Loss of Invariant Chain Protects Nonobese Diabetic Mice against Type 1 Diabetes

CD74 in Kidney Disease

CD74 (invariant MHC class II) regulates protein trafficking and is a receptor for macrophage migration inhibitory factor (MIF) and d-dopachrome tautomerase (d-DT/MIF-2). CD74 expression is increased in tubular cells and/or glomerular podocytes and parietal cells in human metabolic nephropathies, polycystic kidney disease, graft rejection and kidney cancer and in experimental diabetic nephropathy and glomerulonephritis. Stressors like abnormal metabolite (glucose, lyso-Gb3) levels and inflammatory cytokines increase kidney cell CD74. MIF activates CD74 to increase inflammatory cytokines in podocytes and tubular cells and proliferation in glomerular parietal epithelial cells and cyst cells. MIF overexpression promotes while MIF targeting protects from experimental glomerular injury and kidney cysts, and interference with MIF/CD74 signaling or CD74 deficiency protected from crescentic glomerulonephritis. However, CD74 may protect from interstitial kidney fibrosis. Furthermore, CD74 expression by stressed kidney cells raises questions about the kidney safety of cancer therapy strategies delivering lethal immunoconjugates to CD74-expressing cells. Thus, understanding CD74 biology in kidney cells is relevant for kidney therapeutics.

Exposing the Specific Roles of the Invariant Chain Isoforms in Shaping the MHC Class II Peptidome

The peptide repertoire (peptidome) associated with MHC class II molecules (MHCIIs) is influenced by the polymorphic nature of the peptide binding groove but also by cell-intrinsic factors. The invariant chain (Ii) chaperones MHCIIs, affecting their folding and trafficking. Recent discoveries relating to Ii functions have provided insights as to how it edits the MHCII peptidome. In humans, the Ii gene encodes four different isoforms for which structure-function analyses have highlighted common properties but also some non-redundant roles. Another layer of complexity arises from the fact that Ii heterotrimerizes, a characteristic that has the potential to affect the maturation of associated MHCIIs in many different ways, depending on the isoform combinations. Here, we emphasize the peptide editing properties of Ii and discuss the impact of the various isoforms on the MHCII peptidome.

MHC Class II Protein Turnover In vivo and Its Relevance for Autoimmunity in Non-Obese Diabetic Mice

Major histocompatibility complex class II (MHCII) proteins are loaded with endosomal peptides and reside at the surface of antigen-presenting cells (APCs) for a time before being degraded. In vitro, MHCII protein levels and turnover are affected by peptide loading and by rates of ubiquitin-dependent internalization from the cell surface, which is in turn affected by APC type and activation state. Prior work suggested that fast turnover of disease-associated MHCII alleles may contribute to autoimmunity. We recently developed novel stable isotope tracer techniques to test this hypothesis in vivo. In non-obese diabetic (NOD) mice, a model of type 1 diabetes (T1D), MHCII turnover was affected by APC type, but unaffected by disease-associated structural polymorphism. Differences in MHCII turnover were observed between NOD colonies with high and low T1D incidence, but fast turnover was dispensable for autoimmunity. Moreover, NOD mice with gene knockouts of peptide loading cofactors do not develop T1D. Thus, fast turnover does not appear pathogenic, and conventional antigen presentation is critical for autoimmunity in NOD mice. However, shared environmental factors may underpin colony differences in MHCII protein turnover, immune regulation, and pathogenesis.

Structural Characteristics of HLA-DQ that May Impact DM Editing and Susceptibility to Type-1 Diabetes

Autoreactive CD4+ T cells initiate the chronic autoimmune disease Type-1 diabetes (T1D), in which multiple environmental and genetic factors are involved. The association of HLA, especially the DR-DQ loci, with risk for T1D is well documented. However, the molecular mechanisms are poorly understood. In this review, we explore the structural characteristics of HLA-DQ and the role of HLA-DM function as they may contribute to an understanding of autoreactive T cell development in T1D.

The nonconventional MHC class II molecule DM governs diabetes susceptibility in NOD mice

The spontaneous destruction of insulin producing pancreatic beta cells in non-obese diabetic (NOD) mice provides a valuable model of type 1 diabetes. As in humans, disease susceptibility is controlled by the classical MHC class II genes that guide CD4⁺ T cell responses to self and foreign antigens. It has long been suspected that the dedicated class II chaperone designated HLA-DM in humans or H-2M in mice also makes an important contribution, but due to tight linkage within the MHC, a possible role played by DM peptide editing has not been previously tested by conventional genetic approaches. Here we exploited newly established germ-line competent NOD ES cells to engineer a loss of function allele. DM deficient NOD mice display defective class II peptide occupancy and surface expression, and are completely protected against type 1 diabetes. Interestingly the mutation results in increased proportional representation of CD4⁺Foxp3⁺ regulatory T cells and the absence of pathogenic CD4⁺ T effectors. Overall, this striking phenotype establishes that DM-mediated peptide selection plays an essential role in the development of autoimmune diabetes in NOD mice.

MIF, CD74 and other partners in kidney disease: tales of a promiscuous couple

Macrophage migration inhibitory factor (MIF) is increased in kidney and urine during kidney disease. MIF binds to and activates CD74 and chemokine receptors CXCR2 and CXCR4. CD74 is a protein trafficking regulator and a cell membrane receptor for MIF, D-dopachrome tautomerase (D-DT/MIF-2) and bacterial proteins. MIF signaling through CD74 requires CD44. CD74, CD44 and CXCR4 are upregulated in renal cells in diseased kidneys and MIF activation of CD74 in kidney cells promotes an inflammatory response. MIF or CXCR2 targeting protects from experimental kidney injury, CD44 deficiency modulates kidney injury and CXCR4 activation promotes glomerular injury. However, the contribution of MIF or MIF-2 to these actions of MIF receptors has not been explored. The safety and efficacy of strategies targeting MIF, CD74, CD44 and CXCR4 are under study in humans.

On the perils of poor editing: regulation of peptide loading by HLA-DQ and H2-A molecules associated with celiac disease and type 1 diabetes

This review discusses mechanisms that link allelic variants of major histocompatibility complex (MHC) class II molecules (MHCII) to immune pathology. We focus on HLA (human leukocyte antigen)-DQ (DQ) alleles associated with celiac disease (CD) and type 1 diabetes (T1D) and the role of the murine DQ-like allele, H2-Ag7 (I-Ag7 or Ag7), in murine T1D. MHCII molecules bind peptides, and alleles vary in their peptide-binding specificity. Disease-associated alleles permit binding of disease-inducing peptides, such as gluten-derived, Glu-/Pro-rich gliadin peptides in CD and peptides from islet autoantigens, including insulin, in T1D. In addition, the CD-associated DQ2.5 and DQ8 alleles are unusual in their interactions with factors that regulate their peptide loading, invariant chain (Ii) and HLA-DM (DM). The same alleles, as well as other T1D DQ risk alleles (and Ag7), share nonpolar residues in place of Asp at β57 and prefer peptides that place acidic side chains in a pocket in the MHCII groove (P9). Antigen-presenting cells from T1D-susceptible mice and humans retain CLIP because of poor DM editing, although underlying mechanisms differ between species. We propose that these effects on peptide presentation make key contributions to CD and T1D pathogenesis.

CD74: an emerging opportunity as a therapeutic target in cancer and autoimmune disease

INTRODUCTION

CD74, also known as the invariant chain, participates in several key processes of the immune system, including antigen presentation, B-cell differentiation and inflammatory signaling. Despite being described more than 3 decades ago, new functions and novel interactions for this evolutionarily conserved molecule are still being unraveled. As a participant in several immunological processes and an indicator of disease in some conditions, it has potential as a therapeutic target.

AREAS COVERED

The relationship between the structure of CD74 variants and their physiological functions is detailed in this review. The function of CD74 in several cell lineages is examined with a focus on the interactions with cathepsins and, in an inflammatory milieu, the pro-inflammatory cytokine macrophage migratory inhibitory factor. The role of CD74 signaling in inflammatory and carcinogenic processes is outlined as is the use of CD74 as a therapeutic target (in cancer) and tool (as a vaccine).

EXPERT OPINION

CD74 has several roles within the cell and throughout the immune system. Most prominent amongst these are the complex relationships with MIF and cathepsins. Modulation of CD74 function shows promise for the effective amelioration of disease.

Targeted regulation of self-peptide presentation prevents type I diabetes in mice without disrupting general immunocompetence

Peptide loading of MHC class II (MHCII) molecules is directly catalyzed by the MHCII-like molecule HLA-DM (DM). Another MHCII-like molecule, HLA-DO (DO), associates with DM, thereby modulating DM function. The biological role of DO-mediated regulation of DM activity in vivo remains unknown; however, it has been postulated that DO expression dampens presentation of self antigens, thereby preventing inappropriate T cell activation that ultimately leads to autoimmunity. To test the idea that DO modulation of the MHCII self-peptide repertoire mediates self tolerance, we generated NOD mice that constitutively overexpressed DO in DCs (referred to herein as NOD.DO mice). NOD mice are a mouse model for type 1 diabetes, an autoimmune disease mediated by the destruction of insulin-secreting pancreatic beta cells. Our studies showed that diabetes development was completely blocked in NOD.DO mice. Similar to NOD mice, NOD.DO animals selected a diabetogenic T cell repertoire, and the numbers and function of Tregs were normal. Indeed, immune system function in NOD.DO mice was equivalent to that in NOD mice. NOD.DO DCs, however, presented an altered MHCII-bound self-peptide repertoire, thereby preventing the activation of diabetogenic T cells and subsequent diabetes development. These studies show that DO expression can shape the overall MHCII self-peptide repertoire to promote T cell tolerance.

Slc11a1 enhances the autoimmune diabetogenic T-cell response by altering processing and presentation of pancreatic islet antigens

OBJECTIVE

Efforts to map non-major histocompatibility complex (MHC) genes causing type 1 diabetes in NOD mice identified Slc11a1, formerly Nramp1, as the leading candidate gene in the Idd5.2 region. Slc11a1 is a membrane transporter of bivalent cations that is expressed in late endosomes and lysosomes of macrophages and dendritic cells (DCs). Because DCs are antigen-presenting cells (APCs) known to be critically involved in the immunopathogenic events leading to type 1 diabetes, we hypothesized that Slc11a1 alters the processing or presentation of islet-derived antigens to T-cells.

RESEARCH DESIGN AND METHODS

NOD mice having wild-type (WT) or mutant Slc11a1 molecules and 129 mice having WT or null Slc11a1 alleles were examined for parameters associated with antigen presentation.

RESULTS

We found that Slc11a1 enhanced the presentation of a diabetes-related T-cell determinant of GAD65, and its function contributed to the activation of a pathogenic T-cell clone, BDC2.5. An enhanced generation of interferon (IFN)-gamma-producing T-cells was also associated with functional Slc11a1. The alteration of immune responsiveness by Slc11a1 genotype did not correlate with altered MHC class II expression in DCs; however, functional Slc11a1 was associated with accelerated phagocytosis and phagosomal acidification in DCs.

CONCLUSIONS

The association of variants encoding Slc11a1 with type 1 diabetes may reflect its function in processing and presentation of islet self-antigens in DCs. Thus, non-MHC genes could affect the MHC-restricted T-cell response through altered antigen processing and presentation.