PD-L2 is expressed on activated human T cells and regulates their function

T-cell activation and proliferation are regulated by cosignaling adhesion molecules involved in positive or negative signals. Programmed death (PD)-1 is one of immune inhibitory molecules that is expressed in activated T cells and is a promising target for immunotherapy. Both PD-1 ligands, PD-L1 and PD-L2 are expressed on antigen presenting cells (APCs) involved in the dialogue between a T cell and an APC. Here, we analysed the expression of these ligands, especially for PD-L2, on T cells. PD-L2 appears to be expressed on activated CD4 and CD8T cell subsets. Moreover, as PD-1 molecule, PD-L2 engagement at the surface of T cells is able to down-modulate cytokine production and cell proliferation. These observations indicate that PD-L2 is expressed following activation and is involved in the regulation of T cell function, highlighting the level of complexity in the T cell cosignaling network.     

PD-1:PD-L inhibitory pathway affects both CD4(+) and CD8(+) T cells and is overcome by IL-2

Programmed death-1 (PD-1) is an immunoreceptor tyrosine-based inhibitory motif (ITIM)-containing receptor expressed upon T cell activation. PD-1(-/-) animals develop autoimmune diseases, suggesting an inhibitory role for PD-1 in immune responses. Members of the B7 family, PD-L1 and PD-L2, are ligands for PD-1. This study examines the functional consequences of PD-1:PD-L engagement on murine CD4 and CD8 T cells and shows that these interactions result in inhibition of proliferation and cytokine production. T cells stimulated with anti-CD3/PD-L1.Fc-coated beads display dramatically decreased proliferation and IL-2 production, while CSFE analysis shows fewer cells cycling and a slower division rate. Costimulation with soluble anti-CD28 mAb can overcome PD-1-mediated inhibition by augmenting IL-2 production. However, PD-1:PD-L interactions inhibit IL-2 production even in the presence of costimulation and, thus, after prolonged activation, the PD-1:PD-L inhibitory pathway dominates. Exogenous IL-2 is able to overcome PD-L1-mediated inhibition at all times, indicating that cells maintain IL-2 responsiveness. Experiments using TCR transgenic CD4(+) or CD8(+) T cells stimulated with antigen-presenting cells expressing PD-L1 show that both T cell subsets are susceptible to this inhibitory pathway. However, CD8(+) T cells may be more sensitive to modulation by the PD-1:PD-L pathway because of their intrinsic inability to produce significant levels of IL-2.     

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+ T cells 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-gamma 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-alpha. 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-gamma secretion and cytolytic activity of CD8+ T cells in response to endothelial cell antigen presentation. These results show that IFN-gamma 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.     

Programmed cell death 1 (PD-1) regulates the effector function of CD8 T cells via PD-L1 expressed on target keratinocytes

Programmed cell death 1 (PD-1) is an inhibitory molecule expressed by activated T cells. Its ligands (PD-L1 and -L2; PD-Ls) are expressed not only by a variety of leukocytes but also by stromal cells. To assess the role of PD-1 in CD8 T cell-mediated diseases, we used PD-1-knockout (KO) OVA-specific T cell-receptor transgenic (Tg) CD8 T cells (OT-I cells) in a murine model of mucocutaneous graft-versus-host disease (GVHD). We found that mice expressing OVA on epidermal keratinocytes (K14-mOVA mice) developed markedly enhanced GVHD-like disease after transfer of PD-1-KO OT-I cells as compared to those mice transferred with wild-type OT-I cells. In addition, K14-mOVA × OT-I double Tg (DTg) mice do not develop GVHD-like disease after adoptive transfer of OT-I cells, while transfer of PD-1-KO OT-I cells caused GVHD-like disease in a Fas/Fas-L independent manner. These results suggest that PD-1/PD-Ls-interactions have stronger inhibitory effects on pathogenic CD8 T cells than does Fas/Fas-L-interactions. Keratinocytes from K14-mOVA mice with GVHD-like skin lesions express PD-L1, while those from mice without the disease do not. These findings reflect the fact that primary keratinocytes express PD-L1 when stimulated by interferon-γ in vitro. When co-cultured with K14-mOVA keratinocytes for 2 days, PD-1-KO OT-I cells exhibited enhanced proliferation and activation compared to wild-type OT-I cells. In addition, knockdown of 50% PD-L1 expression on the keratinocytes with transfection of PD-L1-siRNA enhanced OT-I cell proliferation. In aggregate, our data strongly suggest that PD-L1, expressed on activated target keratinocytes presenting autoantigens, regulates autoaggressive CD8 T cells, and inhibits the development of mucocutaneous autoimmune diseases.     

Antibody-mediated signaling through PD-1 costimulates T cells and enhances CD28-dependent proliferation

Programmed death-1 (PD-1, CD279) is a molecule expressed on activated T, B and myeloid cells. The role of the interaction of PD-1 ligands (PD-L1 and PD-L2) with PD-1 receptor and the type of signals (costimulatory or inhibitory) that are delivered is a subject of intense debate. Our study has characterized two monoclonal antibodies (mAb) against murine PD-1, termed clone 1H10 and clone 4F10, that recognized different epitopes from that of anti-PD-1, clone J43. We showed that neither of them inhibited anti-CD3-mediated proliferation, but 1H10 mAb induced direct T cell proliferation in the absence of any other stimulus. Moreover, PD-1 engagement with 1H10 mAb costimulated anti-CD3-mediated proliferation and enhanced anti-CD3/CD28 proliferation on both CD4+ and CD8+ T cells in the low range of anti-CD3 concentrations. Anti-PD-1-mediated proliferation induced with 1H10 mAb was also observed in vivo on CD4+ and CD8+ T cells, when CFSE-labeled splenocytes were adoptively transferred to irradiated syngeneic and allogeneic recipients. Overall, our data indicate that PD-1 might not only deliver negative signals to T cells upon interaction through one of its ligands, PD-L1 as reported, but also could costimulate T cells, suggesting a dual potential functional activity of the extracellular domains of this receptor.     

No evidence for dualism in function and receptors: PD-L2/B7-DC is an inhibitory regulator of human T cell activation

The B7 family member programmed-death-1-ligand 2 (PD-L2/B7-DC) is a ligand for programmed-death-receptor 1 (PD-1), a receptor involved in negative regulation of T cell activation. Several independent studies have reported that PD-L2, however, can also potently costimulate murine T cells via an additional yet unidentified receptor. In this study, we evaluated the contribution of PD-L2 to the activation of human T cells using a novel system of engineered T cell stimulators that expresses membrane-bound anti-CD3 antibodies. Analyzing early activation markers, cytokine production and proliferation, we found PD-L2 to consistently inhibit T cell activation. PD-L2 inhibition affected CD4+ and CD8+ T cells and was not abrogated by costimulation via CD28. Blocking PD-1 reverted the inhibitory effect of PD-L2, demonstrating involvement of this pathway. In human T cells, we found no evidence for any of the costimulatory effects described for PD-L2 in murine systems. In line with our functional data that do not point to stimulatory PD-L2-ligands, we show that binding of PD-L2-immunoglobulin to activated human T cells is abrogated by PD-1 antibodies. Our results demonstrate that PD-L2 negatively regulates human T cell activation and thus might be a candidate molecule for immunotherapeutic approaches aimed to attenuate pathological immune responses.     

Intrahepatic expression of the co-stimulatory molecules programmed death-1, and its ligands in autoimmune liver disease

Liver-infiltrating T cells play an essential role in the immunopathogenesis of autoimmune liver disease. Programmed death-1 (PD-1) and its ligands, B7-H1/PD-L1 and B7-DC/PD-L2, are new CD28-B7 family members that are involved in the regulation of immune responses. The ligation of PD-1 inhibits T-cell receptor-mediated T cell proliferation and cytokine production, and PD-1-deficient mice develop various organ-specific autoimmune diseases. To investigate the expressions of PD-1 and its ligands in autoimmune liver disease, in particular autoimmune hepatitis (AIH) and primary biliary cirrhosis (PBC), immunohistochemical analysis was performed. Liver biopsy specimens obtained from 17 patients with AIH and PBC were studied. PD-1 was expressed on more than half of the liver-infiltrating T cells within the portal tract. Some of the intrahepatic T cells expressed B7-H1 in patients with AIH and PBC. B7-H1 and B7-DC were mainly expressed on some Kupffer cells (KC) and liver sinusoidal endothelial cells (LSEC) within the sinusoids and their expression was upregulated in autoimmune liver disease. These results suggest that the interaction of PD-1 on T cells with increased expression of B7-H1 and B7-DC on KC and LSEC might be involved in the downregulation of autoreactive lymphocytes and result in the regulation of pathogenesis in autoimmune liver disease.     

Programmed cell death 1 (PD-1) and its ligand PD-L1 are required for allograft tolerance

Programmed cell death-1 (PD-1, CD279) and its widely expressed, inducible ligand, PD-L1 (CD274), together dampen T cell activation, but whether they are essential for allograft tolerance is unknown. We show, using gene-deficient mice and blocking mAbs in wild-type mice, that costimulation blockade is ineffectual in PD-1(-/-) or PD-L1(-/-) allograft recipients, or in wild-type allograft recipients treated with anti-PD-1 or anti-PD-L1 mAb. Alloreactive PD-1(-/-) CD4 and CD8 T cells had enhanced proliferation and cytokine production compared to wild-type controls, and anergy could not be induced in PD-1-deficient CD4 T cells. We conclude that without inhibitory signals from PD-1 ligation, alloantigen-induced T cell proliferation and expansion cannot be regulated by costimulation blockade, and peripheral tolerance induction cannot occur.     

Adenovirus-mediated delivery of soluble ST2 attenuates ovalbumin-induced allergic asthma in mice

Co-stimulatory molecules are important for regulating T cell activation and immune response. CD274 [programmed death ligand 1 (PD-L1), B7-H1] has emerged as an important immune modulator that can block T cell receptor signalling. We have investigated whether PD-L1 and other co-stimulatory ligands could be expressed in human B cells stimulated by cytosine-phosphate-guanosine (CpG)-DNA. CpG-DNA strongly induced the co-inhibitory molecule ligand, PD-L1, of human B cells. Results show that nuclear factor-kappa B (NF-κB) signalling is involved directly in CpG-DNA-induced PD-L1 expression in human B cells. We sought to determine the effect of CpG-DNA-treated B cells on T helper type 2 (Th2) cytokine production in Cry j 1 (Japanese pollen antigen)-stimulated human CD4-positive cells from patients with seasonal allergic rhinitis caused by Japanese cedar pollen. CpG-DNA-treated B cells reduced Cry j 1-induced interleukin (IL)-5 and IL-13 production in CD4-positive cells. When the binding of PD-1 to PD-L1 was inhibited by PD-1-immunoglobulin (Ig), this chimera molecule reversed the previously described reductions in IL-5 and IL-13 production. In contrast, the CpG B-treated B cells increased both interferon (IFN)-γ and IL-12 production in the presence of Cry j 1-stimulated CD4-positive cells. CpG-DNA simultaneously reduced the expression of B7RP-1 [also known as inducible co-stimulator ligand (ICOSL), B7-H2] and the ligand of CD30 (CD30L). These results indicate that CpG-DNA induces co-inhibitory molecule ligand PD-L1 expression in human B cells and PD-L1 can suppress Th2 cytokine production in Cry j 1-stimulated CD4-positive cells, while CpG-DNA increased Th1 cytokine production and reduced the expression of co-stimulatory molecule ligands that can promote Th2 inflammatory responses.     

Reduced tumor-antigen density leads to PD-1/PD-L1-mediated impairment of partially exhausted CD8⁺ T cells

Clinical progression of cancer patients is often observed despite the presence of tumor-reactive T cells. Co-inhibitory ligands of the B7 superfamily have been postulated to play a part in this tumor-immune escape. One of these molecules, PD-L1 (B7-H1, CD274), is widely expressed on tumor cells and has been shown to mediate T-cell inhibition. However, attempts to correlate PD-L1 tumor expression with negative prognosis have been conflicting. To better understand when PD-1/PD-L1-mediated inhibition contributes to the functional impairment of tumor-specific CD8(+) T cells, we varied the levels of antigen density and/or PD-L1 expression at the surface of tumor cells and exposed them to CD8(+) T cells at different levels of functional exhaustion. We found that the gradual reduction of cognate antigen expression by PD-L1-expressing tumor cells increased the susceptibility of partially exhausted T cells to PD-1/PD-L1-mediated inhibition in vitro as well as in vivo. In conclusion, chronically stimulated CD8(+) T cells become sensitive to PD-1/PD-L1-mediated functional inhibition upon low antigen detection; a setting which is likely involved during tumor-immune escape.     

Immune checkpoint inhibitors in cancer therapy

In recent years immune checkpoint inhibitors have garnered attention as being one of the most promising types of immunotherapy on the horizon. There has been particular focus on the immune checkpoint molecules, cytotoxic Tlymphocyte antigen-4 (CTLA-4) and programmed cell death protein 1 (PD-1) which have been shown to have potent immunomodulatory effects through their function as negative regulators of T cell activation. CTLA-4, through engagement with its ligands B7-1 (CD80) and B7-2 (CD86), plays a pivotal role in attenuating the activation of na?ve and memory T cells. In contrast, PD-1 is primarily involved in modulating T cell activity in peripheral tissues via its interaction with PD-L1 and PD-L2. The discovery of these negative regulators of the immune response was crucial in the development of checkpoint inhibitors. This shifted the focus from developing therapies that targeted activation of the host immune system against cancer to checkpoint inhibitors, which aimed to mediate tumor cell destruction through the removal of coinhibitory signals blocking anti tumor T cell responses.     

TLR-mediated induction of negative regulatory ligands on dendritic cells

Dendritic cells (DCs) shape T-cell response patterns and determine early, intermediate, and late outcomes of immune recognition events. They either facilitate immunostimulation or induce tolerance, possibly determined by initial DC activation signals, such as binding Toll-like receptor (TLR) ligands. Here, we report that DC stimulation through the TLR3 ligand dsRNA [poly(I:C)] limits CD4 T-cell proliferation, curtailing adaptive immune responses. CD4+ T cells instructed by either lipopolysaccharide (LPS) or poly(I:C)-conditioned DCs promptly upregulated the activation marker CD69. Whereas LPS-pretreated DCs subsequently sustained T-cell clonal expansion, proliferation of CD4+ T cells exposed to poly(I:C)-pretreated DCs was markedly suppressed. This proliferative defect required DC-T cell contact, was independent of IFN-alpha, and was overcome by exogenous IL-2, indicating T-cell anergy. Coinciding with the downregulation, CD4+ T cells expressed the inhibitory receptor PD-1. Antibodies blocking the PD-1 ligand PD-L1 restored proliferation. dsRNA-stimulated DCs preferentially induced PD-L1, whereas poly(I:C) and LPS both upregulated the costimulatory molecule CD86 to a comparable extent. Poly(dA-dT), a ligand targeting the cytoplasmic RNA helicase pattern-recognition pathway, failed to selectively induce PD-L1 upregulation, assigning this effect to the TLR3 pathway. Poly(I:C)-conditioned DCs promoted accumulation of phosphorylated SHP-2, the intracellular phosphatase mediating PD-1 inhibitory effects. The ability of dsRNA to bias DC differentiation toward providing inhibitory signals to interacting CD4+ T cells may be instrumental in viral immune evasion. Conversely, TLR3 ligands may have therapeutic value in silencing pathogenic immune responses.     

Programmed death 1 is a marker of angioimmunoblastic T-cell lymphoma and B-cell small lymphocytic lymphoma/chronic lymphocytic leukemia

Programmed death 1 (PD-1) is a lymphoid receptor that negatively regulates immune responses. PD-1 expression was recently reported in some T-cell non-Hodgkin lymphoma (NHL) subtypes, but the expression profile of PD-1 and its ligands (PD-L1 and PD-L2) in B-NHLs remains largely to be characterized. To investigate this issue, monoclonal antibodies against PD-1, PD-L1, and PD-L2 were generated by immunization of balb-c mice. A series of 161 lymphoma tissue and 11 blood samples was analyzed using either immunohistochemistry or flow cytometry. In reactive lymph nodes, PD-1 was mainly expressed in follicular T cells. In B-NHLs, PD-1 was mainly expressed in reactive T cells; but expression was also noted in neoplastic B cells from small lymphocytic lymphoma (SLL, 12/13), grade III follicular lymphoma (3/3), and diffuse large cell lymphoma (2/25). In contrast, neoplastic B cells from mantle cell lymphoma (0/11), marginal zone lymphoma (0/12), Burkitt lymphoma (0/3), and grade 1 to 2 follicular lymphoma (0/40) were PD-1 negative. PD-L1 and PD-L2 were negative in small B-cell lymphomas, including B-SLL. Flow cytometry showed that blood cells from chronic lymphocytic leukemia (B-CLL) also displayed PD-1 expression, which could be increased by CD40 stimulation. PD-1 expression in T-NHLs was restricted to the angioimmunoblastic subtype (8/8). These results show that PD-1 expression among B-NHLs is mainly associated with SLL/CLL and is influenced by activation of the CD40/CD40L pathway. Because the anti-PD-1.6.4 antibody works on paraffin sections, it represents a useful tool to differentiate SLL/CLL from other small B-cell lymphomas.     

PD-1 ligands expressed on myeloid-derived APC in the CNS regulate T-cell responses in EAE

Disease progression in experimental autoimmune encephalomyelitis (EAE) is regulated by programmed death receptor 1 (PD-1) and its ligands, B7-H1 (programmed death ligand 1 (PD-L1)) and B7-DC (PD-L2). B7-H1 and B7-DC have negative regulatory effects upon binding PD-1 on activated T cells and B7-H1 deficiency increases severity of both diabetes and EAE. However, the role of PD-L expression on different APC in the CNS in regulating local T-cell function during relapsing EAE has not been examined. Our data show that the majority of CNS CD4+ T cells isolated during acute EAE are PD-1+, and T cells specific for relapse-associated epitopes express PD-1 upon antigen stimulation in the CNS. B7-H1 and B7-DC are differentially expressed on discrete APC populations in the inflamed CNS. B7-H1 and PD-1 have mainly inhibitory functions on CNS T cells. B7-H1 negatively regulates the stimulation of activated PD-1+ T(H) cells, in co-cultures with microglia and different CNS-infiltrating APC presenting endogenously processed peptides. The preponderance of IFN-gamma+ versus IL-17+ T cells in the CNS of B7-H1(-/-) mice suggests that B7-H1 more selectively suppresses T(H)-1 than T(H)-17 responses in vivo. In contrast, blockade of B7-DC has less pronounced regulatory effects. Overall, the results demonstrate that B7-H1 expressed by CNS myeloid APC negatively regulates T-cell activation during acute relapsing EAE.     

Programmed death-1 ligand 1 interacts specifically with the B7-1 costimulatory molecule to inhibit T cell responses

Pathways in the B7:CD28 family of costimulatory molecules regulate T cell activation and tolerance. B7-dependent responses in Cd28(-/-)Ctla4(-/-) T cells together with reports of stimulatory and inhibitory functions for Programmed Death-1 Ligand 1 or 2 molecules (PD-L1 or PD-L2) have suggested additional receptors for these B7 family members. We show that B7-1 and PD-L1 interacted with affinity intermediate to that of B7-1:CD28 and B7-1:CTLA-4. The PD-L1:B7-1 interface overlapped with the B7-1:CTLA-4 and PD-L1:PD-1 (Programmed Death-1) interfaces. T cell activation and cytokine production were inhibited by the interaction of B7-1 with PD-L1. The responses of PD-1-deficient versus PD-1,B7-1 double-deficient T cells to PD-L1 and of CD28,CTLA-4 double-deficient versus CD28,CTLA-4,PD-L1 triple-deficient T cells to B7-1 demonstrated that PD-L1 and B7-1 interact specifically to inhibit T cell activation. Our findings point to a substantial bidirectional inhibitory interaction between B7-1 and PD-L1 and add an additional dimension to immunoregulatory functions of the B7:CD28 family.     

Paradoxical role of programmed death-1 ligand 2 in Th2 immune responses in vitro and in a mouse asthma model in vivo

Programmed death-1 ligand 2 (PD-L2) is a ligand for programmed death-1 (PD-1), a receptor that plays an inhibitory role in T cell activation. Since previous studies have shown up-regulation of PD-L2 expression by Th2 cytokines, and asthma is driven by a Th2 response, we hypothesized that PD-L2 might be involved in regulation of the immune response in this disease. We have found that lungs from asthmatic mice had sustained up-regulation of PD-1 and PD-L2, with PD-L2 primarily on dendritic cells. Although addition of PD-L2-Fc in vitro led to decreased T cell proliferation and cytokine production, administration of PD-L2-Fc in vivo in a mouse asthma model resulted in elevated serum IgE levels, increased eosinophilic and lymphocytic infiltration into bronchoalveolar lavage fluid, higher number of cells in the draining lymph nodes, and production of IL-5 and IL-13 from these cells. Although PD-1 was expressed on regulatory T cells, PD-L2-Fc did not affect regulatory T cell activity in vitro. This study provides in vivo evidence of an exacerbated inflammatory response following PD-L2-Fc administration and indicates a potential role for this molecule in Th2-mediated diseases such as asthma.     

Expression of programmed-death receptor ligands 1 and 2 may contribute to the poor stimulatory potential of murine immature dendritic cells

Recent data have revealed that Ag presentation by immature dendritic cells (imDCs) plays a role in establishing and maintaining T-cell tolerance, but the mechanism remains unclear. PD-L1 and PD-L2, ligands for programmed-death receptor 1 (PD-1), members of the expanding B7 family, were highlighted for their inhibitory role in T-cell responses. Here, we show that blockade of PD-1 ligands on imDCs resulted in enhanced T-cell proliferation, which is perhaps due to the enhancement of IL-2 production from DC-stimulated T cells. PD-1 ligands blockade on mDCs did not show a significant stimulatory effect as markedly as imDCs. The inhibitory effects of PD-1 ligands would be dependent on maturation status of DCs, where attenuated positive costimulatory molecules provided the opportunity for PD-1 ligands to exert their strong capacity. Our data are consistent with the hypothesis that imDCs have an inhibitory bias, and indicate that PD-L1 and PD-L2 contribute to the poor stimulatory capacity of imDCs.     

Fine-tuned expression of programmed death 1 ligands in mature dendritic cells stimulated by CD40 ligand is critical for the induction of an efficient tumor specific immune response

During maturation, murine myeloid dendritic cells （DCs） upregulated the expressions of CDllc, CD25, CD40, CD80, CD86, MHC Ⅱ and programmed death 1 ligands 1 and 2 （PD-L1 and PD-L2）. Differential expression patterns of PD-L1 and PD-L2 were found when DCs were triggered by CD40 ligand and TNF-α. PD-L1 expression was repressed and PD-L2 expression remained unchanged in mature CD40-ligated DCs, whereas TNF-α stimulated DCs kept high expression of PD-L1 and significantly enhanced PD-L2 expression on DCs. Proliferations of T lymphocytes stimulated by immature DCs were enhanced by blockade of the PD-1 and PD-1 ligand interaction. But inhibitive effects were found in T lymphocytes stimulated by CD40-ligated DCs. With the fine-tuned expressions of PD-L1 and PD-L2, CD40-1igated DCs could sustain a longer activation period and elicit a more efficient T lymphocyte activation. Cellular ＆ Molecular Immunology.     

Tumor PD-L1 co-stimulates primary human CD8(+) cytotoxic T cells modified to express a PD1:CD28 chimeric receptor

Tumors exploit immunoregulatory checkpoints that serve to attenuate T cell responses as a means of circumventing immunologic rejection. Programmed death ligand 1 (PD-L1) is a negative regulator of T cell function and is frequently expressed by solid tumors. By engaging programmed death 1 (PD-1) on activated T cells, PD-L1(+) tumors directly render tumor-specific T cells, including adoptively transferred T cells, functionally exhausted. As a strategy to overcome tumor PD-L1 effects on adoptively transferred T cells, we sought to convert PD-1 to a T cell costimulatory receptor by exchanging its transmembrane and cytoplasmic tail with that of CD28. Rather than becoming exhausted upon engagement of PD-L1(+) tumors, we hypothesized that CD8(+) cytotoxic T lymphocytes (CTL) genetically modified to express this PD1:CD28 chimera would exhibit enhanced functional attributes. Here we show that cell surface expressed PD1:CD28 retains the capacity to bind PD-L1 resulting in T cell costimulation as evidenced by increased levels of ERK phosphorylation, augmentation of cytokine secretion, increased proliferative capacity, and enhanced expression of effector molecule Granzyme B. We provide evidence that this chimera could serve as a novel engineering strategy to overcome PD-L1 mediated immunosuppression.