Cathepsin L is essential for onset of autoimmune diabetes in NOD mice

Specialized roles for cysteine cathepsins in health and disease

Cathepsins were originally identified as proteases that act in the lysosome. Recent work has uncovered nontraditional roles for cathepsins in the extracellular space as well as in the cytosol and nucleus. There is strong evidence that subspecialized and compartmentalized cathepsins participate in many physiologic and pathophysiologic cellular processes, in which they can act as both digestive and regulatory proteases. In this review, we discuss the transcriptional and translational control of cathepsin expression, the regulation of intracellular sorting of cathepsins, and the structural basis of cathepsin activation and inhibition. In particular, we highlight the emerging roles of various cathepsin forms in disease, particularly those of the cardiac and renal systems.

Cathepsin L inhibition prevents murine autoimmune diabetes via suppression of CD8(+) T cell activity

Background

Type 1 diabetes (T1D) is an autoimmune disease resulting from defects in central and peripheral tolerance and characterized by T cell-mediated destruction of islet β cells. To determine whether specific lysosomal proteases might influence the outcome of a T cell–mediated autoimmune response, we examined the functional significance of cathepsin inhibition on autoimmune T1D-prone non-obese diabetic (NOD) mice.

Methods and Findings

Here it was found that specific inhibition of cathepsin L affords strong protection from cyclophosphamide (CY)-induced insulitis and diabetes of NOD mice at the advanced stage of CD8⁺ T cell infiltration via inhibiting granzyme activity. It was discovered that cathepsin L inhibition prevents cytotoxic activity of CD8⁺ T cells in the pancreatic islets through controlling dipeptidyl peptidase I activity. Moreover, the gene targeting for cathepsin L with application of in vivo siRNA administration successfully prevented CY-induced diabetes of NOD mice. Finally, cathepsin L mRNA expression of peripheral CD8⁺ T cells from NOD mice developing spontaneous T1D was significantly increased compared with that from control mice.

Conclusions

Our results identified a novel function of cathepsin L as an enzyme whose activity is essential for the progression of CD8⁺ T cell-mediated autoimmune diabetes, and inhibition of cathepsin L as a powerful therapeutic strategy for autoimmune diabetes.

The lysosome among targets of metformin: new anti-inflammatory uses for an old drug?

BACKGROUND

Rheumatoid arthritis and type-2 diabetes exhibit progressive co-morbidity. Chloroquine (CQ) reportedly improves both. CQ inhibits lysosomal function in cultured cells at supra-therapeutic concentration; however, this is doubted as target mechanism. Some anti-diabetic biguanides are metal-interactive lysosomal inhibitors; and all bind Zn(2+).

OBJECTIVES

i) To bioassay the potency of CQ using (3)H-leucine release from perfused myocardial tissue. ii) To determine whether metformin (MET) is CQ-mimetic, and interactive with Zn(2+).

RESULTS

Therapeutic CQ concentration (0.1 - 0.5 microM) clearly does cause lysosomal inhibition although delayed and submaximal. MET alone (10 microM) caused sub-maximal inhibition. Supra-physiological extracellular Zn(2+) (5 - 50 microM) alone increased tissue Zn(2+) content, and inhibited lysosomal proteolysis. Physiological equivalent Zn(2+) (approximately 1 microM) had no effect. MET (<or= 25 microM) and Zn(2+) (<or= 1 microM) exhibited astounding 10 - 100 fold anti-lysosomal synergy. Cathepsin B was 50% inhibited by 1 muM Zn(2+), and is reportedly inhibited by gold agents.

INTERPRETATION

MET somehow increases the natural inhibitory action of action of Zn(2+) against cysteinyl proteases. TNF-alpha activates lysosomal function; and CatB is among post-receptor players. MET might decrease antigen processing in specialized cells, and lysosomal hyper-catabolism in other cells.

CONCLUSIONS

Trials of MET for new use as an anti-inflammatory agent are suggested. Guanidylguanidine is a practical pharmacophore for synthesis of future anti-lysosomal agents.

A small-molecule oxocarbazate inhibitor of human cathepsin L blocks severe acute respiratory syndrome and ebola pseudotype virus infection into human embryonic kidney 293T cells

A tetrahydroquinoline oxocarbazate (PubChem CID 23631927) was tested as an inhibitor of human cathepsin L (EC 3.4.22.15) and as an entry blocker of severe acute respiratory syndrome (SARS) coronavirus and Ebola pseudotype virus. In the cathepsin L inhibition assay, the oxocarbazate caused a time-dependent 17-fold drop in IC(50) from 6.9 nM (no preincubation) to 0.4 nM (4-h preincubation). Slowly reversible inhibition was demonstrated in a dilution assay. A transient kinetic analysis using a single-step competitive inhibition model provided rate constants of k(on) = 153,000 M(-1)s(-1) and k(off) = 4.40 x 10(-5) s(-1) (K(i) = 0.29 nM). The compound also displayed cathepsin L/B selectivity of >700-fold and was nontoxic to human aortic endothelial cells at 100 muM. The oxocarbazate and a related thiocarbazate (PubChem CID 16725315) were tested in a SARS coronavirus (CoV) and Ebola virus-pseudotype infection assay with the oxocarbazate but not the thiocarbazate, demonstrating activity in blocking both SARS-CoV (IC(50) = 273 +/- 49 nM) and Ebola virus (IC(50) = 193 +/- 39 nM) entry into human embryonic kidney 293T cells. To trace the intracellular action of the inhibitors with intracellular cathepsin L, the activity-based probe biotin-Lys-C5 alkyl linker-Tyr-Leu-epoxide (DCG-04) was used to label the active site of cysteine proteases in 293T lysates. The reduction in active cathepsin L in inhibitor-treated cells correlated well with the observed potency of inhibitors observed in the virus pseudotype infection assay. Overall, the oxocarbazate CID 23631927 was a subnanomolar, slow-binding, reversible inhibitor of human cathepsin L that blocked SARS-CoV and Ebola pseudotype virus entry in human cells.

Processing and presentation of (pro)-insulin in the MHC class II pathway: the generation of antigen-based immunomodulators in the context of type 1 diabetes mellitus

Both CD4(+) and CD8(+) T lymphocytes play a crucial role in the autoimmune process leading to T1D. Dendritic cells take up foreign antigens and autoantigens; within their endocytic compartments, proteases degrade exogenous antigens for subsequent presentation to CD4(+) T cells via MHC class II molecules. A detailed understanding of autoantigen processing and the identification of autoantigenic T cell epitopes are crucial for the development of antigen-based specific immunomodulators. APL are peptide analogues of auto-immunodominant T cell epitopes that bind to MHC class II molecules and can mediate T cell activation. However, APL can be rapidly degraded by proteases occurring in the extracellular space and inside cells, substantially weakening their efficiency. By contrast, protease-resistant APL function as specific immunomodulators and can be used at low doses to examine the functional plasticity of T cells and to potentially interfere with autoimmune responses. Here, we review the latest achievements in (pro)-insulin processing in the MHC class II pathway and the generation of APL to mitigate autoreactive T cells and to activate Treg cells.

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.

Expression of human cathepsin L or human cathepsin V in mouse thymus mediates positive selection of T helper cells in cathepsin L knock-out mice

A genetic deficiency of the cysteine protease cathepsin L (Ctsl) in mice results in impaired positive selection of conventional CD4+ T helper cells as a result of an incomplete processing of the MHC class II associated invariant chain or incomplete proteolytic generation of positively selecting peptide ligands. The human genome encodes, in contrast to the mouse genome, for two cathepsin L proteases, namely cathepsin L (CTSL) and cathepsin V (CTSV; alternatively cathepsin L2). In the human thymic cortex, CTSV is the predominately expressed protease as compared to CTSL or other cysteine cathepsins. In order to analyze the functions of CTSL and CTSV in the positive selection of CD4+ T cells we employed Ctsl knock-out mice crossed either with transgenic mice expressing CTSL under the control of its genuine human promoter or with transgenic mice expressing CTSV under the control of the keratin 14 (K14) promoter, which drives expression to the cortical epithelium. Both human proteases are expressed in the thymus of the transgenic mice, and independent expression of both CTSL and CTSV rescues the reduced frequency of CD4+ T cells in Ctsl-deficient mice. Moreover, the expression of the human cathepsins does not change the number of CD4+CD25+Foxp3+ regulatory T cells, but the normalization of the frequency of conventional CD4+ T cell in the transgenic mice results in a rebalancing of conventional T cells and regulatory T cells. We conclude that the functional differences of CTSL and CTSV in vivo are not mainly determined by their inherent biochemical properties, but rather by their tissue specific expression pattern.

Acquisition of T regulatory function in cathepsin L-inhibited T cells by eye-derived CTLA-2alpha during inflammatory conditions

Pigment epithelium isolated from the eye possesses immunosuppressive properties such as regulatory T (Treg) cell induction; e.g., cultured retinal pigment epithelium (RPE) converts CD4(+) T cells into Treg cells in vitro. RPE constitutively expresses a novel immunosuppressive factor, CTLA-2alpha, which is a cathepsin L (CathL) inhibitor, and this molecule acts via RPE to induce Treg cells. To clarify CTLA-2alpha's role in the T cell response to RPE in ocular inflammation, we used the experimental autoimmune uveitis (EAU) animal model to examine this new immunosuppressive property of RPE. In EAU models, TGF-beta, but not IFN-gamma inflammatory cytokines, promotes the up-regulation of the expression of CTLA-2alpha in RPE. Similarly, CTLA-2alpha via RPE was able to promote TGF-beta production by the CD4(+) T cells. The RPE-exposed T cells (RPE-induced Treg cells) greatly produced TGF-beta and suppressed bystander effector T cells. There was less expression of CathL by the RPE-exposed T cells, and CathL-inhibited T cells were able to acquire the Treg phenotype. Moreover, CathL-deficient mice spontaneously produced Treg cells, with the increase in T cells potentially providing protection against ocular inflammation. More importantly, CD4(+) T cells from EAU in CathL knockout mice or rCTLA-2alpha from EAU animals were found to contain a high population of forkhead box p3(+) T cells. In both EAU models, there was significant suppression of the ocular inflammation. These results indicate that RPE secretes CTLA-2alpha, thereby enabling the bystander T cells to be converted into Treg cells via TGF-beta promotion.

Employing a recombinant HLA-DR3 expression system to dissect major histocompatibility complex II-thyroglobulin peptide dynamism: a genetic, biochemical, and reverse immunological perspective

Previously, we have shown that statistical synergism between amino acid variants in thyroglobulin (Tg) and specific HLA-DR3 pocket sequence signatures conferred a high risk for autoimmune thyroid disease (AITD). Therefore, we hypothesized that this statistical synergism mirrors a biochemical interaction between Tg peptides and HLA-DR3, which is key to the pathoetiology of AITD. To test this hypothesis, we designed a recombinant HLA-DR3 expression system that was used to express HLA-DR molecules harboring either AITD susceptibility or resistance DR pocket sequences. Next, we biochemically generated the potential Tg peptidic repertoire available to HLA-DR3 by separately treating 20 purified human thyroglobulin samples with cathepsins B, D, or L, lysosomal proteases that are involved in antigen processing and thyroid biology. Sequences of the cathepsin-generated peptides were then determined by matrix-assisted laser desorption ionization time-of-flight-mass spectroscopy, and algorithmic means were employed to identify putative AITD-susceptible HLA-DR3 binders. From four predicted peptides, we identified two novel peptides that bound strongly and specifically to both recombinant AITD-susceptible HLA-DR3 protein and HLA-DR3 molecules expressed on stably transfected cells. Intriguingly, the HLA-DR3-binding peptides we identified had a marked preference for the AITD-susceptibility DR signatures and not to those signatures that were AITD-protective. Structural analyses demonstrated the profound influence that the pocket signatures have on the interaction of HLA-DR molecules with Tg peptides. Our study suggests that interactions between Tg and discrete HLA-DR pocket signatures contribute to the initiation of AITD.

Roles for cathepsins S, L, and B in insulitis and diabetes in the NOD mouse

We developed a panel of non-obese diabetic (NOD) mice deficient in major lysosomal cysteine proteases (cathepsins S, L and B) to identify protease enzymes essential for autoimmune diabetes. Null alleles for cathepsins (Cts) S, L or B were introgressed onto the NOD genetic background with 19 Idd markers at homozygosity. Diabetes onset was determined among females aged up to 6 months. We evaluated insulitis and sialadenitis in tissues using histology and computer assisted morphology. NOD mice deficient in Ctss or Ctsb were partially protected from diabetes with incidence at 33% and 28%, respectively, versus wild-type NOD (69%; p < 0.00001). NODs lacking cathepsin L (Ctsl-/-) are completely protected from IDDM, as originally shown by others. Ctsl, Ctss, or Ctsb heterozygous mice were able to develop IDDM, although incidence levels were significantly lower for Ctsb+/- (50%) and Ctsl+/- (55%) as compared to NODs (69%; p < 0.03). Ctsl-/- mice contain functional, diabetogenic T cells and an enriched Foxp3+ regulatory T cell population, and diabetes resistance was due to the presence of an expanded population of regulatory T cells. These data provide additional information about the potency of the diabetogenic T cell population in Ctsl-/- mice which were comparable in potency to wild-type NOD mice. These data illustrate the critical contribution of each of these proteases in determining IDDM in the NOD mouse and provide a useful set of models for further studies.

Genetic and pharmacologic alteration of cathepsin expression influences reovirus pathogenesis

The cathepsin family of endosomal proteases is required for proteolytic processing of several viruses during entry into host cells. Mammalian reoviruses utilize cathepsins B (Ctsb), L (Ctsl), and S (Ctss) for disassembly of the virus outer capsid and activation of the membrane penetration machinery. To determine whether cathepsins contribute to reovirus tropism, spread, and disease outcome, we infected 3-day-old wild-type (wt), Ctsb(-/-), Ctsl(-/-), and Ctss(-/-) mice with the virulent reovirus strain T3SA+. The survival rate of Ctsb(-/-) mice was enhanced in comparison to that of wt mice, whereas the survival rates of Ctsl(-/-) and Ctss(-/-) mice were diminished. Peak titers at sites of secondary replication in all strains of cathepsin-deficient mice were lower than those in wt mice. Clearance of the virus was delayed in Ctsl(-/-) and Ctss(-/-) mice in comparison to the levels for wt and Ctsb(-/-) mice, consistent with a defect in cell-mediated immunity in mice lacking cathepsin L or S. Cathepsin expression was dispensable for establishment of viremia, but cathepsin L was required for maximal reovirus growth in the brain. Treatment of wt mice with an inhibitor of cathepsin L led to amelioration of reovirus infection. Collectively, these data indicate that cathepsins B, L, and S influence reovirus pathogenesis and suggest that pharmacologic modulation of cathepsin activity diminishes reovirus disease severity.

Identification of a major susceptibility locus for lethal graft-versus-host disease in MHC-matched mice

Graft-vs-host disease (GVHD) is the major cause of morbidity and mortality after allogeneic hemopoietic cell transplantation. From a genetic perspective, GVHD is a complex phenotypic trait. Although it is understood that susceptibility results from interacting polymorphisms of genes encoding histocompatibility Ags and immune regulatory molecules, a detailed and integrative understanding of the genetic background underlying GVHD remains lacking. To gain insight regarding these issues, we performed a forward genetic study. A MHC-matched mouse model was used in which irradiated recipient BALB.K and B10.BR mice demonstrate differential susceptibility to lethal GHVD when transplanted using AKR/J donors. Assessment of GVHD in (B10.BR x BALB.K)F(1) mice revealed that susceptibility is a dominant trait and conferred by deleterious alleles from the BALB.K strain. To identify the alleles responsible for GVHD susceptibility, a genome-scanning approach was taken using (B10.BR x BALB.K)F(1) x B10.BR backcross mice as recipients. A major susceptibility locus, termed the Gvh1 locus, was identified on chromosome 16 using linkage analysis (logarithm of the odds, 9.1). A second locus was found on chromosome 13, named Gvh2, which had additive but protective effects. Further identification of Gvh genes by positional cloning may yield new insight into genetic control mechanisms regulating GVHD and potentially reveal novel approaches for effective GVHD therapy.

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.

Retinal pigment epithelium-derived CTLA-2alpha induces TGFbeta-producing T regulatory cells

T cells that encounter ocular pigment epithelium in vitro are inhibited from undergoing TCR-triggered activation, and instead acquire the capacity to suppress the activation of bystander T cells. Because retinal pigment epithelial (RPE) cells suppress T cell activation by releasing soluble inhibitory factors, we studied whether soluble factors also promote the generation of T regulatory (Treg) cells. We found that RPE converted CD4(+) T cells into Treg cells by producing and secreting CTLA-2alpha, a cathepsin L (CathL) inhibitor. Mouse rCTLA-2alpha converted CD4(+) T cells into Treg cells in vitro, and CTLA-2alpha small interfering RNA-transfected RPE cells failed to induce the Treg generation. RPE CTLA-2alpha induced CD4(+)CD25(+)Foxp3(+) Treg cells that produced TGFbeta in vitro. Moreover, CTLA-2alpha produced by RPE cells inhibited CathL activity in the T cells, and losing CathL activity led to differentiation to Treg cells in some populations of CD4(+) T cells. In addition, T cells in the presence of CathL inhibitor increased the expression of Foxp3. The CTLA-2alpha effect on Treg cell induction occurred through TGFbeta signaling, because CTLA-2alpha promoted activation of TGFbeta in the eye. These results show that immunosuppressive factors derived from RPE cells participate in T cell suppression. The results are compatible with the hypothesis that the eye-derived Treg cells acquire functions that participate in the establishment of immune tolerance in the posterior segment of the eye.

Cathepsin L inhibition suppresses drug resistance in vitro and in vivo: a putative mechanism

Cathepsin L is a lysosomal enzyme thought to play a key role in malignant transformation. Recent work from our laboratory has demonstrated that this enzyme may also regulate cancer cell resistance to chemotherapy. The present study was undertaken to define the relevance of targeting cathepsin L in the suppression of drug resistance in vitro and in vivo and also to understand the mechanism(s) of its action. In vitro experiments indicated that cancer cell adaptation to increased amounts of doxorubicin over time was prevented in the presence of a cathepsin L inhibitor, suggesting that inhibition of this enzyme not only reverses but also prevents the development of drug resistance. The combination of the cathepsin L inhibitor with doxorubicin also strongly suppressed the proliferation of drug-resistant tumors in nude mice. An investigation of the underlying mechanism(s) led to the finding that the active form of this enzyme shuttles between the cytoplasm and nucleus. As a result, its inhibition stabilizes and enhances the availability of cytoplasmic and nuclear protein drug targets including estrogen receptor-alpha, Bcr-Abl, topoisomerase-IIalpha, histone deacetylase 1, and the androgen receptor. In support of this, the cellular response to doxorubicin, tamoxifen, imatinib, trichostatin A, and flutamide increased in the presence of the cathepsin L inhibitor. Together, these findings provided evidence for the potential role of cathepsin L as a target to suppress cancer resistance to chemotherapy and uncovered a novel mechanism by which protease inhibition-mediated drug target stabilization may enhance cellular visibility and, thus, susceptibility to anticancer agents.

Polymorphisms in the cathepsin L2 (CTSL2) gene show association with type 1 diabetes and early-onset myasthenia gravis

Type 1 diabetes (T1D) is an autoimmune disease characterized by loss of beta cells in the pancreas. The CTSL2 gene encodes the cysteine protease cathepsin V involved in antigen presentation in human cortical thymic epithelial cells, and involvement of the protease in autoimmunity has been suggested. This study aimed to evaluate CTSL2 as a candidate gene for T1D, and test whether the gene predisposes more generally to autoimmune diseases. Four polymorphisms aiming at tagging the CTSL2 locus were genotyped in 421 T1D families, and subsequently in 861 rheumatoid arthritis patients, 530 juvenile idiopathic arthritis patients, and 559 controls of Norwegian origin. Additionally, DNA from 83 German myasthenia gravis (MG) patients and 244 controls were investigated. A polymorphism, rs16919034, situated downstream of CTSL2 was associated with T1D (60.8%T, p = 0.008; p(c) = 0.03). An association with early-onset MG (45% in cases vs 36.6% in controls; p = 0.03) was observed for another polymorphism (rs4361859) situated upstream of the gene, but within the same linkage disequilibrium block. No association was observed in rheumatoid arthritis or juvenile idiopathic arthritis. Our findings suggest that the CTSL2 gene is associated with T1D and with early-onset MG.

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

The invariant (Ii) chain acts as an essential chaperone to promote MHC class II surface expression, Ag presentation, and selection of CD4(+) T cells. We have examined its role in the development of type 1 diabetes in NOD mice and show that Ii chain-deficient NOD mice fail to develop type 1 diabetes. Surprisingly, Ii chain functional loss fails to disrupt in vitro presentation of islet Ags, in the context of NOD I-A(g7) molecules. Moreover, pathogenic effector cells could be shown to be present in Ii chain-deficient NOD mice because they were able to transfer diabetes to NOD.scid recipients. The ability of these cells to transfer diabetes was markedly enhanced by depletion of CD25 cells coupled with in vivo anti-CD25 treatment of recipient mice. The numbers of CD4(+)CD25(+)Foxp3(+) T cells in thymus and periphery of Ii chain-deficient NOD mice were similar to those found in normal NOD mice, in contrast to conventional CD4(+) T cells whose numbers were reduced. This suggests that regulatory T cells are unaffected in their selection and survival by the absence of Ii chain and that an alteration in the balance of effector to regulatory T cells contributes to diabetes prevention.

Nonobese diabetic mice express aspects of both type 1 and type 2 diabetes

Before the onset of autoimmune destruction, type 1 diabetic patients and an animal model, the nonobese diabetic (NOD) mouse, show morphological and functional abnormalities in target organs, which may act as inciting events for leukocyte infiltration. To better understand these abnormalities, but without the complications associated with lymphocytic infiltrates, we examined genes expressed in autoimmune target tissues of NOD/severe combined immunodeficient (scid) mice and of autoimmune-resistant C57BL/6/scid mice. Our results suggest that the NOD genetic background may predispose them to diabetic complications, including insulin resistance in the absence of high circulating glucose levels and without autoimmune destruction of their beta cells. Several of these genes lie within known type 1 and 2 diabetes loci. These data suggest that the NOD mouse may be a good candidate to study an interface between type 1 and type 2 diabetes.

Endosomal proteases in antigen presentation

Processing of cytoplasmic and endocytized foreign antigens were once thought to be completely separate pathways of peptide generation for MHC class I and class II peptide display. However, recent studies show actually that there is extensive overlap owing, in part, to the positioning of endosomal proteases at sites in which MHC class I and class II paths cross. The questions of how antigens access and persist at sites of endosomal processing are proving to be as important as those regarding the proteases themselves. However, individual proteases within the set of endosomal enzymes have emerged both as targets for microbial evasion from MHC-class-II-dependent immunity and as required elements in autoimmunity. Repeated implication of cathepsins in processing medically important antigens holds promise for therapeutic manipulation of proteases in diseases in which there is disordered adaptive immunity.