Expression of intra-MHC transporter (Ham) genes and class I antigens in diabetes-susceptible NOD mice

Major histocompatibility complex class I shedding and programmed cell death stimulated through the proinflammatory P2X7 receptor: a candidate susceptibility gene for NOD diabetes

It has been hypothesized that type 1 diabetes is initiated by neonatal physiological pancreatic beta-cell death, indicating that the early stages of this autoimmune response may reflect a dysregulated response to immune "danger" signals. One potential danger signal is ATP, high concentrations of which stimulate the purinergic receptor P2X7 on hematopoietic cells. We compared the sensitivity of lymphocytes from model type 1 diabetic (NOD) and control (C57BL/10) mice to activation of this pathway. Stimulation of the P2X7 receptor of NOD mice resulted in more pronounced shedding of the lymphocyte homing receptor CD62L and in increased programmed cell death. Levels of major histocompatibility complex class I molecules, which have previously been reported to be poorly expressed on NOD lymphocytes, were initially normal, but the molecules were shed preferentially from NOD cells after P2X7 receptor stimulation. Thus, although NOD lymphocytes have been considered resistant to programmed cell death, they are highly sensitive to that stimulated through the P2X7 receptor. Because NOD mice express a low activation threshold allele of the P2X7 receptor and the P2X7 gene maps to a locus associated with disease, P2X7 is a good candidate susceptibility gene for NOD diabetes.

Antigen processing and autoimmunity. Evaluation of mRNA abundance and function of HLA-linked genes

Quantitative defects in the density of conformationally correct human lymphocyte antigen (HLA) class I complexes on the surface of lymphocytes are apparent in patients with diverse HLA-linked autoimmune diseases, including Type I diabetes and Sjögren's syndrome. First, HLA class I expression was investigated in individuals with two rare and genetically divergent polyglandular autoimmune diseases. Polyglandular failure patients whose disease showed HLA linkage, but not those whose disease was not HLA linked, exhibited decreased HLA class I expression on the surface of their lymphocytes as well as a reduced abundance of transcripts of the HLA-linked genes Tap1 and Tap2, both of which encode proteins that contribute to HLA class I processing. Second, lymphocytes from patients with insulin-dependent diabetes mellitus (IDDM), Sjögren's syndrome, Graves' disease, and Hashimoto's disease showed varying degrees of decreased abundance of mRNAs that encode Tap1, Tap2, Lmp2, or Lmp7 (the latter two proteins also contribute to HLA class I processing). Third, in twins discordant for IDDM, reduced transcript abundance was preferential to diabetic subjects. Fourth, functional assays of isolated diabetic proteasomes, the peptide cutting complex containing LMP2 and LMP7 proteins, revealed altered peptidase activity. These data suggest that defective transcription of HLA class I-processing genes could contribute to the quantitative defect in cell-surface expression in autoimmune lymphocytes of HLA-controlled disease.

Subcongenic Analysis of the Idd13 Locus in NOD/Lt Mice: Evidence for Several Susceptibility Genes Including a Possible Diabetogenic Role for β2-Microglobulin

Although they share ∼88% of their genome with NOD mice including the H2g7 haplotype, NOR mice remain free of T cell-mediated autoimmune diabetes (IDDM), due to non-MHC genes of C57BLKS/J (BKS) origin. NOR IDDM resistance was previously found to be largely controlled by the Idd13 locus within an ∼24 cM segment on Chromosome 2 encompassing BKS-derived alleles for H3a, B2m, Il1, and Pcna. NOD stocks carrying subcongenic intervals of NOR Chromosome 2 were utilized to more finely map and determine possible functions of Idd13. NOR- derived H3a-Il1 (∼6.0 cM) and Il1-Pcna (∼1.2 cM) intervals both contribute components of IDDM resistance. Hence, the Idd13 locus is more complex than originally thought, since it consists of at least two genes. B2m variants within the H3a-Il1 interval may represent one of these. Monoclonal Ab binding demonstrated that dimerizing with the β2ma (NOD type) vs β2mb isoform (NOR type) alters the structural conformation, but not total expression levels of H2g7 class I molecules (e.g. Kd, Db). β2m-induced alterations in H2g7 class I conformation may partially explain findings from bone marrow chimera analyses that Idd13 modulates IDDM development at the level of non-hematopoietically derived cell types controlling selection of diabetogenic T cells and/or pancreatic β cells targeted by these effectors. Since trans-interactions between relatively common and functionally normal allelic variants may contribute to IDDM in NOD mice, the search for Idd genes in humans should not be limited to functionally defective variants.

Major histocompatibility complex class I molecule expression is normal on peripheral blood lymphocytes from patients with insulin-dependent diabetes mellitus

Recent work from one laboratory has shown, in both nonobese diabetic mice and humans, an association between insulin-dependent diabetes mellitus (IDDM) and quantitative difference in MHC class I molecule expression. This reported decrease in MHC class I molecule expression is very controversial in the nonobese diabetic mouse model of IDDM, but to our knowledge, it has not been evaluated by another group in human IDDM. To evaluate this question, we studied 30 patients with IDDM and 30 age- and sex-matched normal controls. MHC class I molecule expression was measured by flow cytometry with conformational-dependent MHC class I mAbs. The mean antigen density of MHC class I molecule expression in IDDM vs. normal control is 454+/-34 vs. 440+/-28 for lymphocytes and 1,440+/-117 vs. 1,494+/- 117 for monocytes, both P > 0.05. Three conformational-dependent MHC class I antibodies showed consistent results. To estimate the biological variation of MHC class I molecule expression in normal controls, we also studied 10 age- and sex-matched normal control pairs. Using X +/-SD of the percentage difference of mean antigen density in the normal control pairs as our definition of normal, we found that 70% (21/30) of IDDM patients had normal, 13% (4/30) of IDDM patients had decreased, and 17% (5/30) of IDDM patients had increased MHC class I molecule expression on lymphocytes. All IDDM patients showed normal MHC class I expression on monocytes. In conclusion, we find that there is no consistent decrease in MHC class I molecule expression on either lymphocytes or monocytes from patients with IDDM. The MHC class I molecule expression observed in IDDM patients is largely within the expected biological variation of MHC class I molecule expression that has been observed in normal controls.

Molecular mechanisms of class I major histocompatibility complex antigen processing and presentation

The presentation of antigenic peptides by class I major histocompatibility complex molecules plays a central role in the cellular immune response, since immune surveillance for detection of viral infections or malignant transformations is achieved by CD8+ T lymphocytes which inspect peptides, derived from intracellular proteins, bind to class I molecules on the surface of most cells. The transporter associated with antigen processing selectively translocates cytoplasmically derived peptides of appropriate sequence and length into the lumen of the endoplasmic reticulum where they associate with newly synthesized class I molecules. The translocated peptides are generated by multicatalytic and multisubunit proteasomes which degrade cytoplasmic proteins in a ATP-ubiquitin-dependent manner. This review discusses our current molecular understanding of class I antigen processing and presentation.

Immunology taught by viruses

The survival of viruses depends on the survival of susceptible hosts. The vertebrate immune system and viruses have therefore coevolved complementary facets. Evidence from various balanced virus-host relationships illustrates that immunological specificity and memory may best be defined biologically and that the mature immune system does not discriminate between "self" and "nonself." Rather, B cells distinguish antigen patterns, whereas T cell responses depend on localization, transport, and kinetics of antigen within lymphatic organs.

Genetic and pathogenic basis of autoimmune diabetes in NOD mice

Non-obese diabetic (NOD) mice are an excellent model of T-cell mediated autoimmune insulin-dependent diabetes in humans. Recent studies in NOD mice have shown that this disease is a result of epistatic interactions between multiple genes, both inside and outside the major histocompatibility complex (MHC), generating T cells reactive against an expanding repertoire of autoantigens.

Defective major histocompatibility complex class I expression on lymphoid cells in autoimmunity

Lymphocytes from patients with insulin-dependent diabetes mellitus (IDDM), a chronic autoimmune disease, have recently been shown to have decreased surface expression of MHC class I antigens. Since IDDM and other autoimmune diseases share a strong genetic association with MHC class II genes, which may in turn be linked to genes that affect MHC class I expression, we studied other autoimmune diseases to determine whether MHC class I expression is abnormal. Fresh PBLs were isolated from patients with IDDM, Hashimoto's thyroiditis, Graves' disease, systemic lupus erythematosis, rheumatoid arthritis, and Sjogren's syndrome. Nondiabetic and non-insulin-dependent diabetes mellitus patients served as controls. MHC class I expression was measured with a conformationally dependent monoclonal antibody, W6/32. Freshly prepared PBLs from the autoimmune diseases studied and the corresponding fresh EBV-transformed B cell lines had decreased MHC class I expression compared with PBLs from normal volunteers and non-insulin-dependent (nonautoimmune) diabetic patients. Only 3 of more than 180 donors without IDDM or other clinically recognized autoimmune disease had persistently decreased MHC class I expression; one patient was treated with immunosuppressive drugs, and subsequent screening of the other two patients revealed high titers of autoantibodies, revealing clinically occult autoimmunity. Patients with nonautoimmune inflammation (osteomyelitis or tuberculosis) had normal MHC class I expression. Autoimmune diseases are characterized by decreased expression of MHC class I on lymphocytes. MHC class I expression may be necessary for self-tolerance, and abnormalities in such expression may lead to autoimmunity.

MHC class I-deficient mice

A great deal has already been learned from the analysis of beta 2m-mutant mice, but it is clear that a great deal remains to be learned. A significant (though unanticipated) problem with this model system is that it is functionally leaky: residual functional class I expression can be detected in beta 2m- mice, and small numbers of functional CD8+ lymphocytes are present in the animals. In many cases, this has frustrated the initial attempts at obtaining immediate definitive resolution of important questions regarding the function of class I molecules. This has occurred primarily in instances in which the class I-deficient mice fail to express an expected phenotype--for example, in studies showing that beta 2m- mice make adequate protective immune responses against certain intracellular pathogens, and are able to reject some allogeneic tissues with a relatively normal pace. On the other hand, it appears that combining the use of beta 2m- mice with other methods (for example, antibody-mediated depletion of CD8+ T cells) is usually adequate to circumvent these difficulties. It remains to be seen whether other better class I deficiencies can be engineered--for example, large deletions of class I genes or mutations in transcription factors essential for class I gene expression. The extent of immunocompetence of beta 2m- mice was somewhat surprising. It was widely expected that class I-deficient mice would be exquisitely sensitive to many viral infections, though the results indicate that sensitivity varies dramatically with the virus and conditions of infection. However, it appears that in lieu of one major arm of the immune system, compensatory immune mechanisms are in many cases able to deal with infection. Similar conclusions are developing from the analysis of several other recently generated mutant mice. Nevertheless, the results indicate a very important role for class I-directed responses in clearing infections mediated by various viral and parasitic agents, particularly in the case of more severe conditions of infection. Although the class I-deficient mice were initially considered primarily a vehicle for analysis of the role of CD8+ T cells, evidence is accumulating that they manifest deficiencies in several other types of lymphocytes, including NK cells, TCR alpha beta+CD4-CD8- cells, and a subset of TCR gamma delta+ cells. This has been a boon for analysis of the development of these cells, but at the same time it has created difficulties in assigning a biological effect of the mutation to a specific lymphocyte deficiency.(ABSTRACT TRUNCATED AT 400 WORDS)