Rae1 and H60 ligands of the NKG2D receptor stimulate tumour immunity

Natural killer (NK) cells attack many tumour cell lines, and are thought to have a critical role in anti-tumour immunity; however, the interaction between NK cells and tumour targets is poorly understood. The stimulatory lectin-like NKG2D receptor is expressed by NK cells, activated CD8+ T cells and by activated macrophages in mice. Several distinct cell-surface ligands that are related to class I major histocompatibility complex molecules have been identified, some of which are expressed at high levels by tumour cells but not by normal cells in adults. However, no direct evidence links the expression of these 'induced self' ligands with tumour cell rejection. Here we demonstrate that ectopic expression of the murine NKG2D ligands Rae1beta or H60 in several tumour cell lines results in potent rejection of the tumour cells by syngeneic mice. Rejection is mediated by NK cells and/or CD8+ T cells. The ligand-expressing tumour cells induce potent priming of cytotoxic T cells and sensitization of NK cells in vivo. Mice that are exposed to live or irradiated tumour cells expressing Rae1 or H60 are specifically immune to subsequent challenge with tumour cells that lack NKG2D ligands, suggesting application of the ligands in the design of tumour vaccines.     

Expression and function of interleukin-2 receptors on immature thymocytes

T-cell differentiation represents a unique system for studying mechanisms of lymphoid development because it occurs in a segregated site, the thymus, in which distinct subpopulations of thymocytes at various stages of differentiation can be defined on the basis of the differential expression of T-cell surface antigens as well as topography. There is particular interest in thymocyte differentiation because the genotype of radioresistant thymus cells influences the specificity repertoire of the pool of T cells that mature therein: that is, the major histocompatibility complex (MHC) antigens expressed by thymus cells bias the pool of maturing T cells towards recognition of antigens in the 'context' of the products of that MHC haplotype ('thymus education'; refs 1-3). Immature T cells with affinity for thymus MHC antigens are generally thought to undergo a stage of positive selection in the thymus. Here we report that 30% of cells in the least mature adult thymocyte subpopulation yet defined, as well as 50% of immature fetal thymocytes, express receptors for interleukin-2 (IL-2, the T-cell growth factor) without in vitro induction, and will proliferate vigorously in an IL-2-dependent fashion if provided with co-stimulating mitogen.     

Inefficient positive selection of T cells directed by haematopoietic cells.

Intrathymic differentiation of alpha beta TCR+ T cells depends on positive selection of CD4+CD8+ thymocytes by thymic major histocompatibility complex (MHC) molecules. Positive selection allows the maturation of only those T cells capable of restricted antigen recognition in the context of the hosts' MHC alleles. Studies of normal or T-cell receptor-transgenic mice engrafted with MHC-different bone marrow or thymuses support the conclusion that positive selection is directed by MHC molecules expressed on non-haematopoietic cells, presumably thymic epithelial cells. Here we, present contrary evidence that class I MHC molecules expressed by haematopoietic cell types direct positive selection of CD8+ T cells, though at a reduced rate compared with positive selection directed by thymic epithelial cells. The identity of cell types that direct positive selection bears directly on mechanistic models of the process, including the idea that thymic epithelial cell MHC molecules uniquely present specialized peptides that mediate positive selection, and the notion that thymic epithelial cells express unique differentiation-inducing cell surface molecules.     

Beta 2-microglobulin deficient mice lack CD4-8+ cytolytic T cells

Mice homozygous for a beta 2-microglobulin gene disruption do not express any detectable beta 2-m protein. They express little if any functional major histocompatibility complex (MHC) class I antigen on the cell surface yet are fertile and apparently healthy. They show a normal distribution of gamma delta, CD4+8+ and CD4+8- T cells, but have no mature CD4-8+ T cells and are defective in CD4-8+ T cell-mediated cytotoxicity. Our results strongly support earlier evidence that MHC class I molecules are crucial for positive selection of T cell antigen receptor alpha beta+ CD4-8+ T cells in the thymus and call into question the non-immune functions that have been ascribed to MHC class I molecules.     

The DNA damage pathway regulates innate immune system ligands of the NKG2D receptor

Some stimulatory receptors of the innate immune system, such as the NKG2D receptor (also called KLRK1) expressed by natural killer cells and activated CD8(+)T cells, recognize self-molecules that are upregulated in diseased cells by poorly understood mechanisms. Here we show that mouse and human NKG2D ligands are upregulated in non-tumour cell lines by genotoxic stress and stalled DNA replication, conditions known to activate a major DNA damage checkpoint pathway initiated by ATM (ataxia telangiectasia, mutated) or ATR (ATM- and Rad3-related) protein kinases. Ligand upregulation was prevented by pharmacological or genetic inhibition of ATR, ATM or Chk1 (a downstream transducer kinase in the pathway). Furthermore, constitutive ligand expression by a tumour cell line was inhibited by targeting short interfering RNA to ATM, suggesting that ligand expression in established tumour cells, which often harbour genomic irregularities, may be due to chronic activation of the DNA damage response pathway. Thus, the DNA damage response, previously shown to arrest the cell cycle and enhance DNA repair functions, or to trigger apoptosis, may also participate in alerting the immune system to the presence of potentially dangerous cells.     

Developmental regulation of T-cell receptor gene expression

In contrast to B cells or their antibody products, T lymphocytes have a dual specificity, for both the eliciting foreign antigen and for polymorphic determinants on cell surface glycoproteins encoded in the major histocompatibility complex (MHC restriction). The recent identification of T-cell receptor glycoproteins as well as the genes encoding T-cell receptor subunits will help to elucidate whether MHC proteins and foreign antigens are recognized by two T-cell receptors or by a single receptor. An important feature of MHC restriction is that it appears to be largely acquired by a differentiating T-cell population under the influence of MHC antigens expressed in the thymus, suggesting that precursor T cells are selected on the basis of their reactivity with MHC determinants expressed in the host thymus. To understand this process of 'thymus education', knowledge of the developmental regulation of T-cell receptor gene expression is necessary. Here we report that whereas messenger RNAs encoding the beta-and gamma-subunits are relatively abundant in immature thymocytes, alpha mRNA levels are very low. Interestingly, whereas alpha mRNA levels increase during further development and beta mRNA levels stay roughly constant, gamma mRNA falls to very low levels in mature T cells, suggesting a role for the gamma gene in T-cell differentiation.     

Rejection of class I MHC-deficient haemopoietic cells by irradiated MHC-matched mice

Irradiated MHC-heterozygous mice often reject bone marrow cells transplanted from one of the homozygous parental strains, a phenomenon ('hybrid resistance') that appears to violate the laws of transplantation. Rejection of parental and allogeneic marrow cells also differs from conventional T cell-mediated rejection mechanisms as it is effected by NK1.1+ cells. To account for the unusual specificity of bone marrow rejection, it has been proposed that NK1.1+ cells destroy marrow cells that fail to express the full complement of self MHC class I (MHC-I) molecules. We show here that NK1.1+ cells in normal mice reject haemopoietic transplants from mice that are deficient for normal cell-surface MHC-I expression because of a targeted mutation in the beta 2-microglobulin gene. These findings demonstrate that deficient expression of MHC-I molecules renders marrow cells susceptible to rejection.