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Presentation of Antigens to T Lymphocyte
Summary: Luc Van Kaer is interested in the molecular mechanisms that control the presentation of antigens to T lymphocytes.
T lymphocytes are critical mediators of the immune response against infectious agents and tumors. These cells recognize foreign antigens in the context of self-proteins, termed major histocompatibility complex (MHC) molecules. MHC proteins, which were originally identified as mediators of tissue graft rejection, are expressed on the surface of a variety of cells in the body.
Three classes of MHC molecule that present distinct antigens to three different subsets of T cells have been identified: (1) MHC class I molecules bind with short protein fragments, termed peptides, derived from pathogens that live in the cytoplasm of cells, and present these peptides to CD8-expressing T cells (CD8 T cells); (2) MHC class II molecules bind with peptides derived from microorganisms that live inside of intracellular vesicles or outside of cells and present these peptides to CD4 T cells; and (3) CD1d molecules present lipid antigens to a group of regulatory T cells, termed natural killer T (NKT) cells.
Assembly of MHC Class I and Class II Molecules with Peptides
The loading of peptides onto MHC molecules involves a variety of accessory proteins. Our recent studies have focused on the role of the accessory proteins H2-DM and tapasin for assembly of peptides with MHC class II and class I molecules, respectively.
The assembly of MHC class II molecules initiates in the endoplasmic reticulum (ER) of the cell, by association of MHC class II heterodimers with invariant chain (Ii). Ii occupies the peptide-binding portion of MHC class II and transports this heterodimer to endosomal compartments in the cell. There, Ii is degraded by proteases until only a small fragment, the class II–associated Ii peptide (CLIP), remains bound with the class II heterodimer. Peptides, generated by degradation of proteins in endosomal compartments, are then loaded onto MHC class II molecules with the help of the peptide exchange factor H2-DM.
To evaluate the role of H2-DM for MHC class II assembly in vivo, we generated mice that are genetically deficient in H2-DM. In these animals, CLIP remains tightly bound with MHC class II heterodimers and is stably expressed at the cell surface. Consequently, H2-DM mutant mice have defects in the generation of CD4 T cell responses. Our results revealed that H2-DM has a critical role in the assembly of peptides with MHC class II molecules.
The majority of peptides that are presented by MHC class I molecules are generated in the cytoplasm of the cell by a large proteolytic complex, termed the proteasome. Peptides are then translocated into the ER by the TAP transporter, loaded onto MHC class I molecules, and transported to the cell surface for presentation to CD8 T cells.
Peptide-receptive MHC class I molecules in the ER are physically associated with the TAP transporter via the accessory protein tapasin. To evaluate the in vivo role of tapasin for presentation of peptides by MHC class I molecules, we generated tapasin-deficient mice. In these animals, MHC class I molecules fail to be loaded with high-affinity peptides. Despite their peptide-loading defect, MHC class I molecules are transported to the surface of tapasin-deficient cells at a normal rate. Our findings indicate that tapasin is critically important for the loading of MHC class I molecules with high-affinity peptides. This proposed role of tapasin in MHC class I assembly is similar to the proposed function of H2-DM in MHC class II assembly. In keeping with the critical role of MHC class I molecules for CD8 T cell activation, we found that tapasin-knockout mice are defective in CD8 T cell responses.
Regulation of Immune Responses by CD1d-Restricted NKT Cells
NKT cells represent a small population of T cells with unusual specificities and effector functions. Unlike conventional T cells, which recognize peptide antigens in the context of MHC class I or class II molecules, NKT cells recognize lipid antigens bound with the MHC molecule CD1d. The physiologically relevant lipid antigens that are recognized by NKT cells remain to be identified. While their precise function remains unknown, NKT cells have been implicated in immune responses against pathogens and tumors, and in the regulation of autoimmune responses and allergic reactions.
To evaluate the immunological role of the CD1d antigen presentation pathway, we generated CD1d-deficient mice. These animals lack NKT cells, indicating that CD1d expression is required for NKT cell development. In collaboration with a number of different laboratories, we compared wild-type and CD1d-knockout mice for protective immune responses against a variety of pathogens. These studies revealed that mutant animals generate relatively normal immune responses against most pathogens, including influenza virus, Mycobacterium tuberculosis, and malaria parasites. However, we found that mutant mice generate reduced immune responses against respiratory syncytial virus, an important pathogen in children and the elderly. Our findings indicate that NKT cells play a more subtle role in the regulation of immune responses than was previously thought.
The NKT Cell Ligand α-Galactosylceramide Has Potent Immunotherapeutic Activities
Several years ago, investigators at Kirin Brewery Company (Gunma, Japan) isolated the glycolipid α-galactosylceramide (α-GalCer) from a marine sponge as an agent with profound antimetastatic activities in mice. Other investigators subsequently showed that α-GalCer binds with CD1d and activates NKT cells. To investigate whether the antimetastatic activities of α-GalCer require CD1d expression, we injected wild-type and CD1d mutant mice with metastatic tumors and treated these animals with α-GalCer. Our results revealed that α-GalCer clears tumors from wild-type but not CD1d-deficient mice, demonstrating that the antimetastatic activities of this agent are mediated by NKT cells.
α-GalCer–activated NKT cells produce a variety of factors, including the cytokine interleukin-4 (IL-4), that modulate immune responses mediated by conventional T cells. We therefore tested whether α-GalCer administration to mice influences the generation of immune responses mediated by conventional T cells. We found that α-GalCer–activated NKT cells promote the generation of conventional CD4 T cells that produce suppressive cytokines such as IL-4 and IL-10.
Because suppressive cytokines counteract the pathogenic immune responses that are responsible for autoimmune diseases such as type 1 diabetes and multiple sclerosis, we tested the ability of α-GalCer to modulate the progression of these diseases in experimental models. We found that α-GalCer prevents the development of type 1 diabetes in non-obese diabetic (NOD) mice, a well-characterized mouse model for human type 1 diabetes. We further demonstrated that α-GalCer prevents development of experimental autoimmune encephalomyelitis (EAE), an experimental model for multiple sclerosis, in mice. As α-GalCer was unable to prevent diabetes and EAE in CD1d-deficient animals, these activities of α-GalCer require NKT cell activation. Our studies have identified NKT cells as novel targets for immunotherapy of disease.
One interesting aspect of the CD1d antigen presentation system is its high degree of conservation among different species. Thus human CD1d binds with α-GalCer and activates human NKT cells. Our studies performed in mice are therefore directly relevant to treatment of human diseases.
Last updated October 09, 2001
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