The CD1 family: a third lineage of antigen-presenting molecules

The distribution of CD1 molecules in inflammatory neuropathy

The CD1 molecules have been shown to present non-protein antigens, such as complex lipids to Mycobacteria, and may be important in presenting glycolipids which are involved in inflammatory neuropathies. To study the expression of CD1 molecules in peripheral nerve, we examined nerve biopsies from two patients with acute inflammatory demyelinating polyradiculoneuropathy (AIDP), five with acute axonal neuropathy, six with chronic inflammatory demyelinating polyradiculoneuropathy (CIDP), nine with chronic axonal neuropathy, six with vasculitic neuropathy and three with no histological abnormality. Immunocytochemical studies showed strong labelling of CD1b on endoneurial macrophages (CD68+) and on myelinated nerve fibres in both AIDP patients, but it was rarely observed in the other patients. Weaker staining was seen on endoneurial macrophages and/or other endoneurial cells in some of the patients with other peripheral neuropathies, but none of the control nerves. CD1a had a weaker, but similar pattern. There was endoneurial infiltration of CD4+ and CD8+ T cells in the AIDP and CIDP nerves and sometimes in the other peripheral neuropathy nerves, but not in the normal nerves. Most T cells had alpha beta+ T cell receptors (TCR), but gamma delta+ TCR T cells were found in the nerves of both AIDP patients and sometimes in the nerves of other patients with peripheral neuropathy. Staining for mannose receptor was almost universal, being more intense in AIDP, chronic axonal neuropathy and vasculitis patients. We conclude that CD1 molecule expression is upregulated in peripheral neuropathy, especially in association with inflammation.

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.

Tissue distribution, regulation and intracellular localization of murine CD1 molecules

CD1 molecules are MHC-unlinked class Ib molecules consisting of classical (human CD 1a-c) and non-classical subsets (human CD1d and murine CD1). The characterization of non-classical subsets of CD1 is limited due to the lack of reagents. In this study, we have generated two new anti-mouse CD1 monoclonal antibodies, 3H3 and 5C6, by immunization of hamsters with purified CD1 protein. These antibodies recognize CD1-transfected cells and have no reactivity to cells isolated from CD1-/- mice. Both antibodies precipitate the 52 kDa heavy chain and 12 kDa beta2m from thymocytes and splenocytes by radio-immunoprecipitation. Deglycosylation of CD1 reduces molecular mass of the heavy chain by 7.5 kDa, which can be detected by 3H3 but not 5C6. 3H3 and 5C6 detect surface CD1 expression on cells from the thymus, spleen, lymph node and bone marrow, but not on intestinal epithelial cells. Developmentally, CD1 is expressed on thymocytes prior to TCR rearrangement and remains constant throughout thymic development. CD1 is expressed early in the fetal liver (day 14) and remains expressed in hepatocytes postnatally. These data support evidence of a role for CD1 in the selection and/or expansion of NK1- T cells of both thymic origin and extrathymic origin. Unlike classical class I molecules, murine CD1 levels are not affected by IFN-gamma, but like human CD1b can be up-regulated by IL-4 and GM-CSF although only moderately. Similar to human CD1b, murine CD1 is found by immunofluorescence microscopy on the cell surface, and in various intracellular vesicles, including early and late endosomes. Localization in endocytic compartments indicates that murine CD1 may be capable of binding endocytosed antigens.

CD1--a new paradigm for antigen presentation and T cell activation

Despite identification of the CD1 family of molecules in the late 1970s, the function of CD1 was undetermined for more than a decade. Recent evidence has established that CD1 molecules comprise a novel lineage of antigen-presenting molecules, distinct from major histocompatibility complex (MHC) class I and class II molecules. Unlike the MHC molecules, which bind short peptides in their antigen-binding groove for presentation to either CD4+ or CD8+ T cells bearing alpha beta T cell receptors, the CD1 molecules appear to accommodate lipid and glycolipid antigens in their hydrophobic cavity for presentation to a wide variety of T cells, including double-negative alpha beta and gamma delta T cells and CD8+ alpha beta T cells. By using a unique cytoplasmic signal, some CD1 molecules traffic to endosomal compartments for sampling mycobacteria-derived lipid antigens, and subsequently lipid antigen-loaded CD1 molecules are expressed on the cell surface to activate specific T cells. These CD1-restricted T cells kill mycobacteria-infected cells and secrete interferon-gamma, indicating a potential role of CD1-mediated T cell responses in clearing mycobacterial infection. The identification of an MHC-independent antigen presentation pathway for nonpeptide antigens provides new insights into immunoregulation and host defense.

The tyrosine-containing cytoplasmic tail of CD1b is essential for its efficient presentation of bacterial lipid antigens

CD1b is an antigen-presenting molecule that mediates recognition of bacterial lipid and glycolipid antigens by specific T cells. We demonstrate that the nine-amino acid cytoplasmic tail of CD1b contains all of the signals required for its normal endosomal targeting, and that the single cytoplasmic tyrosine is a critical component of the targeting motif. Mutant forms of CD1b lacking the endosomal targeting motif are expressed at high levels on the cell surface but are unable to efficiently present lipid antigens acquired either exogenously or from live intracellular organisms. These results define the functional role of the CD1b targeting motif in a physiologic setting and demonstrate its importance in delivery of this antigen-presenting molecule to appropriate intracellular compartments.

Rifampin increases cytokine-induced expression of the CD1b molecule in human peripheral blood monocytes

In recent years, it has been shown that a nonclassical, major histocompatibility complex-independent system (i.e., CD1-restricted T-cell responses) is involved in T-cell immunity against nonpeptide antigens. The CD1 system appears to function by presenting microbial lipid antigens to specific T cells, and the antigens so far identified include several known constituents of mycobacterial cell walls. Among the four known human CD1 isoforms, the CD1b protein is the best characterized with regard to its antigen-presenting function. Expression of CD1b is upregulated on human blood monocytes upon exposure to granulocyte/macrophage-colony stimulating factor, alone or in combination with interleukin-4 (IL-4) (S. A. Porcelli, Adv. Immunol. 59:1-98, 1995). Rifampin (RFP) and its derivatives are widely used for chemoprophylaxis or chemotherapy against Mycobacterium tuberculosis. However, this agent was found to reduce the mitogen responsiveness of human B and T lymphocytes, chemotaxis, and delayed-type hypersensitivity. The present study extends the immunopharmacological profile of RFP by examining its effects on CD1b expression by human peripheral blood monocytes exposed to GM-CSF plus IL-4. The results showed that clinically attainable concentrations (i.e., 2 or 10 microg/ml for 24 h) of the agent produced a marked increase in CD1b expression on the plasma membrane, as evaluated by fluorescence-activated cell sorter analysis, whereas it had no effect on cytosolic fractions, as indicated by Western blot analysis. This was found to be the result of increased CD1b gene expression, as shown by Northern blot analysis of CD1b mRNA. These results suggest that RFP could be of potential value in augmenting the CD1b-restricted antigen recognition system, thereby enhancing protective cellular immunity to M. tuberculosis.

Molecular interaction of CD1b with lipoglycan antigens

The ability of human CD1b molecules to present nonpeptide antigens is suggested by the T cell recognition of microbial lipids and lipoglycans in the presence of CD1b-expressing antigen-presenting cells. We demonstrate the high-affinity interaction of CD1b molecules with the acyl side chains of known T cell antigens, lipoarabinomannan, phosphatidylinositol mannoside, and glucose monomycolate. Furthermore, CD1b-antigen binding was optimal at acidic pH, consistent with the known requirement for endosomal acidification in CD1b-restricted antigen presentation. The mechanism for CD1b-ligand interaction involves the partial unfolding of the alpha helices of CD1b at acidic pH, revealing a hydrophobic binding site that could accommodate lipid. These data provide direct evidence that the CD1b molecule has evolved unique biochemical properties that enable the binding of lipid-containing antigens from intracellular pathogens.

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.

Unusual MHC-like molecules: CD1, Fc receptor, the hemochromatosis gene product, and viral homologs

The MHC fold, with its well-characterized peptide-binding groove, can perform other functions in addition to presentation of antigenic peptides to T cells. Homologs of MHC molecules have diverse roles that include presentation of lipid antigens (by CD1), transport of immunoglobulins (by the neonatal Fc receptor), regulation of iron metabolism (by the hemochromatosis gene product, HFE), and deception of the host immune system (by viral homologs). Recent crystal structures of two of these non-standard MHC-like molecules have allowed comparison of the recognition properties of classical. MHC molecules with those of their unusual homologs.

Mycobacterial lipoarabinomannan: an extraordinary lipoheteroglycan with profound physiological effects

Detailed structural and functional studies over the last decade have led to current recognition of the mycobacterial lipoarabinomannan (LAM) as a phosphatidylinositol anchored lipoglycan with diverse biological activities. Fatty acylation has been demonstrated to be essential for LAM to maintain its functional integrity although the focus has largely been on the arabinan motifs and the terminal capping function. It has recently been shown that the mannose caps may be involved not only in attenuating host immune response, but also in mediating the binding of mycobacteria to and subsequent entry into macrophages. This may further be linked to an intracellular trafficking pathway through which LAM is thought to be presented by CD1 to subsets of T-cells. The implication of LAM as major histocompatibility complex (MHC)-independent T-cell epitope and the ensuing immune response is an area of intensive studies. Another recent focus of research is the biosynthesis of arabinan which has been shown to be inhibitable by the anti-tuberculosis drug, ethambutol. The phenomenon of truncated LAM as synthesized by ethambutol resistant strains provides an invaluable handle for dissecting the array of arabinosyltransferases involved, as well as generating much needed structural variants for further structural and functional studies. It is hoped that with more systematic investigations based on clinical isolates and human cell lines, the true significance of LAM in the immunopathogenesis of tuberculosis and leprosy can eventually be explained.

Monoclonal antibodies putatively identifying porcine B cells

Comparison was made of the binding of 38 test and three standard monoclonal antibodies (mAbs) to B cells from various pig lymphoid tissues by flow cytometry (FCM) and immunohistochemistry. Some mAbs were also tested on B cells from foetal pig tissues. Twenty of the new mAbs bound, though to variable degrees, to porcine B cells but only three were given cluster assignations: C35 (#147) and BB6-11C9 (#167) were assigned to wCD21 and 2F6/8 (#057) was assigned to SWC7.

Human CD8+ TCR-alpha beta(+) and TCR-gamma delta(+) cells modulate autologous autoreactive neuroantigen-specific CD4+ T-cells by different mechanisms

To investigate the regulatory interactions among autologous T-cells during the course of multiple sclerosis (MS), proteolipid protein peptide-specific CD4+ T-cell clones (TCCs) were irradiated and used as immunogens to stimulate purified populations of autologous CD8+ TCR-alpha beta+ and TCR-gamma delta+ T-cells isolated from the peripheral blood of MS patients, patients with other non-inflammatory neurological diseases, and healthy blood donors. The resulting blasts were expanded in the presence of hIL-2 and then cloned by limiting dilution. Two different groups of CD8+ TCCs were revealed. A first group of CD8+ TCCs recognized autologous CD4+ T-cells based in their TCRV beta structures (anti-idiotypic responsiveness). A second group of CD8+ TCCs recognized Ag activated autologous CD4+ TCCs irrespective of their Ag specificity or TCRV beta expression (anti-ergotypic responsiveness). Both groups showed MHC class I restricted cytotoxicity against CD4+ T-cells and were able to secrete IFN-gamma, TNF alpha/beta and TGF-beta. TCR-gamma delta+ TCCs isolated in response to stimulation with autologous peptide-specific CD4+ TCCs showed only anti-ergotypic cytotoxicity, which was not inhibited by anti-MHC class Ia monoclonal antibodies. Moreover, they were able to secrete IFN-gamma and TNF alpha/beta, but not TGF-beta. These data demonstrate that regulatory mechanisms among human autologous T-cells can be mediated by cytolytic interactions or by the release of specific cytokines. Furthermore, they provide evidence that CD8+ TCR-alpha beta+ and TCR-gamma delta+ cells differ in their patterns of recognition and in their abilities to modulate the immune response mediated by autologous autoreactive CD4+ T-cells.

IL-13 induces CD34+ cells isolated from G-CSF mobilized blood to differentiate in vitro into potent antigen presenting cells

Dendritic cells (DCs), which are antigen presenting cells of potential use in human antitumor vaccination trials, are presently the subject of intense investigation. Many recent studies have reported the possibility of generating ex vivo large numbers of DCs with high antigen presenting capacity by the culture of bone marrow or blood progenitors. In this study, we examined the differentiation into DCs of CD34+ progenitors isolated from the G-CSF mobilized blood of 3 healthy donors and 5 patients with breast cancer and cultured in the presence of GM-CSF + IL-13. The characteristics of the cells were compared to those of cells obtained in the presence of GM-CSF + TNF alpha. By day 15, one third of the bulk cells cultured with IL-13 were CD1a+/CD14- and strongly expressed CD1c, CD40, CD80 and HLA-DR. In contrast, cells obtained with TNF alpha expressed CD1a on one in three cells but with a considerably lower fluorescence intensity than on IL-13-cultured cells and strongly expressed CD14 on more than 50% of cells. CD1a+/CD14- cells emerged in IL-13 cultures at day 5, while in TNF alpha cultures CD14+ cells appeared before CD1a+ cells. Cells grown in the presence of IL-13 had an increased capacity to present antigens to autologous lymphocytes and to stimulate allogeneic T-lymphocytes. This effect was greater than that of cells grown in the presence of TNF alpha. These cells should therefore have greater effector potential in any therapeutic applications in humans.

Structural requirements for glycolipid antigen recognition by CD1b-restricted T cells

The human CD1b protein presents lipid antigens to T cells, but the molecular mechanism is unknown. Identification of mycobacterial glucose monomycolate (GMM) as a CD1b-presented glycolipid allowed determination of the structural requirements for its recognition by T cells. Presentation of GMM to CD1b-restricted T cells was not affected by substantial variations in its lipid tails, but was extremely sensitive to chemical alterations in its carbohydrate or other polar substituents. These findings support the view that the recently demonstrated hydrophobic CD1 groove binds the acyl chains of lipid antigens relatively nonspecifically, thereby positioning the hydrophilic components for highly specific interactions with T cell antigen receptors.

Formation of Birbeck granule-like structures in vascular dendritic cells in human atherosclerotic aorta. Lag-antibody to epidermal Langerhans cells recognizes cells in the aortic wall

It has previously been demonstrated that vascular dendritic cells reside in the arterial intima and are involved in human atherogenesis. During the present ultrastructural examination of aortic atherosclerotic lesions, pentalaminal structures, similar to Birbeck granules which uniquely present in Langerhans cells, were found in the cytoplasm of vascular dendritic cells and the formation of these Birbeck granule-like structures from dense granules was identified. To find out how Birbeck granule-like structures might relate to Birbeck granules of Langerhans cells, we used Lag-antibody which specifically stains Birbeck granules and Birbeck granule-associated structures in Langerhans cells. Lag-positive cells were found in the aortic wall. Our observations suggest a close relationship between vascular dendritic cells and Langerhans cells and this may imply that mechanisms of antigen presentation known for Langerhans cells might be similar to those involved in atherosclerosis.

Interleukin-4 and listeriosis

Experimental infection of mice with Listeria monocytogenes (L. monocytogenes) has served as an appropriate model for analyzing Th1-cell-driven immune responses. Generally, Th2 responses are absent and IL-4 is not detectable. Here, we describe experimental settings under which IL-4 is detectable in listeriosis. Our data suggest that IL-4 is rapidly produced after infection. This prompt IL-4 burst seems to stimulate chemokine responses and, therefore, may participate in the regulation of the early antilisterial host response. Soon thereafter, IL-4 production wanes. At least partially this seems to be caused by downregulation of IL-4-producing CD4+ NK1+ TCR alpha beta int lymphocytes by IL-12. In the absence of IFN-gamma responsiveness, IL-4 production is demonstrable during acquired immunity against L monocytogenes, and this elevated IL-4 production apparently contributes to disease exacerbation. In conclusion, the data are consistent with a detrimental role of IL-4 in listeriosis and active control of IL-4 synthesis by the antilisterial immune response. The rapid, but transient, IL-4 burst in listeriosis probably contributes to host defense without impairing development of the acquired T-cell response because of its shortness.

Antigen presentation: mixing oil and water

Studies of the cellular immunology and protein structure of human and mouse CD1 proteins reveal these to be a family of antigen-presenting molecules that allow T cells to recognize lipid and glycolipid antigens of microbial pathogens.

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.

Crystal structure of mouse CD1: An MHC-like fold with a large hydrophobic binding groove

CD1 represents a third lineage of antigen-presenting molecules that are distantly related to major histocompatibility complex (MHC) molecules in the immune system. The crystal structure of mouse CD1d1, corresponding to human CD1d, at 2.8 resolution shows that CD1 adopts an MHC fold that is more closely related to that of MHC class I than to that of MHC class II. The binding groove, although significantly narrower, is substantially larger because of increased depth and it has only two major pockets that are almost completely hydrophobic. The extreme hydrophobicity and shape of the binding site are consistent with observations that human CD1b and CD1c can present mycobacterial cell wall antigens, such as mycolic acid and lipoarabinomannans. However, mouse CD1d1 can present very hydrophobic peptides, but must do so in a very different way from MHC class Ia and class II molecules.

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.

Leishmania vaccines: Old and New

The approach to the development of a Leishmania vaccine has undergone a revolution since its early beginnings with the ancient practice of leishmazation: the inoculation of infectious parasites from an active lesion in order to produce a self-healing lesion in a healthy individual. Controlled infection has been followed by injection of killed parasites and has now progressed to subunit and naked DNA vaccines. Emanuela Handman here discusses the current studies and the future prospects for a Leishmania vaccine with a focus on cutaneous leishmaniasis. Unfortunately, what J.F. Williams said about antiparasite vaccines in 1987 (Ref. 1) is still true in 1997: 'the reasons for optimism are less evident than the reasons for enthusiasm'.

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.

Dendritic cells: therapeutic potentials

Dendritic cells (DCs) are leukocytes that are specialized to capture antigens and initiate T-cell-mediated immune responses. After capture of antigens, DCs, then in an immature stage, leave their tissue of residence and migrate through the lymph/blood into secondary lymphoid organs where they differentiate into mature cells. Because DCs can prime animals in the absence of any other adjuvant, they have been termed 'nature's adjuvant'. Large numbers of DCs can now be generated from circulating monocytes or from CD34 hematopoietic progenitors in response to GM-CSF in combination with either IL4 or TNF alpha. In mice, tumor antigen loaded DCs have been shown to prevent the development of tumors and even to induce the regression of established tumors. DCs therapy represents a very promising approach to the treatment of cancer and infectious diseases. Early studies indicate the existence of DC populations that can induce tolerance and may prove useful in organ transplantation.

A novel inhibitory receptor (ILT3) expressed on monocytes, macrophages, and dendritic cells involved in antigen processing

Immunoglobulin-like transcript (ILT) 3 is a novel cell surface molecule of the immunoglobulin superfamily, which is selectively expressed by myeloid antigen presenting cells (APCs) such as monocytes, macrophages, and dendritic cells. The cytoplasmic region of ILT3 contains putative immunoreceptor tyrosine-based inhibitory motifs that suggest an inhibitory function of ILT3. Indeed, co-ligation of ILT3 to stimulatory receptors expressed by APCs results in a dramatic blunting of the increased [Ca2+]i and tyrosine phosphorylation triggered by these receptors. Signal extinction involves SH2-containing protein tyrosine phosphatase 1, which is recruited by ILT3 upon cross-linking. ILT3 can also function in antigen capture and presentation. It is efficiently internalized upon cross-linking, and delivers its ligand to an intracellular compartment where it is processed and presented to T cells. Thus, ILT3 is a novel inhibitory receptor that can negatively regulate activation of APCs and can be used by APCs for antigen uptake.

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.

CD1d1 mutant mice are deficient in natural T cells that promptly produce IL-4

Murine CD1 has been implicated in the development and function of an unusual subset of T cells, termed natural T (NT) cells, that coexpress the T cell receptor (TCR) and the natural killer cell receptor NK1.1. Activated NT cells promptly produce large amounts of IL-4, suggesting that these cells can influence the differentiation of CD4+ effector T cell subsets. We have generated mice that carry a mutant CD1d1 gene. NT cell numbers in the thymus, spleen, and liver of these mice were dramatically reduced. Activated splenocytes from mutant mice did not produce IL-4, whereas similarly treated wild-type splenocytes secreted large amounts of this cytokine. These results demonstrate a critical role for CD1 in the positive selection and function of NT cells.

Cloning of novel immunoglobulin superfamily receptors expressed on human myeloid and lymphoid cells: structural evidence for new stimulatory and inhibitory pathways

We have identified two novel human cDNA encoding transmembrane proteins of the immunoglobulin superfamily (IgSF). The two cDNA, called immunoglobulin-like transcripts 1 and 2 (ILT1 and ILT2), are expressed in myeloid and lymphoid cells and are homologous to bovine Fc gamma2R, human killer cell inhibitory receptors (KIR), human Fc alphaR, and mouse gp49. Furthermore, ILT1 and ILT2 are encoded on chromosome 19, as are Fc alphaR and KIR. While the ILT1 and ILT2 extracellular domains are homologous, the transmembrane and cytoplasmic domains differ substantially. ILT1 has an arginine within the transmembrane region, followed by a short cytoplasmic tail, similar to human Fc alphaRI and bovine Fc gamma2R. ILT2 has a long cytoplasmic tail, which contains two YxxV and two YxxL pairs similar to the immunoreceptor tyrosine-based inhibitory motifs in KIR that are known to bind the phosphotyrosine phosphatase SHP-1. These cytoplasmic features suggest that ILT1 and ILT2 may mediate novel transmembrane signals by which myeloid and lymphoid cell responses can be either activated or inhibited.

The mannose receptor delivers lipoglycan antigens to endosomes for presentation to T cells by CD1b molecules

We have characterized the CD1b-mediated presentation pathway for the mycobacterial lipoglycan lipoarabinomannan (LAM) in monocyte-derived antigen-presenting cells. The macrophage mannose receptor (MR) was responsible for uptake of LAM. Antagonism of MR function inhibited both the internalization of LAM and the presentation of this antigen to LAM-reactive T cells. Intracellular MRs were most abundant in early endosomes, but they also were located in the compartment for MHC class II antigen loading (MIIC). Internalized LAM was transported to late endosomes, lysosomes, and MIICs. MRs colocalized with CD1b molecules, suggesting that the MR could deliver LAM to late endosomes for loading onto CD1b. LAM and CD1b colocalized in organelles that may be sites of lipoglycan antigen loading. This pathway links recognition of microbial antigens by a receptor of the innate immune system to the induction of adaptive T cell responses.

Cytokine-induced expression of CD1b molecules by peripheral blood monocytes: influence of 3'-azido-3'-deoxythymidine

CD1b is a nonpolymorphic, MHC-like molecule, capable of presenting non-peptide antigens (Ags) to CD3+, CD4-, CD8-, alphabeta or gammadelta T lymphocytes. Previous studies have shown that CD1b can be induced in monocytes/macrophages by GM-CSF+IL-4, and can restrict their presentation of Mycobacterium tuberculosis antigen (Ag) to Ag-specific T cells. Since a number of HIV-positive subjects undergo mycobacterial infections, preliminary studies have been performed to explore whether anti-HIV chemotherapy would influence cytokine-induced CD1b expression in peripheral blood monocytes. The results obtained by treating monocytes with GM-CSF+IL-4, in presence or absence of 3'-azido-3'-deoxythymidine (AZT) showed that: (a) the majority of adherent mononuclear cells (AMNC) collected from peripheral blood of healthy donors, express CD1b molecule on the cell membrane, upon treatment with GM-CSF+IL-4; (b) CD1b appearance is mainly due to the de novo induction of CD1b gene expression (as confirmed by Northern blot analysis), rather than to migration of the molecule from the cytoplasm to the plasma membrane (as suggested by Western blot analysis); (c) AZT does not alter the percentage of CD1b+ AMNC treated with the cytokines; (d) however, AZT inhibits cytokine-induced proliferation of AMNC, thus reducing the overall Ag-presenting potential of the host. Our results suggest that the anti-proliferative effect of AZT could depress anti-mycobacteria immunity in AZT-treated subjects, which may have important implication for the clinical outcome of patients harbouring inadequately treated mycobacterial infections.

Bacille Calmette Guérin and interleukin-12 down-modulate interleukin-4-producing CD4+ NK1+ T lymphocytes

Early production of interleukin-12 (IL-12) by macrophages and of IL-4 from CD4+ NK1+ T cells influence development of the acquired immune response against infectious agents, namely differentiation of interferon-gamma-secreting T helper 1 (Th1) cells against intracellular pathogens and of IL-4-producing Th2 cells against helminths. Evidence has been presented for transient convertibility of Th1 and Th2 cells in the presence of the polarizing cytokines IL-4 or IL-12, respectively. Moreover, it is likely that IL-4 dominates over IL-12, suggesting that Th2 cell development is preferred in the presence of both cytokines. Mycobacterium bovis Bacille Calmette Guerin (BCG) and IL-12 are potent inducers of Th1 responses. Here we show that BCG and IL-12 down-modulate IL-4-producing CD4+ NK1+ TCR alpha/beta(intermediate) liver lymphocytes. Our data provide further insights into the mechanisms by which BCG and IL-12 may promote unrestricted development of Th1 responses in vivo: BCG and IL-12 not only provide the positive stimuli for Th1 cell differentiation, but also interfere with antagonizing signals.

T-cell recognition of non-peptide antigens

Studies of two distinct human T-cell systems have provided the exciting finding that T cells are able to recognize non-peptide antigens: gammadelta T cells have been shown to recognize isopentenyl pyrophosphate and related structures and human CD1 has been shown to present microbial lipids and lipoglycans such as mycolic acids and lipoarabinomannan to T cells. T-cell responses to these non-peptide antigens should provide a strategic target for immunologic intervention in infectious disease.

Cytoplasmic tail-dependent localization of CD1b antigen-presenting molecules to MIICs

CD1 proteins have been implicated as antigen-presenting molecules for T cell-mediated immune responses, but their intracellular localization and trafficking remain uncharacterized. CD1b, a member of this family that presents microbial lipid antigens of exogenous origin, was found to localize to endocytic compartments that included the same specialized subset of endosomes in which major histocompatibility complex (MHC) class II molecules are proposed to bind endocytosed antigens. Unlike MHC class II molecules, which traffic to antigen-loading endosomal compartments [MHC class II compartments (MIICs)] primarily as a consequence of their association with the invariant chain, localization of CD1b to these compartments was dependent on a tyrosine-based motif in its own cytoplasmic tail.

CD1b is expressed in multiple sclerosis lesions

Recent observations have shown that CD1 molecules act as restriction elements in the presentation of antigens to specialized subsets of T cells. To examine the expression of CD1 molecules in multiple sclerosis (MS) lesions, frozen sections of central nervous system (CNS) tissues from nine MS and three other neurological disease (OND) patients, one patient with Wilson's disease, and one non-neurological control were stained by immunocytochemistry. In chronic-active MS lesions, CD1b immunoreactivity was prominent on perivascular inflammatory cells whereas macrophages within the lesion showed little reactivity. At the lesion edge, intense immunoreactivity for CD1b was found on hypertrophic astrocytes. High level expression of CD1b in MS lesions was found to colocalize with the presence of GM-CSF in astrocytes. In chronic-silent lesions, CD1b expression was found on only a few perivascular astrocytic foot processes and the occasional perivascular macrophage. CD1b was not found in the tissues studied for control purposes. In contrast, MHC class II expression was detected on microglia in all tissues examined. The relatively low level expression of CD1b in normal-appearing tissues, chronic-silent lesions and in the OND controls supports the conclusion that the expression of CD1b in active MS lesions is significantly upregulated and could contribute to lesion development.

gamma/delta and other unconventional T lymphocytes: what do they see and what do they do?

T lymphocytes recognize specific ligands by clonally distributed T-cell receptors (TCR). In humans and most animals, the vast majority of T cells express a TCR composed of an alpha chain and a beta chain, whereas a minor T-cell population is characterized by the TCR gamma/delta. Almost all of our knowledge about T cells stems from alpha/beta T cells and only now are we beginning to understand gamma/delta T cells. In contrast to conventional alpha/beta T cells, which are specific for antigenic peptides presented by gene products of the major histocompatibility complex, gamma/delta T cells directly recognize proteins and even nonproteinacious phospholigands. These findings reveal that gamma/delta T cells and alpha/beta T cells recognize antigen in a fundamentally different way and hence mitigate the dogma of exclusive peptide-major histocompatibility complex recognition by T cells. A role for gamma/delta T cells in antimicrobial immunity has been firmly established. Although some gamma/delta T cells perform effector functions, regulation of the professional and the nonprofessional immune system seems to be of at least equal importance. The prominent residence of gamma/delta T cells in epithelial tissues and the rapid mobilization of gamma/delta T cells in response to infection are consistent with such regulatory activities under physiological and pathologic conditions. Thus, although gamma/delta T cells are a minor fraction of all T cells, they are not just uninfluential kin of alpha/beta T cells but have their unique raison d'être.

Influence of beta 2-microglobulin expression on gamma interferon secretion and target cell lysis by intraepithelial lymphocytes during intestinal Listeria monocytogenes infection

Numerous microbial pathogens, including Listeria monocytogenes, enter the host through the intestine. Although relatively little is known about the biological functions of intestinal intraepithelial lymphocytes (i-IEL), they are generally considered a first line of defense against intestinal infections. In the mouse, the vast majority of i-IEL express the CD8 coreceptor either as a CD8 alpha/alpha homodimer or as a CD8 alpha/beta heterodimer. The CD8 receptor of T-cell receptor TcR gamma/delta i-IEL is exclusively homodimeric, whereas the CD8-expressing TcR alpha/beta i-IEL segregate into equal fractions of CD8 alpha/alpha and CD8 alpha/beta cells. We infected beta 2-microglobulin (beta 2m)+/- mice (possessing all i-IEL populations) and beta 2m -/- mutant mice (lacking all CD8 alpha/beta + i-IEL and having few CD8 alpha/alpha + TcR alpha/beta i-IEL) with L. monocytogenes per os and determined their biological functions after TcR ligation with monoclonal antibodies. Cytolytic activities of TcR alpha/beta and TcR gamma/delta i-IEL from beta 2m +/- mice were not influenced by intestinal listeriosis. Cytolytic activities of TcR alpha/beta i-IEL were impaired in uninfected beta 2m -/- mice, but this reduction was reestablished as a consequence of intestinal listeriosis. Frequencies of gamma interferon (IFN-gamma)-producing TcR alpha/beta i-IEL in uninfected beta 2m -/- mice were reduced, compared with that in their heterozygous controls. Equally low frequencies of IFN-gamma-producing TcR gamma/delta i-IEL in beta 2M +/- and beta 2m-/- mutants were found. Listeriosis increased frequencies of INF-gamma-producing TcR alpha/beta and TcR gamma/delta i-IEL in both mouse strains. Most remarkably, the proportion of IFN-gamma-producing TcR gamma/delta i-IEL was elevated 10-fold in listeria-infected beta 2M -/- mice. Our findings show that the beta 2m-independent CD8 beta- i-IEL expressing either TcR alpha/beta or TcR gamma/delta are stimulated by intestinal listeriosis independent of regional beta 2m expression. We conclude that the three major CD8+ i-IEL populations are stimulated by intestinal listeriosis and that CD8 beta- i-IEL compensate for the total lack of CD8 beta+ i-IEL in beta 2M -/- mutant mice. Hence, in contrast to the peripheral immune system, which crucially depends on CD8 alpha/beta + TcR alpha/beta lymphocytes, the mucosal immune system can rely on additional lymphocytes expressing the CD8 alpha/alpha homodimer.

Antigen presentation by CD1 and MHC-encoded class I-like molecules

Three known lineages of antigen-presenting molecules restrict T-cell responses to microbial antigens: MHC class I and MHC encoded class I like molecules present peptides derived from the proteolysis of intracellular pathogens, MHC class ii molecules present peptides derived from the proteolysis of extracellular pathogens and CD1 molecules present unique microbial lipids and glycolipids. Recent studies have indicated that CD1 molecules mediate a novel system of antigen presentation and that MHC-encoded class I-like molecules can present unique subsets of intracellularly derived peptides.

A pathway of costimulation that prevents anergy in CD28- T cells: B7-independent costimulation of CD1-restricted T cells

A class of molecules that is expressed on antigen presenting cells, exemplified by CD80 (B7), has been found to provide a necessary costimulatory signal for T cell activation and proliferation. CD28 and CTLA4 are the B7 counterreceptors and are expressed on the majority of human CD4+ T cells and many CD8+ T cells. The signal these molecules mediate is distinguished from other costimulatory signals by the finding that T cell recognition of antigen results in a prolonged state of T cell unresponsiveness or anergy, unless these costimulatory molecules are engaged. However, nearly half of the CD8+ and CD4-CD8- T cells lack CD28, and the costimulatory signals required for the activation of such cells are unknown. To understand the pathways of activation used by CD28- T cells, we have examined the costimulatory requirements of antigen-specific CD4-CD8- TCR(+)-alpha/beta circulating T cells that lack the expression of CD28. We have characterized two T cell lines, DN1 and DN6, that recognize a mycobacterial antigen, and are restricted not by major histocompatibility complex class I or II, but by CD1b or CD1c, two members of a family of major histocompatibility complex-related molecules that have been recently implicated in a distinct pathway for antigen presentation. Comparison of antigen-specific cytolytic responses of the DN1 and DN6 T cell lines against antigen-pulsed CD1+ monocytes or CD1+ B lymphoblastoid cell lines (B-LCL) demonstrated that these T cells recognized antigen presented by both types of cells. However, T cell proliferation occurred only when antigen was presented by CD1+ monocytes, indicating that the CD1+ monocytes expressed a costimulatory molecule that the B-LCL transfectants lacked. This hypothesis was confirmed by demonstrating that the T cells became anergic when incubated with the CD1(+)-transfected B-LCL in the presence of antigen, but not in the absence of antigen. The required costimulatory signal occurred by a CD28-independent mechanism since both the CD1+ monocytes and CD1+ B-LCL transfectants expressed B7-1 and B7-2, and DN1 and DN6 lacked surface expression of CD28. We propose that these data define a previously unrecognized pathway of costimulation for T cells distinct from that involving CD28 and its counterreceptors. We suggest that this B7-independent pathway plays a crucial role in the activation and maintenance of tolerance of at least a subset of CD28- T cells.