Direct recognition of cytomegalovirus by activating and inhibitory NK cell receptors

Natural killer (NK) cells express inhibitory receptors for major histocompatibility complex (MHC) class I antigens, preventing attack against healthy cells. Mouse cytomegalovirus (MCMV) encodes an MHC-like protein (m157) that binds to an inhibitory NK cell receptor in certain MCMV-susceptible mice. In MCMV-resistant mice, this viral protein engages a related activating receptor (Ly49H) and confers host protection. These activating and inhibitory receptors are highly homologous, suggesting the possibility that one evolved from the other in response to selective pressure imposed by the pathogen.     

Influence of SHIP on the NK repertoire and allogeneic bone marrow transplantation

Natural killer cell (NK) receptors for major histocompatibility complex (MHC) class I influence engraftment and graft-versus-tumor effects after allogeneic bone marrow transplantation. We find that SH2-containing inositol phosphatase (SHIP) influences the repertoire of NK receptors. In adult SHIP-/- mice, the NK compartment is dominated by cells that express two inhibitory receptors capable of binding either self or allogeneic MHC ligands. This promiscuous repertoire has significant functional consequences, because SHIP-/- mice fail to reject fully mismatched allogeneic marrow grafts and show enhanced survival after such transplants. Thus, SHIP plays an important role in two processes that limit the success of allogeneic marrow transplantation: graft rejection and graft-versus-host disease.     

Persistence of memory CD8 T cells in MHC class I-deficient mice

An understanding of how T cell memory is maintained is crucial for the rational design of vaccines. Memory T cells were shown to persist indefinitely in major histocompatibility complex (MHC) class I-deficient mice and retained the ability to make rapid cytokine responses upon reencounter with antigen. In addition, memory CD8 T cells, unlike na?ve cells, divided without MHC-T cell receptor interactions. This "homeostatic" proliferation is likely to be important in maintaining memory T cell numbers in the periphery. Thus, after na?ve CD8 T cells differentiate into memory cells, they evolve an MHC class I-independent "life-style" and do not require further stimulation with specific or cross-reactive antigen for their maintenance.     

Surface expression of HLA-E, an inhibitor of natural killer cells, enhanced by human cytomegalovirus gpUL40

The nonclassical major histocompatibility complex (MHC) class I molecule HLA-E inhibits natural killer (NK) cell-mediated lysis by interacting with CD94/NKG2A receptors. Surface expression of HLA-E depends on binding of conserved peptides derived from MHC class I molecules. The same peptide is present in the leader sequence of the human cytomegalovirus (HCMV) glycoprotein UL40 (gpUL40). It is shown that, independently of the transporter associated with antigen processing, gpUL40 can up-regulate expression of HLA-E, which protects targets from NK cell lysis. While classical MHC class I molecules are down-regulated, HLA-E is up-regulated by HCMV. Induction of HLA-E surface expression by gpUL40 may represent an escape route for HCMV.     

Limit of T cell tolerance to self proteins by peptide presentation

Cytotoxic T lymphocytes (CTLs) recognize foreign peptides bound to major histocompatibility complex (MHC) class I molecules. MHC molecules can also bind endogenous self peptides, to which T cells are tolerant. Normal mice contained CTLs specific for self peptides that were from proteins of ubiquitous or tissue-restricted expression. In vivo, these endogenous self peptides are not naturally presented in sufficient density by somatic cells expressing MHC class I molecules. They can, however, be presented if added exogenously. Thus, our data imply that CTLs are only tolerant of those endogenous self peptide sequences that are presented by MHC class I-positive cells in a physiological manner.     

Homeostasis of the antibody response: immunoregulation by NK cells

When injected into mice, the synthetic double-stranded polynucleotide poly(inosinic) X poly(cytidylic) acid induces high natural killer (NK) cell activity within 4 to 12 hours. Induction of NK activity in mice immunized 2 or 3 days previously, or the addition of NK cells to cultures immunized in vitro 2 or 3 days previously, promotes early termination of the ongoing primary immunoglobulin M antibody response. A target for NK cells is a population of accessory cells that has interacted with antigen and is necessary for sustaining the antibody response. The inference is strong that NK cells induced normally by immunization also terminate the usual antibody response in vivo by elimination of antigen-exposed accessory cells.     

Prevention of allogeneic bone marrow graft rejection by H-2 transgene in donor mice

Rejection of bone marrow grafts in irradiated mice is mediated by natural killer (NK) cells and is controlled by genes linked to the major histocompatibility complex (MHC). It has, however, not been possible to identify the genes or their products. An MHC class I (Dd) transgene introduced in C57BL donors prevented the rejection of their bone marrow by NK cells in irradiated allogeneic and F1 hybrid mice expressing the Dd gene. Conversely, H-2Dd transgenic C57BL recipients acquired the ability to reject bone marrow from C57BL donors but not from H-2Dd transgenic C57BL donors. These results provide formal evidence that NK cells are part of a system capable of rejecting cells because they lack normal genes of the host type, in contrast to T cells, which recognize cells that contain abnormal or novel sequences of non-host type.     

Self-nonself discrimination by T cells

The alpha beta T cell receptor (TCR) recognizes antigens that are presented by major histocompatibility complex (MHC)-encoded cell surface molecules by binding to both the antigen and the MHC molecules. Discrimination of self from nonself antigens and MHC molecules is achieved by negative and positive selection of T cells in the thymus: potentially harmful T cells with receptors that bind to self antigens plus self MHC molecules are deleted before they can mount immune responses. In contrast, the maturation of useful T cells with receptors that bind foreign antigens plus self MHC molecules requires the binding of their receptor to MHC molecules on thymic epithelium in the absence of foreign antigen. The binding of the TCR to either class I or class II MHC molecules directs differentiation of the selected cells into either CD4-8+ (killer) or CD4+8- (helper) T cells, respectively.     

Defective presentation of endogenous antigen by a cell line expressing class I molecules

Cytotoxic T lymphocytes (CTLs) recognize class I major histocompatibility complex (MHC) molecules associated with antigenic peptides derived from endogenously synthesized proteins. Binding to such peptides is a requirement for class I assembly in the endoplasmic reticulum (ER). A mutant human cell line, T2, assembles and transports to its surface some, but not all, class I MHC molecules. The class I molecules expressed on the surface of T2 do not present peptides derived from cytosolic antigens, although they can present exogenously added peptides to CTL. The transported class I molecules may interact weakly with an unknown retaining factor in the ER such that they can assemble despite the relative shortage of peptides.     

Depletion of CD4+ T cells in major histocompatibility complex class II-deficient mice

The maturation of T cells in the thymus is dependent on the expression of major histocompatibility complex (MHC) molecules. By disruption of the MHC class II Ab beta gene in embryonic stem cells, mice were generated that lack cell surface expression of class II molecules. These MHC class II-deficient mice were depleted of mature CD4+ T cells and were deficient in cell-mediated immune responses. These results provide genetic evidence that class II molecules are required for the maturation and function of mature CD4+ T cells.     

Requirement for positive selection of gamma delta receptor-bearing T cells.

The alpha beta and gamma delta T cell receptors for antigen (TCR) delineate distinct T cell populations. TCR alpha beta-bearing thymocytes must be positively selected by binding of the TCR to major histocompatibility complex (MHC) molecules on thymic epithelium. To examine the requirement for positive selection of TCR gamma delta T cells, mice bearing a class I MHC-specific gamma delta transgene (Tg) were crossed to mice with disrupted beta 2 microglobulin (beta 2M) genes. The Tg+beta 2M- (class I MHC-) offspring had Tg+ thymocytes that did not proliferate to antigen or Tg-specific monoclonal antibody and few peripheral Tg+ cells. This is evidence for positive selection within the gamma delta T cell subset.     

Transport protein genes in the murine MHC: possible implications for antigen processing

T lymphocyte activation requires recognition by the T cell of peptide fragments of foreign antigen bound to a self major histocompatibility complex (MHC) molecule. Genetic evidence suggests that part of the class II region of the MHC influences the expression, in trans, of MHC class I antigens on the cell surface, by regulating the availability of peptides that bind to and stabilize the class I molecule. Two closely related genes in this region, HAM1 and HAM2, were cloned and had sequence similarities to a superfamily of genes involved in the ATP-dependent transport of a variety of substrates across cell membranes. Thus, these MHC-linked transport protein genes may be involved in transporting antigen, or peptide fragments thereof, from the cytoplasm into a membrane-bounded compartment containing newly synthesized MHC molecules.     

Brefeldin A specifically inhibits presentation of protein antigens to cytotoxic T lymphocytes

Cytotoxic T lymphocytes (CTLs) recognize foreign antigens, including viral proteins, in association with major histocompatibility complex (MHC) class I molecules. Brefeldin A, a specific inhibitor of exocytosis, completely and reversibly inhibited the presentation of viral proteins, but not exogenous peptides, to MHC class I-restricted CTLs directed against influenza virus antigens. The effect of brefeldin A on antigen presentation correlated with its inhibition of intracellular transport of newly synthesized class I molecules. Brefeldin A is thus a specific inhibitor of antigen processing for class I-restricted T cell recognition. Its effect on antigen presentation supports the idea that exogenous peptide antigens associate with cell surface class I molecules, whereas protein antigens processed via the cytosolic route associate with nascent class I molecules before they leave the trans-Golgi complex.     

Functional requirement for class I MHC in CNS development and plasticity

Class I major histocompatibility complex (class I MHC) molecules, known to be important for immune responses to antigen, are expressed also by neurons that undergo activity-dependent, long-term structural and synaptic modifications. Here, we show that in mice genetically deficient for cell surface class I MHC or for a class I MHC receptor component, CD3zeta, refinement of connections between retina and central targets during development is incomplete. In the hippocampus of adult mutants, N-methyl-D-aspartate receptor-dependent long-term potentiation (LTP) is enhanced, and long-term depression (LTD) is absent. Specific class I MHC messenger RNAs are expressed by distinct mosaics of neurons, reflecting a potential for diverse neuronal functions. These results demonstrate an important role for these molecules in the activity-dependent remodeling and plasticity of connections in the developing and mature mammalian central nervous system (CNS).     

HLA and NK cell inhibitory receptor genes in resolving hepatitis C virus infection

Natural killer (NK) cells provide a central defense against viral infection by using inhibitory and activation receptors for major histocompatibility complex class I molecules as a means of controlling their activity. We show that genes encoding the inhibitory NK cell receptor KIR2DL3 and its human leukocyte antigen C group 1 (HLA-C1) ligand directly influence resolution of hepatitis C virus (HCV) infection. This effect was observed in Caucasians and African Americans with expected low infectious doses of HCV but not in those with high-dose exposure, in whom the innate immune response is likely overwhelmed. The data strongly suggest that inhibitory NK cell interactions are important in determining antiviral immunity and that diminished inhibitory responses confer protection against HCV.     

Regulation of CD8+ T cell development by thymus-specific proteasomes

Proteasomes are responsible for generating peptides presented by the class I major histocompatibility complex (MHC) molecules of the immune system. Here, we report the identification of a previously unrecognized catalytic subunit called beta5t. beta5t is expressed exclusively in cortical thymic epithelial cells, which are responsible for the positive selection of developing thymocytes. Although the chymotrypsin-like activity of proteasomes is considered to be important for the production of peptides with high affinities for MHC class I clefts, incorporation of beta5t into proteasomes in place of beta5 or beta5i selectively reduces this activity. We also found that beta5t-deficient mice displayed defective development of CD8(+) T cells in the thymus. Our results suggest a key role for beta5t in generating the MHC class I-restricted CD8(+) T cell repertoire during thymic selection.     

Viral persistence in neurons explained by lack of major histocompatibility class I expression

Viruses frequently persist in neurons, suggesting that these cells can evade immune surveillance. In a mouse model, 5 x 10(6) cytotoxic T lymphocytes (CTLs), specific for lymphocytic choriomeningitis virus (LCMV), did not lyse infected neurons or cause immunopathologic injury. In contrast, intracerebral injection of less than 10(3) CTL caused disease and death when viral antigens were expressed on leptomeningeal and choroid plexus cells of the nervous system. The neuronal cell line OBL21 expresses little or no major histocompatibility (MHC) class I surface glycoproteins and when infected with LCMV, resisted lysis by virus-specific CTLs. Expression of MHC heavy chain messenger RNA was limited, but beta 2-microglobulin messenger RNA and protein was made normally. OBL21 cells were made sensitive to CTL lysis by transfection with a fusion gene encoding another MHC class I molecule. Hence, neuronal cells probably evade immune surveillance by failing to express MHC class I molecules.     

Antigen presentation requires transport of MHC class I molecules from the endoplasmic reticulum

The role of exocytosis of major histocompatibility complex (MHC) class I molecules in the presentation of antigens to mouse cytotoxic T lymphocytes (CTLs) was examined by use of a recombinant vaccinia virus that expresses the E19 glycoprotein from adenovirus. E19 blocked the presentation of vaccinia and influenza virus proteins to CTLs in a MHC class I allele-specific manner identical to its inhibition of MHC class I transport from the endoplasmic reticulum. This finding indicates that (i) the relevant parameter for antigen presentation is the rate of MHC class I molecule exocytosis, not the level of class I cell surface expression, and (ii) association of class I molecules with antigen is likely to occur within the endoplasmic reticulum.     

Transport of peptide-MHC class II complexes in developing dendritic cells

Major histocompatibility complex class II (MHC II) molecules capture peptides within the endocytic pathway to generate T cell receptor (TCR) ligands. Immature dendritic cells (DCs) sequester intact antigens in lysosomes, processing and converting antigens into peptide-MHC II complexes upon induction of DC maturation. The complexes then accumulate in distinctive, nonlysosomal MHC II+ vesicles that appear to migrate to the cell surface. Although the vesicles exclude soluble lysosomal contents and antigen-processing machinery, many contain MHC I and B7 costimulatory molecules. After arrival at the cell surface, the MHC and costimulatory molecules remain clustered. Thus, transport of peptide-MHC II complexes by DCs not only accomplishes transfer from late endocytic compartments to the plasma membrane, but does so in a manner that selectively concentrates TCR ligands and costimulatory molecules for T cell contact.