Rat natural killer cell antigen, NKR-P1, related to C-type animal lectins is a carbohydrate-binding protein

Recombinant Expression, In Vitro Refolding and Characterizing Disulfide Bonds of a Mouse Inhibitory C-Type Lectin-Like Receptor Nkrp1b

As a part of the innate immunity, NK (Natural Killer) cells provide an early immune response to different stimuli, e.g. viral infections and tumor growths. However, their functions are more complex; they play an important role in reproduction, alloimmunity, autoimmunity and allergic diseases. NK cell activities require an intricate system of regulation that is ensured by many different receptors on a cell surface which integrate signals from interacting cells and soluble factors. One way to understand NK cell biology is through the structure of NK receptors, which can reveal ligand binding conditions. We present a modified protocol for recombinant expression in Escherichia coli and in vitro refolding of the ligand-binding domain of the inhibitory Nkrp1b (SJL/J) protein. Nkrp1b identity and folding was confirmed using mass spectrometry (accurate mass of the intact protein and evaluation of disulfide bonds) and one-dimensional nuclear magnetic resonance spectroscopy. The intention is to provide the basis for conducting structural studies of the inhibitory Nkrp1b protein, since only the activating Nkrp1a receptor structure is known.

Nkrp1 family, from lectins to protein interacting molecules

The C-type lectin-like receptors include the Nkrp1 protein family that regulates the activity of natural killer (NK) cells. Rat Nkrp1a was reported to bind monosaccharide moieties in a Ca²⁺-dependent manner in preference order of GalNac > GlcNAc >> Fuc >> Gal > Man. These findings established for rat Nkrp1a have been extrapolated to all additional Nkrp1 receptors and have been supported by numerous studies over the past two decades. However, since 1996 there has been controversy and another article showed lack of interactions with saccharides in 1999. Nevertheless, several high affinity saccharide ligands were synthesized in order to utilize their potential in antitumor therapy. Subsequently, protein ligands were introduced as specific binders for Nkrp1 proteins and three dimensional models of receptor/protein ligand interaction were derived from crystallographic data. Finally, for at least some members of the NK cell C-type lectin-like proteins, the “sweet story” was impaired by two reports in recent years. It has been shown that the rat Nkrp1a and CD69 do not bind saccharide ligands such as GlcNAc, GalNAc, chitotetraose and saccharide derivatives (GlcNAc-PAMAM) do not directly and specifically influence cytotoxic activity of NK cells as it was previously described.

Complexity and Diversity of the NKR-P1:Clr (Klrb1:Clec2) Recognition Systems

The NKR-P1 receptors were identified as prototypical natural killer (NK) cell surface antigens and later shown to be conserved from rodents to humans on NK cells and subsets of T cells. C-type lectin-like in nature, they were originally shown to be capable of activating NK cell function and to recognize ligands on tumor cells. However, certain family members have subsequently been shown to be capable of inhibiting NK cell activity, and to recognize proteins encoded by a family of genetically linked C-type lectin-related ligands. Some of these ligands are expressed by normal, healthy cells, and modulated during transformation, infection, and cellular stress, while other ligands are upregulated during the immune response and during pathological circumstances. Here, we discuss historical and recent developments in NKR-P1 biology that demonstrate this NK receptor–ligand system to be far more complex and diverse than originally anticipated.

Re-evaluation of binding properties of recombinant lymphocyte receptors NKR-P1A and CD69 to chemically synthesized glycans and peptides

The binding of monosaccharides and short peptides to lymphocyte receptors (human CD69 and rat NKR-P1A) was first reported in 1994 and then in a number of subsequent publications. Based on this observation, numerous potentially high-affinity saccharide ligands have been synthesized over the last two decades in order to utilize their potential in antitumor therapy. Due to significant inconsistencies in their reported binding properties, we decided to re-examine the interaction between multiple ligands and CD69 or NKR-P1A. Using NMR titration and isothermal titration calorimetry we were unable to detect the binding of the tested ligands such as N-acetyl-d-hexosamines and oligopeptides to both receptors, which contradicts the previous observations published in more than twenty papers over the last fifteen years.

Re-evaluation of the involvement of NK cells and C-type lectin-like NK receptors in modulation of immune responses by multivalent GlcNAc-terminated oligosaccharides

Recognition of glycosylation patterns is one of the basic features of innate immunity. Ability of C-type lectin-like receptors such as NKR-P1 to bind saccharide moieties has become recently a controversial issue. In the present study, binding assay with soluble fluorescently labeled recombinant rat NKR-P1A and mouse NKR-P1C proteins revealed apparently no affinity to the various neoglycoproteins. Lack of functional linkage between NKR-P1 and previously described saccharide binder was supported by the fact, that synthetic N-acetyl-D-glucosamine octabranched dendrimer on polyamidoamine scaffold (GN8P) did not change gene expression of NKR-P1 isoforms in C57BL/6 and BALB/c mice divergent in the NK gene complex (both in vitro and in vivo). Surprisingly, N-acetyl-D-glucosamine-coated tetrabranched polyamido-amine dendrimer specifically binds to NKT cells and macrophages but not to NK cells (consistently with changes in cytokine patterns). Despite the fact that GN8P has been tested as an immunomodulator in anti-cancer treatment animal models for many years, surprisingly no changes in cytokine profiles in serum relevant to anti-cancer responses using B16F10 and CT26 harboring mouse strains C57BL/6 and BALB/c are observed. Our results indicate possible indirect involvement of NK cells in GN8P mediated immune responses.

Synthetic N-acetyl-D-glucosamine based fully branched tetrasaccharide, a mimetic of the endogenous ligand for CD69, activates CD69+ killer lymphocytes upon dimerization via a hydrophilic flexible linker

On the basis of the highly branched ovomucoid-type undecasaccharide that had been shown previously to be an endogenous ligand for CD69 leukocyte receptor, a systematic investigation of smaller oligosaccharide mimetics was performed based on linear and branched N-acetyl-d-hexosamine homooligomers prepared synthetically using hitherto unexplored reaction schemes. The systematic structure-activity studies revealed the tetrasaccharide GlcNAcbeta1-3(GlcNAcbeta1-4)(GlcNAcbeta1-6)GlcNAc (compound 52) and its alpha-benzyl derivative 49 as the best ligand for CD69 with IC(50) as high as 10(-9) M. This compound thus approaches the affinity of the classical high-affinity neoglycoprotein ligand GlcNAc(23)BSA. Compound 68, GlcNAc tetrasaccharide 52 dimerized through a hydrophilic flexible linker, turned out to be effective in activating CD69(+) lymphocytes. It also proved efficient in enhancing natural killing in vitro, decreasing the growth of tumors in vivo, and activating the CD69(+) tumor infiltrating lymphocytes examined ex vivo. This compound is thus a candidate for carbohydrate-based immunomodulators with promising antitumor potential.

Identification and characterization of multiple beta-glucan binding proteins in the Pacific oyster, Crassostrea gigas

The present study reports on the characterization of two cDNAs coding beta-glucan binding proteins (betaGBPs), designated as Cg-betaGBP-1 and Cg-betaGBP-2, from the Pacific oyster, Crassostrea gigas. Cg-betaGBP-1 consists of 555 amino acid residues and possesses two possible integrin recognition sites. The other protein, Cg-betaGBP-2, is composed of 447 amino acid residues without integrin recognition sites. Domain structures of both Cg-betaGBPs are similar to other invertebrate betaGBPs, but phylogenetic positions and major expression tissues for these proteins are different. Cg-betaGBP-1 is expressed in circulatory hemocytes and Cg-betaGBP-2 in digestive glands. Functional assays using recombinant proteins revealed that Cg-betaGBP-2 enhanced the phenoloxidase (PO) activity of hemocyte suspensions under the presence of laminarin, but Cg-betaGBP-1 did not show this enhancement. It is suggested that Cg-betaGBPs in the Pacific oyster have evolved to obtain different immunological functions. Cg-betaGBP-1 possibly evolved for hemocyte-related functions through integrin, and Cg-betaGBP-2 for the PO activation system.

RETRACTED: N-Acetyl-d-glucosamine substituted calix[4]arenes as stimulators of NK cell-mediated antitumor immune response

A series of calixarenes substituted with 2-acetamido-2-deoxy-beta-D-glucopyranose linked by a thiourea spacer was prepared and tested for binding activity to heterogeneously expressed activation receptors of the rat natural killer cells NKR-P1, and the receptor CD69 (human NK cells, macrophages). In the case of NKR-P1, the binding affinity of beta-D-GlcNAc-substituted calixarenes carrying two or four sugar units was in a good agreement with the inhibitory potencies of the linear chitooligomers (chitobiose to chitotetraose) reported previously. The influence of GlcNAc substitution of the calixarene skeleton on binding affinity for CD69 receptor was more profound and the 5,11,17,23-tetrakis[N-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-thioureido]-25,26,27,28-tetrapropoxycalix[4]arene (cone) (1) proved to be the best CD69 ligand identified to date. Lower GlcNAc substitution led to dramatic decrease of the binding activity (by about 1.5 order of magnitude per one GlcNAc unit). The immunostimulating activity results with the newly synthesized GlcNAc tetramers on calixarene scaffolds exhibited stimulation of natural cytotoxicity of human PBMC in concentrations 10(-4) and 10(-8)M. These calix-sugar compounds were superior to the previously tested PAMAM-GlcNAc(8)5.

Comblike dendrimers containing Tn antigen modulate natural killing and induce the production of Tn specific antibodies

Comblike glycodendrimers were prepared by the chemoselective ligation of cysteine-modified glycopeptides (1-7) with a 3-maleimidopropionate-modified linear synthetic carrier (8). Glycodendrimers bearing mono-, di-, or tri-Tn clusters (9-11) were tested as inhibitors using plant and mammalian lectins. In the former group, the Codium fragile lectin showed moderate discrimination among 9, 10, and 11. In the latter group, A and B isoforms of rat NKR-P1 lectin strongly discriminated between 9 and 10. 10 caused a 4-fold increase in killing of the NK resistant tumor cell lines at concentrations as low as 10(-8) M. Surprisingly, 11 interacted exclusively with the rat NKR-P1B isoform and inhibited efficiently natural killing in both rats and humans, even in the presence of the activating compounds 9 and 10. Dinitrophenol haptenization or influenza virus hemagglutinin T-cell epitope conjugation increased the immunogenicity of the parent compounds and resulted in the production of Tn specific antibodies.

Recognition of a carbohydrate xenoepitope by human NKRP1A (CD161)

BACKGROUND

Many immunologically important interactions are mediated by leukocyte recognition of carbohydrates via cell surface receptors. Uncharacterized receptors on human natural killer (NK) cells interact with ligands containing the terminal Galalpha(1,3)Gal xenoepitope. The aim of this work was to isolate and characterize carbohydrate binding proteins from NK cells that bind alphaGal or other potential xenoepitopes, such as N-acetyllactosamine (NAcLac), created by the deletion of alpha1,3galactosyltransferase (GT) in animals.

METHODS AND RESULTS

Initial analysis suggested the human C-type lectin NKRP1A bound to a pool of glycoconjugates, the majority of which contained the terminal Galalpha(1,3)Gal epitope. This was confirmed by high level binding of cells expressing NKRP1A to mouse laminin, which contains a large number of N-linked oligosaccharides with the Galalpha(1,3)Gal structure. The consequence of removing the terminal alphaGal was then investigated. Elevated NAcLac levels were observed on thymocytes from GT-/- mice. Exposing NAcLac on laminin, by alpha-galactosidase treatment, resulted in a significant increase in NKRP1A binding.

CONCLUSIONS

NKRPIA binds to the alphaGal epitope. Moreover, exposing NAcLac by removal of alphaGal resulted in an increase in binding. This may be relevant in the later phases of xenotransplant rejection if GT-/- pigs, like GT-/- mice, display increased NAcLac expression.

Lymphocyte activation receptors: new structural paradigms in group V of C-type animal lectins

The structure–function relationship in group V of C-type animal lectins remains incompletely understood despite the new structures of NK (natural killer) cell receptors that have been solved recently. Recombinant, soluble forms of rat and human NKR-P1 and CD69 that we obtained after in vitro refolding were analysed by Fourier transform–ion cyclotron resonance MS and heteronuclear NMR (1H-15N correlation). In NKR-P1, calcium may not be removed by chelating agents because of the very high affinity of binding. In CD69, incorporation of calcium causes a structural shift in several amino acids important for the interaction with carbohydrates. Structural studies have also allowed us to understand an interesting preference of these receptors for either linear (NKR-P1) or branched (CD69) carbohydrate sequences.

Delta-Short Consensus Repeat 4-Decay Accelerating Factor (DAF: CD55) Inhibits Complement-Mediated Cytolysis but Not NK Cell-Mediated Cytolysis

NK cells play a critical role in the rejection of xenografts. In this study, we report on an investigation of the effect of complement regulatory protein, a decay accelerating factor (DAF: CD55), in particular, on NK cell-mediated cytolysis. Amelioration of human NK cell-mediated pig endothelial cell (PEC) and pig fibroblast cell lyses by various deletion mutants and point substitutions of DAF was tested, and compared with their complement regulatory function. Although wild-type DAF and the delta-short consensus repeat (SCR) 1-DAF showed clear inhibition of both complement-mediated and NK-mediated PEC lyses, delta-SCR2-DAF and delta-SCR3-DAF failed to suppress either process. However, delta-SCR4-DAF showed a clear complement regulatory effect, but had no effect on NK cells. Conversely, the point substitution of DAF (L147 x F148 to SS and KKK(125-127) to TTT) was half down-regulated in complement inhibitory function, but the inhibition of NK-mediated PEC lysis remained unchanged. Other complement regulatory proteins, such as the cell membrane-bound form factor H, fH-PI, and C1-inactivator, C1-INH-PI, and CD59 were also assessed, but no suppressive effect on NK cell-mediated PEC lysis was found. These data suggest, for DAF to function on NK cells, SCR2-4 is required but no relation to its complement regulatory function exists.

Enzymatic synthesis of complex glycosaminotrioses and study of their molecular recognition by hevein domains

Hevein, a protein found in Hevea brasiliensis, has a CRD domain, which is known to bind chitin and GlcNAc-containing oligosaccharides. By using NMR and molecular modeling as major tools we have demonstrated that trisaccharides containing GalNAc and ManNAc residues are also recognized by hevein domains. Thus far unknown trisaccharides GlcNAcbeta(1-->4)GlcNAcbeta(1-->4)ManNAc (1) and GalNAcbeta(1-->4)GlcNAcbeta(1-->4)ManNAc (2) were synthesized with the use of beta-N-acetylhexosaminidase from Aspergillus oryzae. This method is based on the rather unique phenomenon that some fungal beta-N-acetylhexosaminidases cannot hydrolyze disaccharide GlcNAcbeta(1-->4)ManNAc (5) contrary to chitobiose GlcNAcbeta(1-->4)GlcNAc (4) that is cleaved and, therefore, cannot be used as an acceptor for further transglycosylation. Both trisaccharides 1 and 2 were prepared by transglycosylation from disaccharidic acceptor in good yields ranging from 35% to 40%. Our observations strongly indicate that the present nature of the modifications of chitotriose (GlcNAcbeta(1-->lcNAcbeta(1-->4)GlcNAc, 3) at either the non-reducing end (GalNAc instead of GlcNAc) or at the reducing end (ManNAc instead of GlcNAc) do not modify the mode of binding of the trisaccharide to hevein. The association constant values indicate that chitotriose (3) binding is better than that of 1 and 2, and that the binding of (with ManNAc at the reducing end) is favored with respect to that of 2 (with ManNAc at the reducing end with a non-reducing GalNAc moiety).

Missing self-recognition of Ocil/Clr-b by inhibitory NKR-P1 natural killer cell receptors

The NKR-P1 family of C-type lectin-like receptors are expressed on natural killer (NK) cells and NKT cells. We report the cloning and characterization of a cognate ligand for the inhibitory mouse NK receptors (NKR)-P1B and NKR-P1D (CD161b/d). The NKR-P1B/D ligand is osteoclast inhibitory lectin (Ocil), also known as Clr-b, a member of a previously cloned group of C-type lectin-related (Clr) proteins linked to the NKR-P1 receptors in the mouse NK gene complex (NKC). Expression of Ocil/Clr-b on mouse tumor cell lines inhibits NK cell-mediated killing. Inhibition is blocked with a new mAb (4A6) specific for Ocil/Clr-b. By using 4A6 mAb, we demonstrate that Ocil/Clr-b is displayed at high levels on nearly all hematopoietic cells, with the exception of erythrocytes, in a pattern that is similar to that of class I MHC molecules. Remarkably, Ocil/Clr-b is frequently down-regulated on mouse tumor cell lines, indicating a role for this receptor-ligand system in a new form of "missing self-recognition" of tumor cells.

Carbohydrate and Non-Carbohydrate Ligands for the C-Type Lectin-Like Receptors of Natural Killer Cells. A Review

The superfamily of C-type animal lectins is defined by a sequence motif of the carbohydrate- recognition domains (CRDs) and comprises seven groups of molecules. The soluble proteins are group I proteoglycans, group III collectins, and group VII containing the isolated CRDs. Type I membrane proteins include group IV selectins and group VI macrophage receptors and related molecules. Type II membrane proteins are group II hepatic lectins and group V natural killer cell receptors. The latter group has recently attracted considerable attention of the biomedical community. These receptors are arranged at the surface of lymphocytes as homo- or heterodimers composed of two polypeptides consisting of N-terminal peptide tails responsible for signaling, transmembrane domain, neck regions of varying length, and C-terminal lectin-like domains (CTLDs). Since this group is evolutionarily most distant from the rest of C-type animal lectins, the sequence of the C-terminal ligand-binding domain has diversified to accommodate other ligands than calcium or carbohydrates. These domains are referred to as natural killer domains (NKDs) forming a large percentage of CTLDs in vertebrates. Here are summarized the data indicating that calcium, carbohydrates, peptides, and large proteins such as major histocompatibility complex (MHC) class I can all be ligands for NKDs. The wide range of ligands that can be recognized by NKDs includes some new, unexpected compounds such as signal peptide-derived fragments, heat shock proteins, or oxidized lipids. The biological importance of this extended range of recognition abilities is also discussed. A review with 134 references.

Synthesis of Linear and Branched Regioisomeric Chitooligosaccharides as Potential Mimetics of Natural Oligosaccharide Ligands of Natural Killer Cells NKR-P1 and CD69 Lectin Receptors

Regioisomer of chitobiose13with β(1→3) glycosidic bond and branched analog of chitotriose25having β(1→4) and β(1→3) glycosidic bonds, were prepared and tested as potential mimetics of natural oligosaccharide ligands for activating lectin receptors NKR-P1A and CD69 of natural killer (NK) cells. The structural requirements of NKR-P1 lectin receptor on effective mimetics of its natural ligands has been discussed. A significant binding activity of the branched trisaccharide25to the receptor CD69 was observed.

The Isoforms of Rat Natural Killer Cell Receptor NKR-P1 Display a Distinct Binding of Complex Saccharide Ligands - RETRACTED

We wish to retract this article due to the confirmed scientific misconduct of Karel Bezouška. Based on the results of investigation of a joint ethical committee established by the Institute of Microbiology, Academy of Sciences of the Czech Republic and the Faculty of Science, Charles University in Prague, we can conclude that experiments describing binding of natural killer cell receptors to carbohydrate ligands were manipulated by Karel Bezouška. Therefore all authors, except L.M. who cannot be traced, agreed on the retraction of this paper. We apologize to all affected parties.

Molecular characterization of binding of calcium and carbohydrates by an early activation antigen of lymphocytes CD69

CD69 is the earliest leukocyte activation antigen playing a pivotal role in cellular signaling. Here, we show that a globular C-terminal domain of CD69 belonging to C-type lectins binds calcium through Asp 171, Glu 185, and Glu 187 with K(d) approximately 54 microM. Closure of the calcium-binding site results in a conformational shift of Thr 107 and Lys 172. Interestingly, structural changes in all of these amino acids lead to the formation of high-affinity binding sites for N-acetyl-D-glucosamine. Similarly, a structural change in Glu 185 and Glu 187 contributes to a high-affinity site for N-acetyl-D-galactosamine. Site-directed mutagenesis and molecular modeling allowed us to describe the structural details of binding sites for both carbohydrates. These studies explain the importance of calcium for recognition of carbohydrates by CD69 and provide an important paradigm for the role of weak interactions in the immune system.

Co-effect of HLA-G1 and glycosyltransferases in reducing NK cell-mediated pig endothelial cell lysis

Natural killer (NK) cells play an important role in xenograft rejection. The aim of this study was to evaluate the co-effect of human leukocyte antigen (HLA)-G1 expression and the remodeling of glycoantigens such as the alpha-Gal epitope, Galalpha1,3Galbeta1,4GlcNAc-R, by the introduction of glycosyltransferase genes related to NK cell-mediated direct cytotoxicity. Human peripheral blood mononuclear cells or an NK-like cell line, YT cells, was used as an effector and pig endothelial cells (PEC) as the target. A PEC transfectant with HLA-G1 was first prepared by the transfection of HLA-G1 and human beta2 microglobulin. Several new transfectants were then established by the transfection of glycosyltransferase to the HLA-G1 transfectant. The effect of HLA-G1 on NK cell-mediated PEC lysis was lower than that by the glycosyltransferases. Therefore, in the case of the co-transfectants except for HLA-G1+alpha2,6sialyltransferase, such as HLA-G1+N-acetylglucosaminyltransferase-III and HLA-G1+alpha1,2fucosyltransferase, the effect of HLA-G1 expression on NK-mediated killing appeared to be accounted for by the transfected glycosyltransferase activities and the reduced alpha-Gal expression on the cell surface. However, these transfectants showed significant reductions in direct NK cell-mediated cytotoxicity, compared with the single HLA-G1 transfectant. The results herein suggest that a combination of HLA-G1 and glycosyltransferases has considerable potential for the downregulation of NK cell-mediated cytolysis.

Design, functional evaluation and biomedical applications of carbohydrate dendrimers (glycodendrimers)

Receptors for carbohydrates of the lectin type are multisubunit and multivalent proteins with many important biological functions. In order to put their unique biological activities into use in biotechnology and biomedicine, efficient carbohydrate ligands of the glycodendrimer type have been constructed. Although these compounds may be branched into the multiple generations, structures bearing four to 16 terminal carbohydrate substituents have proved to be efficient ligands in most lectin systems. These compounds are rapidly finding important practical applications as antitumor and antiinfective compounds.

Toward an optimal oligosaccharide ligand for rat natural killer cell activation receptor NKR-P1

Aminosugars have a good affinity for the NKR-P1A protein, the major activating receptor at the surface of rat natural killer cells. We have systematically investigated the structural requirements of the recombinant soluble dimeric form of the receptor for its optimal carbohydrate ligands. While N-acetylD-mannosamine was the best neutral monosaccharide ligand, its participation in the context of an extended oligosaccharide sequence was equally important. The IC(50) value for the GalNAcbeta1 --> ManNAc disaccharide was nearly 10(-10) M with a further possible increase depending on the type of the glycosidic linkage and the aglycon nature. From the point of view of its availability, stability, and affinity for the receptor and a potential in vivo use, these studies are pivotal for the design of an oligosaccharide or glycomimetics suitable for further clustering into the multivalent glycodendrimers.

Differentially induced expression of C-type lectins in activated lymphocytes

The human NK gene complex encodes for the leucocyte C-type lectins, CD69, AICL (activation-induced C-type lectin), LLT1 (lectin-like transcript), CD161/NKR-P1A, CD94, and for NKG-2 molecules. These gene products have been implicated in the regulation of the function of natural killer (NK) cells and other lymphocytes. In this study the expression of C-type lectins during the early activation of PMA-stimulated peripheral blood lymphocytes was examined. To investigate the influence of de novo protein synthesis on activation-dependent expression of C-type lectins, cells were cultured in presence of cycloheximide (CHX) and mRNA levels were analyzed by semi-quantitative reverse transcription-polymerase chain reaction. Upregulated levels of CD69, AICL, and LLT1, but less pronounced changes of CD161/NKR-P1A and CD94 mRNA were found at early time points of cellular activation. CD69 was superinduced by CHX at the nuclear precursor transcript and the mRNA level suggesting that regulation of transcriptional activity and mRNA stability contribute to extent of CD69 mRNA accumulation. CHX treatment resulted also in an overexpression of AICL, LLT1, and CD161/NKR-P1A mRNAs. Conversely, CHX blocked CD94 mRNA expression in PMA-stimulated cells, demonstrating that this process is dependent on new protein synthesis. Expression kinetics in context with susceptibility to CHX indicate that the mechanisms responsible for upregulated CD69, AICL, and LLT1 expression are distinct from those which control CD161/NKR-P1A or CD94 expression. J. Cell. Biochem. Suppl. 36: 201-208, 2001.

Target cell susceptibility to lysis by human natural killer cells is augmented by alpha(1,3)-galactosyltransferase and reduced by alpha(1, 2)-fucosyltransferase

Susceptibility of porcine endothelial cells to human natural killer (NK) cell lysis was found to reflect surface expression of ligands containing Gal alpha(1,3)Gal beta(1,4)GlcNAc [corrected], the principal antigen on porcine endothelium recognized by xenoreactive human antibodies. Genetically modifying expression of this epitope on porcine endothelium by transfection with the alpha(1,2)-fucosyltransferase gene reduced susceptibility to human NK lysis. These results indicate that surface carbohydrate remodeling profoundly affects target cell susceptibility to NK lysis, and suggest that successful transgenic strategies to limit xenograft rejection by NK cells and xenoreactive antibodies will need to incorporate carbohydrate remodeling.

Coordinated binding of sugar, calcium, and antibody to macrophage C-type lectin

Mouse macrophage galactose/N-acetylgalactosamine-specific C-type lectin (MMGL) is a type II transmembrane glycoprotein belonging to the C-type lectin family. Our development of monoclonal antibodies led us to discover that a calcium-dependent conformational change is detected by an antibody (termed mAb LOM-11) and that the antibody's binding to the respective site locks the lectin in an active conformation. These findings correspond to the divalent cation-mediated regulatory mechanisms in a family of cell adhesion molecule integrins that have gained much attention. We now provide direct evidence that mAb LOM-11 increases the affinity of the lectin for calcium ions as a mechanism for the conformational lock using a soluble recombinant form of MMGL (rML) produced in bacteria. Furthermore, we discovered by using an enzyme-linked immunosorbent assay that specific monosaccharides induced a binding site for mAb LOM-11 on the immobilized rML under low calcium environments. We also demonstrated that cell surface MMGL on a transfectant cell line underwent a conformational change upon addition of calcium or ligands, as detected by the binding of mAb LOM-11. These properties are reminiscent of ligand-induced binding sites defined for integrins. The present results suggest a possibility that the mAb LOM-11 binding site on the lectin may be a site at which protein-protein interaction helps to fine tune the specificity of the C-type lectins by means of coordinated recognition mechanisms.

The C-type lectin superfamily in the immune system

Protein-carbohydrate interactions serve multiple functions in the immune system. Many animal lectins (sugar-binding proteins) mediate both pathogen recognition and cell-cell interactions using structurally related Ca(2+)-dependent carbohydrate-recognition domains (C-type CRDs). Pathogen recognition by soluble collections such as serum mannose-binding protein and pulmonary surfactant proteins, and also the macrophage cell-surface mannose receptor, is effected by binding of terminal monosaccharide residues characteristic of bacterial and fungal cell surfaces. The broad selectivity of the monosaccharide-binding site and the geometrical arrangement of multiple CRDs in the intact lectins explains the ability of the proteins to mediate discrimination between self and non-self. In contrast, the much narrower binding specificity of selectin cell adhesion molecules results from an extended binding site within a single CRD. Other proteins, particularly receptors on the surface of natural killer cells, contain C-type lectin-like domains (CTLDs) that are evolutionarily divergent from the C-type lectins and which would be predicted to function through different mechanisms.

NK cell receptors

NK cells are regulated by opposing signals from receptors that activate and inhibit effector function. While positive stimulation may be initiated by an array of costimulatory receptors, specificity is provided by inhibitory signals transduced by receptors for MHC class I. Three distinct receptor families, Ly49, CD94/NKG2, and KIR, are involved in NK cell recognition of polymorphic MHC class I molecules. A common pathway of inhibitory signaling is provided by ITIM sequences in the cytoplasmic domains of these otherwise structurally diverse receptors. Upon ligand binding and activation, the inhibitory NK cell receptors become tyrosine phosphorylated and recruit tyrosine phosphatases, SHP-1 and possibly SHP-2, resulting in inhibition of NK cell-mediated cytotoxicity and cytokine expression. Recent studies suggest these inhibitory NK cell receptors are members of a larger superfamily containing ITIM sequences, the inhibitory receptor superfamily (IRS).

GlcNAc-terminated glycodendrimers form defined precipitates with the soluble dimeric receptor of rat natural killer cells, sNKR-P1A

Synthetic GlcNAc-terminated thiourea-bridged glycoclusters were found to be potent inhibitors of binding of the soluble dimeric receptor of rat natural killer cells, sNKR-P1A protein, to its high affinity ligand. Moreover, we have shown here that characteristic precipitation curves can be recorded upon mixing of the GlcNAc glycoclusters with sNKR-P1A. For the GlcNAc8 glycocluster the precipitation curve is biphasic, with high affinity and low affinity precipitates differing in their sensitivity towards GlcNAc-mediated inhibition of precipitation. Quantitative analyses of the precipitates indicate the occurrence of a single sugar binding site per sNKR-P1A subunit, and lead to a model of the most possible spatial arrangements of the glycocluster-receptor lattices. These results provide new tools for further studies on carbohydrate recognition by NKR-P1A.

Phenotype of rat monocytes during acute kidney allograft rejection: increased expression of NKR-P1 and reduction of CD43

Monocytes and macrophages mediate cytotoxicity after appropriate activation and thus represent effectors secondary to T lymphocytes and natural killer (NK) cells. However, very little is known about the role of activated monocytes in organ allograft rejection. In this study, isogeneic (LEW to LEW) and fully allogeneic (DA to LEW) rat kidneys were grafted to bilaterally nephrectomized recipients. Four days after transplantation a comprehensive sample of leucocytes was recovered by perfusion of the recipients' vasculature with phosphate-buffered saline containing EDTA (PBS/EDTA). Monocytes were enriched by density gradient centrifugation and their physical parameters and immunophenotype were investigated by flow cytometry in comparison with untreated, specified pathogen-free (SPF) LEW rats. Isotransplantation and allotransplantation of kidneys dramatically increased the absolute number of intravascular monocytes in the recipient. Analysis of NKR-P1 (CD161), CD4, CD62L, CD43, CD11a, CD18 and MHC class II expression identified at least two monocyte populations in all experimental groups. In graft recipients it was evident that activated monocytes were able to express and upregulate NKR-P1 and CD8, which have not been detected in these cells to date. If activated monocytes utilize NKR-P1 and CD8, by analogy to NK cells and lymphocytes these cells may be endowed with additional pathways to upregulate cytolytic functions and effect allograft damage.

beta-Glucosylation of chitooligomers by galactosyltransferase

Galactosyltransferase from bovine milk was found to be able to utilise UDP-Glc to transfer Glc onto GlcNAc and chitooligomers[-beta-GlcNAc-(1-->4)-]n, n = 2-4. beta-Glucosylated products were used in binding studies with NKR-P1A protein cloned from rat natural killer cells.

The mast cell function-associated antigen exhibits saccharide binding capacity

The mast cell function-associated antigen (MAFA) is a membrane glycoprotein first identified on rat mucosal type mast cells (line RBL-2H3) and known to inhibit the Fc epsilon RI-mediated secretory response. In its extracellular domain, an amino acid stretch homologous to the carbohydrate binding domain of calcium-dependent animal lectins has been found. To investigate its carbohydrate binding capacity, the MAFA has been expressed in the Spodoptera frugiperda insect cell line (Sf9) using the baculovirus expression system. Analysis by flow cytometry and surface labeling with 125I showed that the recombinant MAFA (rMAFA) was expressed as a monomeric and disulfide-linked homodimeric glycoprotein in the membrane of the insect cells, and both forms exhibited the same epitopes as the protein isolated from RBL-2H3 cells. Immunoaffinity-purified rMAFA was then employed for studies of its saccharide binding capacity by using different neoglycans and glycoproteins. The rMAFA was found to bind specifically terminal mannose residues in a Ca(2+)-dependent manner. These results support the notion that the extracellular domain of the MAFA is indeed able to bind ligands, which may be modulatory for the mast cell response.

Natural killer cell-target cell interactions

Research into the molecular mechanisms of target cell recognition by natural killer (NK) cells has recently progressed rapidly. NK cells express several MHC I recognizing receptors that inactivate the NK cells' functions. In pathological alterations of MHC I expression, the NK cell inhibitory receptors do not engage and thus permit the lysis of the target cell. The receptors that trigger the cytolytic machinery of NK cells, after the permission of lysis from the inhibitory receptors, are poorly characterized. Some candidate triggering receptors have been identified and it seems that triggering of NK cell killing is mediated by multiple receptors, as is the inhibition of cytotoxicity.

Rat spleen dendritic cells express natural killer cell receptor protein 1 (NKR-P1) and have cytotoxic activity to select targets via a Ca2+-dependent mechanism

Dendritic cells (DC) are a subset of leukocytes whose major function is antigen presentation. We investigated the phenotype and function of enriched (95-98.5%) rat DC. We show that both spleen and thymus DC express the natural killer cell receptor protein 1 (NKR-P1) as a disulfide linked homodimer of 60 kD. Freshly isolated DC express a low level of NKR-P1, which is strongly upregulated after overnight culture. Spleen, but not thymus DC, were able to kill the NK-sensitive YAC-1 cell line in vitro, and since this killing was Ca2+ dependent, a Fas ligand-Fas interaction was probably not involved. Besides their potent antigen-presenting function, DC can thus be cytotoxic for some tumor targets.

Animal lectins

Protein and lipid glycosylation is no longer considered as a topic whose appeal is restricted to a limited number of analytical experts perseveringly pursuing the comprehensive cataloguing of structural variants. It is in fact arousing curiosity in various areas of basic and applied bioscience. Well founded by the conspicuous coding potential of the sugar part of cellular glycoconjugates which surpasses the storage capacity of oligonucleotide- or oligopeptide-based code systems, recognition of distinct oligosaccharide ligands by endogenous receptors, i.e. lectins and sugar-binding enzymes or antibodies, is increasingly being discovered to play salient roles in animal physiology. Having inevitably started with a descriptive stage, research on animal lectins has now undubitably reached maturity. Besides listing the current categories for lectin classification and providing presentations of the individual families and their presently delineated physiological significance, this review places special emphasis on tracing common structural and functional themes which appear to reverberate in nominally separated lectin and animal categories as well as lines of research which may come to fruition for medical sciences.

The natural killer gene complex: a genetic basis for understanding natural killer cell function and innate immunity

The natural killer gene complex encodes proteins, some of which are structurally unrelated, that impact on NK-cell function. Detailed analyses have indicated that these molecules are involved in NK-cell recognition, activation, and inhibition. The importance of this genomic region is highlighted by studies indicating that NKC-associated genes significantly influence NK cell-mediated innate host defense against life-threatening pathogens and that the NKC is conserved among diverse species. Thus, further elucidation of the NKC and its gene products will provide a genetic basis for understanding innate immunity and NK-cell activity at the molecular level.

Transduction of cytotoxic signals in natural killer cells: a general model of fine tuning between activatory and inhibitory pathways in lymphocytes

NK-cells are large granular lymphocytes, which are capable of exerting two major types of effector function, cell cytotoxicity and lymphokine secretion. NK-cells can exert cell cytotoxicity in one of two ways. First, NK-cells are able to recognize and to induce the lysis of antibody-coated target cells during antibody-dependent cell cytotoxicity (ADCC). Second, during natural cytotoxicity NK-cells are also able to recognize and to induce the lysis of a variety of target cells, including primarily virus-infected cells as well as tumor cells. Recently, a novel mechanism has been elucidated which controls NK-cell-activation programs and which is based on the cell surface expression of killer-cell inhibitory receptors (KIR). We will review here the molecular dissection of this inhibitory signalling pathway which utilizes immunoreceptor tyrosine-based inhibition motifs (ITIM) expressed in KIR intracytoplasmic domain. We will also show that this strategy used by NK-cells to regulate their effector functions is a general decision mechanism which exists not only in T- and B-lymphocytes, but also in a variety of other hematopoietic cells.