Edward A. Greenfield

PD-L2 is a second ligand for PD-1 and inhibits T cell activation.

Programmed death 1 (PD-1)–deficient mice develop a variety of autoimmune-like diseases, which suggests that this immunoinhibitory receptor plays an important role in tolerance. We identify here PD-1 ligand 2 (PD-L2) as a second ligand for PD-1 and compare the function and expression of PD-L1 and PD-L2. Engagement of PD-1 by PD-L2 dramatically inhibits T cell receptor (TCR)-mediated proliferation and cytokine production by CD4+ T cells. At low antigen concentrations, PD-L2–PD-1 interactions inhibit strong B7-CD28 signals. In contrast, at high antigen concentrations, PD-L2–PD-1 interactions reduce cytokine production but do not inhibit T cell proliferation. PD-L–PD-1 interactions lead to cell cycle arrest in G0/G1 but do not increase cell death. In addition, ligation of PD-1 + TCR leads to rapid phosphorylation of SHP-2, as compared to TCR ligation alone. PD-L expression was up-regulated on antigen-presenting cells by interferon γ treatment and was also present on some normal tissues and tumor cell lines. Taken together, these studies show overlapping functions of PD-L1 and PD-L2 and indicate a key role for the PD-L–PD-1 pathway in regulating T cell responses.

Antigen-specific regulatory T cells develop via the ICOS-ICOS-ligand pathway and inhibit allergen-induced airway hyperreactivity.

Asthma is caused by T-helper cell 2 (Th2)-driven immune responses, but the immunological mechanisms that protect against asthma development are poorly understood. T-cell tolerance, induced by respiratory exposure to allergen, can inhibit the development of airway hyperreactivity (AHR), a cardinal feature of asthma, and we show here that regulatory T (TR) cells can mediate this protective effect. Mature pulmonary dendritic cells in the bronchial lymph nodes of mice exposed to respiratory allergen induced the development of TR cells, in a process that required T-cell costimulation via the inducible costimulator (ICOS)–ICOS-ligand pathway. The TR cells produced IL-10, and had potent inhibitory activity; when adoptively transferred into sensitized mice,* TR cells blocked the development of AHR. Both the development and the inhibitory function of regulatory cells were dependent on the presence of IL-10 and on ICOS–ICOS-ligand interactions. These studies demonstrate that TR cells and the ICOS–ICOS-ligand signaling pathway are critically involved in respiratory tolerance and in downregulating pulmonary inflammation in asthma.*There was an error in the AOP version of this article. The sentence in the abstract that read The TR cells produced IL-10, and had potent inhibitory activity; when adoptively transferred into sensitized mouse TR cells, blocked the development of AHR was worded incorrectly. The following sentence is correct: The TR cells produced IL-10, and had potent inhibitory activity; when adoptively transferred into sensitized mice, TR cells blocked the development of AHR. This has been corrected in the HTML and the PDF. We regret this error.

Blockade of Programmed Death-1 Ligands on Dendritic Cells Enhances T Cell Activation and Cytokine Production

Programmed death-1 ligand (PD-L)1 and PD-L2 are ligands for programmed death-1 (PD-1), a member of the CD28/CTLA4 family expressed on activated lymphoid cells. PD-1 contains an immunoreceptor tyrosine-based inhibitory motif and mice deficient in PD-1 develop autoimmune disorders suggesting a defect in peripheral tolerance. Human PD-L1 and PD-L2 are expressed on immature dendritic cells (iDC) and mature dendritic cells (mDC), IFN-γ-treated monocytes, and follicular dendritic cells. Using mAbs, we show that blockade of PD-L2 on dendritic cells results in enhanced T cell proliferation and cytokine production, including that of IFN-γ and IL-10, while blockade of PD-L1 results in similar, more modest, effects. Blockade of both PD-L1 and PD-L2 showed an additive effect. Both whole mAb and Fab enhanced T cell activation, showing that PD-L1 and PD-L2 function to inhibit T cell activation. Enhancement of T cell activation was most pronounced with weak APC, such as iDCs and IL-10-pretreated mDCs, and less pronounced with strong APC such as mDCs. These data are consistent with the hypothesis that iDC have a balance of stimulatory vs inhibitory molecules that favors inhibition, and indicate that PD-L1 and PD-L2 contribute to the poor stimulatory capacity of iDC. PD-L1 expression differs from PD-L2 in that PD-L1 is expressed on activated T cells, placental trophoblasts, myocardial endothelium, and cortical thymic epithelial cells. In contrast, PD-L2 is expressed on placental endothelium and medullary thymic epithelial cells. PD-L1 is also highly expressed on most carcinomas but minimally expressed on adjacent normal tissue suggesting a role in attenuating antitumor immune responses.

Mouse Inducible Costimulatory Molecule (ICOS) Expression Is Enhanced by CD28 Costimulation and Regulates Differentiation of CD4+ T Cells

The inducible costimulatory (ICOS) molecule is expressed by activated T cells and has homology to CD28 and CD152. ICOS binds B7h, a molecule expressed by APC with homology to CD80 and CD86. To investigate regulation of ICOS expression and its role in Th responses we developed anti-mouse ICOS mAbs and ICOS-Ig fusion protein. Little ICOS is expressed by freshly isolated mouse T cells, but ICOS is rapidly up-regulated on most CD4+ and CD8+ T cells following stimulation of the TCR. Strikingly, ICOS up-regulation is significantly reduced in the absence of CD80 and CD86 and can be restored by CD28 stimulation, suggesting that CD28-CD80/CD86 interactions may optimize ICOS expression. Interestingly, TCR-transgenic T cells differentiated into Th2 expressed significantly more ICOS than cells differentiated into Th1. We used two methods to investigate the role of ICOS in activation of CD4+ T cells. First, CD4+ cells were stimulated with beads coated with anti-CD3 and either B7h-Ig fusion protein or control Ig fusion protein. ICOS stimulation enhanced proliferation of CD4+ cells and production of IFN-γ, IL-4, and IL-10, but not IL-2. Second, TCR-transgenic CD4+ T cells were stimulated with peptide and APC in the presence of ICOS-Ig or control Ig. When the ICOS:B7h interaction was blocked by ICOS-Ig, CD4+ T cells produced more IFN-γ and less IL-4 and IL-10 than CD4+ cells differentiated with control Ig. These results demonstrate that ICOS stimulation is important in T cell activation and that ICOS may have a particularly important role in development of Th2 cells.

Ox40-ligand has a critical costimulatory role in dendritic cell: T cell interactions

The tumor necrosis factor family molecule Ox40-ligand (Ox40L) has been identified as a potential costimulatory molecule and also has been implicated in T cell homing and B cell activation. To ascertain the essential functions of Ox40L, we generated and characterized Ox40L-deficient mice. Mice lacking Ox40L exhibit an impaired contact hypersensitivity response, a dendritic cell-dependent T cell-mediated response, due to defects in T cell priming and cytokine production. In contrast, Ox40L-deficient mice do not have defects in T cell homing or humoral immune responses. In vitro, Ox40L-deficient dendritic cells are defective in costimulating T cell cytokine production. Thus, Ox40L has a critical costimulatory function in vitro and in vivo for dendritic cell:T cell interactions.

The BCR/ABL Tyrosine Kinase Induces Production of Reactive Oxygen Species in Hematopoietic Cells

The BCR/ABL oncogene causes chronic myelogenous leukemia, a myeloproliferative disorder characterized by clonal expansion of hematopoietic progenitor cells and myeloid cells. It is shown here that transformation of the hematopoietic cell lines Ba/F3, 32Dcl3, and MO7e with BCR/ABL results in an increase in reactive oxygen species (ROS) compared with quiescent, untransformed cells. The increase in ROS was directly due to BCR/ABL because it was blocked by the ABL-specific tyrosine kinase inhibitor STI571. Oxidative stress through ROS is believed to have many biochemical effects, including the potential ability to inhibit protein-tyrosine phosphatases (PTPases). To understand the significance of increased production of ROS, a model system was established in which hydrogen peroxide (H2O2) was added to untransformed cells to mimic the increase in ROS induced constitutively by BCR/ABL. H2O2 substantially reduced total cellular PTPase activity to a degree approximately equivalent to that of pervanadate, a well known PTPase inhibitor. Further, stimulation of untransformed cells with H2O2 or pervanadate increased tyrosine phosphorylation of each of the most prominent known substrates of BCR/ABL, including c-ABL, c-CBL, SHC, and SHP-2. Treatment of the BCR/ABL-expressing cell line MO7/p210 with the reducing agents pyrrolidine dithiocarbamate orN-acetylcysteine reduced the accumulation of ROS and also decreased tyrosine phosphorylation of cellular proteins. Further, treatment of MO7e cells with H2O2 or pervanadate increased the tyrosine kinase activity of c-ABL. Drugs that alter ROS metabolism or reactivate PTPases may antagonize BCR/ABL transformation.

Endothelial expression of PD-L1 and PD-L2 down-regulates CD8+ T cell activation and cytolysis.

Interactions between CD8+ T cells and endothelial cells are important in both protective and pathologic immune responses. Endothelial cells regulate the recruitment of CD8+ Tcells into tissues, and the activation of CD8+ T cells by antigen presentation and costimulatory signals. PD‐L1 and PD‐L2 are recently described B7‐family molecules which bind to PD‐1 on activated lymphocytes and down‐regulate T cell activation. We found that PD‐L1 is expressed on interferon‐γ stimulated cultured human and mouse endothelial cells, while PD‐L2 was found on stimulated human but not mouse endothelial cells. Expression was further up‐regulated by TNF‐α. Antibody blockade of endothelial cell PD‐L1 and PD‐L2 enhanced endothelial cell costimulation of PHA‐activated human CD8+ T cells. Antibody blockade of mouse endothelial cell PD‐L1 enhanced both IFN‐γ secretion and cytolytic activity of CD8+ T cells in response to endothelial cellantigen presentation. These results show that IFN‐γ activated endothelial cells can inhibit T cell activation via expression of the immunoinhibitory PD‐L1 and PD‐L2 molecules. Endothelial expression of PD‐ligands would allow activation and extravasation of T cells without excessive vessel damage. Our findings highlight a potentially important pathway by which endothelial cells down‐regulate CD8+ T cell‐mediated immune responses.

Expression and Regulation of the PD‐L1 Immunoinhibitory Molecule on Microvascular Endothelial Cells

Objective: To evaluate the expression and regulation of a novel B7‐like protein, PD‐L1, the ligand for the immunoinhibitory receptor PD‐1 expressed on activated T‐cells, on microvascular endothelial cells (ECs)

TIM-3 is Expressed on Activated Human CD4+ T Cells and Regulates Th1 and Th17 Cytokines

TIM‐3 is a molecule selectively expressed on a subset of murine IFN‐γ‐secreting T helper 1 (Th1) cells but not Th2 cells, and regulates Th1 immunity and tolerance in vivo. At this time little is known about the role of TIM‐3 on human T cells. To determine if TIM‐3 similarly identifies and regulates Th1 cells in humans, we generated a panel of mAb specific for human TIM‐3. We report that TIM‐3 is expressed by a subset of activated CD4+ cells, and that anti‐CD3/anti‐CD28 stimulation increases both the level of expression as well as the number of TIM‐3+ T cells. We also find that TIM‐3 is expressed at high levels on in vitro polarized Th1 cells, and is expressed at lower levels on Th17 cells. In addition, human CD4+ T cells secreted elevated levels of IFN‐γ, IL‐17, IL‐2, and IL‐6, but not IL‐10, IL‐4, or TNF‐α, when stimulated with anti‐CD3/anti‐CD28 in the presence of TIM‐3‐specific, putative antagonistic antibodies. This was not mediated by differences in proliferation or cell death, but rather by induction of cytokines at the transcriptional level. These results suggest that TIM‐3 is a negative regulator of human T cells and regulates Th1 and Th17 cytokine secretion.

The Related Adhesion Focal Tyrosine Kinase Forms a Complex with Paxillin in Hematopoietic Cells

Related adhesion focal tyrosine kinase (RAFTK), also known as proline-rich tyrosine kinase 2 and cellular adhesion kinase β, has been recently cloned and characterized as a member of the focal adhesion kinase (FAK) subfamily. RAFTK has an overall 48% amino acid homology to p125FAK and contains a kinase domain but lacks a transmembrane region, myristylation sites, and Src homology region 2 and 3 domains. By Northern blot analysis, RAFTK is expressed in myeloid, lymphoid, and megakaryocytic hematopoietic cells. Like p125FAK, we found that RAFTK interacts with the focal adhesion protein paxillin. In the lymphoid cell line BaF3 and the myeloid cell line 32Dcl3, RAFTK coprecipitates with paxillin. Using in vitro binding assays, RAFTK and paxillin were shown to bind directly, through a segment of paxillin that required amino acids 100-227 and a domain in the C terminus of RAFTK. In vitro, RAFTK could phosphorylate paxillin on tyrosine residues. These results suggest that RAFTK, as well as p125FAK, may be important in phosphotyrosine-signaling events within the focal adhesion.