The identification of a novel T cell activation state controlled by a diabetogenic gene

Negative regulators in homeostasis of naïve peripheral T cells

It is now apparent that naïve peripheral T cells are a dynamic population where active processes prevent inappropriate activation while supporting survival. The process of thymic education makes naïve peripheral T cells dependent on interactions with self-MHC for survival. However, as these signals can potentially result in inappropriate activation, various non-redundant, intrinsic negative regulatory molecules including Tob, Nfatc2, and Smad3 actively enforce T cell quiescence. Interactions among these pathways are only now coming to light and may include positive or negative crosstalk. In the case of positive crosstalk, self-MHC initiated signals and intrinsic negative regulatory factors may cooperate to dampen T cell activation and sustain peripheral tolerance in a binary fashion (on-off). In the case of negative crosstalk, self-MHC signals may promote survival through partial activation while intrinsic negative regulatory factors act as rheostats to restrain cell cycle entry and prevent T cells from crossing a threshold that would break tolerance.

Eosinophilic bowel disease controlled by the BB rat-derived lymphopenia/Gimap5 gene

BACKGROUND & AIMS

Many models of autoimmunity are associated with lymphopenia. Most involve a T-helper cell (Th)1-type disease, including the diabetic BioBreeding (BB) rat. To investigate the roles of identified susceptibility loci in disease pathogenesis, we bred PVG-RT1(u), lymphopenia (lyp)/lyp rats, congenic for the iddm1 (RT1(u)) and iddm2 (lyp, Gimap5(-/-)) diabetes susceptibility loci on the PVG background. Surprisingly, these rats developed a spontaneous, progressive, inflammatory bowel disease. To understand the disease pathogenesis, we undertook investigations at the genetic, histologic, and cellular levels.

METHODS

Genetically lymphopenic rats and congenic wild-type partners were compared for gross pathologic, histologic, and immunologic parameters, the latter including cytokines and autoantibodies.

RESULTS

Genetic analysis demonstrated that homozygosity at the lyp locus was required for disease. All rats developed disease, and the median age at humane killing was approximately 36 weeks. This panintestinal disease showed a conspicuous eosinophilic infiltrate in the submucosa and muscle layers, but the villi were unaffected. Diseased rats showed splenomegaly and massive enlargement of the mesenteric lymph nodes. This pathology resembles human eosinophilic gastroenteritis, and several further features indicate a Th2 basis. The rats developed high serum IgE and made IgG autoantibodies that detected a nonleukocytic cell present in the intestinal wall of all rats (including germ free).

CONCLUSIONS

The T-lymphopenic state associated with GIMAP5 deficiency renders rats generally susceptible to T-cell-mediated autoimmunity, but the immunoregulatory bias (Th1/Th2) of any disease depends on other genetic (or environmental) factors. In the present model, we suggest that defective peripheral tolerance to an intestine-specific autoantigen leads to uncontrolled inflammation of the intestinal wall.

Loss of a gimap/ian gene leads to activation of NF-kappaB through a MAPK-dependent pathway

The diabetes-prone biobreeding (BB-DP) rat contains the lyp mutation which results in lymphopenia and promotes the progression of a T cell-mediated autoimmune attack of the pancreas in certain rat strains. This mutation has been mapped to a gene which bears homology to human Gimap5/Ian5 and results in the truncation and loss of activity of this protein. The lymphopenic state induced by the loss of this protein has led to the proposal that Gimap5 has an anti-apoptotic function. Previously we described an additional phenotype of incomplete activation mediated by the loss of Gimap5 function. Here we further characterize this incomplete activation phenotype and map a potential signal transduction pathway leading to activation. We show that CD5 expression on peripheral T cells is elevated in Gimap5 animals, while thymocyte expression remains similar between the two strains. Additionally, we show that NF-kappaB but not NFAT is activated in unstimulated Gimap5 mutant T cells as compared to unstimulated wild type T cells. Mapping this activation to its upstream source we show that activation of NF-kappaB is correlated with an activation of IKK. Using a variety of kinase inhibitors we further map this increase in IKK to an increase in MEK activation. Finally, to counter the possibility that activation is an indirect consequence of the lymphopenic environment, we created bone marrow chimeras in which Gimap5 mutant T cells developed in a normal environment and show that these cells retain their activated phenotype. Together, we interpret these data as demonstrating that the activation caused by loss of Gimap5 is a cell intrinsic phenomenon caused, in part, by a MEK-dependent activation of IKK. This, in turn, would suggest that Gimap5 functions to promote both T cell survival and quiescence and that these pathways are biochemically linked.

Expression of the Ian family of putative GTPases during T cell development and description of an Ian with three sets of GTP/GDP-binding motifs

Reports suggest that two members of the novel immune-associated nucleotide (Ian) GTPase family, Ian1 and Ian5, play roles in T cell development. We performed real-time PCR analysis of the expression of Ian genes of the rat during T cell maturation, in macrophages and in cell lines. We found that all of the genes were expressed at relatively low levels at the early double-negative thymocyte stage but were expressed more strongly at later cell stages. Our study also revealed the fact that the previously reported Ian9, Ian10 and Ian11 genes are, instead, parts of a single gene for which we retain the name Ian9, potentially encoding a GTPase with a highly unusual triplicated structure. Antisera were developed against both Ian1 and Ian9. We established that Ian9 is produced as an approximately 75-kDa protein in both T cells and thymocytes. We observed that levels of both Ian1 and Ian9 proteins are profoundly reduced in T cells from lymphopenic rats as compared with wild-type rats. It was demonstrated that thymocytes and B cells from lymphopenic rats (Ian5 null) did not show enhanced sensitivity to gamma-irradiation-induced apoptosis.

Measurement of IKK activity in primary rat T cells: rapid activation and inactivation

The translocation of the transcription factor NF-kappaB into the nucleus plays a critical role in many physiological events. In unstimulated cells, NF-kappaB is sequestered in the cytosol, bound to its inhibitor IkappaB. Activation primarily occurs via the IkappaB kinase (IKK) complex which phosphorylates IkappaBalpha at serines 32 and 36, creating a recognition site for IkappaB ubiquitination which then targets IkappaB for degradation. Often it is useful to measure IKK activity to assess upstream signaling events leading to NF-kappaB activation. Current methods of assessing IKK activity are limited to IKK isoforms which are recognized by available IKK antibodies. Here, we describe a procedure to qualitatively assess the overall IKK activity in a cell lysate which can be used on any IKK isoform capable of phosphorylating human IkappaBalpha. This nonradioactive assay is based on measurement of the ability of the cell lysate to phosphorylate GST-IkappaBalpha, as measured by Western blotting, using an anti-phospho-IkappaBalpha antibody. We have used this assay to observe the kinetics of TCR-mediated activation of IKK as compared to PMA/ionomycin in primary rat T cells. PMA/ionomycin induces maximal IKK activity within 1 min of stimulation and this activity remains elevated for over 20 min. In comparison, TCR ligation induces maximal IKK activity after 5 min of stimulation and this activity rapidly diminishes to background levels. These data indicate that different stimuli can activate and inactivate IKK with different kinetics and suggest that TCR-mediated activation of IKK is closely linked to the rapid phosphorylation and dephosphorylation, respectively, of TCR-associated kinases.

T-cell education in autoimmune diabetes: teachers and students

Type 1 diabetes mellitus is a classical example of a T-cell-mediated autoimmune disease. Several aberrations in immune regulation have been described in both human diabetes patients and animal models of type 1 diabetes. In this review, we summarize how proposed immune defects might be implicated in the loss of T-cell tolerance towards self in autoimmune diabetes in humans, nonobese diabetic (NOD) mice and Biobreeding (BB) rats. For this purpose, we will discuss the tolerance-inducing mechanisms that an autoreactive T cell should encounter from its genesis to its pathogenic role in the pancreas, in order of appearance. These comprise central tolerance mechanisms (i.e. positive and negative selection in the thymus) and those mechanisms operative in the periphery (i.e. activation-induced cell death and regulatory T cells).

Genetic control of T and B lymphocyte activation in nonobese diabetic mice

Type 1 diabetes in nonobese diabetic (NOD) mice is characterized by the infiltration of T and B cells into pancreatic islets. T cells bearing the TCR Vbeta3 chain are disproportionately represented in the earliest stages of islet infiltration (insulitis) despite clonal deletion of most Vbeta3(+) immature thymocytes by the mammary tumor virus-3 (Mtv-3) superantigen (SAg). In this report we showed that a high frequency of NOD Vbeta3(+) T cells that escape deletion are activated in vivo and that this phenotype is linked to the Mtv-3 locus. One potential mechanism of SAg presentation to peripheral T cells is by activated B cells. Consistent with this idea, we found that NOD mice harbor a significantly higher frequency of activated B cells than nondiabetes-prone strains. These activated NOD B cells expressed cell surface molecules consistent with APC function. At the molecular level, the IgH repertoire of activated B cells in NOD mice was equivalent to resting B cells, suggesting a polyclonal response in vivo. Genetic analysis of the activated B cell phenotype showed linkage to Idd1, the NOD MHC haplotype (H-2(g7)). Finally, Vbeta3(+) thymocyte deletion and peripheral T cell activation did not require B cells, suggesting that other APC populations are sufficient to generate both Mtv-3-linked phenotypes. These data provide insight into the genetic regulation of NOD autoreactive lymphocyte activation that may contribute to failure of peripheral tolerance and the pathogenesis of type I diabetes.