Unopposed production of granulocyte-macrophage colony-stimulating factor by tumors inhibits CD8+ T cell responses by dysregulating antigen-presenting cell maturation.
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Tumor cells gene-modified to produce GM-CSF potently stimulate antitumor immune responses, in part, by causing the growth and differentiation of dendritic cells (DC). However, GM-CSF-modified tumor cells must be gamma-irradiated or they will grow progressively, killing the host. We observed that 23 of 75 (31%) human tumor lines and two commonly used mouse tumor lines spontaneously produced GM-CSF. In mice, chronic GM-CSF production by tumors suppressed Ag-specific CD8+ T cell responses. Interestingly, an inhibitory population of adherent CD11b(Mac-1)/Gr-1 double-positive cells caused the observed impairment of CD8+ T cell function upon direct cell-to-cell contact. The inhibitory cells were positive for some markers associated with Ag presenting cells, like F4/80, but were negative for markers associated with fully mature DC like DEC205, B7. 2, and MHC class II. We have previously reported that a similar or identical population of inhibitory "immature" APC was elicited after immunization with powerful recombinant immunogens. We show here that these inhibitory cells can be elicited by the administration of recombinant GM-CSF alone, and, furthermore, that they can be differentiated ex vivo into "mature" APC by the addition of IL-4 and GM-CSF. Thus, tumors may be able to escape from immune detection by producing "unopposed" GM-CSF, thereby disrupting the balance of cytokines needed for the maturation of fully functional DC. Further, CD11b/Gr-1 double-positive cells may function as "inhibitory" APC under the influence of GM-CSF alone. (+info)
Limiting TCR expression leads to quantitative but not qualitative changes in thymic selection.
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Thymic selection is controlled in part by the avidity of the interaction between thymocytes and APCs. In agreement, the selective outcome can be modulated by altering the expression levels of selecting ligands on APCs. Here we test the converse proposition, i. e., whether changing TCR levels on thymocytes can alter the selective outcome. To this end, we have generated mice in which all thymocytes express two transgenic TCRs simultaneously (dual TCR-expressing (DTE) mice), the class I-restricted HY TCR and the class II-restricted AND TCR. Due to mutual dilution, surface expression levels of the two individual transgenic TCRs are diminished in DTE relative to single TCR-expressing mice. We find that thymic selection is highly sensitive to these reductions in TCR surface expression. Positive selection mediated by the AND and HY TCRs is severely impaired or abolished, respectively. Negative selection of the HY TCR in male DTE mice is also partly blocked, leading to the appearance of significant numbers of double positive thymocytes. Also, in the periphery of male, but not female, DTE mice, substantial numbers of single positive CD8bright cells accumulate, which are positively selected in the thymus but by a highly inefficient hemopoietic cell-dependent process. Overall our results favor the interpretation that the outcome of thymic selection is not determined solely by avidity and the resulting signal intensity, but is also constrained by other factors such as the nature of the ligand and/or its presentation by different subsets of APCs. (+info)
Functional and phenotypic analysis of thymic CD34+CD1a- progenitor-derived dendritic cells: predominance of CD1a+ differentiation pathway.
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Whether thymic dendritic cells (DC) are phenotypically and functionally distinct from the monocyte lineage DC is an important question. Human thymic progenitors differentiate into T, NK, and DC. The latter induce clonal deletion of autoreactive thymocytes and therefore might be different from their monocyte-derived counterparts. The cytokines needed for the differentiation of DC from thymic progenitors were also questioned, particularly the need for GM-CSF. We show that various cytokine combinations with or without GM-CSF generated DC from CD34+CD1a- but not from CD34+CD1a+ thymocytes. CD34+ thymic cells generated far fewer DC than their counterparts from the cord blood. The requirement for IL-7 was strict whereas GM-CSF was dispensable but nonetheless improved the yield of DC. CD14+ monocytic intermediates were not detected in these cultures unless macrophage-CSF (M-CSF) was added. Cultures in M-CSF generated CD14-CD1a+ DC precursors but also CD14+CD1a- cells. When sorted and recultured in GM-CSF, CD14+ cells down-regulated CD14 and up-regulated CD1a. TNF-alpha accelerated the differentiation of progenitors into DC and augmented MHC class II transport to the membrane, resulting in improved capacity to induce MLR. The trafficking of MHC class II molecules was studied by metabolic labeling and immunoprecipitation. MHC class II molecules were transported to the membrane in association with invariant chain isoforms in CD14+ (monocyte)-derived and in CD1a+ thymic-derived DC but not in monocytes. Thus, thymic progenitors can differentiate into DC along a preferential CD1a+ pathway but have conserved a CD14+ maturation capacity under M-CSF. Finally, CD1a+-derived thymic DC and monocyte-derived DC share very close Ag-processing machinery. (+info)
The TCR zeta-chain immunoreceptor tyrosine-based activation motifs are sufficient for the activation and differentiation of primary T lymphocytes.
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The TCR complex signals through a set of 10 intracytoplasmic motifs, termed immunoreceptor tyrosine-based activation motifs (ITAMs), contained within the gamma-, delta-, epsilon-, and zeta-chains. The need for this number of ITAMs is uncertain. Limited and contradictory studies have examined the ability of subsets of the TCR's ITAMs to signal into postthymic primary T lymphocytes. To study signaling by a restricted set of ITAMs, we expressed in transgenic mice a chimeric construct containing the IAs class II MHC extracellular and transmembrane domains linked to the cytoplasmic domain of the TCR zeta-chain. Tyrosine phosphorylation and receptor cocapping studies indicate that this chimeric receptor signals T cells independently of the remainder of the TCR. We show that CD4+ and CD8+ primary T cells, as well as naive and memory T cells, are fully responsive to stimulation through the IAs-zeta receptor. Further, IAs-zeta stimulation can induce primary T cell differentiation into CTL, Th1, and Th2 type cells. These results show that the zeta-chain ITAMs, in the absence of the gamma, delta, and epsilon ITAMs, are sufficient for the activation and functional maturation of primary T lymphocytes. It also supports the isolated use of the zeta-chain ITAMs in the development of surrogate TCRs for therapeutic purposes. (+info)
Requirement for efficient interactions between CD4 and MHC class II molecules for survival of resting CD4+ T lymphocytes in vivo and for activation-induced cell death.
(69/5762)
Regulation of homeostasis in the immune system includes mechanisms that promote survival of resting T lymphocytes, and others that control activation-induced cell death (AICD). In this study, we report on the use of a transgenic mouse model to test the role of CD4-MHC class II interactions for the susceptibility of CD4+ T lymphocytes to AICD, and for the survival of resting CD4+ T cells in peripheral lymphoid organs. The only I-Abeta gene expressed in these mice is an Abetak transgene with a mutation that prevents MHC class II molecules from interacting with CD4. We show increased apoptosis in CD4+ T lymphocytes derived from wild-type, but not from mutant Abetak transgenic mice following stimulation with staphylococcal enterotoxin A. Therefore, AICD may be impaired in CD4+ T cells derived from mutant Abetak transgenic mice. Importantly, we observed much higher apoptosis in resting CD4+ T cells from mutant Abetak transgenic mice than from wild-type mice. Furthermore, resting CD4+ T cells from mutant Abetak transgenic mice expressed higher levels of cell surface CD95 (Fas, APO-1). Ab-mediated cross-linking of CD95 further increased apoptosis in CD4+ T cells from mutant Abetak transgenic mice, but not from wild-type mice, suggesting apoptosis involved CD95 signaling. When cocultured with APC-expressing wild-type MHC class II molecules, apoptosis in resting CD4+ T lymphocytes from mutant Abetak transgenic mice was reduced. Our results show for the first time that interactions between CD4 and MHC class II molecules are required for the survival of resting CD4+ T cells in peripheral lymphoid organs. (+info)
Expression of major histocompatibility class II antigens on polymorphonuclear neutrophils in patients with Wegener's granulomatosis.
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BACKGROUND: Wegener's granulomatosis is a systemic inflammatory disease of unknown etiology. Many studies suggest that autoimmune reactions are involved, and there is good evidence for the participation of immunocompetent cells. In that context, we examined the activation of polymorphonuclear neutrophils (PMNs) of patients with Wegener's granulomatosis. METHODS: In a prospective study, the expression on the surface of PMNs of CD64 and of the major histocompatibility class II (MHC II) antigen was measured by cytofluorometry in whole blood. The expression of those antigens was correlated to disease activity. RESULTS: Up to 15% of the peripheral PMNs of patients with active disease expressed MHC II. Follow-up studies showed that expression correlated closely with disease activity and that it decreased rapidly under immunosuppressive therapy. Expression of CD64 was seen in approximately 50% of the patients, regardless of disease activity. CONCLUSION: MHC II expression on PMNs might serve as a novel diagnostic marker for active disease and appears to be suitable for monitoring immunotherapy. Moreover, our data provide evidence that PMNs, which are normally MHC II negative, acquire MHC II antigens in the course of disease and may be an unrecognized function within the afferent limb of the immune response. (+info)
Chronic rejection of mouse kidney allografts.
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BACKGROUND: Chronic renal allograft rejection is the leading cause of late graft failure. However, its pathogenesis has not been defined. METHODS: To explore the pathogenesis of chronic rejection, we studied a mouse model of kidney transplantation and examined the effects of altering the expression of donor major histocompatibility complex (MHC) antigens on the development of chronic rejection. RESULTS: We found that long-surviving mouse kidney allografts develop pathological abnormalities that resemble chronic rejection in humans. Furthermore, the absence of MHC class I or class II antigens did not prevent the loss of graft function nor alter the pathological characteristics of chronic rejection. Expression of transforming growth factor-beta (TGF-beta), a pleiotropic cytokine suggested to play a role in chronic rejection, was markedly enhanced in control allografts compared with isografts. However, TGF-beta up-regulation was significantly blunted in MHC-deficient grafts. Nonetheless, these differences in TGF-beta expression did not affect the character of chronic rejection, including intrarenal accumulation of collagens. CONCLUSIONS: Reduced expression of either class I or II direct allorecognition pathways is insufficient to prevent the development of chronic rejection, despite a reduction in the levels of TGF-beta expressed in the allograft. This suggests that the severity of chronic rejection is independent of the level of MHC disparity between donor and recipient and the level of TGF-beta expression within the allograft. (+info)
Dexamethasone inhibits IFNgamma-induced MHC class II expression of intestinal epithelial cells independently of the TGF-beta1 regulatory pathway.
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BACKGROUND: In the presence of inflammation, an increased expression of enterocyte MHC class II is observed, leading to altered mucosal antigen handling. Corticosteroids are potent anti-inflammatory drugs, widely used in treating inflammatory bowel disorders. However, their diverse mechanisms of action are only partially understood. AIM: To evaluate effect and mechanisms of corticosteroids on intestinal crypt epithelial cell MHC class II. METHODS: The effect of dexamethasone treatment on cytokine-induced MHC class II expression was measured in IEC-6 cells by immunofluorescence and flow cytometry. To determine the role of the TGF-beta1 regulatory pathway in mediating the effects of dexamethasone, neutralizing anti-TGF-beta antibodies were used. Additionally, endogenous and dexamethasone-stimulated IEC-6 cell TGF-beta1 production was measured by ELISA. RESULTS: Dexamethasone potently down-regulated IFNgamma-induced class II expression on IEC-6 cells, in a dose-dependent manner. TGF-beta1 had a similar inhibitory effect on class II expression. However, neutralizing anti-TGF-beta antibodies did not alter the effect of dexamethasone. Furthermore, dexamethasone reduced endogenous TGF-beta1 synthesis. CONCLUSIONS: Corticosteroids inhibit cytokine-induced MHC class II expression on IEC-6 cells in a TGF-beta1 independent way. This effect may markedly alter enterocytic antigen presentation, reducing the aberrant state of activation of mucosal immune cells. (+info)