CD1 expression is not affected by human peptide transporter deficiency

Crystal structures of lysophospholipid-bound MHC class I molecules

Newly synthesized major histocompatibility complex (MHC) class I proteins are stabilized in the endoplasmic reticulum (ER) by binding 8-10-mer-long self-peptide antigens that are provided by transporter associated with antigen processing (TAP). These MHC class I:peptide complexes then exit the ER and reach the plasma membrane, serving to sustain the steady-state MHC class I expression on the cell surface. A novel subset of MHC class I molecules that preferentially bind lipid-containing ligands rather than conventional peptides was recently identified. The primate classical MHC class I allomorphs, Mamu-B*098 and Mamu-B*05104, are capable of binding the N-myristoylated 5-mer (C14-Gly-Gly-Ala-Ile-Ser) or 4-mer (C14-Gly-Gly-Ala-Ile) lipopeptides derived from the N-myristoylated SIV Nef protein, respectively, and of activating lipopeptide antigen-specific cytotoxic T lymphocytes. We herein demonstrate that Mamu-B*098 samples lysophosphatidylethanolamine and lysophosphatidylcholine containing up to a C20 fatty acid in the ER. The X-ray crystal structures of Mamu-B*098 and Mamu-B*05104 complexed with lysophospholipids at high resolution revealed that the B and D pockets in the antigen-binding grooves of these MHC class I molecules accommodate these lipids through a monoacylglycerol moiety. Consistent with the capacity to bind cellular lipid ligands, these two MHC class I molecules did not require TAP function for cell-surface expression. Collectively, these results indicate that peptide- and lipopeptide-presenting MHC class I subsets use distinct sources of endogenous ligands.

The role of MHC class Ib-restricted T cells during infection

Even though major histocompatibility complex (MHC) class Ia and many Ib molecules have similarities in structure, MHC class Ib molecules tend to have more specialized functions, which include the presentation of non-peptidic antigens to non-classical T cells. Likewise, non-classical T cells also have unique characteristics, including an innate-like phenotype in naïve animals and rapid effector functions. In this review, we discuss the role of MAIT and NKT cells during infection but also the contribution of less studied MHC class Ib-restricted T cells such as Qa-1-, Qa-2-, and M3-restricted T cells. We focus on describing the types of antigens presented to non-classical T cells, their response and cytokine profile following infection, as well as the overall impact of these T cells to the immune system.

Structural and functional aspects of lipid binding by CD1 molecules

Over the past ten years, investigators have shown that T lymphocytes can recognize not only peptides in the context of MHC class I and class II molecules but also foreign and self-lipids in association with the nonclassical MHC class I molecules the CD1 proteins. We describe the events that have led to the discovery of the role of CD1 molecules, their pattern of intracellular trafficking, and their ability to sample different intracellular compartments for self- and foreign lipids. Structural and functional aspects of lipid presentation by CD1 molecules are presented in the context of the function of CD1-restricted T cells in antimicrobial responses, antitumor immunity, and the regulation of the tolerance and autoimmunity immunoregulatory axis. Particular emphasis is on invariant NKT (iNKT) cells and their ability to modulate innate and adaptive immune responses.

CD1 Antigen Presentation by Human Dendritic Cells as a Target for Herpes Simplex Virus Immune Evasion

In contrast to MHC molecules, which present peptides, the CD1 molecules have been discovered to present lipid Ags to T cells. CD1-restricted T lymphocytes have been recently associated with resistance to virus infection. The mechanisms underlying activation of CD1-restricted T cells in the course of virus infection are not defined. In this study, we wanted to investigate the interaction of HSV with the antiviral CD1 Ag presentation system in human dendritic cells (DC). In response to low titers of HSV, the surface expression of CD1b and CD1d on human DC was up-regulated. These phenotypic changes enhanced the capacity of infected DC to stimulate proliferation of CD1-restricted T lymphocytes. High titers of HSV, however, lead to strong down-regulation of all surface CD1 molecules. This modulation of surface expression was associated with intracellular accumulation, colocalization with viral proteins, and disruption of the CD1 recycling machinery. Finally, even at low titers HSV interfered with the capacity of infected DC to stimulate the release of important cytokines by CD1d-restricted NKT cells. Thus, we demonstrate both the existence of a CD1 pathway allowing human DC to react to viral infection, as well as its blockage by a human herpesvirus.

CD1 tetramers: a powerful tool for the analysis of glycolipid-reactive T cells

CD1 proteins constitute a third class of antigen-presenting molecules. They bind lipids rather than peptides, and the T cells reactive to lipids presented by CD1 have been implicated in the protection against autoimmune diseases and infectious microorganisms and in the immune surveillance for tumors. Thus, the ability to identify, purify, and track the response of CD1-reactive cells is of paramount importance. Previously existing methods for identifying these T cells were not based on TCR specificity, and therefore the data obtained by these methods were in some cases difficult to interpret. The recent generation of tetramers of alpha-galactosyl ceramide (alpha-GalCer) with CD1d has already permitted significant insight into the biology of NKT cells. Tetramers constructed from other CD1 molecules also have been obtained during the previous year. Collectively, these new reagents promise to greatly expand knowledge of the functions of lipid-reactive T cells, with potential use in monitoring the response to lipid-based vaccines and other treatments and in the diagnosis of autoimmune diseases.

Glycosylphosphatidylinositol-Anchored Mucin-Like Glycoproteins from Trypanosoma cruzi Bind to CD1d but Do Not Elicit Dominant Innate or Adaptive Immune Responses Via the CD1d/NKT Cell Pathway

It has been proposed that self and protozoan-derived GPI anchors are natural ligands of CD1d. In this study, we investigated the ability of GPI anchors from Trypanosoma cruzi to bind to CD1d and mediate activation of NKT cells. We observed that GPI-anchored mucin-like glycoproteins (GPI mucins), glycoinositolphospholipids (GIPLs), and their phosphatidylinositol moieties bind to rCD1d and inhibit the stimulation of a NKT hybridoma by the alpha-galactosylceramide-CD1 complex. However, these GPI anchors and related structures were unable to activate NKT cells in vitro or in vivo. We found that high titers of Ab anti-GPI mucins, but not anti-GIPLs, were detected in sera from wild-type as well as in TAP1(-/-), CD1d(-/-), and MHC class II(-/-) mice after immunization. However, T-dependent anti-GPI mucin Ab isotypes, such as IgG1, IgG2a, IgG2b, and IgG3, were absent on MHC class II(-/-), but were conserved in CD1d(-/-) and TAP1(-/-) mice. Furthermore, we found that CD1d(-/-) mice presented a robust cytokine as well as anti-GPI mucins and anti-GIPL Ab responses, upon infection with T. cruzi parasites. These results indicate that, despite binding to CD1d, GPI mucins and related structures expressed by T. cruzi appear not to evoke dominant CD1d-restricted immune responses in vivo. In contrast, MHC class II is critical for the production of the major Ig G isotypes against GPI mucins from T. cruzi parasites.

Role of IL-12-independent and IL-12-dependent pathways in regulating generation of the IFN-gamma component of T cell responses to Salmonella typhimurium

Clearance of facultative intracellular pathogens such as Salmonella requires IFN-gamma from CD4 T cells. Mechanisms linking intracellular pathogen recognition with induction of IFN-gamma-producing T cells are still poorly understood. We show in this study that IL-12 is not required for commitment to the IFN-gamma-producing T cell response in infection with Salmonella typhimurium, but is needed for its maintenance. The IL-12-independent signals required for commitment depend on events during the first hour of infection and are related to Ag presentation. Even transient attenuation of Ag presentation early during infection specifically abrogates the IFN-gamma component of the resulting CD4 T cell response. The IL-12 needed for maintenance is also better induced by live rather than dead bacteria in vivo, and this difference is due to specific suppression of IL-12 induction by dead bacteria. Presence of exogenous IL-4 down-modulates IL-12 production by macrophages activated in vitro. Furthermore, macrophages from IL-4-null mice secrete high levels of both IL-12 and IL-18 in response to stimulation in vivo even with dead bacteria, but this does not lead to induction of IFN-gamma-secreting T cells in response to immunization with dead S. typhimurium. Early IL-4 is contributed by triggering of CD4 NK T cells by dead, but not live, bacteria. Thus, Ag presentation-related IL-12-independent events and IL-4-sensitive IL-12-dependent events play crucial complementary roles in the generation of the IFN-gamma-committed CD4 T cell component of the immune response in Salmonella infection.

The molecular biology of CD1

CD1 were the first human differentiation antigens to be identified by monoclonal antibodies. In this review, we summarize some key results from the molecular study of CD1, with particular reference to their relationship to MHC antigens, and to the existence of two distinct groups of CD1 molecules.

Comparative contribution of CD1 on the development of CD4+ and CD8+ T cell compartments

CD1 molecules are MHC class I-like glycoproteins whose expression is essential for the development of a unique subset of T cells, the NK T cells. To evaluate to what extent CD1 contributes to the development of CD4+ and CD8+ T cells, we generated CD1oIIo and CD1oTAPo mice and compared the generation of T cells in these double-mutant mice and IIo or TAPo mice. FACS analysis showed that the number of CD4+ T cells in CD1oIIo mice was reduced significantly compared with the corresponding population in IIo mice. Both CD4+ NK1.1+ and the CD4+ NK1.1- population were reduced in CD1oIIo mice, suggesting that CD1 can select not only CD4+ NK1.1+ T cells but also some NK1.1- CD4+ T cells. Functional analysis showed that the residual CD4+ cells in CD1oIIo can secrete large amounts of IFN-gamma and a significant amount of IL-4 during primary stimulation with anti-CD3, suggesting that this population may be enriched for NK T cells restricted by other class I molecules. In contrast to the CD4+ population, no significant differences in the CD8+ T cell compartment can be detected between TAPo and CD1oTAPo mice in all lymphoid tissues tested, including intestinal intraepithelial lymphocytes. Our data suggest that, unlike other MHC class I molecules, CD1 does not contribute in a major way to the development of CD8+ T cells.

Binding and antigen presentation of ceramide-containing glycolipids by soluble mouse and human CD1d molecules

We have purified soluble mouse and human CD1d molecules to assess the structural requirements for lipid antigen presentation by CD1. Plate-bound CD1d molecules from either species can present the glycolipid alpha-galactosyl ceramide (alpha-GalCer) to mouse natural killer T cells, formally demonstrating both the in vitro formation of antigenic complexes, and the presentation of alpha-GalCer by these two CD1d molecules. Using surface plasmon resonance, we show that at neutral pH, mouse CD1 and human CD1d bind to immobilized alpha-GalCer, unlike human CD1b, which requires acidic pH for lipid antigen binding. The CD1d molecules can also bind both to the nonantigenic beta-GalCer and to phosphatidylethanolamine, indicating that diverse lipids can bind to CD1d. These studies provide the first quantitative analysis of monomeric lipid antigen-CD1 interactions, and they demonstrate that the orientation of the galactose, or even the nature of the polar head group, are likely to be more important for T cell receptor contact than CD1d binding.

Susceptibility of mice deficient in CD1D or TAP1 to infection with Mycobacterium tuberculosis

Cellular immunity against Mycobacterium tuberculosis controls infection in the majority of infected humans. Studies in mice have delineated an important role for CD4(+) T cells and cytokines including interferon gamma and tumor necrosis factor alpha in the response to infection with mycobacteria. Recently, the identification of CD8(+) CD1-restricted T cells that kill M. tuberculosis organisms via granulysin and the rapid death after infection of beta2 microglobulin deficient mice in humans has drawn attention to a critical role for CD8(+) T cells. The nature of mycobacterial-specific CD8(+) T cells has been an enigma because few have been identified in any species. Here, we delineate the contribution of class I MHC-restricted T cells in the defense against tuberculosis as transporter associated with antigen processing (TAP)1-deficient mice died rapidly, bore a greater bacterial burden, and had more severe tissue pathology than control mice. In contrast, CD1D-/- mice were not significantly different in their susceptibility to infection than control mice. This data demonstrates a critical role for TAP-dependent peptide antigen presentation and provides further evidence that class I MHC-restricted CD8(+) T cells, the major T cell subset activated by this antigen processing pathway, play an essential role in immunity to tuberculosis.

The CD1 system: antigen-presenting molecules for T cell recognition of lipids and glycolipids

Recent studies have identified the CD1 family of proteins as novel antigen-presenting molecules encoded by genes located outside of the major histocompatibility complex. CD1 proteins are conserved in all mammalian species so far examined and are prominently expressed on cells involved in antigen presentation, which suggests a role in activation of cell-mediated immunity. This has now been confirmed by functional studies demonstrating the ability of CD1 proteins to restrict the antigen-specific responses of T cells in humans and mice. Identification of naturally occurring antigens presented by CD1 has revealed the surprising finding that these are predominantly a variety of foreign lipids and glycolipids, including several found prominently in the cell walls and membranes of pathogenic mycobacteria. Structural, biochemical, and biophysical studies support the view that CD1 proteins bind the hydrophobic alkyl portions of these antigens directly and position the polar or hydrophilic head groups of bound lipids and glycolipids for highly specific interactions with T cell antigen receptors. Presentation of antigens by CD1 proteins requires uptake and intracellular processing by antigen presenting cells, and evidence exists for cellular pathways leading to the presentation of both exogenous and endogenous lipid antigens. T cells recognizing antigens presented by CD1 have a range of functional activities that suggest they are likely to mediate an important component of antimicrobial immunity and may also contribute to autoimmunity and host responses against neoplastic cells.

HLA class I deficiencies due to mutations in subunit 1 of the peptide transporter TAP1

The transporter associated with antigen processing (TAP), which is composed of two subunits (TAP1 and TAP2) that have different biochemical and functional properties, plays a key role in peptide loading and the cell surface expression of HLA class I molecules. Three cases of HLA class I deficiency have previously been shown to result from the absence of a functional TAP2 subunit. In the present study, we analyzed two cases displaying not only the typical lung syndrome of HLA class I deficiency but also skin lesions, and found these patients to be TAP1-deficient. This defect leads to unstable HLA class I molecules and their retention in the endoplasmic reticulum. However, the absence of TAP1 is compatible with life and does not seem to result in higher susceptibility to viral infections than TAP2 deficiency. This work also reveals that vasculitis is often observed in HLA class I-deficient patients.

Immunohistochemical localization of CD1a-positive putative dendritic cells in human breast tumours

The presence of a high number of infiltrating CD1a+ cells in malignant neoplasms has been reported to be associated with an improved prognosis, reduced tumour recurrence and fewer metastases. This study identified a population of CD1a+ cells within the lymphoid cell infiltrate in human ductal breast carcinoma (n = 52), which was significantly different from normal breast tissue, in which only two out of nine cases expressed CD1a+ cells (P = 0.0192). In the majority of cases, the infiltrate was low compared with the number of macrophages and T cells present (results not shown). There was no correlation between the number of CD1a+ cells and tumour grade, with all tumour grades expressing similar numbers of infiltrating CD1a+ cells. There was clear evidence, however, that the CD1a+ cells were closely associated with tumour cells. It is likely that CD1a+ cells have a role in antigen capture and presentation in human tumours, and this study documents the density of CD1a+ cells in a large sample of all histological grades of human breast carcinomas.

Antigen-presenting function of mouse CD1: one molecule with two different kinds of antigenic ligands

Mouse CD1 (mCD1) is an antigen-presenting molecule that is constitutively expressed by most bone marrow-derived cells. Peptides with a hydrophobic binding motif can bind to mCD1, and the peptide-CD1 complex is recognized by CD8+ cytolytic T cells. In contrast, NK1.1+ T cells, which are CD8-, are autoreactive for mCD1 molecules. This autoreactivity, along with the ability of NK T cells to rapidly produce large amounts of cytokine, has led to the suggestion that these cells may be immunoregulatory. We have shown that the mCD1-autoreactive T cells can distinguish between different cell types that express similar levels of mCD1, suggesting that mCD1-bound autologous ligands may be critical for T-cell stimulation. Consistent with this, some of these mCD1-restricted T cells can recognize the glycolipid alpha-galactosylceramide presented by mCD1, while others do not respond. The mCD1 crystal structure reveals a deep and narrow hydrophobic antigen-binding site which can more easily bind lipid antigens than the long hydrophobic peptides that we have defined as mCD1 antigens. The ability of mCD1 to bind and present two different types of ligands raises the question as to how mCD1 can accommodate both types of antigens.

Induction of an antigen-specific, CD1-restricted cytotoxic T lymphocyte response In vivo

The majority of T cell responses are restricted to peptide antigens bound by polymorphic major histocompatibility complex (MHC) molecules. However, peptide antigens can be presented to T cells by murine non-MHC-encoded CD1d (mCD1) molecules, and human T cell lines specific for nonpeptide antigens presented on CD1 isoforms have been identified. It is shown here that antigen-specific, mCD1-restricted lymphocytes can be generated in vivo by immunizing mice with a combination of plasmids encoding chicken ovalbumin, murine CD1d, and costimulatory molecules. Splenocytes from immunized mice have CD1d-restricted, MHC- unrestricted, ovalbumin-specific cytolytic activity that can be inhibited by anti-CD1 antibodies as well as a competing CD1-binding peptide. These results suggest a physiologic role for murine CD1d to present exogenous protein antigens.

The role of CD1 molecules in immune responses to infection

Recent work on CD1 molecules has demonstrated that human CD1b and a lipoglycan from mycobacteria that CD1b presents colocalize to late endosomes. Presentation of this lipoglycan by CD1b requires antigen uptake via the mannose receptor. CD8(+) CD1-restricted T cells can decrease the load of intracellular mycobacteria by granule release. TCR-transgenic and CD1-deficient mice have provided insights into the role of CD1 in the T helper responses required for the clearance of some microorganisms.

Requirements for CD1d recognition by human invariant Valpha24+ CD4-CD8- T cells

A subset of human CD4-CD8- T cells that expresses an invariant Valpha24-JalphaQ T cell receptor (TCR)-alpha chain, paired predominantly with Vbeta11, has been identified. A series of these Valpha24 Vbeta11 clones were shown to have TCR-beta CDR3 diversity and express the natural killer (NK) locus-encoded C-type lectins NKR-P1A, CD94, and CD69. However, in contrast to NK cells, they did not express killer inhibitory receptors, CD16, CD56, or CD57. All invariant Valpha24(+) clones recognized the MHC class I-like CD16 molecule and discriminated between CD1d and other closely related human CD1 proteins, indicating that recognition was TCR-mediated. Recognition was not dependent upon an endosomal targeting motif in the cytoplasmic tail of CD1d. Upon activation by anti-CD3 or CD1d, the clones produced both Th1 and Th2 cytokines. These results demonstrate that human invariant Valpha24+ CD4-CD8- T cells, and presumably the homologous murine NK1+ T cell population, are CD1d reactive and functionally distinct from NK cells. The conservation of this cell population and of the CD1d ligand across species indicates an important immunological function.

Analysis of the requirement for beta 2-microglobulin for expression and formation of human CD1 antigens

Human CD1 form a group of nonpolymorphic leukocyte surface molecules with homology to major histocompatibility complex (MHC) proteins. Recent findings in human and in mouse demonstrate the capacity of CD1 molecules to present nonpeptide components like lipids or lipoglycans as well as peptides. We studied the involvement of beta 2-microglobulin (beta 2m) in expression of the classic human CD1 proteins CD1a, CD1b, and CD1c. The beta 2m-deficient human melanoma cell line FO-1 was transiently transfected with either CD1a, CD1b, or CD1c DNA alone, or in combination with beta 2m using the adenovirus-enhanced receptor-mediated transfer infection system. Only co-transfection of FO-1 cells with CD1+ beta 2m resulted in the detection of CD1 Ag by monoclonal antibodies (mAb). This indicated that CD1 mAb recognized determinants are dependent on beta 2m and raised the question whether beta 2m-free forms of CD1 can be expressed. Therefore, to visualize CD1 molecule expression independently of beta 2m, we expressed tagged recombinant forms. A full-length CD1b construct tagged at the very C terminus with a small peptide was transported to the plasma membrane only when beta 2m was co-transfected. beta 2m involvement in the transport of CD1 was confirmed by expression of soluble forms of CD1a, CD1b, and CD1c in three different cell types. Analogous to tagged full-length CD1b, secretion of the soluble CD1 constructs was strictly dependent on beta 2m. The soluble CD1 chimeras were secreted as complexes with endogenous beta 2m. Thus, similar to its role for MHC class I expression, beta 2m is essential for processing and surface transport of the classic human CD1 molecules CD1a, CD1b, and CD1c.

Impaired NK1+ T cell development and early IL-4 production in CD1-deficient mice

The MHC class lb molecule, CD1, has been conserved throughout mammalian evolution. To assess the function of CD1 in lymphocyte development, we generated mice with targeted disruption of the CD1.1 and CD1.2 genes. CD1-deficient mice have normal numbers of CD4+ and CD8+ T cells but marked reduction in NK1.1-bearing T cells, particularly those with a canonical gene rearrangement of V alpha14-J alpha281. CD1-deficient mice are unable to generate a rapid IL-4 response following systemic T cell activation but can generate effective antigen-specific Th2 responses. Thus, CD1 is required for the development of a specialized subset of T lymphocytes with a monomorphic antigen receptor. The rapid effector cytokine secretion of these T cells suggests that CD1 educates adaptive immune cells to subserve functions of innate immunity.

Cell stress-regulated human major histocompatibility complex class I gene expressed in gastrointestinal epithelium

Conventional major histocompatibility complex (MHC) class I genes encode molecules that present intracellular peptide antigens to T cells. They are ubiquitously expressed and regulated by interferon gamma. Two highly divergent human MHC class I genes, MICA and MICB, are regulated by promoter heat shock elements similar to those of HSP70 genes. MICA encodes a cell surface glycoprotein, which is not associated with beta 2-microglobulin, is conformationally stable independent of conventional class I peptide ligands, and almost exclusively expressed in gastrointestinal epithelium. Thus, this MHC class I molecule may function as an indicator of cell stress and may be recognized by a subset of gut mucosal T cells in an unusual interaction.

Antigen-presenting function of the mouse CD1 molecule

CD1 molecules are distantly related to major histocompatibility complex (MHC)-encoded class I molecules, and they are coexpressed with beta2 microglobulin (beta2m). In the mouse, CD1 is expressed by intestinal epithelial cells and also by some cells in spleen and lymph node. We have shown that surface expression of mouse CD1 (mCD1) is not dependent upon a functional transporter associated with antigen processing (TAP). This, and other data, suggest that mCD1 may acquire peptides in an intracellular compartment other than the endoplasmic reticulum, where classical class I molecules bind peptide. mCD1 molecules also are distinct from classical class I molecules with regard to the types of peptides that they bind. We have demonstrated that mCD1 molecules preferentially bind peptides much longer than the 8-9 amino acids typical of the peptides that bind to classical class I molecules. The sequence motif for mCD1 peptide binding is characterized by the presence of bulky and hydrophobic amino acid side chains. We have generated mCD1-restricted and peptide-specific T-cell lines, thereby demonstrating the immunologic relevance of peptide binding to mCD1. The reactive T cells are TCR alphabeta+ and CD8+, a phenotype typical of many lymphocytes in both lymph node and intestinal mucosae. We speculate that mCD1 molecules may be capable of sampling peptides from the gut lumen and presenting them to mucosal T lymphocytes. In this way, they may function in the maintenance of normal mucosal homeostasis, and perhaps also in the induction of systemic tolerance to antigens delivered by the oral route. In summary, CD1 molecules are a novel category of antigen-presenting molecules that have features in common with class I molecules, features in common with class II, and properties distinct from either subset of antigen-presenting molecules. Further studies of the antigen-presenting function of these molecules are certain to yield new insight into immune regulation and perhaps also into the mechanism of oral tolerance.

Structure and function of the CD1 family of MHC-like cell surface proteins

The CD1 family of proteins are structurally related to MHC class I proteins, but are only distantly related to the class I proteins or other MHC-linked class I-like proteins. Sequence comparisons indicate that the CD1 proteins have evolved into two subfamilies, those which are similar to human CD1a, b, and c and those which are similar to human CD1d. The CD1A-, B-, and C-like genes were deleted from rodents and the CD1D gene was duplicated. CD1a, b, and c are expressed by thymocytes, dendritic cells, activated monocytes, and B cells (CD1c), a tissue distribution which strongly suggests a role in antigen presentation. In contrast, CD1d and its murine homologues are expressed by many cells outside of the lymphoid and myeloid lineages. The CD1 proteins are in most cases expressed as beta 2mg-associated membrane glycoproteins, but may associate with additional proteins. CD1d is expressed on the surface of intestinal epithelial cells in a nonglycosylvated form without beta 2mg. Whether the CD1 proteins function as antigen-presenting molecules is unresolved, but it is unlikely that they present conventional peptide antigens. Strong evidence indicates that murine CD1 proteins are recognized by a population of NK1.1+, CD4+ or CD4-CD8- (double negative, DN) T cells which express an invariant TCR alpha chain. CD1d is most likely recognized by the homologous T cell population in humans. DN alpha beta T cells which recognize CD1a, b, or c have been isolated, including clones which recognize a lipid antigen from mycobacteria presented by CD1b. A third potential population of CD1 reactive cells are CD8+ T cells in the intestinal epithelium. Taken together, these observations indicate that CD1 proteins interact with several specialized populations of T cells. The precise biological functions mediated through these interactions remain to be determined.