Nanocomplejos del receptor para el antígeno de linfocitos T: agruparse para cooperar / T cell receptor nanocomplexes: cooperating clusters

El receptor para antígeno de los linfocitos T (TCR) es un complejo multiproteíco responsable de la activación y regulación de la respuesta inmune adaptativa. Este receptor muestra una gran especificidad y sensibilidad, a la vez que tiene una baja afinidad por su ligando. El ligando del TCR es un complejo formado por el péptido antigénico y una molécula del complejo principal de histocompatibilidad (MHC). Es más, los linfocitos T responden a antígeno en un rango muy amplio de concentraciones. Esto es, los linfocitos T continúan dando una respuesta aumentada a concentraciones de antígeno que exceden en varios órdenes de magnitud la concentración activadora mínima. La estequiometría y organización del TCR en la membrana han estado bajo intenso escrutinio porque pueden ser clave para explicar sus propiedades paradójicas. Esta revisión subraya la existencia de nuevos datos que indican que el TCR se presenta en linfocitos T intactos y en reposo como una mezcla variable de formas monovalentes (con sólo un sitio de unión para el ligando) y formas multivalentes de distinto grado. Estos resultados contrastan con datos anteriores de estequiometría del TCR obtenidos por procedimientos bioquímicos. No obstante, la mayor parte de estas discrepancias pueden deberse al efecto de distintos detergentes en la integridad del receptor. Aquí discutimos un modelo donde los complejos multivalentes del TCR son los responsables de dotar a los linfocitos T de sensibilidad a antígeno porque son activados por bajas concentraciones de antígeno, mientras que los complejos monovalentes son los responsables del amplio rango dinámico (AU)

The T cell antigen receptor (TCR·CD3) is a multi-subunit complex responsible for triggering an adaptive immune response. It shows high specificity and sensitivity while having a low affinity for the ligand. Furthermore, T cells respond to antigen over a wide concentration range. The stoichiometry and architecture of TCR·CD3 in the membrane have been under intense scrutiny because they might be key to explaining its paradoxical properties. This review highlights new evidence that TCR·CD3 is found on intact, unstimulated T cells in monovalent (one ligand-binding site per receptor) as well as in several distinct multivalent forms. This is in contrast to the TCR·CD3 stoichiometries determined by several biochemical means, but these data can be explained by the effects of different detergents on the integrity of the receptor. Here, we discuss a model in which the multivalent receptors are important for the detection of low concentrations of ligand, and therefore confer sensitivity, whereas the co-expressed monovalent TCR·CD3s allow a wide dynamic range (AU)

Cutting edge: CD95 maintains effector T cell homeostasis in chronic immune activation.

The elimination of activated T cells is important to maintain homeostasis and avoid immunopathology. CD95 (Fas/APO-1) has been identified as a death mediator for activated T cells in vitro but the function of CD95 in death of mature T cells in vivo is still controversial. Here we show that triggering of the costimulatory TNF receptor family member CD27 sensitized T cells for CD95-induced apoptosis. CD95-deficient (lpr/lpr) T cells massively expanded and differentiated into IFN-gamma-secreting effector cells in transgenic mice that constitutively express the CD27 ligand, CD70. Concomitantly, CD95-deficient CD70 transgenic mice became moribund by 4 wk of age with severe liver pathology and bone marrow failure. These findings establish that CD95 is a critical regulator of effector T cell homeostasis in chronic immune activation.

Differential expression of dendritic cell markers by all-trans retinoic acid on human acute promyelocytic leukemic cell line.

Dendritic cells (DCs) are the most potent antigen-presenting cells (APCs) for naive T cells and play an important role in cancer immunology. All-trans retinoic acid (ATRA) is known to be a differentiating agent in the treatment of acute promyelocytic leukemia (APL). In this study, we investigated whether ATRA can differentiate the retinoic acid (RA)-sensitive promyelocytic leukemic cell line, NB4, to DC-like cells and whether these differentiated cells can activate T cells. NB4 cells were differentiated to myeloid cells by 4, 6, and 8 days of ATRA treatment. NB4 cells up-regulated markers found in DCs, including HLA-DR, costimulatory molecules (CD80 and CD86), adhesion molecules (CD40), and chemokine receptors (CCR6) when cultured for 8 days in the presence of 1 microM ATRA. Upregulation of CD83 was also detected on the surface of ATRA-treated NB4 cells versus untreated cells. The addition of cytokines alone, such as GM-CSF or CD40 ligand, did not affect the expression of CD83 in untreated NB4 cells but they up-regulated CD83 in ATRA-treated cells. CD11b was coexpressed with CD80, CD83, and CD86 in ATRA-treated NB4 cells. In a functional assay, ATRA-treated NB4 cells stimulated T cell proliferation when challenged with Staphylococcus enterotoxin B. These results suggest that the differentiation of NB4 cells by ATRA causes the cells to express DC markers, and that ATRA-differentiated NB4 cells are able to present antigens to T cells.

OX40 signaling renders adult T-cell leukemia cells resistant to Fas-induced apoptosis.

We reported previously that OX40, a member of the tumor necrosis factor receptor family, is expressed constitutively on fresh leukemia/lymphoma cells isolated from patients with adult T-cell leukemia (ATL). In this study, we tested whether OX40 signaling affects the Fas-mediated apoptosis of fresh ATL cells isolated from 7 patients (3 acute type, 3 chronic type, and 1 smoldering type). In all these patients, the coculture of ATL cells with MMCE/OX40 ligand gp34, a stable human gp34 transfectant of a mouse epithelial cell line, resulted in a decrease in the percentage of apoptotic cells after treatment with anti-Fas monoclonal antibody, compared to coculture with MMCE/mock controls. Similar findings were obtained in OX40(+)- human T-cell leukemia virus type I-transformed T-cell lines. To elucidate the molecular mechanism of this phenomenon, we used Kit225/OX40, a stable OX40 transfectant of an IL-2-dependent T-cell line, and its deletion mutant, Kit225/del-OX40, in which the intracytoplasmic domain of OX40 had been deleted. Coculture with MMCE/gp34 inhibited the apoptosis of Kit225/OX40, but Kit225/del-OX40 apoptosis was hardly affected. These results suggest that ATL cells may escape Fas-mediated destruction of the immune system through OX40 signaling.

In vitro and in vivo activities of OX40 (CD134)-IgG fusion protein isoforms with different levels of immune-effector functions.

Recombinant fusion proteins consisting of the extracellular domain of immunoregulatory proteins and the constant domain of immunoglobulin G (IgG) are a novel class of human therapeutics. IgG isoforms exert different levels of immune effector functions, such as complement lysis and antibody-dependent cell cytotoxicity (ADCC). Several OX40-Ig fusion proteins were generated and compared in their potency to inhibit immune reactions. OX40-IgG fusion proteins act as decoys and inhibit T cell costimulation and extravasation induced by OX40 ligand-expressing antigen-presenting cells (APC) and vascular endothelial cells, respectively. In addition, OX40-IgG1 protein induces ADCC and complement lysis in OX40 ligand-expressing cells. Replacement of the IgG1 by the IgG4 domain (OX40-IgG4) eliminated complement lysis and reduced ADCC by half. Mutation of Leu(235) to Glu in IgG4 eliminated the remaining ADCC activity and generated a protein devoid of immune effector functions (OX40-IgG4mut). In vitro, OX40-IgG1 was more potent in inhibiting proliferation and cytokine release by peripheral blood mononuclear cells than OX40-IgG4mut, as OX40-IgG1 induced cell death in APC. However, both proteins reduced T cell-mediated colitis in mice to the same extent, indicating that in vivo neutralization of OX40L is sufficient. This study also demonstrates that effector functions of antibodies are retained and can be rationally designed in receptor-IgG fusion proteins.

Intracellular signaling of gp34, the OX40 ligand: induction of c-jun and c-fos mRNA expression through gp34 upon binding of its receptor, OX40.

We investigated the intracellular signaling events of OX40 ligand (gp34), a member of the TNF family. To elucidate the intracellular signaling via gp34, we prepared a model system in which a human gp34-transfected mouse epithelial cell line was stimulated with a recombinant soluble form of OX40. We demonstrated that OX40 binding resulted in increase in c-jun and c-fos mRNA levels in this transfectant by Northern blot analysis, which was blocked by the pretreatment with anti-gp34 Ab. The studies with various gp34 deletion mutants showed that the cytoplasmic portion including the amino acid sequence 16-21 (RPRFER) was required for the induction of c-jun and c-fos mRNA expression. Furthermore, OX40 binding induced c-jun mRNA expression also in HUVECs, which in our previous study have been shown to express gp34 and interact with activated T cells through the OX40/gp34 pathway. On the other hand, c-fos mRNA was detectable neither in unstimulated HUVECs nor in gp34-stimulated HUVECs. These results indicate that the OX40/gp34 system generates two-way signals and may elicit biological effects on vascular endothelial cells.

Reciprocal expression of CD70 and of its receptor, CD27, in human long term-activated T and natural killer (NK) cells: inverse regulation by cytokines and role in induction of cytotoxicity.

By transfection of COS cells with an expression vector containing CD70 cDNA we demonstrate that two previously described MoAbs (ED6 and LD6) recognize CD70. By means of these MoAbs, we show that the surface expression of CD70 inversely correlates with the expression of its receptor, CD27, on activated T and NK cell populations and clones, although a subpopulation of cells expressing low density of both molecules exists. In addition, culture in the presence of IL-4 significantly enhances CD27 and reduces CD70 surface expression in phytohaemagglutinin (PHA)-activated peripheral blood lymphocytes (PBL), while tumour necrosis factor-alpha (TNF-alpha) displays opposite effects, indicating that receptor and ligand are reciprocally regulated by these cytokines. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis of CD27 and CD70 mRNA suggests a transcriptional control of CD27 antigen expression in T cell clones. In addition, we show by the use of a re-directed killing assay that in cytotoxic T cell receptor (TCR) alpha/beta+ T cell clones, CD27 molecule may be involved in the regulation of cytolytic functions and may act synergistically with CD2. Finally, CD70 also acts as a signal-transducing molecule in some activated CD70+ TCR gamma/delta+ T or NK cell clones. In conclusion, our data indicate that CD27 and CD70 molecules are differentially expressed and regulated on long term-activated T and NK cells and are involved in the control of cellular functions.

Direct demonstration of the CD27 molecule involved in the negative regulatory effect on T cell activation.

CD27 is a member of the nerve growth factor/tumor necrosis factor receptor family and appears to play an important role in the process of T cell activation. We have previously shown that perturbation of the CD27 molecule by anti-1A4 (CD27) mAb inhibits T cell proliferation induced by various stimuli. To further determine the molecular basis of this inhibition of T cell proliferation by anti-1A4 mAb, we have established CD27-transfectant lines. Introduction of the human CD27 cDNA to ovalbumin-specific I-Ad-restricted murine T cell hybridomas significantly reduced their antigen-specific IL-2 production. In addition, treatment of these human CD27 transfectants with an anti-CD27 mAb (anti-1A4) further reduced their IL-2 production. These results provide direct evidence that CD27 plays a crucial role in the process of T cell activation.