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In 1995, tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) was identified based on its sequence homology to other TNF family members (1). Among members of the TNF family, TRAIL shares the highest sequence homology with Fas ligand (FasL, CD95L). However, unlike FasL, TRAIL appears to induce apoptosis of tumor cells but not most normal cells (2). To date, five receptors for TRAIL have been cloned: TRAIL-R1 (DR4, Apo2A) (3), TRAIL-R2 (DR5, TRICK, Killer) (3, 4, 5, 6, 7, 8), TRAIL-R3 (DcR1, TRID, LIT) (6, 7, 8, 9), TRAIL-R4 (DcR2, TRUNDD) (10), and osteoprotegerin (OPG) (11). Unlike other TRAIL receptors, which bind only to TRAIL, osteoprotegerin also binds to osteoprotegerin ligand (OPGL), TRANCE, and RANK ligand (RANKL). TRAIL-R1 and TRAIL-R2 contain intracellular death domains and induce, via coupling with intracellular adaptor proteins, the proteolytic cleavage of caspase-8 (12). Caspase-8 activation initiates the extrinsic and intrinsic apoptotic pathways, resulting in caspase-3 cleavage, which is an irreversible step in a cell’s commitment to apoptosis. Other TRAIL receptors do not generate death signals because TRAIL-R3 does not contain a death domain and is attached to the membrane by a glycolipid anchor (6,9), whereas the death domain of TRAIL-R4 is not functional (10) and OPG exists only in a soluble form (11). Therefore, TRAIL-R3 and TRAIL-R4 might act as decoy receptors by competing with other TRAIL receptors for TRAIL. The expression pattern of TRAIL receptors on certain cell lines might determine their sensitivity to TRAIL. However, the expression of TRAIL decoy receptors is not always related to a cell’s resistance to TRAIL-induced apoptosis (13). Other factors may, therefore, play more decisive roles in determining a cell’s sensitivity to TRAIL. In this review, we will focus on NF-κB and PPAR-γ, two transcription factors that were recently found to play important roles in TRAIL-induced apoptosis.

Intercellular adhesion molecule (ICAM)-1, or CD54, is a member of the immunoglobulin superfamily that binds to lymphocyte function-associated antigen-1 and macrophage-1 antigen. ICAM-1:LFA-1/Mac-1 interaction may be involved in both activation and extravasation of leukocytes. To determine the roles of ICAM-1 in the development of autoimmune disease, we studied experimental autoimmune encephalomyelitis (EAE) in mice deficient in ICAM-1. We found that T cell proliferation and TH1-type cytokine production in response to myelin antigen were significantly reduced in ICAM-1-deficient mice, whereas TH2-type cytokine IL-10 was increased. Unexpectedly, EAE induced by either myelin oligodendrocyte glycoprotein or myelin basic protein was significantly enhanced in mice deficient in ICAM-1. The enhancement was evidenced primarily by the increase in disease severity, mortality, and the degree of central nervous system inflammation. The cellular composition of the inflammatory infiltrates in the central nervous system is similar in control and ICAM-1-deficient mice. These results suggest that (1) ICAM-1 is involved in the activation of autoreactive TH-1, but not TH2 cells, and (2) ICAM-1 plays an important role in down-regulating autoimmune inflammation in the central nervous system.

Experimental autoimmune encephalomyelitis (EAE) is an inflammatory disease of the central nervous system (CNS) which is often used as an animal model for human multiple sclerosis (MS). The disease is mediated by autoreactive lymphocytes recognizing myelin self-antigens. The autoreactive lymphocytes elicit autoimmune inflammation in the CNS and lead to demyelination and loss of neurological functions. Although autoimmune encephalomyelitis can lead to irreversible nervous tissue injury and demise of animals, EAE is often characterized by spontaneous disease recovery or remission. It is not known how EAE progression is regulated, nor is it clear how autoimmune inflammation in the CNS can resolve while the myelin-specific lymphocytes and myelin self-antigens remain in the animals. Cytokines, especially TH2-type cytokines, have long been suggested to play a role in regulating EAE. However, experiments using recombinant cytokines or neutralizing antibodies to cytokines have generated conflicting results. To determine the roles of interleukin (IL)-4 and IL-10 in experimental autoimmune encephalomyelitis, we have studied mice deficient in IL-4 or IL-10. We found that IL-10- but not IL-4-deficient mice had accelerated EAE following immunization with myelin oligodendrocyte glycoprotein (MOG). Importantly, spontaneous recovery from EAE occurred in normal and IL-4-deficient mice, but not in mice deficient in IL-10. Furthermore, we established that the acceleration of EAE in IL-10-deficient mice was associated with a decrease in IL-4 and an increase in IFN-γ production in response to MOG antigen. These results strongly suggest that IL-10 plays a crucial role in the progression and recovery of autoimmune encephalomyelitis.

Oral administration of antigen induces a systemic hyporesponsiveness termed oral tolerance. High doses of oral antigen lead to deletion or anergy of T-cells whereas low doses induce regulatory T-cells that secrete Th2 cytokines (IL-4/IL-10) and TGF-beta. The initiating events associated with oral tolerance have not been well characterized. We investigated the induction phase of oral tolerance by adoptively transferring ovalbulumin (OVA) p (323-339) TcR specific transgenic (Tg+) T-cells into BALB/c recipients that were then fed either a high (5 mg x 5) or a low (0.1 mg x 5) dose of OVA 323-329 peptide. The frequency of Tg+ T-cells in lymphoid tissues was determined by flow cytometry using an anti-clonotypic monoclonal antibody. In high-dose-fed animals, Tg+ cells increased six- to eightfold in Peyer's patches after one feeding and then progressively decreased to 44% of those in the control by Day 20. In contrast, a biphasic-type response was observed in lymph node and spleen where Tg+ cells decreased after the first feeding, returned to the control level, and then decreased to 36-63% of the control level by Day 20. In low-dose-fed animals, changes in Tg+ T cells were only observed in Peyer's patches after five feedings, where cells increased approximately twofold. T-cell activation as measured by proliferation and IFN-gamma secretion occurred in both low- and high-dose-fed animals after only one feeding and then declined whereas secretion of Th2 cytokines and TGF-beta remained high even 10 days after the last feeding in low-dose-fed animals. Immunization with OVA/CFA demonstrated peripheral tolerance as measured by decreased proliferation and IFN-gamma secretion and was associated with increased production of TGF-beta and IL-10. These results suggest that the inductive phase of oral tolerance is characterized by an activation of antigen-specific T-cells that involves the initial secretion of IFN-gamma followed by prolonged secretion of Th2 cytokines and TGF-beta.

Nonhybridized CD8+ Ts cell clones were generated from individual spleen cells of a B6D2F1 mouse, which had been immunosuppressed in an antigen-specific manner by administration of tolerogenic conjugates of ovalbumin (OVA) and monomethoxypolyethylene glycol. The cloned Ts cells were shown to suppress both in vivo and in vitro anti-OVA antibody formation in an antigen-specific and isotype-nonspecific manner, i.e., IgM, IgG1, and IgG2a anti-OVA antibodies were suppressed. The cytokine profile of three Ts cell clones was determined by bioassays, Western blot, and polymerase chain reaction analyses. It was shown that all the Ts cell clones produced IL-2, IL-4, IFN-gamma, TGF-beta 1, LT, and TNF-alpha upon activation with hamster anti-CD3 monoclonal antibody (mAb) or antigen plus APC. However, neither the mAbs to IFN-gamma, TGF-beta, or LT/TNF-alpha, nor the recombinant IL-2 was able to abrogate the suppression of in vitro antibody production by cloned Ts cells. These data are taken to indicate that (i) the cloned Ts cells suppress anti-OVA antibody production both in vivo and in vitro in an isotype-nonrestricted manner, (ii) the cytokine profile of these cloned Ts cells is similar to that of Th0 cells, and (iii) the immunosuppression mediated by these T cells is not directly related to the cytokines produced by cloned Ts cells.

The induction of antigen-specific tolerance in mice by conjugates of ovalbumin (OVA) and monomethoxypolyethylene glycol (mPEG) previously had been shown to be associated with the generation of antigen-specific suppressor T (Ts) cells. For the elucidation of the nature of these Ts cells, five nonhybridized OVA-specific Ts cell clones were generated from the spleen cells of a BDF1 mouse which had been immunosuppressed by the tolerogenic conjugate, OVA(mPEG) 12 . The cloned Ts cells were maintained in vitro by periodic stimulation with OVA and feeder cells and were able to suppress the in vitro antibody production in an OVA-specific and MHC class I (H-2K d or H-2D d )-restricted manner. All these Ts cell clones were shown to be Thyl.2 + , CD4 − , CD5 − , CD8 + , and to express CD3 and the αβ heterodimer of the T cell receptor. The cell-free extracts of these cells contained soluble suppressor factors which could mimic in vitro the suppressive activity of the intact cells. In contrast to cytotoxic T lymphocytes (CTL), none of the cloned Ts cells were endowed with cytolytic activity as revealed in the perforin-mediated microhemolysis and in the 18-hr 51 Cr release assays. These results demonstrate that (i) OVA-specific Ts cell clones can be generated from mice pretreated with OVA(mPEG) 12 by employing conventional T cell culture techniques, and (ii) these Ts cells are functionally different from conventional CD8 + CTL.

The findings of previous studies in this laboratory demonstrating that conjugates of human monoclonal (myeloma) IgG (HIgG) and monomethoxypolyethylene glycol (mPEG) were able to induce in mice antigen-specific tolerance and CD8+ suppressor T (Ts) cells were confirmed in the present study. An extract (TsF) of a nonhybridized clone of Ts cells (viz., clone 23.32), which had been derived from spleen cells of mice tolerized with HIgG(mPEG)26, was shown to possess antigen-specific suppressive activity. This monoclonal TsF was able to specifically suppress in vitro antibody formation only if it was present from the beginning of the culture. From the results of the cellular dissection of the system used it was concluded that (i) the TsF had no effect on fully differentiated primed B cells or plasma cells, and (ii) the TsF inactivated carrier-primed Th cells when the culture contained concomitantly naive CD8+ T cells, accessory cells, and antigen. These data support the view that the monoclonal TsF exerted its downregulating effect on Th cells only if it could first interact with a CD8+ T cell, in the presence of accessory cells and antigen.