Cross-talk between T cells and thymic epithelium
⇓ T cell maturation within the thymus involves complex interactions between thymic epithelial cells and developing thymocytes. However, the signals involved in maturation and in emigration of mature naive T cells from the thymus are not fully known. Hüe et al. (p. 1909 ) focused on the interaction of the NKG2D activating receptor, expressed on αβ CD8+ T cells, γδ T cells and NK cells, with one of its ligands, the nonclassical MHC class I molecule MICA. PE-labeled MICA tetramers stained CD3brightCD4−CD8+ thymocytes from normal thymuses of 2-day- to 4-year-old children. This population of NKG2D+ cells had surface markers consistent with a fully mature naive CD8+ T cell phenotype, thus representing a late stage of thymopoiesis. Immunohistochemical analyses using FITC-labeled MICA tetramers and an anti-CD8 mAb showed that the NKG2D-expressing cells colocalized with CD8+ T cells in the thymic medulla but not in the cortex. An anti-MICA mAb stained epithelial cells in normal human gut and in the outer walls of Hassal’s corpuscles and in mesenchymal cells in human thymic medulla. MICA proteins were over-expressed in both the medulla and cortex of human thymomas, whereas MICA tetramers detected fewer NKG2D+ cells in the thymoma compared with adjacent normal thymus. The results suggest that NKG2D/MICA interactions are involved in CD8+ T cell maturation and that the NKG2D signal might play a role in migration of mature CD8+ T cells from the thymus.
Overcoming neonatal tolerance
Mice tolerized as neonates to a proteolipid protein-1 (PLP1) peptide expressed on an Ig molecule (Ig-PLP1) develop only a mild, transient form of experimental allergic encephalomyelitis (EAE) when challenged at 7 wk with the same PLP1 peptide in CFA. Bell et al. (p. 1801 ) set out to eliminate the residual disease that appeared after Ag challenge. Instead, they restored EAE in the tolerized mice. Treatment of tolerized mice with anti-B7.1 or anti-B7.2 mAb at the time of PLP1 challenge resulted in a fatal paralysis in 100% and 29%, respectively, of the animals. Tolerized mice treated with no Ab or with a rat IgG isotype developed the transient disease but fully recovered. Proliferative activity and IFN-γ production were restored in anergic splenic Th1 cells from tolerized mice when either anti-B7.1 or anti-B7.2 mAb was administered at the time of immunization with PLP1. Treatment with control rat IgG had no effect. However, if mice were given a mixture of the two anti-B7 mAbs at the time of Ag challenge, the pattern of transient EAE reappeared. The addition of anti-IL-12 mAb with one of the anti-B7 mAbs abolished the reactivation of the anergic T cells. This phenomenon occurred by neutralization of the IL-12 produced primarily by the CD8α− subset of splenic dendritic cells in response to binding of B7 by anti-B7 mAb. The data are consistent with a dual function of B7 molecules, both as costimulators of splenic Th1 cells and as inducers of IL-12 by dendritic cells.
Thrombospondin-1 and rheumatoid arthritis
⇓ The CD47 ligand thrombospondin-1 (TSP1) is expressed on injured and/or repairing tissue, as well as on a subpopulation of fibroblast-like synoviocytes (FLS) that induce T cell proliferation. However, the contributions of CD47 costimulation and TSP1 expression in chronic tissue inflammation are not clear. Vallejo et al. (p. 1732 ) used immunohistochemistry to detect abundant expression of TSP1 in the synovial tissues of patients with rheumatoid arthritis (RA). (TSP1 appears to act as a bridging molecule between CD47 and the receptor CD36, expressed on different cell types.) TSP-1 colocalized with its receptor CD36 on the endothelial lining of synovial capillaries and in FLS scattered throughout synovial tissue parenchyma. FLS lines established from RA patients expressed HLA-DR, TSP1, and the TSP1 receptor CD36 and were adhesive for autologous T cells. A synthetic peptide corresponding to the CD47-binding domain of TSP1 inhibited T cell adhesion to FLS, whereas a mutated peptide did not. TSP1+ FLS were stimulatory for autologous T cells for 8 wk in the absence of exogenous factors. Expression profiles of T cells costimulated with anti-CD3 and anti-CD47 mAbs were distinct from those seen after costimulation with anti-CD3 and anti-CD28 mAbs. L-ferritin, a component of the acute-phase response, was highly expressed in T cells costimulated with anti-CD47 and augmented proliferation of T cells cultured with suboptimal amounts of anti-CD3 mAb. The data are consistent with active participation of FLS in inflammatory synovitis in the RA joint through TSP1-CD47 interaction and L-ferritin production.
The ICAM-1 cytoplasmic tail: tyrosine phosphorylation not required
The adhesion molecule ICAM-1 facilitates the migration of leukocytes across the walls of endothelial cells (EC) and transduces signals in response to leukocyte adhesion. To date, the manner in which EC control these functions is unknown. Greenwood et al. (p. 2099 ) expressed wild-type human ICAM-1 (WT-hICAM-1) and three mutated molecules in rat brain EC. Transendothelial migration of T lymphocytes doubled in brain EC transfected with a vector expressing WT-hICAM-1 over that seen in EC cells transfected with an empty vector control. Expression of GPI-hICAM-1 (a C-terminal truncation at the membrane-spanning domain fused to the GPI anchor of LFA3) failed to increase lymphocyte migration. Expression of hICAM-1ΔC (lacking an intracellular C-terminal domain) led to a modest increase, whereas expression of hICAM-1Y512F (a C-terminal domain with a phenylalanine amino acid substitution at the conserved tyrosine residue codon 512) had a more pronounced increase. Lymphocyte adhesion was increased greatly to EC expressing WT-hICAM-1 and somewhat to EC expressing hICAM-1Y512F compared with the vector control. In contrast, EC expressing hICAM-1ΔC or GPI-hICAM-1 had reduced cell adhesion compared with the vector control. Treatment of EC with phosphorylated and nonphosphorylated peptides comprising the membrane proximal part of the intracellular domain of the ICAM-1 sequence reduced the increased migration of T lymphocytes in cells expressing WT-hICAM-1 but did not alter T cell adhesion. Thus, the C-terminal intracellular domain is critically important in EC transmigration and in the adhesion of lymphocytes. Phosphorylation of tyrosine 512 is not required.
The IL-4Ra chain cytoplasmic tail: tyrosine phosphorylation not required (redux)
⇓ Interleukin-4 is critical for the differentiation of activated CD4+ T cells into effector Th2 cells. Previous studies indicated that the transcription factor STAT6 bound to phosphorylated tyrosines in the cytoplasmic tail of the IL-4 receptor α-chain (IL-4Rα). That model is challenged by Mora et al. (p. 1891 ). The authors transfected freshly activated mouse IL-4Rα-deficient T lymphocytes with vectors expressing wild type and mutated IL-4R proteins. Cells carrying a mutant receptor lacking most of the cytoplasmic tail were unable to produce IL-4 upon restimulation, whereas cells carrying the wild type or other receptor mutants could. Surprisingly, cells expressing mouse IL-4Rα in which critical tyrosine residues were mutated or deleted were able to produce levels of IL-4 after stimulation comparable to levels seen with the wild type receptor. STAT6 activation and binding activity were detected in cells expressing either the wild-type receptor or the tyrosine mutant but only if the transduced cells were stimulated with IL-4. Similar results were seen following transfections of the tissue culture cell line Jurkat but only after pretreatment of the transduced cells either by anti-CD3, by a calcium ionophore or by PMA. Chemical inhibition of the extracellular-regulated kinase (ERK) pathway prevented PMA induction of STAT6, but blockage of p38 mitogen-activated protein kinase did not. Deletion of a block of conserved serines/acidic residues in the cytoplasmic tail crippled the ability of IL-4Rα to induce STAT6 phosphorylation. The data indicate that use of the ERK pathway required activated T lymphocytes and the conserved region of amino acids in the cytoplasmic tail of IL-4Rα, and that the ERK pathway was necessary and sufficient for receptor potentiation.
Active T cell anergy
⇓ Peripheral T cell tolerance is critical in preventing autoimmunity, graft rejection and graft-vs-host disease. Although many studies have been conducted on T cell anergy, it is unclear why anergic cells fail to respond to antigenic stimulation. Tzachanis et al. (p. 1691 ) used suppression subtractive hybridization to detect increased expression of the GTPase ADP-ribosylation factor-6 (ARF6) in human T cells rendered anergic by culture with anti-CD3 mAb. In productively stimulated T cells, anti-ARF6 mAb stained scattered internal structures, the known location of ARF6 in the GDP-bound form (ARF6-GDP). However, in anergic T cells, anti-ARF6 mAb stained plasma membranes, the location of ARF6 in the GTP-bound form (ARF6-GTP). Jurkat T cells and human T cells from peripheral blood were transfected with an empty vector, a mutated form of ARF6 that remained in the GTP-bound state, or a mutated form of ARF6 that remained in the GDP-bound state. Anti-CD3 mAb stimulation of the Jurkat transfected cells showed that only the ARF6-GTP transfected cells failed to redistribute F-actin at the contact site, did not activate extracellular-regulated kinases-1 and -2, and lacked transcription of IL-2. Anti-CD3 mAb stimulated primary T cells transfected with ARF6-GTP did not activate extracellular-regulated kinases-1 and -2 and were inhibited in proliferative responses. ARF6 expression was detectable in the plasma membranes of T cells isolated from peripheral blood of healthy individuals. The results show that anergy correlates with localization of AFR6-GTP in plasma membranes of T cells.
Summaries written by Dorothy L. Buchhagen, Ph.D.
- Copyright © 2003 by The American Association of Immunologists