or absence of thrombomodulin. Based on these results, we have established the positions of some monoclonal antibody-binding sites on the protein C molecule.     

Probing the C4-binding site on C1s with monoclonal antibodies. Evidence for a C4/C4b-binding site on the gamma-domain

The catalytic site for C4 of C1s has been presumed to consist of a C4-binding domain and a proteolytic domain. A mAb to C1s, M81, blocked C4 activation and C4 binding to C1s. M81 recognized the H chain of C1s. Using M81 as a probe, we tried to define C4-binding site on C1s. Plasmin digestion of C1s generated four products of Mr 58,000 (P1), 48,000 (P2), 37,000 (P3), and 27,000 (P4). These products, except for P2, all possessed a 26,000-Da H chain fragment (26k-HF) connected to variable-sized L chain pieces. 26k-HF alone had an ability to interact with M81. Amino-terminal amino acid analysis of 26k-HF mapped the epitope for M81 to domain IV and/or V of gamma-domain of C1s. The gamma-domain therefore contains the C4-binding site. The confirm and further elucidate the role of the C4-binding site for C4, we used a substrate-blotting technique in which labeled C4 was incubated with nitrocellulose membrane-fixed C1s and its fragments. C4 was successfully blotted onto C1s and P1, but not P2-P4; i.e., further degradation of the L chain led to the loss of C4-binding. During the incubation, most of the added C4 was converted to C4b. The binding was augmented, if the proteolytic activity of C1s and P1 was blocked, so that the added C4 remained intact. Although C4b also bound to C1s and P1, its binding was less effective and abolished by the addition of cold C4. Based on these results, the gamma-domain and the L chain constitute the catalytic site of C1s to activate C4 to C4b. Moreover, the generated C4b, although it still has weak affinity for C1s, can be replaced by newly coming C4.     

Botulism: the agent,mode of action of the botulinum neurotoxins,forms of acquisition,treatment and prevention

The botulinum neurotoxins are produced by anaerobic, spore-forming bacteria belonging to the Clostridium genus. They are synthesised as a single chain protein (150 kDa), which is not or weakly active. The active form results from a proteolysis cleaving the precursor in a light chain (about 50 kDa) and a heavy chain (about 100 kDa), which are linked by a disulfide bridge. The heavy chain is involved in the recognition of a specific neuronal surface receptor and mediates the internalization of the light chain into the cytosol. The light chain is responsible for the intracellular activity. It catalyses the proteolysis of SNARE proteins, which are involved in the exocytosis of synaptic vesicles containing acetylcholine. Hence, the release of acetylcholine at the neuromuscular junction is blocked, leading to a flaccid paralysis. Human botulism, usually type A, B or E, is associated with intoxination, ingestion of preformed toxin in food, with digestive toxi-infection, mainly in newborns (infant botulism), or with wound contamination (wound botulism). The treatment of botulism is usually symptomatic. The specific treatment is based on the serotherapy or on the use of purified specific antibodies. The vaccination against botulism is efficient. However, since the botulinum neurotoxins are widely used for the treatment of numerous dystonias, a generalised vaccination is not conceivable.     

Preparation and properties of monoclonal antibodies against lipopolysaccharide-sensitive serine protease zymogen, factor C, from horseshoe crab (Tachypleus tridentatus) hemocytes.

Seventeen murine monoclonal antibodies (mAbs) against horseshoe crab clotting factor, factor C, were prepared and characterized. When the binding sites of these mAbs were analyzed by immunoblotting, ten mAbs recognized nonreduced factor C, five mAbs were directed against the heavy chain, and two mAbs were directed against the B chain. Three mAbs, 1H4, 2C12, and 2A7, one selected from each group, were used for further study. The mAb 1H4, which recognized only nonreduced factor C molecule, inhibited the factor C activity in a dose-dependent manner. It also inhibited lipopolysaccharide (LPS)- and alpha-chymotrypsin-mediated activations of the zymogen factor C, suggesting that 1H4 binds close to the active site and/or the substrate-binding site located in the serine protease domain (B chain) of factor C. On the other hand, 2C12 and 2A7 recognized, respectively, an epitope located in the heavy and the B chains, and inhibited LPS-mediated activation of factor C, but not alpha-chymotrypsin-mediated activation of factor C or factor C activity. Both F(ab')2 and Fab' fragments derived from 2C12 inhibited LPS-mediated activation in the same manner. These three mAbs did not bind with LPS, although a factor C-mAb complex was able to bind LPS, suggesting that the LPS-mediated activation of the zymogen factor C was induced through intermolecular interaction between the LPS-bound factor C molecules. The dissociation constants (Kd) for 1H4, 2C12, and 2A7 binding to factor C were determined as 1.9 x 10(-9), 0.6 x 10(-10), and 1.8 x 10(-10) M, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Calcium-sensitive thermal transitions and domain structure of human complement subcomponent C1r

Fluorescent probes and other methods have been used to investigate the thermal stability of activated C1r and functionally intact fragments isolated from tryptic digests of the protein. This enzyme exhibits two irreversible transitions that differ with respect to their sensitivity to metal ions. The high-temperature transition occurs with a midpoint near 53 degrees C in 0.02 M tris(hydroxymethyl)aminomethane buffer and 0.15 M NaCl, pH 7.4. It is relatively insensitive to Ca2+ and ionic strength and is accompanied by a loss of catalytic activity. The low-temperature transition is most easily observed in the presence of ethylenediaminetetraacetic acid and is completely abolished by 100 microM Ca2+. Its midpoint varies between 26 degrees C at low ionic strength and 40 degrees C in the presence of 0.5 M NaCl. The low-temperature transition results in extensive polymerization of the protein without loss of the esterolytic activity or the ability to react with C1 inhibitor; however, the ability to reconstitute hemolytically active C1 or even bind to C1s in the presence of Ca2+ is destroyed. A highly purified N-terminal fragment generated by tryptic digestion of C1r in the presence of Ca2+ retained its ability to interact with C1s, disrupting the formation of C1s dimers in the presence of Ca2+. In the absence of Ca2+, this fragment displays only a low-temperature transition that is very similar to the one observed with the whole protein and that destroys its ability to bind to C1s. Addition of Ca2+ stabilizes this fragment, shifting the midpoint of its melting transition upward by more than 20 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)     

Isolation and functional characterization of the heavy and light chains of human tissue-type plasminogen activator

Two-chain tissue-type plasminogen activator (t-PA), which consists of a heavy chain (Mr congruent to 38,000) and a light chain (Mr congruent to 31,000) connected by a disulfide bridge, was reduced with 2-mercaptoethanol and then air-reoxidized at a low protein concentration and carboxamidomethylated. The two chains were separated by means of zinc chelate-agarose, which was found to bind the light chain selectively. The light chain was fully active on the tripeptide substrate H-D-isoleucyl-L-prolyl-L-arginine p-nitroanilide (S-2288) and partially active on plasminogen. The plasminogen activator activity of the light chain was, in contrast to that of two-chain t-PA, not stimulated by fibrin or fibrinogen fragments. Fibrin-agarose chromatography of radiolabeled chains showed that only the heavy chain bound to fibrin. These results indicate that the active site-containing light chain in t-PA needs the heavy chain for fibrin stimulation of its plasminogen activator activity.     

Proteolysis of the heavy chain of major histocompatibility complex class I antigens by complement component C1s.

The major histocompatibility complex (MHC) class I antigens contain a light chain, beta 2-microglobulin, non-covalently associated to the transmembrane heavy alpha-chain carrying the allotypic determinants. Since the C1q complement component is known to associate with beta 2-microglobulin, and we recently found that activated C1s complement was capable of cleaving beta 2-microglobulin, we decided to investigate the proteolytic activity of C1 complement towards the heavy chain of class I antigens. Our results demonstrate that human C1s complement cleaves the heavy chain of human class I antigens into at least two fragments, with apparent molecular weights of 22,000 and 24,000 g/mol on sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE), under both reducing and non-reducing conditions. The cleavage of the heavy chain is inhibited by the presence of C1 esterase inhibitor. The molecular weights of the fragments are in agreement with the cleavage located in the area between the disulphide loops of the alpha 2-and alpha 3-domains of the heavy chain. In addition human C1s complement is able to cleave H-2 antigens from mouse in a similar fashion but not rat MHC class I antigen or mouse MHC class II antigen (I-Ad). Mouse MHC class I antigen-specific determinants could also be detected in supernatant from mouse spleen cells incubated with C1r and C1s. These results indicate the presence in the body fluids of a non-membrane-bound soluble form of the alpha 1-and alpha 2-domains which represent the binding site for antigenic peptides.     

Substrate specificities of recombinant mannan-binding lectin-associated serine proteases-1 and -2

Mannan-binding lectin (MBL)-associated serine proteases-1 and 2 (MASP-1 and MASP-2) are homologous modular proteases that each interact with MBL, an oligomeric serum lectin involved in innate immunity. To precisely determine their substrate specificity, human MASP-1 and MASP-2, and fragments from their catalytic regions were expressed using a baculovirus/insect cells system. Recombinant MASP-2 displayed a rather wide, C1s-like esterolytic activity, and specifically cleaved complement proteins C2 and C4, with relative efficiencies 3- and 23-fold higher, respectively, than human C1s. MASP-2 also showed very weak C3 cleaving activity. Recombinant MASP-1 had a lower and more restricted esterolytic activity. It showed marginal activity toward C2 and C3, and no activity on C4. The enzymic activity of both MASP-1 and MASP-2 was specifically titrated by C1 inhibitor, and abolished at a 1:1 C1 inhibitor:protease ratio. Taken together with previous findings, these and other data strongly support the hypothesis that MASP-2 is the protease that, in association with MBL, triggers complement activation via the MBL pathway, through combined self-activation and proteolytic properties devoted to C1r and C1s in the C1 complex. In view of the very low activity of MASP-1 on C3 and C2, our data raise questions about the implication of this protease in complement activation.     

Degradation of type I and II collagen by human activated C1-s

The activated first component of human complement, C1-s, was shown to cleave type I and II collagen and gelatin. The proteolytic activity was heat labile and was inhibited by a monoclonal antibody (M241) which recognized light chain of active human C1-s or by a serine protease inhibitor, DFP, but not by a chelating agent.     

Exosite-interactive regions in the A1 and A2 domains of factor VIII facilitate thrombin-catalyzed cleavage of heavy chain

Thrombin catalyzes the proteolytic activation of factor VIII, cleaving two sites in the heavy chain and one site in the light chain of the procofactor. Evaluation of thrombin binding the reaction products from heavy chain cleavage by steady state fluorescence energy transfer using a fluorophore-labeled, active site-modified thrombin as well as by solid phase binding assays using a thrombin Ser(205) --> Ala mutant indicated a high affinity site in the A1 subunit (K(d) approximately 5 nm) that was dependent upon the Na(+)-bound form of thrombin, whereas a moderate affinity site in the A2 subunit (K(d) approximately 100 nm) was observed for both Na(+)-bound and -free forms. The solid phase assay also indicated that hirudin blocked thrombin interaction with the A1 subunit and had little, if any, effect on its interaction with the A2 subunit. Conversely, heparin blocked thrombin interaction with the A2 subunit and showed a marginal effect on A1 binding. Evaluation of the A2 sequence revealed two regions rich in acidic residues that are localized close to the N and C termini of this domain. Peptides encompassing these clustered acidic regions, residues 373-395 and 719-740, blocked thrombin cleavage of the isolated heavy chain at Arg(372) and Arg(740) and inhibited A2 binding to thrombin Ser(205) --> Ala, suggesting that both A2 domain regions potentially support interaction with thrombin. A B-domainless, factor VIII double mutant Asp(392) --> Ala/Asp(394) --> Ala was constructed, expressed, and purified and possessed specific activity equivalent to a severe hemophilia phenotype. This mutant was resistant to cleavage at Arg(740), whereas cleavage at Arg(372) was not affected. These data suggest the acidic region comprising residues 389-394 in factor VIII A2 domain interacts with thrombin via its heparin-binding exosite and facilitates cleavage at Arg(740) during procofactor activation.     

Complement component C1r mediated cleavage of the heavy chain of the major histocompatibility class I antigens.

Apart from cleaving C1s, we demonstrate for the first time that: 1) at concentrations found in serum, the activated forms of the complement components C1r in addition to C1s can cleave the heavy chain of MHC class I antigens, 2) the cleavage by C1r and C1s is seemingly dependent upon a native configuration of the MHC class I antigen, since heat denaturation of the HLA antigens reduce the cleavage. The proteolytic fragments following C1 cleavage were characterized by precipitation with Con A-Sepharose, anti-MHC class I and anti-beta 2-microglobulin antibodies. The proteolysis of the alpha-chain of MHC class I was shown to take place between the alpha 2- and alpha 3- domains as estimated by the Con A-Sepharose precipitation pattern on SDS-PAGE. The alpha 1/alpha 2 fragment was still shown to interact with beta 2-microglobulin as shown by immunoprecipitation.     

Inhibition of acanthamoeba actomyosin-II ATPase activity and mechanochemical function by specific monoclonal antibodies

Monoclonal and polyclonal antibodies that bind to myosin-II were tested for their ability to inhibit myosin ATPase activity, actomyosin ATPase activity, and contraction of cytoplasmic extracts. Numerous antibodies specifically inhibit the actin activated Mg++-ATPase activity of myosin-II in a dose-dependent fashion, but none blocked the ATPase activity of myosin alone. Control antibodies that do not bind to myosin-II and several specific antibodies that do bind have no effect on the actomyosin-II ATPase activity. In most cases, the saturation of a single antigenic site on the myosin-II heavy chain is sufficient for maximal inhibition of function. Numerous monoclonal antibodies also block the contraction of gelled extracts of Acanthamoeba cytoplasm. No polyclonal antibodies tested inhibited ATPase activity or gel contraction. As expected, most antibodies that block actin-activated ATPase activity also block gel contraction. Exceptions were three antibodies M2.2, -15, and -17, that appear to uncouple the ATPase activity from gel contraction: they block gel contraction without influencing ATPase activity. The mechanisms of inhibition of myosin function depends on the location of the antibody-binding sites. Those inhibitory antibodies that bind to the myosin-II heads presumably block actin binding or essential conformational changes in the myosin heads. A subset of the antibodies that bind to the proximal end of the myosin-II tail inhibit actomyosin-II ATPase activity and gel contraction. Although this part of the molecule is presumably some distance from the ATP and actin-binding sites, these antibody effects suggest that structural domains in this region are directly involved with or coupled to catalysis and energy transduction. A subset of the antibodies that bind to the tip of the myosin-II tail appear to inhibit ATPase activity and contraction through their inhibition of filament formation. They provide strong evidence for a substantial enhancement of the ATPase activity of myosin molecules in filamentous form and suggest that the myosin filaments may be required for cell motility.     

Proteolysis of the heavy chain of major histocompatibility complex class I antigens by complement component C1s

The major histocompatibility complex (MHC) class I antigens contain a light chain β₂-microglobulin, non-covalently associated to the transmembrane heavy α-chain carrying the allotypic determinants. Since the C1q complement component is known to associate with β₂-microglobulin, and we recently found that activated C1s complement was capable of cleaving β₂-microglobulin, we decided to investigate the proteolytic activity of C1 complement towards the heavy chain of class I antigens. Our results demonstrate that human C1s complement cleaves the heavy chain of human class I antigens into at least two fragments, with apparent molecular weights of 22 000 and 24 000 g/ mol on sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE), under both reducing and non-reducing conditions. The cleavage of the heavy chain is inhibited by the presence of C1 esterase inhibitor. The molecular weights of the fragments are in agreement with the cleavage located in the area between the disulphide loops of the α₂-andα₃-domains of the heavy chain. In addition human C1s complement is able to cleave H-2 antigens from mouse in a similar fashion but not rat MHC class I antigen or mouse MHC class II antigen (I-A^d). Mouse MHC class I antigen-specific determinants could also be detected in supernatant from mouse spleen cells incubated with C1r and C1s. These results indicate the presence in the body fluids of a non-membrane-bound soluble form of the α₁andα₂-domains which represent the binding site for atnigenic peptide.     

Characterization of monoclonal antibodies to Acanthamoeba myosin-I that cross-react with both myosin-II and low molecular mass nuclear proteins

We characterized nine monoclonal antibodies that bind to the heavy chain of Acanthamoeba myosin-IA. Eight of these antibodies bind to myosin-IB and eight cross-react with Acanthamoeba myosin-II. All but one of the antibodies bind to a 30-kD chymotryptic peptide of myosin-IA that derives from the COOH terminus of the molecule, and to tryptic peptides as small as 17 kD, hence these epitopes are clustered closely together on the heavy chain. None of the antibodies prevent heavy chain phosphorylation by myosin-I heavy chain kinase. One antibody inhibits the K+-EDTA ATPase activity and three antibodies inhibit the actin- activated Mg++-ATPase activity of myosin-I under the set of conditions that we tested. When fluorescent antibody staining of both whole cells and isolated nuclei is done, several of these monoclonal antibodies react strongly with nuclei. These antibodies also stain the cytoplasmic matrix, especially the cortex near the plasma membrane. All nine of the monoclonal antibodies bind to polypeptides of 30-34 kD that are highly enriched in nuclei isolated from Acanthamoeba. There is no myosin-I in the isolated nuclei, so the 30-34-kD polypeptides, not myosin-I, are responsible for the nuclear staining.     

Monoclonal antibodies binding to the tail of Dictyostelium discoideum myosin: their effects on antiparallel and parallel assembly and actin- activated ATPase activity

Eight monoclonal antibodies that bind to specific sites on the tail of Dictyostelium discoideum myosin were tested for their effects on polymerization and ATPase activity. Two antibodies that bind close to the myosin heads inhibited actin activation of the ATPase either partially or completely, without having an effect on polymerization. Two other antibodies bind to sites within the distal portion of the tail that has been shown, by cleavage mapping, to be important for polymerization. One of these antibodies binds close to the sites of heavy chain phosphorylation which is known to regulate both myosin polymerization and actin-activated ATPase activity. Both antibodies showed strong inhibition of polymerization accompanied by complete inhibition of the actin-activated ATPase activity. A unique effect was obtained with an antibody that binds to the end of the myosin tail. This antibody prevented the formation of bipolar filaments. It caused myosin to assemble into unipolar filaments with heads at one end and the antibody molecules at the other. Only at concentrations higher than required for its effect on polymerization did this antibody show substantial inhibition of the actin-activated ATPase. These results indicate that, using a monoclonal antibody as a blocking agent, parallel assembly of myosin can be dissected out from antiparallel association, and that essentially normal actin-activated ATPase activity could be obtained after significant reductions in filament size.     

Inactivation, subunit dissociation, aggregation, and unfolding of myosin subfragment 1 during guanidine denaturation.

The effect of guanidine hydrochloride on ATPase activity, gel filtration, turbidity, exposure of thiol groups, far-UV circular dichroism, and the fluorescence emission intensity of myosin subfragment 1 (S-1) was studied under equilibrium conditions. It was found that the denaturation process involves several intermediate states. The enzymatic activity of S-1 is at first lost at very low concentrations of GdnHCl (lower than 0.5 M). At a slightly higher GdnHCl concentration (about 0.5 M), the light chains dissociate and this dissociation is closely followed by the formation of aggregates between the naked heavy chains of S-1 molecules in the guanidine hydrochloride range of concentrations 0.5-1 M. At GdnHCl concentrations above 1 M, aggregates gradually disappear and S-1 loses its secondary and tertiary structures. These phenomena are partly reversible, and ATPase activity is only partially recovered under highly limited conditions. These results are discussed in relation to the nature of myosin subunit assembly. The head fragment of 20 kDa is thus suggested to be implicated in the binding of light chain to heavy chain and in the self-association of free heavy chains.     

Variations in the enzymatic properties of human complement subcomponent C1s by treatment with human plasma kallikrein

We have investigated the effect of plasma kallikrein digestion upon hydrolytic activities of human C1s. Incubation of C1s (85 kDa) with plasma kallikrein led to progressive cleavages on the heavy chain to yield C1s-K1 (70 kDa) then C1s-K2 (53 kDa). Although these cleavages caused little change in the C2 hydrolytic and esterase activities of C1s, a marked loss in the C4 hydrolytic activity was observed. C1s-K1 and C1s-K2 were purified by DE-52 chromatography and it was found that the proteolysis of C1s into C1s-K1 was accompanied with a decrease in the C4 hydrolytic activity. Although the turnover numbers for the hydrolysis of C4 by C1s-K1 and C1s-K2 were almost the same as that of intact C1s, the Kms for C4 of C1s-K1 and C1s-K2 were found to be increased to 10 times that of intact C1s. This result suggests that the apparent decrease in the C4 hydrolytic activity upon plasma kallikrein digestion of C1s is not due to disruption in the active site but is due to decrease in the affinity between C4 and the C1s derivatives. In support of this assumption, C1s-K1 was found to be devoid of the ability to bind C4b-Sepharose. C1s is capable of forming a dimer through the C1s-binding domain in the N-terminal side of the heavy chain. Although C1s-K1 is still capable of forming a dimer, C1s-K2 fails to form a dimer, suggesting that the N-terminal C1s-binding site is released during cleavage of C1s-K1 into C1s-K2.(ABSTRACT TRUNCATED AT 250 WORDS)     

Molecular imprint of enzyme active site by camel nanobodies: rapid and efficient approach to produce abzymes with alliinase activity

Screening of inhibitory Ab1 antibodies is a critical step for producing catalytic antibodies in the anti-idiotypic approach. However, the incompatible surface of the active site of the enzyme and the antigen-binding site of heterotetrameric conventional antibodies become the limiting step. Because camelid-derived nanobodies possess the potential to preferentially bind to the active site of enzymes due to their small size and long CDR3, we have developed a novel approach to produce antibodies with alliinase activities by exploiting the molecular mimicry of camel nanobodies. By screening the camelid-derived variable region of the heavy chain cDNA phage display library with alliinase, we obtained an inhibitory nanobody VHHA4 that recognizes the active site. Further screening with VHHA4 from the same variable domain of the heavy chain of a heavy-chain antibody library led to a higher incidence of anti-idiotypic Ab2 abzymes with alliinase activities. One of the abzymes, VHHC10, showed the highest activity that can be inhibited by Ab1 VHHA4 and alliinase competitive inhibitor penicillamine and significantly suppressed the B16 tumor cell growth in the presence of alliin in vitro. The results highlight the feasibility of producing abzymes via anti-idiotypic nanobody approach.     

Cross-reactive idiotypic determinants on antibodies and antigen-specific helper T cell continuous lines

Anti-idiotypic antibodies produced in C57BL/6 mice (H-2b, Igh-1b) against (T,G)-A--L-specific antibodies of C3H.SW mice (H-2b, Igh-1j) were used to probe (T,G)-A--L-specific helper T cell lines and clones for the expression of idiotypic determinants on the cell surface of the monoclonal functional T cells. By using the fluorescence-activated cell sorter (FACS II), anti-idiotypic sera of individual mice that specifically bind C3H.SW anti-(T,G)-A--L antibodies were shown to stain significantly cells of the E-9M(+) continuous T helper line originated from C3H.SW (T,G)-A--L "educated" T cells. The same antisera did not react with a helper T cell line of C3H.SW origin specific to human gamma-globulin. They also did not stain a (T,G)-A--L-specific helper T cell line derived from CWB (H-2b, Igh-1b) mice, which differ from C3H.SW mice only in their heavy chain allotypes. Thus, the expression of the idiotypic determinants on the T cell lines appears to be antigen-specific and linked to the heavy chain allotypic marker as shown for the specific antibodies. Different clones derived from the E-9 M(+) line were tested their reactivity with the individual anti-idiotypic sera. All clones but one (1.11) were stained significantly. The clones were tested for their biologic activity and all of them except clone 1.11 were found to exert helper activity specific to (T,G)-A--L. Thus, individual anti-idiotypic sera against C3H.SW anti-(T,G)-A--L antibodies recognize cross-reactive idiotypic structures on the surface of antigen-specific monoclonal helper T cells.     

Activation of CXCR4 triggers ubiquitination and down-regulation of major histocompatibility complex class I (MHC-I) on epithelioid carcinoma HeLa cells

Many cancer cells display down-regulated major histocompatibility complex (MHC) class I antigen (MHC-I), which seems to enable them to evade immune surveillance, whereas the underlying mechanisms remain incompletely understood. Here, we demonstrate that ligand (CXCL12) stimulation of CXCR4, a major chemokine receptor expressed in many malignant cancer cells, induced MHC-I heavy chain down-regulation from the cell surface of the human epithelioid carcinoma HeLa cells, the human U251 and U87 glioblastoma cells, the human MDA-MD 231 breast cancer cells, and the human SK-N-BE (2) neuroblastoma cells. Activation of CXCR4 also induced MHC-I down-regulation in human peripheral blood mononuclear cells. The internalized MHC-I heavy chain molecules were partially co-localized with Rab7, a later endosomal marker. Activation of CXCR4 induced ubiquitination of MHC-I heavy chain, and mutation of the C-terminal two lysine residues (Lys-332, Lys-337) on one of the MHC-I alleles, HLA.B7, blocked CXCR4-evoked ubiquitination and down-regulation of HLA.B7. Moreover, purified GST-conjugated CXCR4 C terminus directly associated with the purified His-tagged beta2-microglobulin (beta2M), and MHC-I heavy chain was co-immunoprecipitated with CXCR4 in a beta2M-dependent manner. This interaction appears to be critical for CXCR4-evoked down-regulation of MHC-I heavy chain as evidenced by the data that MHC-I heavy chain down-regulation was inhibited by either truncation of the CXCR4 C terminus or knockdown of beta2M. All together, these findings shed new light on the role of CXCR4 in tumor evasion of immune surveillance via inducing MHC-I down-regulation from the cell surface.