The immune dysregulatory compound mercuric chloride induces integrin-mediated T-lymphocyte adhesion

Exposure of Brown Norway rats to mercuric chloride induces systemic autoimmunity, involving T- and B-lymphocyte activation, (auto-)antibody production and multiorgan inflammation. Several divalent metal ions, such as Mg2+ and Mn2+, can activate binding of integrins to their ligands, thus causing lymphocyte adhesion. To test the hypothesis that Hg2+ acts in a similar way, we studied the effect of HgCl2 on integrin-mediated T-cell adhesion. HgCl2 induced cell-cell aggregation of human T lymphoblasts. Exposure of a human T-cell clone to HgCl2 for 1 hr enhanced, in a dose-dependent way, cell binding to fibronectin (FN) and to intercellular adhesion molecules (ICAM) -1, -2 and -3. Furthermore, HgCl2 induced strong binding of Jurkat T cells to FN. These effects of HgCl2 were of similar magnitude as the effects of phorbol 12-myristate 13-acetate (PMA) or MnCl2. Studies using blocking antibodies indicated the involvement of CD11a in binding to ICAMs, and of CD49d, CD49e, and CD29 in binding to FN. Adhesion to FN induced by HgCl2 or by PMA, but not by MnCl2, was dependent on temperature and on extracellular Ca2+ or Mg2+. Addition of cytochalasin B enhanced synergistically the FN adhesion induced by MnCl2, whereas the effects of PMA and HgCl2 were not modified. These results indicate that Hg2+ is a potent activator of T-cell adhesion, mediated by several integrins and ligands. In contrast to the effect of MnCl2, HgCl2-induced cell adhesion probably involves an intracellular pathway. Activation of integrins by HgCl2 may play an important role in activation and migration of leucocytes involved in HgCl2-induced immune dysregulation in vivo.

Strong expression of CD134 (OX40), a member of the TNF receptor family, in a T helper 2-type cytokine environment

CD134 (OX40) is involved in T cell costimulation and T cell-dependent antibody production. We show strongly increased T cell expression of CD134 in a model of T helper 2-mediated systemic autoimmunity, induced by HgCl2. Regulation of CD134 expression on CD4+ T cells was further studied in vitro, identifying CD134 as an early marker of T cell activation. CD134 expression could be induced by interleukin-4, but not by interferon-gamma or tumor necrosis factor-alpha. Effects of interleukin-4 and of phorbol 12-myristate 13-acetate on CD134 expression could be blocked by the protein kinase inhibitor staurosporin. Combination of these stimuli with ionomycin resulted in a strongly synergistic increase of CD134 expression, which was blocked by the calcineurin-inhibitor cyclosporin A. The results demonstrate the involvement of two synergistically acting pathways in induction of CD134 expression. Furthermore, they suggest a role for interleukin-4 in induction of CD134 expression in vivo.

Differential expression of T-cell adhesion molecules and LFA-1-dependent intercellular adhesion in HgCl2-induced autoimmunity and immune suppression

Exposure of Brown Norway (BN) rats to HgCl2 induces Th2-mediated systemic autoimmunity. In contrast, in Lewis rats, HgCl2 induces immune suppression, mediated by CD8+ T cells. HgCl2 was previously found to enhance expression of LFA-1, ICAM-1 and CD134 (OX40) on T cells in BN rats. In the present study, T cells from Lewis rats were studied at day 4 after injection of HgCl2. CD8+ T lymphoblasts were significantly increased, which were predominantly CD45RC(hi), and which showed enhanced LFA-1 expression. Furthermore, CD4+CD45RC(hi) T cells showed increased numbers of ICAM-1+ cells, whereas expression of CD134 and CD26 was relatively decreased in CD4+ T lymphoblasts. Ex vivo experiments demonstrated that HgCl2-exposure of BN rats, but not of Lewis rats, significantly enhances PMA [phorbol 12-myristate 13-acetate]-induced lymphocyte aggregation, mediated by LFA-1 and ICAM-1. In conclusion, HgCl2-injected Lewis rats show early signs of T-lymphocyte activation, predominantly on CD8+ cells. Strain-dependent effects of HgCl2 on cell adhesion molecules and expression of CD134 may play an important role in development of either autoimmunity or immune suppression.

Differential regulation of expression of the MHC class II molecules RT1.B and RT1.D on rat B lymphocytes: effects of interleukin-4, interleukin-13 and interferon-gamma

Susceptibility to induction of both T helper 1- (Th1) and Th2-mediated autoimmunity is multifactorial and involves genetic linkage to the major histocompatibility complex (MHC) class II haplotype. Brown Norway (BN) rats exposed to mercuric chloride develop a Th2-dependent systemic autoimmunity, whereas Lewis rats, which are highly susceptible to Th1-mediated autoimmunity, develop immune suppression after mercuric chloride exposure. Exposure to mercuric chloride is known to enhance B-lymphocyte expression of the MHC class II molecule RT1.B, predominantly in BN rats. We demonstrate that, in contrast, expression of RT1.D was unmodified on these B cells, whereas both RT1.B and RT1.D were up-regulated on epithelial cells. Regulation of B-cell MHC class II isotype expression was further studied in vitro, using BN rat lymph node (LN) cells. Interleukin-4 (IL-4) strongly enhanced B-cell expression of RT1.B (2.8-fold), whereas RT1.D expression was only slightly, although significantly, modified (1.2-fold). B cells from Lewis rats showed a similar IL-4-induced enhancement of RT1.B expression (2.5-fold), whereas, in contrast, RT1.D expression was unmodified. Exposure of LN cells from BN rats to interferon-gamma induced a moderate increase of B-cell MHC class II expression, predominantly of RT1.B. Strong and rapid enhancement of B-cell RT1.D expression was observed after stimulation by phorbol 12-myristate 13-acetate and ionomycin. Rat IL-13 did not modify B-cell MHC class II expression; however, it induced typical morphological changes in peritoneal macrophages. These experiments demonstrate isotype-specific and strain-dependent regulation of MHC class II expression on rat B lymphocytes, which may be of pathophysiological relevance for the strain-dependent susceptibility for Th1- or Th2-mediated autoimmunity.

Thiol levels in CD134-defined subsets of rat T lymphocytes: possible implications for HgCl2-induced immune dysregulation

CD134 (OX40), a member of the tumour necrosis factor receptor family, is expressed on activated T cells and mediates T and B cell costimulation. Its expression is increased after exposure to the thiol-binding compound HgCl2 in BN rats, but not in Lewis rats, in association with induction of a T cell-dependent systemic autoimmune syndrome only in BN rats. Intracellular thiols are involved in regulation of activation and death in T lymphocytes. Therefore, we examined intracellular thiol levels in CD134-defined T cell subsets from BN and Lewis rats. Levels of total thiols and glutathione (GSH) were significantly higher in CD134+CD4+ cells than in CD134+CD4+ cells in both strains. In Lewis rats, total thiol levels in CD4+CD134+ cells, but not in CD4+CD134+ cells, were higher than in BN rats. In contrast, BN rats showed higher GSH levels in CD4+CD134+ cells, but not in CD4+CD134+ cells. In vitro exposure to HgCl2 decreased intracellular thiol levels, predominantly in CD4+CD134+ cells. Furthermore, HgCl2-induced enrichment of CD134+ viable cells was inversely correlated to HgCl2-induced cell death. Strain-dependent differences in thiol levels in CD134-defined subsets of CD4+ lymphocytes and subset-specific modification of thiol levels may contribute to differential lymphocyte activation by oxidizing chemicals.

Signaling through CD44 is mediated by tyrosine kinases. Association with p56lck in T lymphocytes

Evidence from a large body of studies indicates that CD44 is involved in a number of important biological processes, including lymphocyte activation and homing, hematopoiesis, and tumor progression and metastasis. A proper understanding of the role of CD44 in these processes has been severely hampered by a lack of insight into the mode in which CD44 communicates with intracellular signal transduction pathways. In this report, we have addressed this aspect of CD44 functioning by studying CD44 signaling in T lymphocytes. We show that ligation of CD44 by monoclonal antibodies (mAbs) transduces signals to T cells which lead to tyrosine phosphorylation of ZAP-70 and other intracellular proteins. In vitro kinase assays demonstrate that cross-linking of CD44 induces an increase in the intrinsic activity of p56lck. Furthermore, immunoprecipitations show that CD44 is physically associated with p56lck. Our findings suggest that tyrosine kinases, particularly p56lck, play a central role in CD44 mediated signaling.