Characterization of the Fas ligand/Fas-dependent apoptosis of antiretroviral, class I MHC tetramer-defined, CD8+ CTL by in vivo retrovirus-infected cells

B cell clonality in cancer

Carcinogenesis in the process of long-term co-evolution of tumor cells and immune environment essentially becomes possible due to incorrect decisions made, remembered, and reproduced by the immune system at the level of clonal populations of antigen-specific T- and B-lymphocytes. Tumor-immunity interaction determines the nature of such errors and, consequently, delineates the possible ways of successful immunotherapeutic intervention. It is generally recognized that tumor-infiltrating B cells (TIL-B) can play both pro-tumor and anti-tumor roles. However, the exact mechanisms that determine the contribution of clonal B cell lineages with different specificities and functions remain largely unclear. This is due to the variability of cancer types, the molecular heterogeneity of tumor cells, and, to a large extent, the individual pattern of each immune response. Further progress requires detailed investigation of the functional properties and phenotypes of clonally heterogeneous B cells in relation to their antigenic specificities, which determine the functionality of both effector B lymphocytes and immunoglobulins produced in the tumor environment. Based on a real understanding of the role of clonal antigen-specific populations of B lymphocytes in the tumor microenvironment, we need to learn how to develop new methods of targeted immunotherapy, as well as adapt existing treatment options to the specific needs of different patients and patient subgroups. In this review, we will cover B cells functional diversity and their multifaceted roles in the tumor environment.

Emetine Is Not Ipecac: Considerations for Its Use as Treatment for SARS-CoV2

Emetine is a potent antiviral that acts on many viruses in the low-nM range, with several studies in animals and humans demonstrating antiviral activity. Historically, emetine was used to treat patients with Spanish influenza, in the last stages of the pandemic in the early 1900s. Some of these patients were "black" with cyanosis. Emetine rapidly reversed the cyanosis and other symptoms of this disease in 12-24 h. However, emetine also has been shown to have anti-inflammatory properties and it appears it is these anti-inflammatory properties that were responsible for the effects seen in patients with Spanish influenza. Emetine, in the past, has also been used in 10s to 100s of millions of people at a dose of ~60 mg daily to treat amoebiasis. Based on viral inhibition data we can calculate a likely SARS-CoV2 antiviral dose of ~1/10th the amoebiasis dose, which should dramatically reduce the risk of any side effects. While there are no anti-inflammatory dose response data available, based on the potential mode of action, the anti-inflammatory actions may also occur at low doses. This paper also examines the toxicity of emetine seen in clinical practice and that seen in the laboratory, and discusses the methods of administration aimed at reducing side effects if higher doses were found to be necessary. While emetine is a "pure drug" as it is extracted from ipecac, some of the differences between emetine and ipecac are also discussed.

B-cells with a FasL expressing regulatory phenotype are induced following successful anti-tuberculosis treatment

Introduction

Studies show that B‐cells, in addition to producing antibodies and antigen‐presentation, are able to produce cytokines as well. These include regulatory cytokines such as IL‐10 by regulatory B‐cells. Furthermore, a rare regulatory subset of B‐cells have the potential to express FasL, which is a death‐inducing ligand. This subset of B‐cells have a positive role during autoimmune disease, but has not yet been studied during tuberculosis. These FasL‐expressing B‐cells are induced by bacterial LPS and CpG, thus we hypothesized that this phenotype might be induced during tuberculosis as well.

Methods

B‐cells from participants with TB (at diagnosis and during treatment) and controls were collected, and analyzed by means of real‐time PCR and flow cytometry. In addition to this, BAL was collected from TB participants as well and analyzed by means of MAGPix (multi‐cytokine) technology.

Results

Gene expression analysis show that FASL transcript levels increase by the end of treatment. Similarly, phenotypic analysis show that there is a higher frequency of FasL‐expressing B‐cells by the end of treatment.

Conclusion

Collectively, these results indicate that these FasL‐expressing B‐cells are being induced during anti‐TB treatment, and thus may play a positive role. Further studies are required to elucidate this.

Killer B lymphocytes and their fas ligand positive exosomes as inducers of immune tolerance

Induction of immune tolerance is a key process by which the immune system is educated to modulate reactions against benign stimuli such as self-antigens and commensal microbes. Understanding and harnessing the natural mechanisms of immune tolerance may become an increasingly useful strategy for treating many types of allergic and autoimmune diseases, as well as for improving the acceptance of solid organ transplants. Our laboratory and others have been interested in the natural ability of some B lymphocytes to express the death-inducing molecule Fas ligand (FasL), and their ability to kill T helper (T_H) lymphocytes. We have recently shown that experimental transformation of human B cells by a non-replicative variant of Epstein-Barr virus (EBV) consistently resulted in high expression of functional FasL protein. The production and release of FasL⁺ exosomes that co-expressed major histocompatibility complex (MHC) class II molecules and had the capacity to kill antigen-specific T_H cells was also observed. Several lines of evidence indicate that FasL+ B cells and FasL⁺MHCII⁺ exosomes have important roles in natural immune tolerance and have a great deal of therapeutic potential. Taken together, these findings suggest that EBV-immortalized human B lymphoblastoid cell lines could be used as cellular factories for FasL⁺ exosomes, which would be employed to therapeutically establish and/or regain immune tolerance toward specific antigens. The goals of this review are to summarize current knowledge of the roles of FasL⁺ B cells and exosomes in immune regulation, and to suggest methods of manipulating killer B cells and FasL⁺ exosomes for clinical purposes.

CTLs respond with activation and granule secretion when serving as targets for T-cell recognition

Cytotoxic T lymphocytes (CTLs) suppress T cell responses directed against their antigens regardless of their own T cell receptor (TCR) specificity. This makes the use of CTLs promising for tolerance induction in autoimmunity and transplantation. It has been established that binding of the CTL CD8 molecule to the major histocompatibility complex (MHC) class I α3 domain of the recognizing T cell must be permitted for death of the latter cell to ensue. However, the signaling events triggered in the CTL by this molecular interaction in the absence of TCR recognition have never been clarified. Here we use single-cell imaging to study the events occurring in CTLs serving as targets for recognition by specific T cells. We demonstrate that CTLs actively respond to recognition by polarizing their cytotoxic granules to the contact area, releasing their lethal cargo, and vigorously proliferating. Using CTLs from perforin knockout (KO) mice and lymphocyte specific kinase (Lck) knockdown with specific small interfering RNA (siRNA), we show that the killing of the recognizing CD8 T cell is perforin dependent and is initiated by Lck signaling in the CTL. Collectively, these data suggest a novel mechanism in which the entire cascade generally triggered by TCR engagement is "hijacked" in CTLs serving as targets for T cell recognition without TCR ligation.

Deletion of alloreactive T cells by veto cytotoxic T lymphocytes is mediated through extracellular signal-regulated kinase phosphorylation

BACKGROUND

Anti third-party cytotoxic T lymphocytes (CTLs) were shown to exhibit marked veto activity, thereby inducing transplantation tolerance across major histocompatibility antigens. Elimination of effector cells requires co-expression of CD8 and FasL on the veto cells and is mediated through CD8-major histocompatibility complex (MHC) class I interaction and Fas-Fas ligand signaling.

METHODS

To further interrogate the signaling events induced in the effector cells on their interaction with veto cell populations, effector cells from 2C transgenic mice were preincubated with different signaling inhibitors and were subject to fluorescence-activated cell sorting and western blot analysis.

RESULTS

Screening with inhibitors revealed specific inhibition only with the map kinase (MEK)/extracellular signal regulated kinase (ERK) inhibitor, U0126. Accordingly, fluorescence-activated cell sorting and western blot analysis showed that ERK phosphorylation is induced in the effector cells within 1 hr of incubation with the veto cells. ERK phosphorylation had no effect on the Fas expression level, nor was it reduced when using effector cells from Fas KO mice. Examination of ERK phosphorylation in high and low MHC-I expressing effectors revealed marked differences, suggesting that the interaction between CD8 on the veto CTL, and MHC-I on the effector cells is likely responsible for ERK phosphorylation. Furthermore, XIAP in 2C cells is specifically reduced on binding to the cognate veto cells during the mixed lymphocyte reaction but before the appearance of Annexin V reactivity.

CONCLUSIONS

These results suggest that the interaction between CD8 on veto CTL and the MHC class I alpha3 domain on the effector cell, leads to phosphorylation of MEK/ERK in the latter cell, associated with a significant reduction of XIAP levels which, in turn, enables potent triggering of Fas-FasL mediated apoptosis on cognate binding of the veto CTLs.

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.

Fas ligand enhances hematopoietic cell engraftment through abrogation of alloimmune responses and nonimmunogenic interactions

Early after transplantation, donor lineage-negative bone marrow cells (lin(-) BMC) constitutively upregulated their expression of Fas ligand (FasL), suggesting an involvement of the Fas/FasL axis in engraftment. Following the observation of impaired engraftment in the presence of a dysfunctional Fas/FasL axis in FasL-defective (gld) donors or Fas-defective (lpr) recipients, we expressed a noncleavable FasL chimeric protein on the surface of donor lin(-) BMC. Despite a short life span of the protein in vivo, expression of FasL on the surface of all the donor lin(-) BMC improved the efficiency of engraftment twofold. The FasL-coated donor cells efficiently blunted the host alloimmune responses in primary recipients and retained their hematopoietic reconstituting potential in secondary transplants. Surprisingly, FasL protein improved the efficiency of engraftment in syngeneic transplants. The deficient engraftment in lpr recipients was not reversed in chimeric mice with Fas(-) stroma and Fas(+) BMC, demonstrating that the host marrow stroma was also a target of donor cell FasL. Hematopoietic stem and progenitor cells are insensitive to Fas-mediated apoptosis and thus can exploit the constitutive expression of FasL to exert potent veto activities in the early stages of engraftment. Manipulation of the donor cells using ectopic FasL protein accentuated the immunogenic and nonimmunogenic interactions between the donor cells and the host, alleviating the requirement for a megadose of transplanted cells to achieve a potent veto effect. Disclosure of potential conflicts of interest is found at the end of this article.

Apoptosis of epitope-specific antiretroviral cytotoxic T lymphocytes via Fas ligand-Fas interactions

C57BL/6 (B6; H-2b) mice are capable of mounting a vigorous AKR/Gross Murine Leukemia Virus (MuLV)-specific cytotoxic T lymphocyte (CTL) response to AKR/Gross MuLVs whereas AKR.H- 2b congenic mice, although carrying the responder H-2b major histocompatibility haplotype, are specifically nonresponsive. Furthermore, when viable AKR.H-2b spleen cells are cocultured with primed responder B6 antiviral precursor CTLs, the AKR.H-2b cells function as "veto" cells that actively mediate the inhibition by apoptosis of B6 antiviral CTL generation in a contact-dependent, MHC-restricted, and veto cell Fas ligand (FasL)/responder T cell Fas-dependent manner. In the present study we show that antigen-specific, antiviral CTLs that survive apoptotic inhibition by AKR.H-2b veto cells display a less activated cell surface phenotype, and are less able to bind specific MHC-peptide tetramers, including on a per-T cell receptor (TcR) basis. In addition, surviving antiviral CTLs also appeared to be functionally deficient, based on both their reduced ability to lyse specific target cells and to produce interferon (IFN)-gamma. Carboxyfluorescein diacetate succinimidyl ester staining confirmed that AKR/Gross MuLV-specific CTLs proliferated less extensively when AKR.H-2b veto cells were included in cocultures. AKR/Gross MuLV-specific effector CTLs as well as memory CTLs were each efficiently targeted for inhibition by AKR.H-2b veto cells. Attempts to enhance the quality of the priming by multiple in vivo immunizations did not alter the capacity of the AKR.H-2b cells to inhibit the antiviral CTL response. These results further characterize the nature of the interaction between veto cells and antiviral CTLs, and underscore the efficiency of veto cell-mediated inhibition of the CTL response.

Spontaneous in vivo retrovirus-infected T and B cells, but not dendritic cells, mediate antigen-specific Fas ligand/Fas-dependent apoptosis of anti-retroviral CTL

C57BL/6 (H-2b), but not spontaneous virus-expressing AKR.H-2b congenic, mice generate retrovirus-specific CD8+ CTL responses to the immunodominant Kb-restricted epitope, KSPWFTTL. AKR.H-2b non-responsiveness is mediated by a peripheral tolerance mechanism. When co-cultured with primed B6 antiviral pCTL, AKR.H-2b splenocytes are recognized by the antiviral TcR as "veto" cells, which inhibit by an exquisitely virus-specific, MHC-restricted, veto cell FasL/responder T cell Fas, mediated apoptotic mechanism. Here, AKR.H-2b thymus, lymph node, and bone marrow cells are also shown to inhibit antiviral CTL generation. Purified AKR.H-2b CD4+ and CD8+ T cells, and B cells, served effectively as FasL-dependent veto cells. In contrast, AKR.H-2b dendritic cells (DC) did not efficiently veto antiviral CTL responses, despite expressing sufficient MHC class I/viral peptide complexes for TcR recognition. AKR.H-2b DC also expressed FasL mRNA and cell surface protein, albeit at a lower level than AKR.H-2b T and B cells. These findings suggest a fail-safe escape mechanism by virus-infected cells for escape from CTL-mediated immunity.

Tolerance induction by veto CTLs in the TCR transgenic 2C mouse model. II. Deletion of effector cells by Fas-Fas ligand apoptosis

The direct assay of veto CTLs in the 2C mouse model enables monitoring, by FACS, the fate of the TCR transgenic effector CD8(+) T cells, the transgene of which can be stained with clonotypic Ab 1B2. After the addition of veto cells, CD8(+)1B2(+) effector cells increasingly express annexin V, and maximal apoptosis is attained 72 h after initiation of MLR. This veto activity can be partially blocked by anti-CD8 Abs directed against the allele expressed by the veto CTLs, but not by the effector cells. When effector CD8(+) T cells were from 2C mice, which lack Fas expression ((2CX lpr)F(2)), deletion of effector cells was not exhibited by veto cells. The protein levels of the apoptosis inhibitors FLIP and Bcl2 in purified CD8(+)1B2(+) effector cells at different time points after MLR showed an initial up-regulation of these inhibitors, with marked reduction of FLIP, but not of Bcl2, by 48 h after initiation of culture. Taken together, these results are in accordance with a Fas-FasL-based mechanism in which prolonged binding between the effector cell and the veto cell might be required to allow FLIP to be down-regulated. Such prolonged interaction might be afforded through the interaction of CD8 molecules on the veto cell with the alpha3 domain of H2 class 1 on the effector cell.

Single-nucleotide polymorphism g.1548G > A (E469K) in human ICAM-1 gene affects mRNA splicing pattern and TPA-induced apoptosis

The single-nucleotide polymorphism (SNP) g.1548G > A (E469K) in the human intercellular adhesion molecule-1 (ICAM-1) gene has been suggested to have an association with several types of inflammatory diseases. The polymorphism is located at the three-base position upstream of the splice donor site that produces an alternatively spliced short isoform (ICAM-1-S). To clarify its functional relevance, we studied RNA splicing patterns by comparing cells with different genotype (G/G cells and A/A cells). G/G cells expressed a lower amount of ICAM-1-S mRNA than A/A cells. Since ICAM-1-S has no transmembrane or intracellular domain, ICAM-1 signal transduction and cell-cell contact including Fas-FasL interaction may be influenced. In addition, we studied the effect of this change on FLIP-L mRNA and apoptosis. FLIP-L mRNA tended to decrease, while cell death induced by phorbol 12-myristate 13-acetate was increased. These results suggest that the g.1548 polymorphism modifies inflammatory immune responses by changing cell-cell interaction and then regulating apoptosis.

Microanatomic relationships between CD8+ cells and HIV-1-producing cells in human lymphoid tissue in vivo

OBJECTIVE

Host immune responses are unable to fully suppress HIV-1 replication in lymphoid tissues. Microanatomic relationships between HIV-1-producing cells and CD8+ cells in lymphoid tissues were analyzed to determine whether there was evidence for an immune privileged site or impaired recognition of virus-producing cells.

METHODS

CD8+ cell phenotypes were determined on disaggregated inguinal lymph node cells by flow cytometry for seven untreated HIV-1-infected subjects. Microanatomic relationships between HIV-1-producing cells and CD8+ cells were analyzed in lymph node sections from 15 HIV-1-infected individuals using in situ hybridization and immunohistochemical staining.

RESULTS

Most (median, 96%) lymph node CD8+ cells coexpressed CD3. Frequencies of virus-producing cells detected by in situ hybridization correlated with plasma HIV-1 RNA concentration (Spearman rho = 0.70; p =.02; n = 11). The percentage of lymph node cells adjacent to virus-producing cells that were CD8+ (median, 29%) was not statistically different from the percentage of CD8+ cells in lymphoid tissue overall (median, 34%; p =.09).

CONCLUSIONS

Multiple explanations could account for the observation that CD8+ cells do not preferentially accumulate around virus-producing cells including the possibility that HIV-1-specific CD8+ cells cannot recognize virus-producing cells. Further studies are necessary to determine whether HIV-1-specific CD8+ T cells aggregate around virus-producing cells in lymphoid tissue.