Distinct Regulation of T-Cell Death by CD28 Depending on Both Its Aggregation and T-Cell Receptor Triggering: A Role for Fas-FasL

CTLA-4 controls the thymic development of both conventional and regulatory T cells through modulation of the TCR repertoire

Cytotoxic T lymphocyte-associated antigen-4 (CTLA-4; CD152) is of pivotal importance for self-tolerance, with deficiency or unfavorable polymorphisms leading to autoimmune disease. Tolerance to self-antigens is achieved through thymic deletion of highly autoreactive conventional T (Tconv) cells and generation of FoxP3(+) regulatory T (Treg) cells. The main costimulatory molecule, CD28, augments the negative selection of Tconv cells and promotes the generation of FoxP3(+) Treg cells. The role of its antagonistic homolog CTLA-4, however, remains a topic of debate. To address this topic, we investigated the thymic development of T cells in the presence and absence of CTLA-4 in a T-cell receptor (TCR) transgenic mouse model specific for the myelin basic protein peptide Ac1-9. We reveal that CTLA-4 is expressed in the corticomedullary region of the thymus. Its absence alters the response of CD4(+)CD8(-) thymocytes to self-antigen recognition, which affects the quantity of the Treg cells generated and broadens the repertoire of peripheral Tconv cells. T-cell repertoire alteration after deletion of CTLA-4 results from changes in TCR Vα and Jα segment selection as well as CDR3α composition in Tconv and Treg cells. CTLA-4, therefore, regulates the early development of self-reactive T cells in the thymus and plays a key role in central tolerance.

Live Mycobacterium avium subsp. paratuberculosis and a killed-bacterium vaccine induce distinct subcutaneous granulomas, with unique cellular and cytokine profiles

Type II (lepromatous) granulomas are characterized by a lack of organization, with large numbers of macrophages heavily burdened with bacilli and disorganized lymphocyte infiltrations. Type II granulomas are a characteristic feature of the enteric lesions that develop during clinical Mycobacterium avium subsp. paratuberculosis infection in the bovine. Considering the poor organization and function of these granulomas, it is our hypothesis that dendritic cell (DC) function within the granuloma is impaired during initial infection. In order to test our hypothesis, we used a subcutaneous M. avium subsp. paratuberculosis infection model to examine early DC function within M. avium subsp. paratuberculosis-induced granulomas. In this model, we first characterized the morphology, cellular composition, and cytokine profiles of subcutaneous granulomas that develop 7 days after subcutaneous inoculation with either vaccine or live M. avium subsp. paratuberculosis. Second, we isolated CD11c(+) cells from within granulomas and measured their maturation status and ability to induce T-cell responses. Our results demonstrate that M. avium subsp. paratuberculosis or vaccine administration resulted in the formation of distinct granulomas with unique cellular and cytokine profiles. These distinct profiles corresponded to significant differences in the phenotypes and functional responses of DCs from within the granulomas. Specifically, the DCs from the M. avium subsp. paratuberculosis-induced granulomas had lower levels of expression of costimulatory and chemokine receptors, suggesting limited maturation. This DC phenotype was associated with weaker induction of T-cell proliferation. Taken together, these findings suggest that M. avium subsp. paratuberculosis infection in vivo influences DC function, which may shape the developing granuloma and initial local protection.

Autologous designer antigen-presenting cells by gene modification of T lymphocyte blasts with IL-7 and IL-12

An effective immune response to antigen requires professional antigen-presenting cell (APC), which not only present antigen, but also provide costimulation and cytokines (eg, IL-12) that drive T cell differentiation down the appropriate effector pathway (Tc1/TH1). For T cell-based immunotherapy protocols, the availability of large numbers of autologous professional APC is a major limitation because professional APC do not proliferate in vitro. T cells themselves can proliferate exponentially in vitro and have the ability to present antigen. They can also express costimulatory molecules after activation. Therefore, we hypothesized that if activated T cells were genetically modified to express proinflammatory cytokines required to polarize T cells toward a Tc1 response, they could fulfill the requirements for an abundant, autologous APC. To test this potential, T cells were activated by CD3/CD28 antibodies and pulsed with model HLA-A2+ peptides derived from CMVpp65, MAGE-3, and MART-1. Activated T-APC readily reactivated CD8 pp65 memory T cells from healthy CMV seropositive donors; however, the activation of MAGE-3 and MART-1-specific CD8 T cells required both IL-7 and IL-12, which could be provided either exogenously or by genetic modification of the T-APC. Responder T cells could be expanded to large numbers with subsequent stimulations using activated, peptide-pulsed T-APC and IL-2. Tumor antigen-specific T cell lines killed both peptide-pulsed target cells and tumor cell lines. Thus, T cells provide a platform for the generation of autologous APC that can be customized to express both antigens and therapeutic molecules for the induction of antigen-specific T cell immunity.

CCL5-CCR5-mediated apoptosis in T cells: Requirement for glycosaminoglycan binding and CCL5 aggregation

CCL5 (RANTES (regulated on activation normal T cell expressed and secreted)) and its cognate receptor, CCR5, have been implicated in T cell activation. CCL5 binding to glycosaminoglycans (GAGs) on the cell surface or in extracellular matrix sequesters CCL5, thereby immobilizing CCL5 to provide the directional signal. In two CCR5-expressing human T cell lines, PM1.CCR5 and MOLT4.CCR5, and in human peripheral blood-derived T cells, micromolar concentrations of CCL5 induce apoptosis. CCL5-induced cell death involves the cytosolic release of cytochrome c, the activation of caspase-9 and caspase-3, and poly(ADP-ribose) polymerase cleavage. CCL5-induced apoptosis is CCR5-dependent, since native PM1 and MOLT4 cells lacking CCR5 expression are resistant to CCL5-induced cell death. Furthermore, we implicate tyrosine 339 as a critical residue involved in CCL5-induced apoptosis, since PM1 cells expressing a tyrosine mutant receptor, CCR5Y339F, do not undergo apoptosis. We show that CCL5-CCR5-mediated apoptosis is dependent on cell surface GAG binding. The addition of exogenous heparin and chondroitin sulfate and GAG digestion from the cell surface protect cells from apoptosis. Moreover, the non-GAG binding variant, (44AANA47)-CCL5, fails to induce apoptosis. To address the role of aggregation in CCL5-mediated apoptosis, nonaggregating CCL5 mutant E66S, which forms dimers, and E26A, which form tetramers at micromolar concentrations, were utilized. Unlike native CCL5, the E66S mutant fails to induce apoptosis, suggesting that tetramers are the minimal higher ordered CCL5 aggregates required for CCL5-induced apoptosis. Viewed altogether, these data suggest that CCL5-GAG binding and CCL5 aggregation are important for CCL5 activity in T cells, specifically in the context of CCR5-mediated apoptosis.

Elimination of activated but not resting primary human CD4 and CD8 T cells by Fas ligand (FasL/CD95L)-expressing Killer-dendritic cells

Dendritic cells (DC) genetically engineered to express high levels of Fas ligand (FasL/CD95L) have been demonstrated to delete T cells in an antigen specific manner in several different animal models in vivo. However, the immunomodulatory capacity of primary human FasL-expressing Killer-DC has not been determined. Therefore, human Killer-DC were generated from mature monocyte-derived DC using the inducible CRE/LoxP adenoviral vector system, and the immunoregulatory capacity of these cells was analyzed in cocultures with primary human T cells in vitro. Combined transductions of DC by AdloxPFasL and AxCANCre resulted in FasL expression in > 70% of DC without affecting the mature phenotype. Proliferation of activated primary human T cells was inhibited up to 80% in cocultures with FasL-expressing DC but not EGFP-transduced DC, which was due to induction of apoptosis in activated but not resting CD4+ and CD8+ T cells. Apoptosis induced by Killer-DC could be blocked by an anti-FasL-antibody in a dose dependent fashion. The present results demonstrate that FasL-expressing Killer-DC eliminate activated but not resting primary human CD4+ and CD8+ T cells by induction of Fas-mediated apoptosis supporting the concept to apply Killer-DC as a novel strategy for the treatment of T cell-dependent autoimmune disease and allograft rejection in humans.

CTLA-4-Fas ligand functions as a trans signal converter protein in bridging antigen-presenting cells and T cells

Co-stimulator blockade and trans inhibitory signaling, using agents such as CTLA-4-Ig and Fas ligand (FasL) respectively have been invoked as alternative strategies for suppressing pathogenic T cells. This study describes a novel hetero-bifunctional fusion protein, CTLA-4-FasL, designed to combine within a single protein both co-stimulator blocking and trans inhibitory signaling potentials. A chimeric expression cassette, in which the ectodomain coding sequences for CTLA-4 and FasL were linked in-frame, was used to produce a CTLA-4-FasL fusion protein. CTLA-4-FasL binding to both B7-1/B7-2-expressing Daudi B cells and Fas-expressing Jurkat T cells was documented by immunofluorescence and flow cytometry. The capacity of CTLA-4-FasL to induce apoptosis in Jurkat targets was markedly enhanced by the addition of Daudi and other B7-1/B7-2(+) B cell lines, which provided a membrane platform for the otherwise soluble CTLA-4-fusion protein. Moreover, in dual-chamber experiments, Daudi cells pre-coated with CTLA-4-FasL demonstrated Jurkat inhibitory activity that was cell-contact dependent. Significantly, when used to inhibit in vitro cellular proliferation of peripheral blood mononuclear cells, CTLA-4-FasL was approximately 1000-fold more potent than the extensively characterized CTLA-4-Ig fusion protein. Furthermore, the degree of inhibition induced by CTLA-4-FasL substantially surpassed that observed for CTLA-4-Ig and a soluble FasL when used in combination. CTLA-4-FasL represents the first of a novel class of fusion proteins, designated here as 'trans signal converter proteins', that combine trans signal masking and direct trans signaling functions.

The C-class chemokine, lymphotactin, impairs the induction of Th1-type lymphokines in human CD4+ T cells

Chemokines are involved in the regulation of leukocyte migration and for some of them, T-cell costimulation. To date, the only direct property of lymphotactin (Lptn), the unique member of the C class of chemokines, consists of T-cell chemoattraction. This report describes a novel function for Lptn in human T-lymphocyte biology, by demonstrating the direct ability of Lptn to both inhibit and costimulate CD4+ and CD8+ T-cell activation, respectively. Lptn but not RANTES inhibited CD4+ T-cell proliferation, through a decreased production of Th1 (interleukin [IL]-2, interferon [IFN]-γ) but not Th2 (IL-4, IL-13) lymphokines, and decreased IL-2R expression. Transfections in Jurkat cells showed a Lptn-mediated transcriptional down-regulation of gene-promoter activities specific for Th1-type lymphokines, as well as of nuclear factor of activated T cells (NF-AT) but not AP-1 or NF-ΚB enhancer activities. This suppressive action of Lptn could be compensated by overexpression of NF-ATc but not NF-ATp. CD4+ T-cell proliferation was completely restored by exogenous IL-2 or reversed by pertussis toxin, wortmannin, and genistein, suggesting the involvement of multiple partners in Lptn signaling. In contrast to CD4+ cells, Lptn exerted a potent costimulatory activity on CD8+ T-cell proliferation and IL-2 secretion. These data provide important insights into the role of Lptn in differential regulation of normal human T-cell activation and its possible implication in immune response disorders.