Fas-dependent elimination of nonselected CD8 cells and lpr disease

CD138 promotes the accumulation and activation of autoreactive T cells in autoimmune MRL/lpr mice

Autoreactive T cells, specifically CD138+ (syndecan-1) T cells produced in Fas-deficient systemic lupus erythematosus (SLE) mouse models, were shown to significantly promote the generation of autoantibodies. In the present study, Murphy Roths Large lymphoproliferative (MRL/lpr) lupus mice were used to investigate the role of CD138 protein expression in T cells in the progression of SLE. Measurement of flow cytometry, immunofluorescence and Luminex were performed to determine the effect of CD138 on T cells in MRL/lpr mice. The results demonstrate that CD138+ T cells induce apoptosis via a Fas-dependent pathway. CD138 protein expression in T cells of MRL/lpr mice significantly reduced T cell apoptosis and contributed to the accumulation of T cells and double negative (DN) T cells, whilst simultaneously promoting T cell activation in Fas-deficient lupus mice. CD138 protein expression in DN T cells also significantly increased the protein expression of Fas ligand to enhance the cytotoxicity of DN T cells. Furthermore, phorbol 12-myristate 13-acetate and ionomycin (PI) stimulation reduced CD138 protein expression in CD3+ T cells and prevented CD138+ T cell accumulation by inducing specific apoptosis. PI stimulation also activated T cells in MRL/lpr mice to increase CD69 protein expression. CD69 protein expression in CD138+ T cells significantly increased the frequency of apoptotic CD138+ T cells. In addition, results from the present study demonstrated that CD138- T cells of MRL/lpr lupus mice had an activation defect. CD138 protein expression in T cells significantly reversed the defective activation and activating T cells could significantly reduce CD138 protein expression in CD3+ T cells of MRL/lpr mice. This suggests that CD138 protein expression in CD3+CD138- T cells of MRL/lpr mice may be a consequence of the impaired activation in autoreactive T cells prior to exposure to self-antigens by the immune system. CD138 expression in autoreactive T cells has a central role in promoting the progression and development of autoimmune response in MRL/lpr mice.

Glucocorticoid prevents CD138 expression in T cells of autoimmune MRL/lpr mice

CD138⁺ T cells, the majority of which are CD4 and CD8 double-negative (DN) T cells, contribute to the production of anti-dsDNA antibodies in a CD4 receptor-dependent way to promote the development of systemic lupus erythematosus (SLE). Accumulation of CD138⁺ T cells in the spleen of MRL/lpr mice was significantly reduced by prednisone. Reduced expression of CD138 in DN T cells induced by prednisone treatment alleviated the accumulation of DN T cells in MRL/lpr mice. The frequency of CD138⁺ cells in CD4⁺ T cells of prednisone-treated MRL/lpr mice was also significantly reduced, which subsequently contributed to reduced production of anti-dsDNA antibody in the prednisone-treated MRL/lpr mice. Additionally, prednisone significantly reduced serum IgG and IgG subsets and simultaneously increased IgM secretion in serum. This suggested that glucocorticoids played a protective role during SLE treatment in MRL/lpr mice by promoting the production of IgM. The present study provides new insights into the mechanism of glucocorticoid for the treatment of SLE.

A distinct CD38+CD45RA+ population of CD4+, CD8+, and double-negative T cells is controlled by FAS

The identification and characterization of rare immune cell populations in humans can be facilitated by their growth advantage in the context of specific genetic diseases. Here, we use autoimmune lymphoproliferative syndrome to identify a population of FAS-controlled TCRαβ⁺ T cells. They include CD4⁺, CD8⁺, and double-negative T cells and can be defined by a CD38⁺CD45RA⁺T-BET⁻ expression pattern. These unconventional T cells are present in healthy individuals, are generated before birth, are enriched in lymphoid tissue, and do not expand during acute viral infection. They are characterized by a unique molecular signature that is unambiguously different from other known T cell differentiation subsets and independent of CD4 or CD8 expression. Functionally, FAS-controlled T cells represent highly proliferative, noncytotoxic T cells with an IL-10 cytokine bias. Mechanistically, regulation of this physiological population is mediated by FAS and CTLA4 signaling, and its survival is enhanced by mTOR and STAT3 signals. Genetic alterations in these pathways result in expansion of FAS-controlled T cells, which can cause significant lymphoproliferative disease.

Abnormally differentiated CD4+ or CD8+ T cells with phenotypic and genetic features of double negative T cells in human Fas deficiency

Lack of KLRG1 and T-bet expression is a unique feature of DNT and subsets of single positive T cells in ALPS patients. Genetic, phenotypic, and transcriptional evidence indicates that DNT in ALPS patients derive from both CD4+ and CD8+ T cells.

Cutting edge: Lymphoproliferation caused by Fas deficiency is dependent on the transcription factor eomesodermin

A hallmark of autoimmune lymphoproliferative syndrome (ALPS), caused by mutation of the Fas death receptor, is massive lymphadenopathy from aberrant expansion of CD4(-)CD8(-) (double-negative [DN]) T cells. Eomesodermin (Eomes) is a member of the T-box family of transcription factors and plays critical roles in effector cell function and memory cell fitness of CD8(+) T lymphocytes. We provide evidence in this study that DN T cells exhibit dysregulated expression of Eomes in humans and mice with ALPS. We also find that T cell-specific deletion of Eomes prevents lymphoid hypertrophy and accumulation of DN T cells in Fas-mutant mice. Although Eomes has critical physiological roles in the function and homeostasis of CD8(+) T cells, overexpression of Eomes appears to enable pathological induction or expansion of unusual CD8-related T cell subsets. Thus, antagonism of Eomes emerges as a therapeutic target for DN T cell ablation in ALPS.

Overview of the immune response

The immune system has evolved to protect the host from a universe of pathogenic microbes that are themselves constantly evolving. The immune system also helps the host eliminate toxic or allergenic substances that enter through mucosal surfaces. Central to the immune system's ability to mobilize a response to an invading pathogen, toxin, or allergen is its ability to distinguish self from nonself. The host uses both innate and adaptive mechanisms to detect and eliminate pathogenic microbes, and both of these mechanisms include self-nonself discrimination. This overview identifies key mechanisms used by the immune system to respond to invading microbes and other exogenous threats and identifies settings in which disturbed immune function exacerbates tissue injury.

Fas-mediated T cell deletion potentiates tumor antigen-specific tolerance in a mouse model of prostate cancer

A pivotal obstacle to cancer immunotherapy is peripheral T cell tolerance to tumor-associated antigens (TAAs). Tolerance induction among mature T cells in the periphery operates through a variety of mechanisms, including anergy and apoptosis. Although Fas-FasL-mediated apoptosis is a well-defined tolerance inducing mechanism, direct evidence of its interference with TAA-specific immunity in vivo is still lacking. In this report, we used the TRAMP mouse, which expresses SV40 large T antigen (Tag) preferentially in the prostate and develops prostate tumors, as a model system to address the role of Fas-mediated apoptosis in regulating peripheral T cell tolerance. Using RT-PCR and tetramer staining to quantify TAA-specific TCR-expressing cytolytic T lymphocytes (CTLs), we have shown the presence of TAA-specific CTLs at higher levels in TRAMP mice than in syngeneic C57Bl/6 mice. Tag-specific immunization led to the expansion of Tag-specific CTLs in C57Bl/6 mice, and to their elimination in TRAMP mice. Interestingly, in TRAMP mice with deficient Fas (Hybrid TRAMP-lpr/lpr), Tag-specific CTL elimination in response to Tag immunization did not take place. The results of cytolytic-function assays were consistent with induction and elimination patterns of TAA-specific CTLs and those of RT-PCR and tetramer staining. In conclusion, our data show that Fas-mediated TAA-specific CTL apoptosis contributes to T cell tolerance and suggest that such tolerance could be potentiated following TAA-specific immunization.

Peripheral survival of naïve CD8+ T cells

Maintenance of a sufficient population of naive CD8+ T cells in the peripheral lymphoid compartment is critical for immunocompetence. Peripheral T cell number is a function of T cell generation, survival, and death. Homeostasis, a critical balance between survival and death, must exist to prevent either lymphopenia or lymphocytosis. In the current review, we discuss known requirements for the survival of naive peripheral CD8+ T cells as well as mechanisms of death when survival signals are lost. We also discuss associations between survival and homeostasis-driven proliferation, and highlight the gaps in our knowledge of these critical processes.

The cytotoxic potential of regulatory T cells: what has been learned from gene knockout model systems?

The mechanisms of T-cell regulation are difficult to elucidate because of their complexity and the numerous subcategories of cell populations. There are two fundamental approaches to address this conundrum. First, it is possible to use a purified cell population and submit these cells to various assays. The second approach is to manipulate a target molecule and determine what effect this has on T-cell homeostasis in vitro and in vivo. This molecular strategy may help characterize multiple regulatory populations that use the same pathway for controlling T-cell function. Through a concerted two-pronged effort, the authors' laboratory and others have attempted to decipher different molecular pathways for regulatory cell function. Several gene knockout mouse models display a phenotype of profound lack of homeostasis in which T cells accumulate, presumably because of a defect in regulation. Dependent on the molecule disrupted, the immune cell subset being examined may no longer be appropriately regulated. Accordingly, the phenotype of exogenously added "putative" regulatory cells can then be examined by assessing their ability to control this aberrant accumulation. By using co-transplantation techniques, much information can be postulated regarding potential regulatory cell phenotype and function. Model systems with target gene manipulations involving Fas ligand, Fas, perforin, interleukin-2, and cytotoxic T-lymphocyte-associated antigen-4 exist and in all cases appear to disrupt critical cytotoxic regulatory cell function.

Biology of FasL

FasL (CD95L) is a well-known and well-characterized death-inducing ligand. Spontaneous mutations in FasL and its cognate receptor Fas (CD95) have helped understand the role of these molecules in the disease. Once thought to be mainly involved in the homeostasis of immune system, the territory of FasL regulation has been expanded to angiogenesis and tumor progression. Here, we review what is currently known about the role of FasL in many areas of biology.

1. Overview of the immune response

Host defense against pathogenic microbes requires dramatically different responses, depending on the character of the pathogen and on the tissue under attack. Central to the immune system's ability to mobilize a response to an invading pathogen is its ability to distinguish self from nonself. The host has evolved both innate and adaptive mechanisms to respond to and eliminate pathogenic microbes. Both of these mechanisms include self-nonself discrimination. This overview describes key mechanisms used by the immune system to respond to invading microbes and identifies settings in which disturbed immune function exacerbates tissue injury.

Syngeneic mixed lymphocyte reaction (SMLR) with dendritic cells: direct visualization of dividing T cell subsets in SMLR

Syngeneic mixed lymphocyte reaction (SMLR) has been considered to represent T cell response to self antigens. In this study using stimulator dendritic cells (DC), we analyzed cellular components responding to the syngeneic DC. It was shown that the predominant dividing cells were CD8(+) T cells although the response of CD4(+) T cells was essential for initiation of SMLR. In spite of the vigorous proliferation and expression of several activation markers, these SMLR-activated CD8(+) T cells hardly killed syngeneic targets and most of the CD8(+) T cells produced no interferon-gamma upon restimulation with DC. Furthermore, in SMLR where CD8(+) T cells were absent or inhibited, a considerable proliferation of CD4(-) CD8(-) double negative-T cells that included TCRalpha/beta(+) natural killer-T cells (NKT cells), TCRgamma/delta(+) NKT cells and TCRgamma/delta(+) T cells was observed.