The A' and F' pockets of human CD1b are both required for optimal presentation of lipid antigens to T cells

Two canine CD1a proteins are differentially expressed in skin

Lipid antigens are presented to T cells by the CD1 family of proteins. In this study, we characterize the complete dog (Canis familiaris) CD1 locus, which is located on chromosome 38. The canine locus contains eight CD1A genes (canCD1A), of which five are pseudogenes, one canCD1B, one canCD1C, one canCD1D, and one canCD1E gene. In vivo expression of canine CD1 proteins was shown for canCD1a6, canCD1a8, and canCD1b, using a panel of anti-CD1 monoclonal antibodies (mAbs). CanCD1a6 and canCD1a8 are recognized by two distinct mAbs. Furthermore, we show differential transcription of the three canCD1A genes in canine tissues. In canine skin, the transcription level of canCD1A8 was higher than that of canCD1A6, and no transcription of canCD1A2 was detected. Based on protein modeling and protein sequence alignment, we predict that both canine CD1a proteins can bind different glycolipids in their groove. Besides differences in ectodomain structure, we observed the unique presence of three types of cytoplasmic tails encoded by canCD1A genes. cDNA sequencing and expressed sequence tag sequences confirmed the existence of a short, human CD1a-like cytoplasmic tail of four amino acids, of an intermediate length form of 15 amino acids, and of a long form of 31 amino acids.

Molecular and supra-molecular structure related differences in toxicity and granulomatogenic activity of mycobacterial cord factor in mice

To establish the structure biological activity relationship of cord factor (trehalose 6,6'-dimycolate, TDM), we compared the molecular or supra-molecular structure of TDM micelles with toxicity, thymic atrophy and granulomatogenicity in lungs and spleen of BALB/c mice. According to the difference in the mycolyl subclass composition, TDM was divided into two groups, one possessing alpha-, methoxy- and keto-mycolates in M. tuberculosis H37Rv, M. bovis BCG and M. kansasii (group A) and the other having alpha-, keto- and wax ester-mycolates in M. avium serotype 4, M. phlei and M. flavescens (group B), although mycolic acid molecular species composition differed in each group considerably. Supra-molecular structure of TDM micelle differed species to species substantially and the micelle size of TDM from M. bovis BCG Connaught was the largest. The highest toxicity was shown with TDM from M. tuberculosis H37Rv which possessed the highest amount of alpha- (47.3%) and methoxy-mycolates (40.8%), while TDM from M. phlei having the low amount of alpha-mycolate (11.6%) showed almost no toxicity with the given doses. The thymic atrophy was observed with TDM from group A, but not with TDM from group B. On the other hand, TDM from group B showed massive lung granulomatogenic activity based on the histological observations and organ indices. Taken together, group A TDM showed a wide variety of micelle sizes and specific surface areas, high to low toxicity and marked to moderate granulomatogenicity, while group B TDM showed smaller sizes of micelles and larger specific surface areas, lower toxicity but higher granulomatogenicity in lungs. Existence of higher amount of longer chain alpha-mycolates in TDM appeared to be essential for high toxicity and thymic apoptotic activity, whereas TDM possessing wax ester-mycolate with smaller sized micelles seemed to be less toxic, but more granulomatogenic in lungs in mice. Thus, the mycolic acid subclass and molecular species composition of TDM affect critically the micelle forms, toxicity and granulomatogenicity in mice, while the relative abundances and carbon chain length of alpha-mycolate affected the toxicity in mice.

Intact molecular characterization of cord factor (trehalose 6,6'-dimycolate) from nine species of mycobacteria by MALDI-TOF mass spectrometry

Cord factor (trehalose 6,6'-dimycolate, TDM) is an unique glycolipid with a trehalose and two molecules of mycolic acids in the mycobacterial cell envelope. Since TDM consists of two molecules of very long branched-chain 3-hydroxy fatty acids, the molecular mass ranges widely and in a complex manner. To characterize the molecular structure of TDM precisely and simply, an attempt was made to determine the mycolic acid subclasses of TDM and the molecular species composition of intact TDM by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry for the first time. The results showed that less than 1 microg mycolic acid methyl ester of TDM from nine representative species of mycobacteria and TDM from the same species was sufficient to obtain well-resolved mass spectra composed of pseudomolecular ions [M+Na]+. Although the mass ion distribution was extremely diverse, the molecular species of each TDM was identified clearly by constructing a molecular ion matrix consisting of the combination of two molecules of mycolic acids. The results showed a marked difference in the molecular structure of TDM among mycobacterial species and subspecies. TDM from Mycobacterium tuberculosis (H37Rv and Aoyama B) showed a distinctive mass pattern and consisted of over 60 molecular ions with alpha-, methoxy- and ketomycolate. TDM from Mycobacterium bovis BCG Tokyo 172 similarly showed over 35 molecular ions, but that from M. bovis BCG Connaught showed simpler molecular ion clusters consisting of less than 35 molecular species due to a complete lack of methoxymycolate. Mass ions due to TDM from M. bovis BCG Connaught and Mycobacterium kansasii showed a biphasic distribution, but the two major peaks of TDM from M. kansasii were shifted up two or three carbon units higher compared with M. bovis BCG Connaught. Within the rapid grower group, in TDM consisting of alpha-, keto- and wax ester mycolate from Mycobacterium phlei and Mycobacterium flavescens, the mass ion distribution due to polar mycolates was shifted lower than that from the Mycobacterium avium-intracellulare group. Since the physico-chemical properties and antigenic structure of mycolic acid of TDM affect the host immune responses profoundly, the molecular characterization of TDM by MALDI-TOF mass analysis may give very useful information on the relationship of glycolipid structure to its biological activity.

Direct molecular mass determination of trehalose monomycolate from 11 species of mycobacteria by MALDI-TOF mass spectrometry

Direct estimation of the molecular mass of single molecular species of trehalose 6-monomycolate (TMM), a ubiquitous cell-wall component of mycobacteria, was performed by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. When less than 1 μg TMM was analysed by MALDI-TOF mass spectrometry, quasimolecular ions [M+Na]+of each molecular species were demonstrated and the numbers of carbons and double bonds (or cyclopropane rings) were determined. Since the introduction of oxygen atoms such as carbonyl, methoxy and ester groups yielded the appropriate shift of mass ions, the major subclasses of mycolic acid (α, methoxy, keto and wax ester) were identified without resorting to hydrolytic procedures. The results showed a marked difference in the molecular species composition of TMM among mycobacterial species. Unexpectedly, differing from other mycoloyl glycolipids, TMM fromMycobacterium tuberculosisshowed a distinctive mass pattern, with abundant odd-carbon-numbered monocyclopropanoic (or monoenoic)α-mycolates besides dicyclopropanoic mycolate, ranging from C75to C85, odd- and even-carbon-numbered methoxymycolates ranging from C83to C94and even- and odd-carbon-numbered ketomycolates ranging from C83to C90. In contrast, TMM fromMycobacterium bovis(wild strain and BCG substrains) possessed even-carbon-numbered dicyclopropanoicα-mycolates. BCG Connaught strain lacked methoxymycolates almost completely. These results were confirmed by MALDI-TOF mass analysis of mycolic acid methyl esters liberated by alkaline hydrolysis and methylation of the original TMM. Wax ester-mycoloyl TMM molecular species were demonstrated for the first time as an intact form in theMycobacterium avium–intracellularegroup,M. phleiandM. flavescens. TheM. avium–intracellularegroup possessed predominantly C85and C87wax ester-mycoloyl TMM, whileM. phleiand the rapid growers tested contained C80, C81, C82and C83wax ester-mycoloyl TMM. This technique has marked advantages in the rapid analysis of not only intact glycolipid TMM, but also the mycolic acid composition of each mycobacterial species, since it does not require any degradation process.

Identifying and structurally characterizing CD1b in Aotus nancymaae owl monkeys

This study reports the molecular characterization and tissue expression of the non-human Aotus nancymaae primate CD1b isoform in the search for an experimental animal model to be used in evaluating the role of non-peptide antigen-presentation molecules in the immune response to infectious agents. CD1b expression on the surface of A. nancymaae peripheral blood monocyte-derived dendritic cells, shown by flow cytometry, was made possible by using human CD1b isoform antibodies. Studying the expression of CD1b-encoded transcripts revealed this molecule's broad distribution in several tissues. The A. nancymaae CD1b transcript-encoded amino-acid sequence showed 95.5% identity with the human sequence. Such high sequence homology was reflected in the identical structural conservation of how pockets A', C' and F' and tunnel T' conforming the antigen's binding site are organized, the similar arrangement of those amino-acids interacting with the T-cell receptor (TCR) during antigen presentation, and the conservation of YQNI-motif sequence in the cytoplasmatic tail (responsible for the molecule's intracellular trafficking in humans). Comparing the structure of human CD1a and CD1b and mouse CD1d proteins with CD1b structure in A. nancymaae obtained by minimization revealed that changes in the latter molecule's alpha1 and alpha2 domains imposed a narrowing of the antigen-binding groove in A. nancymaae CD1b. The high structural similarity between A. nancymaae CD1b and that from humans presented in this study leads to A. nancymaae being proposed as a suitable experimental animal model for analyzing CD1b in vivo, mainly in bacterial and parasite infections such as tuberculosis and malaria, respectively.

The crystal structure of human CD1b with a bound bacterial glycolipid

The human MHC class I-like molecule CD1b is distinctive among CD1 alleles in that it is capable of presenting a set of glycolipid species that show a very broad range of variation in the lengths of their acyl chains. A structure of CD1b complexed with relatively short acyl chain glycolipids plus detergent suggested how an interlinked network of channels within the Ag-binding groove could accommodate acyl chain lengths of up to 80 carbons. The structure of CD1b complexed with glucose monomycolate, reported in this study, confirms this hypothesis and illustrates how the distinctive substituents of intracellular bacterial glycolipids can be accommodated. The Ag-binding groove of CD1b is, uniquely among CD1 alleles, partitioned into channels suitable for the compact accommodation of lengthy acyl chains. The current crystal structure illustrates for the first time the binding of a natural bacterial lipid Ag to CD1b and shows how its novel structural features fit this molecule for its role in the immune response to intracellular bacteria.

Direct Measurement of Antigen Binding Properties of CD1 Proteins Using Fluorescent Lipid Probes

CD1 proteins are antigen-presenting molecules that bind foreign and self-lipids and stimulate specific T cell responses. In the current study, we investigated ligand binding by CD1 proteins by developing a fluorescent probe binding approach using soluble recombinant human CD1 proteins. To increase stability and yield, soluble group 1 CD1 (CD1b and CD1c) and group 2 CD1 (CD1d) proteins were produced as single chain secreted CD1 proteins in which beta2-microglobulin was fused to the N termini of the CD1 heavy chains by a flexible peptide linker sequence. Analysis of ligand binding properties of single chain secreted CD1 proteins by using fluorescent lipid probes indicated significant differences in ligand preference and in pH dependence of binding by group 1 versus group 2 CD1 proteins. Whereas group 1 CD1 isoforms (CD1b and CD1c) show stronger binding of nitrobenzoxadiazole (NBD)-labeled dialkyl-based ligands (phosphatidylcholine, sphingomyelin, and ceramide), group 2 CD1 (CD1d) proteins were stronger binders of small hydrophobic probes such as 1-anilinonaphthalene-8-sulfonic acid and 4,4'-dianilino-1,1'-naphthyl-5,5'-disulfonic acid. Competition studies indicated that binding of fluorescent lipid probes involved association of the probe with the hydrophobic ligand binding groove of CD1 proteins. Analysis of selected alanine substitution mutants of human CD1b known to inhibit antigen presentation showed that NBD-labeled lipid probe binding could be used to distinguish mutations that interfere with ligand binding from those that affect T cell receptor docking. Our findings provide further evidence for the functional specialization of different CD1 isoforms and demonstrate the value of the fluorescent lipid probe binding method for assisting structure-based studies of CD1 function.

Structural features of the acyl chain determine self-phospholipid antigen recognition by a CD1d-restricted invariant NKT (iNKT) cell

Little is known about the antigen specificity of CD1d-restricted T cells, except that they frequently recognize CD1d-expressing antigen-presenting cells in the absence of exogenous antigen. We previously demonstrated that the 24.8.A iNKT cell hybridoma was broadly reactive with CD1d-transfected cell lines and recognized the polar lipid fraction of a tumor cell extract. In the present study, the antigen recognized by the 24.8.A iNKT cell hybridoma was purified to homogeneity and identified as palmitoyl-oleoyl-sn-glycero-3-phosphoethanolamine (16:0-18:1 PE). The 24.8.A iNKT cell hybridoma recognized synthetic 16:0-18:1[cis] PE, confirming that this phospholipid is antigenic. Recognition correlated with the degree of unsaturation of the acyl chains. Using a panel of synthetic PEs, the 24.8.A iNKT cell hybridoma was shown to be activated by PEs that contained at least one unsaturated acyl chain. The configuration of the double bonds was important, as the 24.8.A iNKT cell hybridoma recognized unsaturated acyl chains in the cis, but not the trans, configuration. PEs with multiple double bonds were recognized better than those with a single double bond, and increasing acyl chain unsaturation correlated with increased binding of PE to CD1d. These data illustrate the potential importance of the acyl chain structure for phospholipid antigen binding to CD1d.

Structure of human CD1b with bound ligands at 2.3 A, a maze for alkyl chains

The human genome encodes five nonpolymorphic major histocompatibility complex class I-like glycoproteins, CD1a to CD1e, that present lipid antigens for specific recognition by T lymphocytes. Using single alkyl chain detergents, we developed a protocol to generate recombinant human CD1b-lipid complexes. We present here the crystal structures of CD1b in complex with either phosphatidylinositol or ganglioside GM2 at 2.3 A and 2.8 A resolutions, respectively. The antigen-binding groove houses four interlinked hydrophobic channels that are occupied by the alkyl chains of the glycolipid plus two detergent molecules. A distinct exit beneath the alpha 2 helix further contributes to the plasticity of the binding groove. These structures reveal the mechanism by which two alkyl chain lipids bind to CD1b, and how CD1b can adapt to ligands of different alkyl chain length. They also suggest how very long alkyl chains, such as those of mycolic acid, could be fully contained within the binding groove. These results extend the spectrum of potential CD1b ligands by revealing that, in addition to two alkyl chain lipids, mono-alkyl and triple-alkyl chain lipids can be accommodated in the binding groove.

A new aspect in glycolipid biology: glycosphingolipids as antigens recognized by T lymphocytes

T cells may recognize a large variety of ligands with different chemical structures. Recently, glycosphingolipids have also been shown to stimulate human T lymphocytes. Recognition of glycosphingolipids is restricted by the nonpolymorphic CD1 molecules, expressed by professional antigen-presenting cells and by macrophages infiltrating inflammatory sites. CD1 molecules have a structure resembling that of classical MHC class I molecules, with the terminal extracellular domains characterized by two antiparallel alpha helices placed on two hydrophobic pockets. The glycosphingolipids bound to CD1 insert the lipid tails in the two pockets and position the hydrophilic head on the external part of CD1. The TCR interacts with aminoacids present in the two alpha helices and with residues provided by the carbohydrate moiety of glycosphingolipids and discriminates their structural variations. T cells recognizing self-glycosphingolipids release proinflammatory cytokines and may have a pathogenetic role in autoimmune diseases such as multiple sclerosis.

Immunological properties of trehalose dimycolate (cord factor) and other mycolic acid-containing glycolipids--a review

Mycolic acids are characteristic fatty acids of Mycobacteria and are responsible for the wax-like consistence of these microorganisms. Decades of research revealed that mycolic acid-containing glycolipids, in particular trehalose-6,6'-dimycolate (TDM, cord factor) as their best-studied representative, exert a number of immunomodifying effects. They are able to stimulate innate, early adaptive and both humoral and cellular adaptive immunity. Most functions can be associated with their ability to induce a wide range of chemokines (MCP-1, MIP-1alpha, IL-8) and cytokines (e.g., IL-12, IFN-gamma, TNF-alpha, IL-4, IL-6, IL-10). This review tries to link well-known properties of mycolic acid-containing glycolipids, e.g., stimulation of cellular and humoral immunity, granuloma formation and anti-tumor activity, with recent findings in molecular immunology and to give an outlook on potential practical applications.

Lipid length controls antigen entry into endosomal and nonendosomal pathways for CD1b presentation

CD1 proteins present various glycolipid antigens to T cells, but the cellular mechanisms that control which particular glycolipids generate T cell responses are not understood. We show here that T cell recognition of glucose monomycolate antigens with long (C(80)) alkyl chains involves the delivery of CD1b proteins and antigens to late endosomes in a process that takes several hours. In contrast, analogs of the same antigen with shorter (C(32)) alkyl chains are rapidly, but inefficiently, presented by cell surface CD1b proteins. Dendritic cells (DCs) preferentially present long-chain glycolipids, which results, in part, from their rapid internalization and selective delivery of antigens to endosomal compartments. Nonprofessional antigen-presenting cells, however, preferentially present short-chain glycolipids because of their lack of prominent endosomal presentation pathways. Because long alkyl chain length distinguishes certain microbial glycolipids from common mammalian glycolipids, these findings suggest that DCs use a specialized endosomal-loading pathway to promote preferential recognition of glycolipids with a more intrinsically foreign structure.

Fine specificity of TCR complementarity-determining region residues and lipid antigen hydrophilic moieties in the recognition of a CD1-lipid complex

alphabeta TCR can recognize peptides presented by MHC molecules or lipids and glycolipids presented by CD1 proteins. Whereas the structural basis for peptide/MHC recognition is now clearly understood, it is not known how the TCR can interact with such disparate molecules as lipids. Recently, we demonstrated that the alphabeta TCR confers specificity for both the lipid Ag and CD1 isoform restriction, indicating that the TCR is likely to recognize a lipid/CD1 complex. We hypothesized that lipids may bind to CD1 via their hydrophobic alkyl and acyl chains, exposing the hydrophilic sugar, phosphate, and other polar functions for interaction with the TCR complementarity-determining regions (CDRs). To test this model, we mutated the residues in the CDR3 region of the DN1 TCR beta-chain that were predicted to project between the CD1b alpha helices in a model of the TCR/CD1 complex. In addition, we tested the requirement for the negatively charged and polar functions of mycolic acid for Ag recognition. Our findings indicate that the CDR loops of the TCR form the Ag recognition domain of CD1-restricted TCRs and suggest that the hydrophilic domains of a lipid Ag can form a combinatorial epitope recognized by the TCR.

Lipid protein interactions: the assembly of CD1d1 with cellular phospholipids occurs in the endoplasmic reticulum

CD1d1 is a member of a family of lipid Ag-presenting molecules. The cellular ligands associated with CD1d1 were isolated and characterized by biochemical means as an approach to elucidate the mechanism by which CD1 molecules assemble in vivo. Natural ligands of mouse CD1d1 included cellular phosphatidylinositol and phosphatidylinositol-glycans that are synthesized in the endoplasmic reticulum. Further biochemical data revealed that the two CD1d1 mutants, one defective in recycling from-and-to the plasma membrane and the other in efficiently negotiating the secretory pathway, associated with phosphatidylinositol. Thus phosphatidylinositol associated with CD1d1 in the early secretory pathway. Phosphatidylinositol also associated with CD1d1 in Pig-A-deficient cells that are defective in the first glycosylation step of glycosylphosphatidylinositol biosynthesis. Moreover, cellular phosphatidylinositol-glycans are not Valpha14Jalpha15 natural T cell Ags. Therefore, we predict that cellular lipids occlude the hydrophobic Ag-binding groove of CD1 during assembly until they are exchanged for a glycolipid Ag(s) within the recycling compartment for display on the plasma membrane. In this manner, cellular lipids might play a chaperone-like role in the assembly of CD1d1 in vivo, akin to the function of invariant chain in MHC class II assembly.