CD1 and lipid antigens: intracellular pathways for antigen presentation

Recently, different members of the CD1 family of MHC-like molecules have been shown to sample different intracellular compartments to present lipid and glycolipid antigens to T cells. Emerging models suggest that CD1 may have evolved to monitor the integrity of membrane lipids and/or to present microbial lipid antigens to both alpha beta and gamma delta T cells.     

CD1-mediated immune responses to glycolipids.

Major advances have recently been accomplished in understanding the biochemistry of interactions between lipid antigens and CD1. The diversity of lipid antigens loaded onto CD1 molecules in normal cells, the ligand requirements for CD1 recognition by autoreactive T cells, including NK T cells, and the significance of CD1-mediated autoreactivity, as opposed to pathogen reactivity, remain controversial issues.     

Activation of human T cells by CD1 and self-lipids

Over two decades ago, it was discovered that the human T-cell repertoire contains T cells that do not recognize peptide antigens in the context of MHC molecules but instead respond to lipid antigens presented by CD1 antigen-presenting molecules. The ability of T cells to ‘see’ lipid antigens bound to CD1 enables these lymphocytes to sense changes in the lipid composition of cells and tissues as a result of infections, inflammation, or malignancies. Although foreign lipid antigens have been shown to function as antigens for CD1-restricted T cells, many CD1-restricted T cells do not require foreign antigens for activation but instead can be activated by self-lipids presented by CD1. This review highlights recent developments in the field, including the identification of common mammalian lipids that function as autoantigens for αβ and γδ T cells, a novel mode of T-cell activation whereby CD1a itself rather than lipids serves as the autoantigen, and various mechanisms by which the activation of CD1-autoreactive T cells is regulated. As CD1 can induce T-cell effector functions in the absence of foreign antigens, multiple mechanisms are in place to regulate this self-reactivity, and stimulatory CD1-lipid complexes appear to be tightly controlled in space and time.     

Functional CD1a is stabilized by exogenous lipids

Self-glycosphingolipids bind to surface CD1 molecules and are readily displaced by other CD1 ligands. This capacity to exchange antigens at the cell surface is not common to other antigen-presenting molecules and its physiological importance is unclear. Here we show that a large pool of cell-surface CD1a, but not CD1b molecules, is stabilized by exogenous lipids present in serum. Under serum deprivation CD1a molecules are altered and functionally inactive, as they are unable to present lipid antigens to T cells. Glycosphingolipids and phospholipids bind to, and restore functionality to CD1a without the contribution of newly synthesized and recycling CD1a molecules. The dependence of CD1a stability on exogenous lipids is not related to its intracellular traffic and rather to its antigen-binding pockets. These results indicate a functional dichotomy between CD1a and CD1b molecules and provide new information on how the lipid antigenic repertoire is immunologically sampled.     

CD1 proteins: targets of T cell recognition in innate and adaptive immunity

The CD1 family consists of antigen presenting molecules encoded by genes located outside of the major histocompatibility complex. CD1 proteins are conserved among mammalian species and are expressed on the surface of cells involved in antigen presentation. The CD1 system has been shown to be involved in activation of cell-mediated responses, and T cells specific for either CD1 molecules or antigens presented by CD1 have been isolated. Structural and biochemical analyses demonstrate that antigens presented by CD1 are nonpeptide lipid or glycolipid structures, including examples found in the cell walls of pathogenic mycobacteria. The hydrophobic part of these antigens most likely binds in the CD1 ligand-binding groove, whereas the polar headgroup of these antigens appears to make direct contact with the T cell receptor and determines specific recognition. Presentation of antigens by CD1 molecules requires uptake and intracellular processing by antigen presenting cells and can be achieved for both exogenous and endogenous antigens. T cells recognizing CD1 restricted antigens have a broad range of functional activities that suggest that the CD1 system is involved in both innate and adaptive immune responses against microbial infections.     

CD1d-mediated antigen presentation to natural killer T (NKT) cells

CD1d molecules are lipid antigen-presenting molecules. They are involved in presenting these antigens to a unique subpopulation of T cells called natural killer T (NKT) cells, which have the capacity to produce both T helper (Th) 1 and Th2 cytokines. Thus, it is possible that the antigens presented by CD1d and/or the level at which they are presented could have profound effects on the immunoregulation of autoimmune and infectious diseases, as well as cancer. Because of the ability of CD1d-binding ligands to modulate NKT cell responses, targeting CD1d-mediated antigen presentation as a novel approach for new therapies in these and other diseases holds great promise.     

Induction of cell-associated interleukin 1 through stimulation of the adhesion-promoting proteins LFA-1 (CD11a/CD18) and CR3 (CD11b/CD18) of human monocytes.

Serum-free culture of human monocytes in the presence of monoclonal antibodies to the LFA-1 alpha chain (CD11a), CR3 alpha chain (CD11b) or beta chain (CD18) bound to Sepharose induced the dose-dependent production of cell-associated interleukin (IL) 1 activity and of IL 1 alpha and IL 1 beta antigens, but no release of extracellular IL 1 activity or antigen in the culture medium. Triggering of IL 1 production was also observed with insolubilized anti-CD11/CD18 F(ab')2 antibodies. Two cross-linked antibodies recognizing distinct epitopes on the CD11b molecule induced cell-associated IL 1. Soluble antibodies did not induce IL 1 production. The kinetics of induction of IL 1 by stimulation of adhesion-promoting proteins differed from those of IL 1 induction by adhesion to plastic. The lack of induction of IL 1 release by stimulation of the CD11/CD18 molecules resembled the intracellular accumulation of IL 1 induced by lipid A. Induction of IL 1 by adhesive processes may be a mechanism by which T cells trigger IL 1 production by monocytes during antigen presentation.     

Two classes of CD1 genes

Herein, we report the DNA sequence of two human CD1 genes, R2 and R3, distinct from those encoding the CD1a, -b and -c antigens. Both genes appear to have an exon/intron structure analogous to the previously analyzed CD1 genes and to be functional on the basis of their sequence. Analysis of the variability patterns, potential intramolecular interactions and predicted secondary structure profile on an alignment of all known CD1 alpha chains suggest some shared structural features with major histocompatibility complex class I molecules in the alpha 1 domains but substantial differences in the alpha 2 domains. Sequence comparison shows that, while R2 is most related to CD1a, -b and -c, albeit to a somewhat lower degree than the latter are to themselves, R3 is more homologous to mouse than to human CD1, suggesting the existence of two functional classes within the CD1 gene family. We propose to retain the non-committal R2 and R3 names until the putative antigens have been identified and their tissue distribution has been established.     

Microsomal triglyceride transfer protein regulates endogenous and exogenous antigen presentation by group 1 CD1 molecules

Lipid antigens are presented to T cells by the non-polymorphic MHC class I-related CD1 molecules. Microsomal triglyceride transfer protein (MTP) is an endoplasmic reticulum (ER)-resident chaperone that has been shown to lipidate the group 2 CD1 molecule CD1d and thus to regulate its function. We now report that MTP also regulates the function of group 1 CD1 molecules CD1a, CD1b, and CD1c. Pharmacological inhibition of MTP in monocyte-derived dendritic cells and lymphoblastoid B cell lines transfected with group 1 CD1 resulted in a substantial decrease in endogenous self lipid antigen presentation to several CD1-restricted T cell lines. Silencing MTP expression in CD1c-transfected HeLa cells similarly resulted in decreased self reactivity. Unexpectedly, inhibition of ER-resident MTP, which was confirmed by confocal microscopy, also markedly decreased presentation of exogenous, endosomally loaded, mycobacterial lipid antigens by CD1a and CD1c to T cells. Thus, these studies indicate that MTP, despite its ER localization, regulates endogenous as well as exogenous lipid antigen presentation, and suggest a broad role for MTP in the regulation of CD1 antigen presentation.     

pH-dependent interdomain tethers of CD1b regulate its antigen capture

As CD1 proteins recycle between the cell surface and endosomes, they show altered receptiveness to lipid antigen loading. We hypothesized that changes in proton concentration encountered within distinct endosomal compartments influence the charge state of residues near the entrance to the CD1 groove and thereby control antigen loading. Molecular dynamic models identified flexible areas of the CD1b heavy chain in the superior and lateral walls of the A' pocket. In these same areas, residues that carry charge in a pH-dependent manner (D60, E62) were found to tether the rigid alpha1 helix to flexible areas of the alpha2 helix and the 50-60 loop. After disruption of these tethers with acid pH or mutation, we observed increased association and dissociation of lipids with CD1b and preferential presentation of antigens with bulky lipid tails. We propose that ionic tethers act as molecular switches that respond to pH fluxes during endosomal recycling and regulate the conformation of the CD1 heavy chain to control the size and rate of antigens captured.     

Understanding the function of CD1-restricted T cells

CD1 molecules bind foreign lipid antigens as they survey the endosomal compartments of infected antigen-presenting cells. Unlike T cells that recognize CD1-restricted foreign lipids, CD1-restricted T cells that are self-antigen-reactive function as 'auto-effectors' that are rapidly stimulated to carry out helper and effector functions upon interaction with CD1-expressing antigen-presenting cells. The functional distinctions between subsets of CD1-restricted T cells, and the pathways by which these cells both influence the inflammatory and tolerogenic effects of dendritic cells and activate natural killer cells and other lymphocytes, provide insight into how CD1-restricted T cells regulate antimicrobial responses, antitumor immunity and the balance between tolerance and autoimmunity.     

T lymphocyte recognition of human group 1 CD1 molecules: implications for innate and acquired immunity

Recent evidence has established that non-MHC-encoded molecules of the CD1 family mediate MHC-independent pathways for antigen presentation and T cell activation. Human group 1 CD1 molecules (CD1a, CD1b, CD1c) are expressed mainly on professional antigen-presenting cells, and mediate presentation of microbial lipid and glycolipid antigens to T cells. These group 1 CD1 molecules differentially sample distinct endocytic compartments that may contain different sets of lipid antigens derived from intracellular microbes, and activate antigen-specific T cells. These T cells lyse infected antigen-presenting cells and secrete Th1 cytokines, such as interferon- gamma, and granulysin, a potent antimicrobial protein, and thus can control microbial infection. Reactivity to CD1a, b, and c molecules in the absence of foreign antigen has also been detected in T cells bearing alphabeta and gammadelta TCRs. These T cells may recognize self-lipid antigens and are considered to be autoreactive. In particular, the main tissue subset of gammadelta T cells (V delta 1(+)subset) show prominent reactivity to CD1c, and produce interferon- gamma and granulysin. These CD1c directly reactive T cells may mediate immunity against microbial infection even before antigen-specific T cells differentiate and expand. Together, human CD1a, b and c molecules elicit T cell-dependent immunity to the universe of foreign and self-lipid antigens in both innate and acquired immunity settings.     

Polymorphism of human CD1 genes

Human CD1 genes have been reported to be invariant or to show limited polymorphism. Recently, certain functions of CD1 antigens have been described to include the presentation lipid and glycolipid antigens. These observations prompted a thorough survey of the genetic polymorphism in the five human CD1 genes (CD1a-CD1e). Using polymerase chain reaction-single stranded conformational polymorphism (PCR-SSCP) combined with sequence analyses, exons 2 and 3 from CD1a-CD1e were characterized from a total of 110 unrelated healthy donors. Results showed that all five genes (CD1a-CD1e) are polymorphic in exon 2. Substitutions in CD1b and CD1c are silent, whereas, substitutions in CD1a, CD1d and CD1e result in amino acid replacements in the deduced protein products. CD1a and CD1e polymorphisms are prevalent in the population. The substitutions in CD1a have characteristics that may influence interactions with beta2-microglobulin beta2-m) or accessory molecules. The substitution in CD1e is located in the region predicted to interact with ligands and may differentially impact the ability of CD1e alleles to bind antigen.     

Group 1 CD1-restricted T cells take center stage

The recognition of lipid antigens by T cells is a complex and fascinating phenomenon. The MHC-like molecules of the CD1 family have evolved to present a wide variety of both self and foreign lipids for recognition by T-cell receptors. While much progress has been made in our understanding of the NKT cells that recognize lipids presented by CD1d molecules, our knowledge of the T-cell populations directed at the related group 1 CD1 molecules, i.e. CD1a, CD1b and CD1c, has lagged behind. In this issue of the European Journal of Immunology, a study identifies a surprisingly large portion of human peripheral blood T cells as being autoreactive to the group 1 CD1 proteins. This work and other recent developments highlight the presence of a substantial number of unconventional T cells as part of our normal T-cell repertoire. This interesting finding is discussed in details in this commentary.     

Separate pathways for antigen presentation by CD1 molecules

The ability to sample relevant intracellular compartments is necessary for effective antigen presentation. To detect peptide antigens, MHC class I and II molecules differentially sample cytosolic and endosomal compartments. CD1 constitutes another lineage of lipid antigen-presenting molecules. We show that CD1b traffics deeply into late endosomal compartments, while CD1a is excluded from these compartments and instead traffics independently in the recycling pathway of the early endocytic system. Further, CD1b but not CD1a antigen presentation is dependent upon vesicular acidification. Since lipids and various bacteria are known to traffic differentially, either penetrating deeply into the endocytic system or following the route of recycling endosomes, these findings elucidate efficient monitoring of distinct components of the endocytic compartment by CD1 lipid antigen-presenting molecules.     

The CD1 family of lipid antigen-presenting molecules

The paradigm that T cells recognize peptide antigens presented by major histocompatibility complex class I and class II molecules has been a guiding principle in the development of immunology. Here, Steven Porcelli and colleagues review studies that extend this paradigm by showing that CD1 proteins are a separate lineage of antigen-presenting molecules with unusually hydrophobic ligand-binding grooves that present nonpeptide lipid and glycolipid antigens to T cells.     

The CD1 family and T cell recognition of lipid antigens

For many years it was thought that T lymphocytes recognized only peptide antigens presented by MHC class I or class II molecules. Recently, it has become clear that a wide variety of lipids and glycolipids are also targets of the T cell response. This novel form of cell-mediated immune recognition is mediated by a family of lipid binding and presenting molecules known as CD1. The CD1 proteins represent a small to moderate sized family of beta2-microglobulin-associated transmembrane proteins that are distantly related to MHC class I and class II molecules. They are conserved in most or all mammals, and control the development and function of T cell populations that participate in innate and adaptive immune responses through the recognition of self and foreign lipid antigens. Here we review the current state of our understanding of the structure and function of CD1 proteins, and the role of CD1-restricted T cell responses in the immune system.     

Characterization of guinea-pig group 1 CD1 proteins

CD1 molecules are cell-surface glycoproteins with strong structural similarities to major histocompatibility complex (MHC) class I molecules, and studies in humans and mice have demonstrated that CD1 proteins perform the unique role of presenting lipid antigens to T lymphocytes. Our previous studies have shown that guinea-pigs, unlike the muroid rodents, have an extended family of group 1 CD1 genes. In the current study, we raised monoclonal anibodies (mAbs) against guinea-pig CD1 proteins and generated transfected cell lines expressing individual members of the guinea-pig CD1 family. Our results indicated that multiple members of the guinea-pig CD1 family, including members that are homologous to the human CD1b and CD1c proteins, are expressed at the protein level in transfected cells and in specialized antigen-presenting cells such as monocyte-derived dendritic cells. In addition, CD1 proteins, especially guinea-pig CD1b3, were expressed on a large number of B cells in the guinea-pig, and CD1 expression appeared to be regulated by B-cell maturation or differentiation. Interestingly, three different patterns of intracellular localization were observed for the various guinea-pig CD1 isoforms, a finding that is reminiscent of the distinct patterns of intracellular localization that have been previously demonstrated for human CD1a, CD1b and CD1c. Taken together, these results provide further evidence for substantial similarities between the guinea-pig and human CD1 systems, thus supporting the possibility that the guinea-pig may offer significant advantages as an animal model for the study of the in vivo role of CD1 proteins in infectious and autoimmune diseases.     

The molecular basis of CD 1 -mediated presentation of Iipid antigens

Summary: The CD1 family of proteins mediates a newly described pathway for presentation of lipids and glycolipids for specific recognition by T cells. All four of the known human CD1 proteins (CD1a, CD1b, CD1c and CD1d) as well as murine CD Id have now been shown to mediate T-cell recognition of lipid or glycolipid antigens. These antigens include naturally occuring foreign glycolipids from intracellular pathogens or synthetic glycolipids that are related in structure to mammalian glycolipids. The CD l b and CD I d-presented antigens differ in their fine structures but reveal a general motif in which a rigid hydrophilic cap is botind to two aliphatic hydrocarbon chains. Different T-cell populations recognize individual antigens without cross-reactivity to closely related antigen structures or CD 1 isoforms, documenting the complexity and fine specificity of CD l-mediated T-cell responses. Mapping of the molecular detertninants of recognition for CD 1 b and CD 1d-preseced antigens reveals that T cells discriminate the fine structure of the hydrophilic cap of the antigen, but both the length and structure of the lipid chains may be altered without loss of recognition. This pattern of lipid antigen recognition may be accounted for by a simple molecular mechanism of presentation that parallels the known mechanism for presentation of peptides, but solves the special problems related to the hydrophobic chemical nature of the lipid antigens. We propose that CD 1 binds antigen by accommodating the two lipid tails within the hydrophobic groove of its two membrane distal domains, positioning the rigid hydrophilic cap of the antigen on the solvent-exposed surface of the CD1 protein, where it can directly contact the T-cell antigen receptor. This model provides a molecular basis for recognition of a new and diverse set of T-cell antigens contained within the lipid bilayers of cellular membranes.     

CD1 Expression in Human Atherosclerosis : A Potential Mechanism for T Cell Activation by Foam Cells

Atherosclerotic plaques are chronic inflammatory lesions composed of dysfunctional endothelium, smooth muscle cells, lipid-laden macrophages, and T lymphocytes. This study analyzed atherosclerotic tissue specimens for expression of CD1 molecules, a family of cell surface proteins that present lipid antigens to T cells, and examined the possibility that CD1+ lipid-laden macrophages might present antigen to T cells. Immunohistochemical studies using a panel of specific monoclonal antibodies demonstrated expression of each of the four previously characterized human CD1 proteins (CD1a, -b, -c, and -d) in atherosclerotic plaques. Expression of CD1 was not observed in normal arterial specimens and appeared to be restricted to the CD68+ lipid-laden foam cells of atherosclerotic lesions. CD1 molecules colocalized in areas of the arterial wall that also contained abundant T lymphocytes, suggesting potential interactions between CD1+ cells and plaque-infiltrating lymphocytes in situ. Using CD1-expressing foam cells derived from macrophages in vitro, we demonstrated the ability of such cells to present lipid antigens to CD1 restricted T cells. Given the abundant T cells, CD1+ macrophages, and lipid accumulation in atherosclerotic plaques, we propose a potential role for lipid antigen presentation by CD1 proteins in the generation of the inflammatory component of these lesions.