The ability of immunodominant peptides derived from several antigen systems to compete with each other for T cell activation was studied. Only peptides restricted by a given transplantation antigen are mutually competitive. There is a correlation between haplotype restriction, ability to bind to the appropriate transplantation antigen, and ability to inhibit activation of other T cells restricted by the same transplantation antigen. An exception was noted in that a peptide derived from an antigen, bacteriophage lambda cI repressor, binds to the I-Ed molecule in a specific way, yet is not I-Ed-restricted. Comparison of the sequence of the repressor peptide with that of other peptides able to bind to (and be restricted by) I-Ed and a polymorphic region of the I-Ed molecule itself revealed a significant degree of homology. Thus, peptides restricted by a given class II molecule appear to be homologous to a portion of the class II molecule itself. The repressor-derived peptide is identical at several polymorphic residues at this site, and this may account for the failure of I-Ed to act as a restriction element. Comparison of antigenic peptide sequences with transplantation antigen sequences suggests a model that provides a basis for explaining self, nonself discrimination as well as alloreactivity.

The present study demonstrates the ability of plasma fibronectin or cold-insoluble globulin (CIg) to promote the uptake of ' 25 1-labeled, gelatin-coated latex beads (g-Ltx*) by monolayers of peritoneal macrophages (PM) . The uptake of g-Ltx* by PM was enhanced by CIg in a concentration-dependent fashion and required the presence of heparin (10 U/ml) as an obligatory cofactor for maximal particle uptake . Treatment of PM monolayers with trypsin (1 mg/ml) for 15 min at 37°C after particle uptake removed <15% of the radioactivity incorporated by the monolayers . However, a similar trypsin treatment of the monolayers before the addition of latex particles depressed Clg-dependent uptake by >75% . Pretreatment of PM monolayers with inhibitors of glycolysis effectively reduced the Clg-dependent uptake of latex.Similarly, pretreatment of monolayers with either inhibitors of protein synthesis or agents that disrupt cytoskeletal elements also significantly depressed CIg-dependent particle uptake . Phagocytosis of g-Ltx* by PM in the presence of CIg and heparin was confirmed by electron microscopy . Finally, g-Ltx* could also be effectively opsonized with CIg at 37°C before their addition to the monolayers . These studies suggest that the recognition of g-Ltx* in the presence of CIg required cell surface protein(s) and that subsequent phagocytosis of these particles by PM was energy dependent and required intact intracellular cytoskeleton elements . Thus, PM monolayers provide a suitable system for further studies on the function of Clg in the recognition and phagocytosis of gelatin-coated particles by phagocytic cells.The process of phagocytosis by vertebrate phagocytes is markedly enhanced by humoral recognition factors called opsonins . Immune opsonins are antibody or complement proteins that interact with foreign antigens and with receptors on the surface of phagocytic cells (26, 36) . There also exist opsonins that enhance the uptake of colloidal material by macrophages of the reticuloendothelial system that are not immune proteins (34). One such nonimmune opsonin has been purified from rat (1,2,6,22) and human serum (4) and was originally designated a-2-macroglobulin on account of its electrophoretic mobility and size (2,22) . More recent studies have demonstrated that this nonimmune opsonin isolated from human serum is identical to cold-insoluble globulin (CIg) or fibronectin (5), a high molecular weight adhesive glycoprotein, a form of which is also found on the surface of many cell types (10,38) .Up until now, quantitation of the opsonic activity of CIg has depended largely on a liver slice assay utilizing radiolabeled,

Hypoxia-inducible factor 1α (HIF-1α) and HIF-2α are master transcription factors that regulate cellular responses to hypoxia, but the exact function in regulatory T (Treg) cells is controversial. Here, we show that Treg cell development is normal in mice with Foxp3-specific knockout (KO) of HIF-1α or HIF-2α. However, HIF-2α-KO (but not HIF-1α-KO) Treg cells are functionally defective in suppressing effector T cell-induced colitis and inhibiting airway hypersensitivity. HIF-2α-KO Treg cells have enhanced reprogramming into IL-17-secreting cells. We show crosstalk between HIF-2α and HIF-1α, and that HIF-2α represses HIF-1α expression. HIF-1α is upregulated in HIF-2α-KO Treg cells and further deletion of HIF-1α restores the inhibitory function of HIF-2α-KO Treg cells. Mice with Foxp3-conditional KO of HIF-2α are resistant to growth of MC38 colon adenocarcinoma and metastases of B16F10 melanoma. Together, these results indicate that targeting HIF-2α to destabilize Treg cells might be an approach for regulating the functional activity of Treg cells.

No abstract

Decoy receptor 3 (DcR3), a soluble receptor for FasL, LIGHT, and TL1A, induces osteoclast formation from monocyte, macrophage, and bone stromal marrow cells. However, the function of DcR3 on bone formation remains largely unknown. To understand the function of DcR3 in bone formation in vivo, transgenic mice overexpressing DcR3 were generated. Bone mineral density (BMD) and bone mineral content (BMC) of total body were significantly lower in DcR3 transgenic mice as compared with wild-type controls. The difference in BMD and BMC between DcR3 transgenic and control mice was confirmed by histomorphometric analysis, which showed a 35.7% decrease in trabecular bone volume in DcR3 transgenic mice in comparison with wild-type controls. The number of osteoclasts increased in DcR3 transgenic mice. In addition, local administration of DcR3 (30 g/ml, 10 l, once/day) into the metaphysis of the tibia via the implantation of a needle cannula significantly decreased the BMD, BMC, and bone volume of secondary spongiosa in tibia. Local injection of DcR3 also increased osteoclast numbers around trabecular bone in tibia. Furthermore, coadminstration of soluble tumor necrosis factor receptor inhibitor/Fc chimera (TNFRSF1A) but not osteoprotegerin inhibited the action of DcR3. In addition, in an assay of osteoclast activity on substrate plates, DcR3 significantly increased the resorption activity of mature osteoclasts. Treatment with higher concentrations of DcR3 slightly increased nodule formation and alkaline phosphatase activity of primary cultured osteoblasts. These results indicate that DcR3 may play an important role in osteoporosis or other bone diseases.Bone is a complex tissue composed of several cell types that are continuously undergoing a process of renewal and repair termed "bone remodeling." Two major cell types responsible for bone remodeling are osteoclasts, which resorb bone, and osteoblasts, which form new bone (1).Osteoclasts are cells derived from the monocyte-macrophage lineage that have an important role in modeling bone during skeletal growth and in remodeling bone during adult life (2). Increased osteoclast activity or uncoupling of osteoclastic bone resorption from bone formation results in focal or generalized bone loss and is a characteristic feature of bone diseases such as osteoporosis, Paget disease of bone, and cancer-associated bone disease (3). The importance of osteoclastic bone resorption in the pathogenesis of these diseases is reflected by the fact that the most successful drug treatment for bone disease works by inhibiting bone resorption (4). Recent findings indicated that two key molecules, macrophage colony-stimulating factor (M-CSF) 3 and the receptor for activation of NF-B ligand (RANKL), are essential and sufficient to promote osteoclastogenesis (5). On the other hand, several systemic hormones and cytokines, including glucocorticoid (6, 11), interleukin-1 (7), interleukin-6 (8), interleukin-11 (9), transforming growth factor-␤ (10), and tumor necrosis factor-␣ (TNF-␣) (12, 13), have been s...

Ceramide has been suggested as the secondary messenger mediating the apoptotic signal for Fas engagement. By using different inhibitors, we demonstrated here that ceramide is unlikely a mediator of Fas-initiated apoptosis. First, cAMP prevented cell death induced by ceramide but not by Fas. Second, ceramide-triggered, but not Fas-triggered, apoptosis was antagonized by the free radical scavenger C60. Third, the metal chelator pyrrolidinedithiocarbamate suppressed ceramide-initiated DNA fragmentation but had no effect on the Fas-induced cell death. Fourth, the SAPK/ERK kinase dominant negative mutant, which attenuated ceramide-induced cell death, did not prevent Fas-induced apoptosis. Finally, activation of NF-κB inhibited ceramide-induced but not Fas-initiated apoptosis. The fact that many antagonists of ceramide-induced apoptosis could not suppress Fas-mediated cell death clearly indicates that ceramide is not the mediator for Fas-initiated apoptotic signal.

No abstract

Decoy receptor 3 (DcR3)/TR6/M68 is a soluble receptor that binds to the Fas ligand LIGHT and TL1A. Elevated levels of DcR3 expression have been found in many tumors. We report an unexpected effect of DcR3 by sensitizing Jurkat and U937 cells to apoptosis induced by tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). Cell death triggered by anti-Fas and tumor necrosis factor was unaffected by DcR3. DcR3 by itself did not stimulate apoptosis. The ability to augment TRAIL-initiated cell death was not observed with soluble lymphotoxin ␤ receptor or soluble death receptor 3, indicating that binding to LIGHT or TL1A alone is insufficient to trigger TRAIL sensitivity. Incubation with DcR3 did not increase the surface expression of TRAIL receptor, and the level of Fas-associated death domain protein and cellular FLICE-like inhibitory protein was not altered. Instead, in the presence of DcR3, TRAIL engagement resulted in an increased activation of caspase-8, an elevated cleavage of Bid, and enhanced release of Smac and cytochrome c from mitochondria to cytosol compared with TRAIL alone. This led to increased activation of caspase-9 and caspase-3. The unusual ability of DcR3 to promote TRAIL-triggered death may be used to potentiate TRAIL efficacy during treatment tumors overexpressing DcR3. Decoy receptor 3 (DcR3)1 /TR6/M68 is a member of the tumor necrosis factor receptor superfamily (1). DcR3 binds to at least three different ligands: Fas ligand (FasL), LIGHT, and TL1A (1-3). The biological functions initiated by these three ligands are blocked by the presence of DcR3. DcR3 is overexpressed in primary lung and colon tumors (1), in gliomas (4), gastrointestinal tract tumors (5, 6), autoimmune tissues (7), and virusassociated leukemia (8). It has been suggested that DcR3 acts as a decoy receptor for tumor cells to neutralize the cytotoxic and regulatory effects of FasL, LIGHT, and TL1A, in that inoculation of rats with glioma cells overexpressing DcR3 resulted in decreased infiltration of T cells and macrophages (4).DcR3 has also been shown to promote angiogenesis through neutralization of TL1A (9).Recent studies have revealed the profound modulatory effects of DcR3 on many cells. DcR3 regulates CD14 ϩ monocyte differentiation into dendritic cells, with down-regulation of HLA-DR, CD56/ICAM-1, and CD80, and up-regulation of CD86 in mature dendritic cells, leading to skewed Th2 differentiation in primed T cells (10). DcR3 attenuates T cell activation and reduces T cell response to mouse heart allograft (11). DcR3 also decreases T cell interaction with antigen-presenting cells (12) and inhibits T cell chemotaxis (13). In addition, DcR3 triggers actin re-organization, increases the adhesion of monocytes (14), and reduces phagocytic activity and proinflammatory cytokine production in macrophages (15).TRAIL induces apoptosis by binding to TRAIL receptor (16, 17). There are five different TRAIL receptors, including TRAIL-R1/DR4, TRAIL-R2/DR5, TRAIL-R3/DcR1, TRAIL-R4/ DcR2, and osteoprotegerin. Binding of DR4 and DR...