Molecular basis for thymic leukemia Ag binding to CD8+ T cells
Mouse thymic leukemia (TL) Ag is expressed on epithelial cells, immature thymocytes, and activated dendritic cells in addition to thymic leukemia cells. TL, encoded by T3 and T18 genes within the MHC region, interacts with CD8αα, but not with CD8αβ, on T lymphocytes to promote their differentiation into memory T cells. However, the molecular basis for the TL-CD8αα interaction is not known. Attinger et al. (p. 3501 ) created Kb and HLA-A2 recombinant chimeric proteins in which the α3 domains were substituted with α3 from T18d and a T18d chimeric protein in which its α3 domain was replaced with that from Kb. Only PE-labeled proteins with the T18d α3 domain stained CD8αα-transfected T cells. The authors engineered TL proteins to contain combinations of Kb amino acids at position 228 within one loop and at 197 and 198 within a second loop. Both loops in the MHC class I molecule previously were shown by x-ray crystallography to make contact with CD8αα homodimers. Changes at all three amino acids, but not those at one or two amino acids, decreased binding of T18d to CD8αα; a Kb molecule mutated to contain the three T18d amino acids had increased CD8αα binding. Stable APC transfectants expressing the T18d triple mutant were unable to activate a CD8αα-transfected cell line, whereas APC transfectants expressing Kb molecules mutated to contain the three T18d amino acids activated CD8αα-transfected cells in three different in vitro systems. The experiments demonstrate that three amino acids within two loops of the α3 domain of TL that contact CD8αα confer specificity for high avidity TL Ag binding to CD8+ T cells.
Anti-inflammatory activity of IL-1R antagonist
The secreted form of the IL-1R antagonist (IL-1Ra) is a competitive inhibitor of IL-1 in the microenvironment of cells. Three splice variants of secreted IL-1Ra mRNA are present in cells, and two intracellular IL-1Ra protein isoforms (icIL-1Ra) of unknown function have been identified. Banda et al. (p. 3608 ) found that the third component of the COP9 signalosome (CSN3) specifically interacted with recombinant icIL-1Ra1 in a yeast two-hybrid screen with HeLa cell lysates. The icIL-1Ra1 and CSN interaction was confirmed by cosedimentation in glycerol gradients, coprecipitation of CSN with GST fusion proteins of IL-1Ra isoforms, and coimmunoprecipitation of rIL-1Ra with CSN from cell lysates. CSN appears to function as an interface between signal transduction pathways and ubiquitin-dependent proteolysis in mammalian cells. In vitro phosphorylation of IκB, p53, and c-Jun, substrates of CSN-associated kinases, was inhibited by icIL-1Ra1. IL-1α-induced production of IL-6 and IL-8 was very low in two keratinocyte cell lines with high endogenous levels of icIL-1Ra1; transfection of one of the cell lines with small interfering RNAs for CSN3 or icIL-1Ra1 increased IL-1α-induced IL-6 and IL-8 production. A third cell line, with low endogenous levels of the inhibitor, produced high levels of the two cytokines after IL-1α treatment; transfection of those cells with a vector expressing icIL-1Ra1 reversed the response. Phosphorylation of p38 MAPK in cells stimulated with IL-1α was inversely related to the amount of icIL-1Ra1. The authors propose that the anti-inflammatory effect of icIL-1Ra1 in keratinocytes occurs through its binding to CSN3 and inhibition of kinases involved in IL-6 and IL-8 production.
Immunizing against systemic lupus erythematosus
Datta and colleagues have shown that systemic lupus erythematosus (SLE)-prone mice immunized with high concentrations of nucleosomal histone peptides have increased life spans. But the concentrations (300 μg i.v.) are too high for human use. In a continuation of their work, Kang et al. (p. 3247 ) injected prenephritic (SWR × NZB)F1 mice (SNF1) biweekly with individual major histone autoepitopes at a dose of 1 μg of peptide/dose s.c. Saline-injected control mice developed disease at 20 wk of age, and all died within 12 mo. Three peptides delayed incidence of disease and reduced mortality. Animals immunized with the most effective peptide, H471–94, developed severe nephritis at 47 wk of age (20%) and 60% survived to 21 mo of age. Mice immunized with any of the three peptides had substantial reductions of serum IgG2a, IgG2b, and IgG3 autoantibodies to dsDNA, ssDNA, nucleosomes, and histones at 3 mo compared with saline controls. T cells from SNF1 mice injected with any of the three peptides produced IFN-γ in response to coculture with APC in the presence of the peptides or nucleosomes; responses were cross-reactive among the peptides. CD4+CD25+ T cells or CD8+ T cells from peptide-tolerized mice suppressed in vitro B cell autoantibody production stimulated by untreated lupus CD4+ T cells. Adoptive transfer of either category of T cells from peptide-treated mice delayed the appearance of serum autoantibodies and nephritis in recipients in response to treatment with a fourth pathogenic histone peptide. Anti-TGF-β Ab or separation by a Transwell membrane abrogated suppression by CD8+ T cells in vitro; CD4+CD25+ T cells required cell contact to suppress. The authors show that immunization with a low dose of a nucleosomal histone peptide protects SNF1 mice against SLE by generating suppressive regulatory T cells.
Kaposi’s sarcoma pathogenesis
Human herpes virus 8 is considered to be the causative agent of Kaposi’s sarcoma (KS). Mice, transgenic for the viral-encoded G protein-coupled receptor (vGPCR), develop highly vascularized lesions and tumors characteristic of KS. However, a direct role for vGPCR in KS pathogenesis has not been demonstrated. Jensen et al. (p. 3686 ) generated double-transgenic mice expressing vGPCR conditionally. One transgene (responder) carried vGPCR under positive control of doxycycline (DOX), a tetracycline analog; the second transgene (activator) carried the tetracycline-controlled transactivator under control of the human CD2 promoter. All double-transgenic mice developed highly vascularized ear lesions within 20-40 days of DOX treatment, whereas wild-type littermate controls did not. At 60 days, lesions in DOX-treated animals contained spindle-shaped cells expressing endothelial cell surface markers. Gene expression analysis on RNAs extracted from ear skin up to 120 days of DOX treatment showed an up-regulation of mRNAs for vGPCR, several angiogenic factors (including CXC chemokines) and their receptors, and several endothelial cell markers. Cessation of DOX treatment at 60 days resulted in regression of ear lesions and reduction of expression of vGPCR and other up-regulated genes. Flow cytometric analysis of cells from ears of DOX-treated transgenic mice carrying a third transgene encoding a DOX-responsive reporter gene showed that only reporter-positive, vGPCR-expressing cells had endothelial cell markers. DOX treatment of double-transgenic mice on a RAG2−/− background resulted in development of KS-like lesions. The experiments provide direct evidence that vGPCR expression in mice induces a KS-like disease involving endothelial cells.
CD1 and autoimmune thyroiditis
Lipid-reactive T cells are associated with progression of human autoimmune diseases. However, a role for CD1 proteins, which present lipid Ags to T cells, has not been established in autoimmune disorders. Roura-Mir et al. (p. 3773 ) found extensive lymphocytic infiltrates in surgical biopsy specimens of patients with Graves’ disease (GD) and Hashimoto’s thyroiditis (HT) that were absent from normal thyroid tissue and multinodal goiters. Two subpopulations of APCs expressing CD1 proteins were identified by immunohistochemistry. CD1c+CD20+IgD+ mantle zone B cells localized to the T cell-B cell interface of lymphoid follicles within GD and HT thyroids. CD1a+CD1b+CD1c+CD83+ tissue dendritic cells were scattered outside the lymphoid follicles. Polyclonal lymphocytes isolated from thyroid biopsy specimens of patients with thyroiditis lysed CD1+ target cells from patients regardless of MHC haplotype. Polyclonal cell lines derived from HT and GD lymphocytes activated in vitro with thyroid-derived APCs strongly lysed CD1a-transfected B cells; weak lysis of CD1c-transfected B cells was increased by treating the transfected cells with a synthetic phospholipid. The data show that CD1-restricted T cells capable of MHC class I-independent lysis of CD1c+ mantle zone B cells and CD1+ tissue dendritic cells, both necessary for an autoimmune response, are present in thyroid glands of GD and HT patients.
Helicobacter pylori-neutrophil interactions
Despite being ingested by neutrophils (PMNs), the Gram-negative bacterium Helicobacter pylori is not killed but, rather, causes gastritis, which can lead to peptic ulceration or cancer. Allen et al. (p. 3658 ) found that H. pylori stimulated a greater respiratory burst by PMNs than two control bacteria or opsonized zymosan. However, as measured by several tests, reactive oxygen species (ROS) failed to accumulate in H. pylori-containing phagosomes in PMNs whereas they did in control cultures. H. pylori, but not the other bacteria, triggered superoxide release from the PMNs ∼10 min after ingestion. A lower amount of the integral membrane component of NADPH oxidase was found by confocal microscopy to associate with or be retained by H. pylori-containing phagosomes compared with controls. Other components of the NADPH oxidase complex were not recruited to the H. pylori phagosome but accumulated in NADPH oxidase complexes in patches at the surface of infected cells. An altered phosphorylation pattern of one subunit correlated with the change in location of the NADPH oxidase complex. H. pylori treated with formalin, but not with heat or ethanol, activated PMNs to release ROS. Serum opsonization of H. pylori reduced the oxidative burst and superoxide release, resulted in relocation of the NADPH oxidase complex to the bacteria-containing phagosomes, and decreased survival of the bacteria. The data show that H. pylori survive and cause host mucosal damage by disrupting intracellular NADPH oxidase complex assembly and by inducing PMN to release ROS.
Summaries written by Dorothy L. Buchhagen, Ph.D.