Thioredoxin (TRX) is a ubiquitous oxidoreductase with strong co-cytokine, chemoattractant and anti-apoptotic activities. TRX expression was found to be particularly elevated in the intestinal mucosa, where its physiologic function is entirely unknown. Here, we demonstrate a high level of TRX expression in lamina propria T cells (LP-T) as opposed to autologous peripheral blood T lymphocytes (PB-T). Addition of recombinant human TRX (rhTRX) to PB-T enhances TRX gene expression. This autoregulation involves the calcineurin signaling pathway, as rhTRX antagonizes the cyclosporine A (CsA)-and tacrolimus-mediated suppression of TRX gene expression. Similarly, rhTRX reverses the suppression of IL-2 mRNA production by CsA and enhances cytokine production preferentially in prestimulated cells. The differential TRX expression in LP-T versus PB-T may thus contribute to the high-level, CsA-resistant IL-2 production characteristic for CD2-stimulated LP-T. Inversely, inactivation of TRX in LP-T through inhibition of TRX reductase abolishes cytokine gene expression. TRX may play a key role in the specialized intestinal microenvironment in amplifying immediate immune responses of LP-T whenever appropriate costimulation of LP-T is provided.
IntroductionThioredoxin (TRX) is a ubiquitous 12-kDa oxidoreductase containing two redox-active cysteine residues in its conserved active-center motif -Cys-Gly-Pro-Cys- [1,2]. Together with the selenoprotein TRX reductase and NADPH, it operates as a protein disulfide-reducing system [3][4][5][6][7][8]. Human PBMC express only minute amounts of TRX, which increase after mitogenic stimulation in vitro [3,9,10]. Oxidative stress through, e.g., hydrogen peroxide or after ischemia-reperfusion enhances the expression of TRX [11,12], which serves as an antioxidant in addition to intracellular glutathione [13][14][15]. Moreover, TRX is involved in the redox control of DNA binding of transcription factors like NF-jB [16,17] and AP-1 [18,19]. TRX stimulates cell growth [20] and is known as an autocrine growth factor for human T lymphotropic virus-1 and EBV-transformed lymphocytes [9]. TRX mediates anti-apoptotic effects [21][22][23], is a strong chemoattractant for neutrophils, monocytes, and T cells [24], and acts like a co-cytokine through stimulation of DNA synthesis in concert with IL-1 [9], IL-2 [4, 9], , and . TRX was suggested to enhance proliferation by increasing the cellular uptake of cystine into PBMC [27] and by sensitizing cells for growth factors [28].Upon stimulation, lymphocytes secrete TRX through a leaderless secretory pathway [29,30]. Extracellular TRX binds to the cell membrane without a high-affinity [25,32]. Because the gut is the largest immunologic organ of the body and harbors the largest T cell compartment (lamina propria), one could assume that TRX is involved in intestinal immune responses.The proliferative activity of lamina propria T lymphocytes (LP-T) towards TCR/CD3-dependent stimuli is extremely low in the normal gut, which may in part be explained by the inability of ...