IFN-γ-inducible lysosomal thiol reductase (GILT) is a unique thiol reductase with optimal enzymatic activity at low pH. GILT plays a crucial role in unfolding the antigenic proteins in preparation for their proteolytic cleavage and presentation of resulting peptides by MHC class II. In this study, we demonstrate that GILT is expressed in T lymphocytes and that it has an APC-nonrelated role in the regulation of T cell activation. Surprisingly, comparison of wild-type and GILT-deficient T cell activation in vitro revealed stronger responsiveness in the absence of GILT. The effect of GILT in reducing the proliferative and cytotoxic responses was endogenous to T cells and resulted from decreased sensitivity at the individual cell level. Therefore, a molecule with primarily lysosomal localization suppresses T cell activation, a process characterized by signal transmission from plasma membrane to cytoplasm and nucleus.

Gamma interferon-inducible thiol reductase (GILT) is an enzyme involved in the initial steps of antigen processing and presentation. Recently we have shown that GILT is also expressed in mouse T cells, where it exerts an inhibitory role on T cell activation. In this study, we identified mitochondrial manganese superoxide dismutase (SOD2) as one of the key intermediaries affected by GILT expression in fibroblasts. Expression and activity of SOD2 is reduced in the absence of GILT because of reduced SOD2 protein stability. The forced increase in SOD2 expression in the absence of GILT restores fibroblast proliferation to wild-type levels. Thus, GILT appears to have a fundamental role in cellular proliferation mediated through its influence on SOD2 protein activity and expression.Enzymes of the thiol reductase family carry out reduction, oxidation, and isomerization of protein disulfide bonds in cytosol (for example, thioredoxin) (1, 2), mitochondria (3), endoplasmic reticulum (protein-disulfide isomerase) (2), and lysosomes (gamma interferon-inducible thiol reductase, GILT).2 The majority of these enzymes are functional at neutral or slightly alkaline conditions (4), they have similar three-dimensional structures, and all feature a conservative active site loop containing two cysteines in the sequence -CGPC-(5). GILT is a unique and unusual member of the thiol reductase family because its optimal enzymatic activity is at a low pH (4.5-5.5) (6 -8) and has an atypical active site (-CGAC-).GILT is synthesized as a 35-kDa soluble glycoprotein precursor and is transported to the endosomal compartment via the mannose-6-P receptor pathway (9). It is processed into the mature form (30 kDa) by proteolytic removal of N-and C-terminal peptides. The protein has an approximate molecular mass of 30 kDa and was therefore initially named IP-30 (6). In addition to endosomal/lysosomal localization, GILT is secreted in the tissue culture medium of the GILT-expressing cell lines (7,10), and is present in mouse sera.3 GILT is constitutively expressed in professional antigen-presenting cells (APCs), but it is also inducible by pro-inflammatory cytokines such as interferon ␥, tumor necrosis factor ␣, and interleukin 1␤ (9).Using GILT Ϫ/Ϫ mice as a model, we have shown that GILT catalyzes initial unfolding of antigenic protein (protein becomes more accessible for further processing by cathepsins) and therefore facilitates protein/peptide binding to MHC class II molecules (10). By changing the redox state of exogenous antigenic proteins with disulfide bonds, GILT initiates the adaptive immune response. However, we have shown that GILT is constitutively expressed in T cells and has a role in the regulation of T cell activation. This is so far the only known GILT function not related to MHC class II processing (11). GILT Ϫ/Ϫ T cells show increased proliferation and cytotoxic T cell activity in response to anti-CD3 stimulation. This observation suggests that GILT has a more fundamental role in cellular processes than just reduction of antigens...

Reactivity to self-peptide/MHC complexes is required for selection of the TCR repertoire in the thymus but can also promote autoimmunity. Reduced TCR sensitivity of mature T cells is thought to help control the autoreactivity in peripheral T cells. The molecular basis for reduced sensitivity of peripheral T cells is not known. We found that peripheral T cells, but not immature thymocytes, lacking IFN-γ-inducible lysosomal thiol reductase (GILT) display increased sensitivity to TCR ligation. GILT−/− peripheral T cells express reduced levels of mitochondrial superoxide dismutase 2 and consequently display higher levels of reactive oxygen radicals and ERK1/2 phosphorylation following activation. The increased sensitivity of GILT-deficient T cells results in a more severe hyperglycemia associated with streptozotocin-induced diabetes. GILT expression levels progressively increase in T cells with maturation. These data suggest that regulation of GILT expression may be a mechanism of T cell differentiation-associated changes in sensitivity to TCR engagement.

Gamma interferon Inducible Lysosomal Thiol reductase (GILT) is a unique lysosomal reductase that reduces disulfide bonds of endocytosed proteins. Lack of GILT clearly decreases CD4 T cell-antigen specific responses against some epitopes of antigens containing disulfide bonds, but not to proteins with few or no disulfide bridges. Hence, global impact of GILT on antigen presentation is currently not well understood. We used Nano-LC-ESI-MS/MS to investigate how GILT affects diversity of self-peptides presented by MHC class II molecules. Surprisingly, the repertoire of self-peptides in the absence of GILT does not appear to be significantly different, as only few peptide species (∼2%) were found to be the unique indicators of GILT's presence or absence. In the absence of GILT about thirty peptide species (∼5%) were found either uniquely or fourteen to hundred fold more abundantly expressed than in the presence of GILT. Our data indicate that GILT has limited yet unexpected effect on self-peptide species presented by MHC class II antigens.

INF-Gamma-Inducible Lysosomal Thiol Reductase (GILT) is a unique member of thiol reductase family with optimal enzymatic activity at low pH. It is located in the endosomal compartment of the cell where the antigen processing takes place. GILT facilitates protein unfolding, which makes protein more accessible to processing by proteases present in endosomes/lysosmes and consequent presentation of resulting peptides by MHCII. In addition, it has been shown that GILT has an APC-unrelated role in the regulation of T cell activation. In this study, we demonstrate that GILT affects the expression and function of manganese superoxide dismutase (SOD2) in mouse fibroblast cell line. In comparison to the wild-type, GILT-deficient mouse ear fibroblasts had significantly lower SOD2 expression level and function. In addition, GILT-deficient mouse fibroblasts have shown increased proliferation. These data raise the possibility that there may be a direct and/or indirect effect of lysosomal protein GILT on mitochondrial SOD2 expression level, and function. This work was supported by Scientist Development Grant, American Heart Association 0535032N