TLR-mediated B cell activation results in ectopic CLIP expression that promotes B cell-dependent inflammation.

Infectious pathogens produce compounds called Toll ligands that activate TLRs on lymphocytes. Acute activation triggered by certain TLRs appears to "jump start" the innate immune response, characterized by the release of inflammatory cytokines and cellular expansion. In some individuals, there is a failure to control acute inflammation, resulting in postinfectious, chronic inflammation. Susceptibility to chronic inflammation is strongly associated with an individual's MHC genes. Recent clinical trials for several autoimmune diseases characterized by chronic inflammation suggest that B lymphocyte depletion therapies dampen chronic immune activation. However, currently, there is no known mechanism that accounts for the correlation among TLR activation, MHC genetics, and a pathological role for B-lymphocytes. Our hypothesis is that TLR-activated B cells (B cells that have been polyclonally activated in the absence of antigen-specific signals) are not controlled properly by T cell-dependent B cell death, thereby causing B cell-dependent chronic inflammation. Here, we show that treatment with Toll ligands results in polyclonal B cell activation accompanied by ectopic expression of CLIP. Furthermore, by adoptively transferring purified CLIP+ B cells in syngeneic animals, we find that CLIP+ B cells induce production of TNF-α by host T cells. Finally, we demonstrate that CLIP-targeted peptide competition results in the death of polyclonally activated CLIP+ B cells.

Endogenous versus exogenous fatty acid availability affects lysosomal acidity and MHC class II expression.

Although the immune system, inflammation, and cellular metabolism are linked to diseases associated with dyslipidemias, the mechanism(s) remain unclear. To determine whether there is a mechanistic link between lipid availability and inflammation/immune activation, we evaluated macrophage cell lines incubated under conditions of altered exogenous and endogenous lipid availability. Limiting exogenous lipids results in decreased lysosomal acidity and decreased lysosomal enzymatic activity. Both lysosomal parameters are restored with the addition of oleoyl-CoA, suggesting that fatty acids play a role in the regulation of lysosomal function. Cell surface expression of major histocompatibility complex (MHC)-encoded molecules is also decreased in the absence of exogenous lipids. Additionally, we observe decreased gamma-interferon stimulation of cell surface MHC class II. Using cerulenin to limit the endogenous synthesis of fatty acids results in decreased cell surface expression of MHC class II but does not appear to alter lysosomal acidity, suggesting that lysosomal acidity is dependent on exogenous, but not endogenous, fatty acid availability. Testing these conclusions in an in vivo mouse model, we observed statistically significant, diet-dependent differences in lysosomal acidity and MHC class II cell surface expression. Collectively, these data demonstrate a mechanistic link between lipid availability and early events in the immune response.

Efficacy and safety of cesarean delivery for prevention of mother-to-child transmission of HIV-1.

BACKGROUND: Cesarean section before labor and before ruptured membranes ("elective cesarean section", or ECS) has been introduced as an intervention for the prevention of mother-to-child transmission (MTCT) of HIV-1. The role of mode of delivery in the management of HIV-1-infected women should be assessed in light of risks as well as benefits, since HIV-1-infected pregnant women must be provided with available information with which to make informed decisions regarding cesarean section and other options to prevent transmission of infection to their children. OBJECTIVES: Our objectives were to assess the efficacy (for prevention of MTCT of HIV-1) and the safety of ECS among HIV-1-infected women. SEARCH STRATEGY: Electronic searches were undertaken using MEDLINE and other databases. Hand searches of reference lists of pertinent reviews and studies, as well as abstracts from relevant conferences, were also conducted. Experts in the field were contacted to locate any other studies. The search strategy was iterative. SELECTION CRITERIA: Randomized clinical trials assessing the efficacy and safety of ECS for prevention of MTCT of HIV-1 were included in the analysis, as were observational studies with relevant data. DATA COLLECTION AND ANALYSIS: Data regarding HIV-1 infection status of infants born to HIV-1-infected women according to mode of delivery were extracted from the reports of the studies. Similarly, data regarding postpartum morbidity (PPM) (including minor (e.g., febrile morbidity, urinary tract infection) and major (e.g., endometritis, thromboembolism) morbidity) of the HIV-1-infected women, and infant morbidity, according to mode of delivery were extracted. MAIN RESULTS: One randomized clinical trial of the efficacy of ECS for prevention of MTCT of HIV-1 was identified. No data regarding infant morbidity according to the HIV-1-infected mother's mode of delivery were available. Data regarding PPM according to mode of delivery were available from this clinical trial as well as from five observational studies. Among HIV-1-infected women not taking antiretrovirals (ARVs) during pregnancy or taking only zidovudine, ECS was found to be efficacious for prevention of MTCT of HIV-1. PPM is generally higher among HIV-1-infected women who undergo cesarean as compared to vaginal delivery, with the risk with ECS being intermediate between that of vaginal delivery and NECS (including emergency procedures). Other factors associated with the risk of PPM among HIV-1-infected women include HIV-1 disease stage (more advanced disease, as manifested by lower CD4 counts and higher viral loads, being associated with a greater risk of PPM) and co-morbid conditions (e.g., diabetes). AUTHORS' CONCLUSIONS: ECS is an efficacious intervention for the prevention of MTCT among HIV-1-infected women not taking ARVs or taking only zidovudine. The risk of PPM with ECS is higher than that associated with vaginal delivery, yet lower than with NECS. Among HIV-1-infected women, more advanced maternal HIV-1 disease stage and concomitant medical conditions (e.g., diabetes) are independent risk factors for PPM. The risk of MTCT of HIV-1 according to mode of delivery among HIV-1-infected women with low viral loads (low either because the woman's HIV-1 disease is not advanced, or because her HIV-1 disease is well-controlled with ARVs) is unclear. Therefore, an important issue to be addressed in one or more large studies (individual studies or an individual patient data meta-analysis combining data from more than one study) is assessment of the effectiveness of ECS for prevention of MTCT of HIV-1 among HIV-1-infected women with undetectable viral loads (with or without receipt of highly active ARV therapy (HAART)).

T helper-cell phenotype regulates atherosclerosis in mice under conditions of mild hypercholesterolemia.

BACKGROUND: T cells are implicated in atherosclerosis, but little is known about the genetic control or molecular pathways, especially under conditions of mild hypercholesterolemia. METHODS AND RESULTS: BALB/c mice, making a CD4+ Th2 (IL-4+) cell response, express both MHC class II antigens (IA(d), IE(d)) and are atherosclerosis-resistant. C57Bl/6 mice produce a CD4+ Th1 (interferon [IFN]gamma+) response, express IA(b) but no IE, and are atherosclerosis-prone. To evaluate T helper-cell phenotype in fatty streak formation, wild-type C57Bl/6 mice (IA(b)+IE-) and transgenic mice, either AB(o), IA(b)-IE-; ABEalpha, IA-IE(k)+; or BL:TG:Ealpha, IA(b)+IE(k)+, were fed a high-cholesterol diet for 16 weeks and evaluated histomorphometrically for aortic lesions. Lesion size in AB(o), ABEalpha, and BL:TG:Ealpha strains was decreased by 54%, 79%, and 82%, respectively, compared with wild-type, correlating with decreased Th1 and increased Th2 expression and suggesting that T helper-cell phenotype is important in fatty lesion development. Decreasing Th1 cells by antibodies (alpha-CD4) or cytokines (IL-4) also caused >/=80% reductions in lesion size. Immunohistology revealed IFN-gamma, but not IL-4, colocalized with activated macrophages. Confirming these findings in a different mouse strain, BALB/c Stat 6 knockout mice (Th2 cell-deficient) developed aortic lesions comparable to C57Bl/6 mice on the same diet. CONCLUSIONS: In mildly hypercholesterolemic C57Bl/6 mice, presence of IA(b) and absence of IE regulated CD4+ T helper-cell phenotype; fatty lesions were proportional to IFNgamma+ Th1 cells in both C57Bl/6 and BALB/c strains. IFN-gamma may participate through macrophage activation, whereas IL-4 may act to limit Th1-cell response.

V gamma 1+ T cells suppress and V gamma 4+ T cells promote susceptibility to coxsackievirus B3-induced myocarditis in mice.

Coxsackievirus B3 infections of C57BL/6 mice, which express the MHC class II IA but not IE Ag, results in virus replication in the heart but minimal myocarditis. In contrast, Bl.Tg.Ealpha mice, which are C57BL/6 mice transgenically induced to express IE Ag, develop significant myocarditis upon Coxsackievirus B3 infection. Despite this difference in inflammatory damage, cardiac virus titers are similar between C57BL/6 and Bl.Tg.Ealpha mice. Removing gammadelta T cells from either strain by genetic manipulation (gammadelta knockout(ko)) changes the disease phenotype. C57BL/6 gammadelta ko mice show increased myocarditis. In contrast, Bl.Tg.Ealpha gammadelta ko mice show decreased cardiac inflammation. Flow cytometry revealed a difference in the gammadelta cell subsets in the two strains, with Vgamma1 dominating in C57BL/6 mice, and Vgamma4 predominating Bl.Tg.Ealpha mice. This suggests that these two Vgamma-defined subsets might have different functions. To test this possibility, we used mAb injection to deplete each subset. Mice depleted of Vgamma1 cells showed enhanced myocarditis, whereas those depleted of Vgamma4 cells suppressed myocarditis. Adoptively transfusing enriched Vgamma4(+) cells to the C57BL/6 and Bl.Tg. Ealpha gammadelta ko strains confirmed that the Vgamma4 subset promoted myocarditis. Th subset analysis suggests that Vgamma1(+) cells biased the CD4(+) T cells to a dominant Th2 cell response, whereas Vgamma4(+) cells biased CD4(+) T cells toward a dominant Th1 cell response.

Fas engagement accelerates liver regeneration after partial hepatectomy.

Fas (CD95) is a receptor involved in induction of apoptotic cell death of Fas-bearing cells, including hepatocytes and T cells. Injection of Fas-specific antibodies into mice leads to fulminant hepatic failure and death. Fas also transduces growth-promoting signals in proliferating T cells, fibroblasts and some tumor cells. Here we show that partial hepatectomy, which triggers the immediate onset of liver regeneration, protected mice against the lethal effects of Fas-specific antibodies and prevented hepatocyte apoptosis in response to Fas engagement in vivo. Furthermore, Fas engagement accelerated liver regeneration after partial hepatectomy. Liver regeneration kinetics were delayed in mutant mice with decreased cell surface Fas expression (lpr mice). In contrast, regeneration was not delayed in lpr-cg mutant mice, which have a Fas mutation that prevents Fas-induced death but not Fas-dependent proliferative stimulation. Our results indicate that Fas engagement on cells in regenerating or healing tissues may promote cell growth.

Rapid early onset lymphocyte cell death in mice resistant, but not susceptible to Leishmania major infection.

Leishmania major (Lm) infection in mice is a prototypical model for the role of immune deviation in disease resistance. Resistant strains of mice develop a Th1 response to Lm infection, distinguished by secretion of IL-12 and interferon gamma. In contrast, susceptible strains display sustained IL-4 expression characteristic of a Th2 response. However, when mechanisms of cell death are blocked, mice display a susceptible phenotype even in the presence of a strong Th1 response, suggesting that cell death, and not cytokine bias, may be an important factor in disease resistance. Here, we investigated this hypothesis by comparing lymphocyte cellularity, cell death and Fas expression in resistant CBA and susceptible BALB/c mice during the course of Lm infection. We found that delayed onset of cell death and late Fas induction correlated with massive lymphocyte accumulation and susceptibility to leishmaniasis, while early cell death and rapid Fas induction occurred in resistant mice.

Estradiol prevents and testosterone promotes Fas-dependent apoptosis in CD4+ Th2 cells by altering Bcl 2 expression.

Coxsackievirus B3 (CVB3) induces myocarditis in male BALB/c mice. Female mice are resistant to viral myocarditis, except in the third trimester of pregnancy and postpartum. Cardiac damage is mediated by T lymphocytes activated during virus infection. Th1 (interferon-gamma+) cell responses promote cardiac injury, while disease resistance correlates to preferential activation of Th2 (interleukin-4+) cell responses. CVB3-specific Th1 and Th2 cell clones were established, treated with between 0 and 100 ng/ml 17beta estradiol and 4-androsten-17beta-ol-one (testosterone) for two days, 51Cr-labeled and cultured on FasL-transfected 3T3 cells to determine susceptibility to Fas-dependent apoptosis. Testosterone treatment enhanced Th2 cell lysis while estradiol treatment was protective. Staining of Th2 cells for Bcl 2, an anti-apoptotic factor, indicates that Bcl 2 expression increased in these cells with estradiol but decreased with testosterone exposure. Hormone-induced changes in Bcl 2 expression likely explain the selective survival of Th2 cells in females and prevention of viral myocarditis.

Dichotomy between naïve and memory CD4(+) T cell responses to Fas engagement.

Engagement of Fas (APO-1, CD95), a member of the tumor necrosis factor receptor superfamily, can induce apoptotic cell death. However, Fas engagement also can costimulate lymphocyte proliferation. The physiologic regulation of these two outcomes is poorly understood. Here, we have used two systems, the first in vitro and the second in vivo, to demonstrate that naïve and memory CD4(+) T cells display dichotomous responses to Fas ligation. Naïve CD4(+) T cells (CD44(lo), CD45RB+, CD62L+) die as a consequence of Fas ligation in the presence of anti-CD3 antibody, whereas memory T cells (CD44(hi), CD45RB-, CD62L-), freshly isolated from the same starting population and subjected to the same stimulation conditions, are costimulated to proliferate by Fas ligation. In vitro, we demonstrate that CD28-mediated signals or T helper 1 and T helper 2 differentiation cytokines alter the response of naïve T cells, but not of memory T cells, to Fas ligation. In vivo experiments in hen egg lysozyme (HEL) T cell receptor transgenic mice show that CD4(+) T cells from HEL-naïve mice are killed by Fas ligation, but CD4(+) T cells from long-term HEL-exposed mice are costimulated by Fas ligation. Thus, the physiological outcome of Fas ligation in CD4(+) T cells is determined primarily by the antigenic history of the T cell.

Increased expression of CD40 on thymocytes and peripheral T cells in autoimmunity: a mechanism for acquiring changes in the peripheral T cell receptor repertoire.

CD40, a cell surface molecule found on B lymphocytes and other antigen presenting cells, can, when engaged by CD40 ligand (CD40L), induce gene rearrangements and isotype switching. We report here that CD40 is also expressed on thymocytes and on up to 50% of peripheral T cells from autoimmune prone strains of mice. In normal animals, CD40 is present on a small population of T cells and thymocytes. CD40 is expressed on most T cell hybridomas. We demonstrate that CD40 engagement on peripheral T cells, T cell hybridomas and thymocytes results in altered TCRValpha expression. That induced expression of different Valpha's results from the activity of the recombinase gene is implied by the observation that CD40 does not induce TCR changes in RAG knock-out mice. Total cell numbers remained unchanged between anti-CD40 treated and untreated populations of thymocytes or T cells indicating that treatment does not induce cell proliferation or cell death. The data presented here suggest a mechanism by which self reactive T cells accumulate peripherally and independently of selective processes of the thymus.

gamma delta+ T cells regulate major histocompatibility complex class II(IA and IE)-dependent susceptibility to coxsackievirus B3-induced autoimmune myocarditis.

Coxsackievirus B3 (CVB3) infection induces myocardial inflammation and myocyte necrosis in some, but not all, strains of mice. C57BL/6 mice, which inherently lack major histocompatibility complex (MHC) class II IE antigen, develop minimal cardiac lesions despite high levels of virus in the heart. The present experiments evaluate the relative roles of class II IA and IE expression on myocarditis susceptibility in four transgenic C57BL/6 mouse strains differing in MHC class II antigen expression. Animals lacking MHC class II IE antigen (C57BL/6 [IA+ IE-] and ABo [IA- IE-]) developed minimal cardiac lesions subsequent to infection despite high concentrations of virus in the heart. In contrast, strains expressing IE (ABo Ealpha [IA- IE+] and Bl.Tg.Ealpha [IA+ IE+]) had substantial cardiac injury. Myocarditis susceptibility correlated to a Th1 (gamma interferon-positive) cell response in the spleen, while disease resistance correlated to a preferential Th2 (interleukin-4-positive) phenotype. Vgamma/Vdelta analysis indicates that distinct subpopulations of gamma delta+ T cells are activated after CVB3 infection of C57BL/6 and Bl.Tg.Ealpha mice. Depletion of gamma delta+ T cells abrogated myocarditis susceptibility in IE+ animals and resulted in a Th1-->Th2 phenotype shift. These studies indicate that the MHC class II antigen haplotype controls myocarditis susceptibility, that this control is most likely mediated through the type of gamma delta T cells activated during CVB3 infection, and finally that different subpopulations of gamma delta+ T cells may either promote or inhibit Th1 cell responses.

Hormonal regulation of CD4(+) T-cell responses in coxsackievirus B3-induced myocarditis in mice.

Coxsackievirus B3 infection causes significant cardiac inflammation in male, but not female, B1.Tg.Ealpha mice. This gender difference in disease susceptibility correlates with selective induction of CD4(+) Th1 (gamma interferon-positive) cell responses in animals with testosterone, whereas estradiol promotes preferential CD4(+) Th2 (interleukin-4 positive [IL-4(+)]) cell responses. Differences in immune deviation of CD4(+) T cells cannot be explained by variation in B7-1 or B7-2 expression. Infection significantly upregulated both molecules, but no differences were detected between estradiol- and testosterone-treated groups. Significantly increased numbers of activated (CD69(+)) T cells expressing the gammadelta T-cell receptor were found in male and testosterone-treated male and female mice. In vivo depletion of gammadelta+ cells by using monoclonal antibodies inhibited myocarditis and resulted in a shift from a Th1 to Th2 response phenotype. Taken together, our results indicate that testosterone promotes a CD4(+) Th1 cell response and myocarditis by promoting increased gammadelta+ cell activation.

Fas-induced B cell apoptosis requires an increase in free cytosolic magnesium as an early event.

Ligation of the Fas molecule expressed on the surface of a cell initiates multiple signaling pathways that result in the apoptotic death of that cell. We have examined Mg2+ mobilization as well as Ca2+ mobilization in B cells undergoing Fas-initiated apoptosis. Our results indicate that cytosolic levels of free (non-complexed) Mg2+ ([Mg2+]i) and Ca2+ ([Ca2+]i) increase in cells undergoing apoptosis. Furthermore, the percentages of cells mobilizing Mg2+, fragmenting DNA, or externalizing phosphatidylserine (PS) increase in parallel as the concentration of anti-Fas monoclonal antibody is raised. Kinetic analysis suggests that Mg2+ mobilization is an early event in apoptosis, clearly preceding DNA fragmentation and probably occurring prior to externalization of PS as well. The source of Mg2+ that produces the increases in [Mg2+]i is intracellular and most likely is the mitochondria. Extended pretreatment of B cells with carbonyl cyanide m-chlorophenylhydrazone, an inhibitor of mitochondrial oxidative phosphorylation, produces proportional decreases in the percentage of cells mobilizing Mg2+, fragmenting DNA, and externalizing PS in response to anti-Fas monoclonal antibody treatment. These observations are consistent with the hypothesis that elevated [Mg2+]i is required for apoptosis. Furthermore, we propose that the increases in [Mg2+]i function not only as cofactors for Mg2+-dependent endonucleases, but also to facilitate the release of cytochrome c from the mitochondria, which drives many of the post-mitochondrial, caspase-mediated events in apoptotic cells.

Does the oxidative/glycolytic ratio determine proliferation or death in immune recognition?

Here we discuss the possibility that the way cells utilize fuel(s) for energy confers the properties that can be recognized by the immune system and, reciprocally, that recognition by the immune system can alter the balance of the cell's energy metabolism. We propose that immune recognition, of somatic cells via MHC can alter the their energy metabolism and induce a metabolic shift. We demonstrate the reciprocal relationship that inducing a shift in metabolism toward glycolysis by supplying glucose and insulin results in the upregulation of immunologically recognizable molecules such as cell surface Fas. Thus, immune recognition can induce metabolic deviation. Metabolic deviation can result in altered immune recognition and ultimately in cell proliferation, cell differentiation, or cell death.

Apoptosis in coxsackievirus B3-induced myocarditis and dilated cardiomyopathy.

Group B coxsackieviruses (CVB), which infect the myocardium, cause myocarditis and dilated cardiomyopathy. However, not all infections of the myocardium result in disease. In the mouse model, CVB infection stimulates autoimmune T cell response to cardiac antigens, and these autoimmune effectors cause myocyte necrosis and cardiomyopathy. Induction of pathogenic autoimmunity depends upon CD4+ Th1 (interferon-gamma positive) cells while Th2 (IL-4 positive) cell responses promote disease resistance. T lymphocytes expressing the gamma-delta T cell receptor (gamma delta +) constitute up to 12% of the inflammatory cells in the heart and are crucial to maintaining a dominant Th1 response phenotype. gamma delta + lymphocytes modulate T cell responses by selectively lysing CD4+ Th2 cells. Th1 cells are not killed by gamma delta + cells. Lysis requires direct cell:cell interaction between the gamma delta + cell and CD4+ Th2 target and is most likely mediated through Fas:FasL interaction. These studies demonstrate a novel mechanism for immune modulation of cytokine responses in vivo.

Fas ligand: receptor or ligand?

In this review, we chronicle the discovery, biochemical characterization, and assignment of Fas (CD95) as receptor and Fas Ligand (FasL, CD95L) as ligand. We review the functional descriptions of the molecules as death-inducing receptor and ligand or as mediators of cell division and/or growth arrest. We suggest that bidirectional mediation of signals in receptor-ligand pairs may be a common and important mechanism regulating cell fate. Bidirectional signaling clearly impacts the interpretation of experimental studies of intercellular communication.

Newly discovered role for Fas ligand in the cell-cycle arrest of CD4+ T cells.

Fas Ligand (FasL) can induce apoptosis of Fas-bearing cells. It is expressed on the cell surface of many tumor cells, immune-privileged tissues and activated lymphocytes. We report here that FasL can itself transduce signals, leading to cell-cycle arrest and cell death in CD4+ T cells. In vitro, FasL engagement inhibited CD4+ T-cell proliferation, cell-cycle progression, and IL-2 secretion. In vivo, FasL engagement prevented superantigen-mediated CD4+, but not CD8+, T-cell expansion. These findings demonstrate that FasL engagement regulates cell-cycle progression, and show that FasL engagement in vivo has a potent anti-inflammatory effect specific for CD4+ T cells.

Drug resistance results in alterations in expression of immune recognition molecules and failure to express Fas (CD95).

It is demonstrated that methotrexate/cisplatin-sensitive L1210 cells express low levels of major histocompatibility complex (MHC) class II relative to the high levels expressed on methotrexate (MTX)/cisplatin-resistant L1210/DDP cells. L1210 cells express cell-surface Fas, while the L1210/DDP cells express no cell-surface Fas. Expression of costimulatory molecules B7-1/B7-2 and Fas is increased on L1210 cells, but not L1210/DDP, in the presence of methotrexate or trimetrexate (TMTX). Therefore, a component of the mechanism of action of some anti-cancer agents may be to facilitate immune recognition and T cell-directed, Fas-induced cell death. Loss of cell-surface Fas expression and failure of Fas (CD95)-dependent apoptotic death has been observed when cells develop drug resistance. The defect in apoptosis can be overcome by anti-cancer agents or experimental manipulation that induce Fas expression on the drug-resistant cells.

Analysis of CD80 and CD86 expression on peripheral blood B lymphocytes reveals increased expression of CD86 in lupus patients.

We screened peripheral blood mononuclear cells from 13 SLE patients, all having quiescent disease at the time of analysis, 12 allergy patients, and 21 normal subjects for the expression of CD80 (B7-1) and CD86 (B7-2, B70) on small (resting) and large (activated) subsets of CD19+ B cells. The percentage of CD86+ cells was significantly higher in all B cell subsets in the SLE patients compared to either normal controls or allergy patients. No differences in the mean percentage CD86+ stained B cells (CD19+) were found when comparing the allergy patients and the normal controls. The percentage of CD80+ cells in the large activated B cell (CD19+) subset of the SLE patient population was significantly higher than in the comparable subset from the normal controls and the allergy patients. Comparison of the small resting B cell subset did not reveal a significant difference in CD80 expression between the normal controls, the allergy patients, and the SLE patients. Our findings suggest that the B7 family of molecules, and CD86 in particular, may reflect immunologic dysregulation in patients with autoimmune disease and may reflect a state facilitating heightened B cell activity and hypergammaglobulinemia that occur in active SLE.

Rescue of thymocytes from glucocorticoid-induced cell death mediated by CD28/CTLA-4 costimulatory interactions with B7-1/B7-2.

During the differentiation of thymocytes to mature T cells the processes of positive and negative selection result in signals that either protect thymocytes from cell death, or delete, through apoptosis, thymocytes with self-reactive T cell receptors (TCR). Glucocorticoids have been shown to induce thymocyte apoptosis and are produced within the thymic microenvironment. Furthermore, steroid-induced apoptosis of thymocytes has been suggested as a potential mechanism for removal of nonselected thymocytes. In this report, we demonstrate that thymocytes can be rescued from glucocorticoid-induced apoptosis by incubation with cells that express high levels of B7-1 or B7-2. In addition, the ability to be rescued by B7-1 and/or B7-2 can precede expression of the TCR. We demonstrate that CD3(+)-depleted or CD3+/ TCR-beta(+)-doubly depleted thymocytes can be rescued from glucocorticoid-induced apoptosis through the interaction of CD28 or CTLA-4 on thymocytes with cells bearing high levels of B7-1 or B7-2. Furthermore, these transfected cells are major histocompatibility complex (MHC) class II negative and, while they may express MHC class I, there is no preferential rescue of CD8+ thymocytes in the presence of glucocorticoids. Together, these data suggest that the rescue of thymocytes from glucocorticoids can be independent of the TCR. We also demonstrate that, in addition to CD28, CTLA-4 is expressed on thymocytes, suggesting that rescue from glucocorticoid-induced cell death can be mediated by both CD28 and CTLA-4. A CTLA-4Ig fusion protein which binds to both B7-1 and B7-2 was shown to completely block the rescue of thymocytes from glucocorticoid-induced cell death. Therefore, we conclude that interactions between B7-1/B7-2 and CD28/CTLA-4 are sufficient and necessary for rescue of thymocytes from glucocorticoid-induced cell death.