Antigen presentation by liver cells controls intrahepatic T cell trapping, whereas bone marrow-derived cells preferentially promote intrahepatic T cell apoptosis.

Systemic activation and proliferation of CD8(+) T cells result in T cell accumulation in the liver, associated with T cell apoptosis and liver injury. However, the role of Ag and APC in such accumulation is not clear. Bone marrow chimeras were constructed to allow Ag presentation in all tissues or alternatively to restrict presentation to either bone marrow-derived or non-bone marrow-derived cells. OVA-specific CD8(+) T cells were introduced by adoptive transfer and then activated using peptide, which resulted in clonal expansion followed by deletion. Ag presentation by liver non-bone marrow-derived cells was responsible for most of the accumulation of activated CD8(+) T cells. In contrast, Ag presentation by bone marrow-derived cells resulted in less accumulation of T cells in the liver, but a higher frequency of apoptotic cells within the intrahepatic T cell population. In unmodified TCR-transgenic mice, Ag-induced T cell deletion and intrahepatic accumulation of CD8(+) T cells result in hepatocyte damage, with the release of aminotransaminases. Our experiments show that such liver injury may occur in the absence of Ag presentation by the hepatocytes themselves, arguing for an indirect mechanism of liver damage.     

TNF-alpha controls intrahepatic T cell apoptosis and peripheral T cell numbers

At the end of an immune response, activated lymphocyte populations contract, leaving only a small memory population. The deletion of CD8(+) T cells from the periphery is associated with an accumulation of CD8(+) T cells in the liver, resulting in both CD8(+) T cell apoptosis and liver damage. After adoptive transfer and in vivo activation of TCR transgenic CD8(+) T cells, an increased number of activated CD8(+) T cells was observed in the lymph nodes, spleen, and liver of mice treated with anti-TNF-alpha. However, caspase activity was decreased only in CD8(+) T cells in the liver, not in those in the lymphoid organs. These results indicate that TNF-alpha is responsible for inducing apoptosis in the liver and suggest that CD8(+) T cells escaping this mechanism of deletion can recirculate into the periphery.     

Passive and active mechanisms trap activated CD8+ T cells in the liver

The liver is a site where activated CD8(+) T cells are trapped and destroyed at the end of an immune response. The intrahepatic accumulation of activated murine TCR transgenic CD8(+) T cells was significantly reduced when either ICAM-1 or VCAM-1 was blocked by specific Ab. These two adhesion mechanisms account for essentially all the trapping of activated CD8(+) T cells in the mouse liver. Although the ICAM-1-mediated trapping depends on the capacity of the vasculature and/or the parenchymal cells to present Ag, the accumulation of cells through VCAM-1 does not require Ag recognition. Thus, Ags expressed by the non-bone marrow-derived cells in the liver actively cause CD8(+) T cell accumulation through TCR-activated ICAM-1 adhesion, but the liver can also passively sequester activated CD8(+) T cells that do not recognize intrahepatic Ag, through VCAM-1 adhesion.     

Type 1 IFN maintains the survival of anergic CD4+ T cells

Anergic T cells have immunoregulatory activity and can survive for extended periods in vivo. It is unclear how anergic T cells escape from deletion, because both anergy and apoptosis can occur after TCR ligation. Stimulation of human CD4+ T cell clones reactive to influenza hemagglutinin peptides can occur in the absence of APCs when MHC class II-expressing, activated T cells present peptide to each other. This T:T peptide presentation can induce CD95-mediated apoptosis, while the cells that do not die are anergic. We found that the death after peptide or anti-CD3 treatment of a panel of CD4+ T cell clones is blocked by IFN-beta secreted by fibroblasts and also by IFN-alpha. This increases cell recovery after stimulation, which is not due to T cell proliferation. This mechanism for apoptosis inhibition rapidly stops protein kinase C-delta translocation from the cytoplasm to the nucleus, which is an early event in the death process. A central observation was that CD4+ T cells that are rescued from apoptosis after T:T presentation of peptide by IFN-alphabeta remain profoundly anergic to rechallenge with Ag-pulsed APCs. However, anergized cells retain the ability to respond to IL-2, showing that they are nonresponsive but functional. The prevention of peptide-induced apoptosis in activated T cells by IFN-alphabeta is a novel mechanism that may enable the survival and maintenance of anergic T cell populations after TCR engagement. This has important implications for the persistence of anergic T cells with the potential for immunoregulatory function in vivo.     

Stimulation by soluble CD70 promotes strong primary and secondary CD8+ cytotoxic T cell responses in vivo

Identification of the signals required for optimal differentiation of naive CD8(+) T cells into effector and memory cells is critical for the design of effective vaccines. In this study we demonstrate that CD27 stimulation by soluble CD70 considerably enhances the magnitude and quality of the CD8(+) T cell response. Stimulation with soluble CD70 in the presence of Ag significantly enhanced the proliferation of CD8(+) T cells and their ability to produce IL-2 and IFN-gamma in vitro. Administration of Ag and soluble CD70 resulted in a massive (>300-fold) expansion of Ag-specific CD8(+) T cells in vivo, which was due to the enhanced proliferation and survival of activated T cells. In mice that received Ag and soluble CD70, CD8(+) T cells developed into effectors with direct ex vivo cytotoxicity. Furthermore, unlike peptide immunization, which resulted in a diminished response after rechallenge, CD27 stimulation during the primary challenge evoked a strong secondary response upon rechallenge with the antigenic peptide. Thus, in addition to increasing the frequency of primed Ag-specific T cells, CD27 signaling during the primary response instills a program of differentiation that allows CD8(+) T cells to overcome a state of unresponsiveness. Taken together these results demonstrate that soluble CD70 has potent in vivo adjuvant effects for CD8(+) T cell responses.     

Gads regulates the expansion phase of CD8+ T cell-mediated immunity

The Gads adaptor protein is critical for TCR-mediated Ca(2+) mobilization. We investigated the effect of Gads deficiency on the proliferation of CD8(+) T cells following peptide stimulation and in the context of infection with an intracellular pathogen. We stimulated CD8(+) T cells from Gads(+/+) OT-I and Gads(-/-) OT-I mice with cognate Ag (SIINFEKL) or altered peptide ligand. In vitro experiments revealed that Gads was required for optimal proliferation of CD8(+) T cells. This defect was most evident at the early time points of proliferation and when low doses of Ag were used as stimuli. Cell cycle analysis demonstrated that Gads(-/-) CD8(+) T cells had impaired TCR-mediated exit from the G(0) phase of the cell cycle. Furthermore, Gads(-/-) CD8(+) T cells had delayed expression of c-myc and CD69 upon the stimulation with SIINFEKL. We then investigated how Gads deficiency would impact CD8(+) T cell-mediated immunity in the context of infection with an intracellular pathogen. At early time points, Gads(+/+) and Gads(-/-) CD8(+) T cells proliferated to a similar extent, despite the fact that expression of CD69 and CD25 was reduced in the absence of Gads. After 5 d postinfection, Gads was required to sustain the expansion phase of the immune response; the peak response of Gads(-/-) cells was significantly lower than for Gads(+/+) cells. However, Gads was not required for the differentiation of naive CD8(+) T cells into memory cells. We conclude that the primary function of Gads is to regulate the sensitivity of the TCR to Ag ligation.     

Selective retention of activated CD8+ T cells by the normal liver.

Activation-induced cell death resulting in peripheral deletion of CD8+ T cells is associated with the accumulation of large numbers of apoptotic T cells in the liver. The hypothesis that this accumulation results from the intrahepatic trapping of T cells from the circulating pool predicts that the liver should retain T cells, which subsequently undergo apoptosis. Here we test this prediction. Perfusion of the liver with lymphocyte mixtures showed retention of activated, but neither resting nor apoptosing, T cells. This trapping was selective for CD8+ cells and was mediated primarily by ICAM-1 constitutively expressed on sinusoidal endothelial cells and Kupffer cells. T cells trapped in the liver became apoptotic. The normal liver is therefore a "sink" for activated T cells.     

Expansion of the antigenic repertoire of a single T cell receptor upon T cell activation.

Activated T cells and their naive precursors display different functional avidities for peptide/MHC, but are thought to have identical antigenic repertoires. We show that, following activation with a cognate mimotope (NRP), diabetogenic CD8(+) T cells expressing a single TCR (8.3) respond vigorously to numerous peptide analogs of NRP that were unable to elicit any responses from naive 8.3-CD8(+) T cells, even at high concentrations. The NRP-reactive, in vivo activated CD8(+) cells arising in pancreatic islets of nonobese diabetic mice are similarly promiscuous for peptide/MHC, and paradoxically this promiscuity expands as the aviditiy of the T cell population for NRP/MHC increases with age. Thus, activation and avidity maturation of T lymphocyte populations can lead to dramatic expansions in the range of peptides that elicit functional T cell responses.     

Modulation of Fas-dependent apoptosis: a dynamic process controlling both the persistence and death of CD4 regulatory T cells and effector T cells

We have previously shown that regulatory CD25(+)CD4(+) T cells are resistant to clonal deletion induced by viral superantigen in vivo. In this work we report that isolated CD25(+)CD4(+) T cells activated in vitro by anti-CD3 Ab are resistant to Fas-induced apoptosis, in contrast to their CD25(-)CD4(+) counterparts. Resistance of CD25(+)CD4(+) T cells to Fas-dependent activation-induced cell death is not linked to their inability to produce IL-2 or to their ability to produce IL-10. The sensitivity of both populations to Fas-induced apoptosis can be modulated in vitro by changing the CD25(+)CD4(+):CD25(-)CD4(+) T cell ratio. The sensitivity of CD25(-)CD4(+) T cells to apoptosis can be reduced, while the sensitivity of CD25(+)CD4(+) T cells can be enhanced. Modulation of Fas-dependent apoptosis is associated with changes in cytokine production. However, while CD25(-)CD4(+) T cell apoptosis is highly dependent on IL-2 (production of which is inhibited by CD25(+)CD4(+) T cells in coculture), modulation of CD25(+)CD4(+) T cell apoptosis is IL-2 independent. Taken together, these results suggest that CD25(+)CD4(+) and CD25(-)CD4(+) T cell sensitivity to Fas-dependent apoptosis is dynamically modulated during immune responses; this modulation appears to help maintain a permanent population of regulatory T cells required to control effector T cells.     

CD8+ CTL priming by exact peptide epitopes in incomplete Freund's adjuvant induces a vanishing CTL response, whereas long peptides induce sustained CTL reactivity

Therapeutic vaccination trials, in which patients with cancer were vaccinated with minimal CTL peptide in oil-in-water formulations, have met with limited success. Many of these studies were based on the promising data of mice studies, showing that vaccination with a short synthetic peptide in IFA results in protective CD8(+) T cell immunity. By use of the highly immunogenic OVA CTL peptide in IFA as a model peptide-based vaccine, we investigated why minimal CTL peptide vaccines in IFA performed so inadequately to allow full optimization of peptide vaccination. Injection of the minimal MHC class I-binding OVA(257-264) peptide in IFA transiently activated CD8(+) effector T cells, which eventually failed to undergo secondary expansion or to kill target cells, as a result of a sustained and systemic presentation of the CTL peptides gradually leaking out of the IFA depot without systemic danger signals. Complementation of this vaccine with the MHC class II-binding Th peptide (OVA(323-339)) restored both secondary expansion and in vivo effector functions of CD8(+) T cells. Simply extending the CTL peptide to a length of 30 aa also preserved these CD8(+) T cell functions, independent of T cell help, because the longer CTL peptide was predominantly presented in the locally inflamed draining lymph node. Importantly, these functional differences were reproduced in two additional model Ag systems. Our data clearly show why priming of CTL with minimal peptide epitopes in IFA is suboptimal, and demonstrate that the use of longer versions of these CTL peptide epitopes ensures the induction of sustained effector CD8(+) T cell reactivity in vivo.     

Increased CD8+ cytotoxic T cell responses to myelin basic protein in multiple sclerosis

Autoreactive T cells of CD4 and CD8 subsets recognizing myelin basic protein (MBP), a candidate myelin autoantigen, are thought to contribute to and play distinct roles in the pathogenesis of multiple sclerosis (MS). In this study we identified four MBP-derived peptides that had high binding affinity to HLA-A2 and HLA-A24 and characterized the CD8(+) T cell responses and their functional properties in patients with MS. There were significantly increased CD8(+) T cell responses to 9-mer MBP peptides, in particular MBP(111-119) and MBP(87-95) peptides that had high binding affinity to HLA-A2, in patients with MS compared with healthy individuals. The resulting CD8(+) T cell lines were of the Th1 phenotype, producing TNF-alpha and IFN-gamma and belonged to a CD45RA(-)/CD45RO(+) memory T cell subset. Further characterization indicated that the CD8(+) T cell lines obtained were stained with MHC class I tetramer (HLA-A2/MBP(111-119)) and exhibited specific cytotoxicity toward autologous target cells pulsed with MBP-derived peptides in the context of MHC class I molecules. These cytotoxic CD8(+) T cell lines derived from MS patients recognized endogenously processed MBP and lysed COS cells transfected with genes encoding MBP and HLA-A2. These findings support the potential role of CD8(+) CTLs recognizing MBP in the injury of oligodendrocytes expressing both MHC class I molecules and MBP.     

Acquisition of MHC:Peptide Complexes by Dendritic Cells Contributes to the Generation of Antiviral CD8+ T Cell Immunity In Vivo

There is an increasing body of evidence suggesting that the transfer of preformed MHC class I:peptide complexes between a virus-infected cell and an uninfected APC, termed cross-dressing, represents an important mechanism of Ag presentation to CD8(+) T cells in host defense. However, although it has been shown that memory CD8(+) T cells can be activated by uninfected dendritic cells (DCs) cross-dressed by Ag from virus-infected parenchymal cells, it is unknown whether conditions exist during virus infection in which naive CD8(+) T cells are primed and differentiate to cytolytic effectors through cross-dressing, and indeed which DC subset would be responsible. In this study, we determine whether the transfer of MHC class I:peptide complexes between infected and uninfected murine DC plays a role in CD8(+) T cell priming to viral Ags in vivo. We show that MHC class I:peptide complexes from peptide-pulsed or virus-infected DCs are indeed acquired by splenic CD8α(-) DCs in vivo. Furthermore, the acquired MHC class I:peptide complexes are functional in that they induced Ag-specific CD8(+) T cell effectors with cytolytic function. As CD8α(-) DCs are poor cross-presenters, this may represent the main mechanism by which CD8α(-) DCs present exogenously encountered Ag to CD8(+) T cells. The sharing of Ag as preformed MHC class I:peptide complexes between infected and uninfected DCs without the restraints of Ag processing may have evolved to accurately amplify the response and also engage multiple DC subsets critical in the generation of strong antiviral immunity.     

Variable immunodominance hierarchies for H2-M3-restricted N-formyl peptides following bacterial infection

H2-M3-restricted presentation of N-formyl methionine (f-Met) peptides to CD8(+) T cells provides a mechanism for selective recognition of bacterial infection. In this report we demonstrate that Listeria monocytogenes infection induces distinct CD8(+) T cell populations specific for each of the known Listeria-derived formyl methionine peptides presented by M3. The sum H2-M3-restricted, Listeria-specific T cell response constitutes a major fraction of the total CD8(+) T cell response to primary infection. H2-M3-restricted T cell populations expand synchronously in vivo and achieve peak frequencies approximately 2 days earlier than MHC class Ia-restricted T cell populations. Although cross-recognition of different f-Met peptides by M3-restricted T cells was previously described, costaining of CD8(+) T cells ex vivo with H2-M3 tetramers complexed with different f-Met peptides shows that the majority of Listeria-specific, M3-restricted CD8(+) T cells are peptide specific. In contrast to the highly predictable size and immunodominance hierarchies of MHC class Ia-restricted T cell responses, the magnitudes of T cell responses specific for H2-M3-restricted peptides are remarkably variable between genetically identical mice. Our findings demonstrate that H2-M3-restricted T cell responses are distinct from classically restricted T cell responses to bacterial infection.     

Protracted protection to Plasmodium berghei malaria is linked to functionally and phenotypically heterogeneous liver memory CD8+ T cells

We previously demonstrated that protection induced by radiation-attenuated (gamma) Plasmodium berghei sporozoites is linked to MHC class I-restricted CD8(+) T cells specific for exoerythrocytic-stage Ags, and that activated intrahepatic memory CD8(+) T cells are associated with protracted protection. In this study, we further investigated intrahepatic memory CD8(+) T cells to elucidate mechanisms required for their maintenance. Using phenotypic markers indicative of activation (CD44, CD45RB), migration (CD62L), and IFN-gamma production, we identified two subsets of intrahepatic memory CD8(+) T cells: the CD44(high)CD45RB(low)CD62L(low)CD122(low) phenotype, representing the dominant effector memory set, and the CD44(high)CD45RB(high)CD62L(low/high)CD122(high) phenotype, representing the central memory set. Only the effector memory CD8(+) T cells responded swiftly to sporozoite challenge by producing sustained IFN-gamma; the central memory T cells responded with delay, and the IFN-gamma reactivity was short-lived. In addition, the subsets of liver memory CD8(+) T cells segregated according to the expression of CD122 (IL-15R) in that only the central memory CD8(+) T cells were CD122(high), whereas the effector memory CD8(+) T cells were CD122(low). Moreover, the effector memory CD8(+) T cells declined as protection waned in mice treated with primaquine, a drug that interferes with the formation of liver-stage Ags. We propose that protracted protection induced by P. berghei radiation-attenuated sporozoites depends in part on a network of interactive liver memory CD8(+) T cell subsets, each representing a different phase of activation or differentiation, and the balance of which is profoundly affected by the repository of liver-stage Ag and IL-15.     

The density of peptides displayed by dendritic cells affects immune responses to human tyrosinase and gp100 in HLA-A2 transgenic mice

Several HLA-A*0201-restricted peptide epitopes that can be used as targets for active immunotherapy have been identified within melanocyte differentiation proteins. However, uncertainty exists as to the most effective way to elicit CD8+ T cells with these epitopes in vivo. We report the use of transgenic mice expressing a derivative of HLA-A*0201, and dendritic cells, to enhance the activation of CD8+ T cells that recognize peptide epitopes derived from human tyrosinase and glycoprotein 100. We find that by altering the cell surface density of the immunizing peptide on the dendritic cells, either by pulsing with higher concentrations of peptide, or by changing the MHC-peptide-binding affinity by generating variants of the parent peptides, the size of the activated CD8+ T cell populations can be modulated in vivo. Significantly, the density of peptide that produced the largest response was less than the maximum density achievable through short-term peptide pulsing. We have also found, however, that while some variant peptides are effective at eliciting both primary and recall CD8+ T cell responses that can recognize the parental epitope, other variant epitopes lead to the outgrowth of CD8+ T cells that only recognize the variant. HLA-A*0201 transgenic mice provide an important model to define which peptide variants are most likely to stimulate CD8+ T cell populations that recognize the parental, melanoma-specific peptide.     

Cutting edge: culture with high doses of viral peptide induces previously unprimed CD8(+) T cells to produce cytokine

Culturing naive T cells with 50 microM selected HIV-1 envelope peptides for 6 days in the presence of IL-2 drives the emergence of a substantial CD8(+) population that secretes IFN-gamma following short-term stimulation with 1 microM peptide. This response is H-2K(b) restricted, epitope specific, and requires the continuing presence of peptide. The same effect was found for known H-2D(b)-restricted peptides from two influenza virus proteins. The great majority of these influenza-specific CD8(+)IFN-gamma(+) T cells neither stained with the cognate tetramer nor expressed the TCR Vbeta bias that is characteristic of the CD8(+) set expanded in vivo during an infection. Thus, multipoint binding of low affinity TCRs on naive CD8(+) T cells can drive peptide-specific cytokine production. However, at least for two influenza-derived epitopes, the avidity of the TCR-MHC peptide interaction appears to be insufficient to stabilize a tetrameric complex of MHC class I glycoprotein plus peptide on the lymphocyte surface.     

Identification of novel CD33 antigen-specific peptides for the generation of cytotoxic T lymphocytes against acute myeloid leukemia

Identification of immunogenic peptides for the generation of cytotoxic T lymphocytes (CTLs) may lead to the development of novel cellular therapies to treat disease relapse in acute myeloid leukemia (AML) patients. The objective of these studies was to evaluate the ability of unique HLA-A2.1-specific nonameric peptides derived from CD33 antigen to generate AML-specific CTLs ex vivo. We present data here on the identification of an immunogeneic HLA-A2.1-specific CD33(65-73) peptide (AIISGDSPV) that was capable of inducing CTLs targeted to AML cells. The CD33-CTLs displayed HLA-A2.1-restricted cytotoxicity against both mononuclear cells from AML patients and the AML cell line. The peptide- as well as AML cell-specificity of CD33-CTLs was demonstrated and the secretion of IFN-gamma by the CTLs was detected in response to CD33(65-73) peptide stimulation. The cultures contained a distinct CD33(65-73) peptide-tetramer(+)/CD8(+) population. Alteration of the native CD33(65-73) peptide at the first amino acid residue from alanine (A) to tyrosine (Y) enhanced the HLA-A2.1 affinity/stability of the modified CD33 peptide (YIISGDSPV) and induced CTLs with increased cytotoxicity against AML cells. These data therefore demonstrate the potential of using immunogenic HLA-A2.1-specific CD33 peptides in developing a cellular immunotherapy for the treatment of AML patients.     

Ex vivo characterization of multiepitopic tumor-specific CD8 T cells in patients with chronic myeloid leukemia: implications for vaccine development and adoptive cellular immunotherapy

Identification of tumor-associated Ags is a prerequisite for vaccine-based and adoptive immune therapies. Some tumor-associated Ags elicit specific CD8 T cells in patients with chronic myeloid leukemia (CML). Here, we characterized ex vivo responses of CD8 T cells from CML patients to extrajunction bcr-abl peptides and telomerase 540-548 hTert, PR1, and WT1 peptides. CML-specific CD8 T cells were present in most treated patients and were usually multiepitopic: WT1, hTert, PR1, and bcr74 tetramer(+) cells were detected in 85, 82, 67, and 61% of patients, respectively. The breadth and magnitude of these responses did not differ significantly according to treatment or disease status. CML-specific tetramer(+) CD8 T cells had a predominantly memory phenotype, an intermediate perforin content, and low intracellular IFN-gamma accumulation in the presence of the relevant peptide. However, in short-term culture with HLA-matched leukemia cells, the patients' memory T cells were specifically reactivated to become IFN-gamma-producing effector cells, suggesting that CD8 T cell precursors with lytic potential are present in vivo and can be activated by appropriate stimulation. In conclusion, this study shows that multiepitopic tumor-specific CD8 T cell responses occur naturally in most CML patients, opening the way to new strategies for enhancing anti-CML immunity, in particular in patients with minimal residual disease.     

Suppression of experimental autoimmune encephalomyelitis using peptide mimics of CD28

The B7:CD28/CTLA-4 costimulatory pathway plays a critical role in regulating the immune response and thus provides an ideal target for therapeutic manipulation of autoimmune disease. Previous studies have shown that blockade of CD28 signaling by mAbs can both prevent and exacerbate experimental autoimmune encephalomyelitis (EAE). In this study, we have designed two CD28 peptide mimics that selectively block B7:CD28 interactions. By surface plasmon resonance, both the end group-blocked CD28 peptide (EL-CD28) and its retro-inverso isomer (RI-CD28) compete effectively with the extracellular domain of CD28 for binding to B7-1. Both the CD28 peptide mimics inhibited expansion of encephalitogenic T cells in vitro. A single administration of EL-CD28 or RI-CD28 peptide significantly reduced disease severity in EAE. Importantly, we show that either CD28 peptide mimic administered during acute disease dramatically improved clinical signs of EAE, suppressing ongoing disease. The ratio of CD80:CD86 expression was significantly lower on CD4(+) and F4/80(+) spleen cells in CD28 peptide-treated mice. Peripheral deletion of Ag-specific CD4(+) T cells occurs following in vivo blockade of CD28 with synthetic CD28 peptides.     

Apoptosis by neglect of CD4+ Th cells in granulomas: a novel effector mechanism involved in the control of egg-induced immunopathology in murine schistosomiasis

In infection with Schistosoma mansoni, parasite eggs precipitate an intrahepatic granulomatous and fibrosing inflammation that is mediated by CD4(+) Th cells. Compared with CBA mice, C57BL/6 mice develop smaller granulomas composed of cells that exhibit reduced proliferative responses to schistosome egg Ags. In the present study, we investigated CD4(+) T cell apoptosis as a possible mechanism that could account for this subdued response. We found throughout the course of several infection weeks a markedly higher proportion of apoptotic CD4(+) T cells in granulomas from C57BL/6 mice than in those from CBA mice ex vivo; the apoptosis further increased upon cell cultivation in vitro. Activation-induced cell death or CD8(+) T cells failed to account for the enhanced apoptosis as infected Fas-, Fas ligand,- and CD8-deficient mice exhibited similar apoptosis to that seen in wild-type counterparts. However, a strikingly lower IL-2 production by schistosome egg Ag-stimulated C57BL/6 granuloma and mesenteric lymph node cells suggested the possibility of apoptosis due to growth factor deprivation. Indeed, the CD4(+) T cell apoptosis was significantly reversed by addition of rIL-2 in vitro, or by injection of rIL-2 in vivo, which also resulted in significant exacerbation of granulomatous inflammation. These findings indicate that apoptosis by neglect can represent a significant means of controlling CD4(+) T cells that mediate the immunopathology in schistosomiasis.