Major histocompatibility complex (MHC) class I molecules present antigen by transporting peptides from intracellularly degraded proteins to the cell surface for scrutiny by cytotoxic T cells. Recent work suggests that peptide binding may be required for efficient assembly and intracellular transport of MHC class I molecules, but it is not clear whether class I molecules can ever assemble in the absence of peptide. We report here that culture of the murine lymphoma mutant cell line RMA-S at reduced temperature (19-33 degrees C) promotes assembly, and results in a high level of cell surface expression of H-2/beta 2-microglobulin complexes that do not present endogenous antigens, and are labile at 37 degrees C. They can be stabilized at 37 degrees C by exposure to specific peptides known to interact with H-2Kb or Db. Our findings suggest that, in the absence of peptides, class I molecules can assemble but are unstable at body temperature. The induction of such molecules at reduced temperature opens new ways to analyse the nature of MHC class I peptide interactions at the cell surface.

Little is known about the events triggering lymphocyte invasion of the pancreatic islets in prelude to autoimmune diabetes. For example, where islet-reactive T cells first encounter antigen has not been identified. We addressed this issue using BDC2.5 T cell receptor transgenic mice, which express a receptor recognizing a natural islet beta cell antigen. In BDC2.5 animals, activated T cells were found only in the islets and the lymph nodes draining them, and there was a close temporal correlation between lymph node T cell activation and islet infiltration. When naive BDC2.5 T cells were transferred into nontransgenic recipients, proliferating cells were observed only in pancreatic lymph nodes, and this occurred significantly before insulitis was detectable. Surprisingly, proliferation was not seen in 10-day-old recipients. This age-dependent dichotomy was reproduced in a second transfer system based on an unrelated antigen artificially expressed on beta cells. We conclude that beta cell antigens are transported specifically to pancreatic lymph nodes, where they trigger reactive T cells to invade the islets. Systemic or extrapancreatic T cell priming, indicative of activation via molecular mimicry or superantigens, was not seen. Compromised presentation of beta cell antigens in the pancreatic lymph nodes of juvenile animals may be the root of a first “checkpoint” in diabetes progression.

NK cells use a variety of receptors to detect abnormal cells, including

IntroductionNatural killer (NK) cells take part in the defense against virusinfected, neoplastic, and allogeneic cells. 1,2 NK-cell function is balanced by signals from inhibitory and activating receptors. If the balance is tipped toward activation, cytolysis and cytokine production are initiated. 3 Murine inhibitory receptors belong to the Ly49 or the NKG2/CD94 family. 4 Their ligands are MHC class I molecules, and MHC class I deficiency, or the presence of MHC class I alleles that fail to bind the inhibitory receptors, therefore results in "missing self" recognition. 5 NK cells from mice and humans without MHC class I expression are incapable of rejecting MHC class I-deficient cells, showing that they have developed hyporesponsiveness in the MHC-deficient environment. [6][7][8] Conversely, NK cells lacking inhibitory receptors for self-MHC class I are hyporesponsive despite normal MHC class I expression. [9][10][11] Studies with MHC class I transgenic mice provided evidence that individual MHC class I alleles deliver educational signals to NK cells, and this conveys novel reactivity to the NK-cell system. 12,13 Altogether, these data imply an education mechanism, in which host MHC class I molecules both secure self-tolerance and convey functional capacity to NK cells. 6,7,9,10 The question of how MHC class I alleles influence NK-cell functionality is important for the understanding of hematopoietic stem cell transplantation across killer cell Ig-like receptor (KIR)/ HLA donor-recipient mismatched barriers, in which NK cells elicit therapeutically beneficial missing self-rejections of recipient leukemic cells. 14 Epidemiologic correlations between certain KIR/HLA genotypes and disease susceptibility and outcome constitute another area in which NK-cell education is of potential importance. [15][16][17] Several models have been proposed to explain the educational effects of MHC class I molecules on NK-cell function.Kim and colleagues suggested that NK cells are initially hyporesponsive and become "licensed" when their Ly49 receptors engage self-MHC class I during maturation. 10,18 In another model, NK cells are responsive by default, and become hyporesponsive, or "disarmed," in the absence of inhibitory input. 9,19 A commonly held view in NK-cell education is that an NK cell is either educated or not, depending on whether its inhibitory receptors are engaged during NK-cell education. This view is based on the notions that Ly49 receptors discriminate sharply between MHC class I alleles 20,21 and that the functional capacity of NK-cell subsets to develop missing self-activity depend on the MHC setup. 22 Our recent studies showed, however, that individual MHC class I alleles are not equally efficient in educating NK cells for the capacity to reject MHC-deficient cells in vivo; some display a good "educating impact" on the NK-cell system, while some MHC class I alleles are less efficient. 23 Here, we provide data that explain the difference in educating impact between MHC class I alleles. We show that the number and ...

Evidence has been accumulating that shows that insulin-dependent diabetes is subject to immunoregulation. To determine whether cytotoxic T lymphocyte–associated antigen 4 (CTLA-4) is involved, we injected anti–CTLA-4 mAb into a TCR transgenic model of diabetes at different stages of disease. When injected into young mice, months before they would normally become diabetic, anti–CTLA-4 induced diabetes rapidly and essentially universally; this was not the result of a global activation of T lymphocytes, but did reflect a much more aggressive T cell infiltrate in the pancreatic islets. These effects were only observed if anti–CTLA-4 was injected during a narrow time window, before the initiation of insulitis. Thus, engagement of CTLA-4 at the time when potentially diabetogenic T cells are first activated is a pivotal event; if engagement is permitted, invasion of the islets occurs, but remains quite innocuous for months, if not, insulitis is much more aggressive, and diabetes quickly ensues.

No abstract

From the early days of natural killer (NK) cell research, it was clear that MHC genes controlled the specificity of mouse NK cell-dependent responses, such as the ability to reject transplanted allogeneic bone marrow and to kill tumour cells. Although several mechanisms that are involved in this 'education' process have been clarified, most of the mechanisms have still to be identified. Here, we review the current understanding of the processes that are involved in NK cell education, including how the host MHC class I molecules regulate responsiveness and receptor repertoire formation in NK cells and the signalling pathways that are involved.

The ability of murine NK cells to reject cells lacking self MHC class I expression results from an in vivo education process. To study the impact of individual MHC class I alleles on this process, we generated mice expressing single MHC class I alleles (Kb, Db, Dd, or Ld) or combinations of two or more alleles. All single MHC class I mice rejected MHC class I–deficient cells in an NK cell–dependent way. Expression of Kb or Dd conveyed strong rejection of MHC class I–deficient cells, whereas the expression of Db or Ld resulted in weaker responses. The educating impact of weak ligands (Db and Ld) was further attenuated by the introduction of additional MHC class I alleles, whereas strong ligands (Kb and Dd) maintained their educating impact under such conditions. An analysis of activating and inhibitory receptors in single MHC class I mice suggested that the educating impact of a given MHC class I molecule was controlled both by the number of NK cells affected and by the strength of each MHC class I–Ly49 receptor interaction, indicating that NK cell education may be regulated by a combination of qualitative and quantitative events.