Expression and function of PD-1 in human γδ T cells that recognize phosphoantigens

Programmed cell death-1 (PD-1) is an inhibitory receptor and plays an important role in the regulation of αβ T cells. Little is known, however, about the role of PD-1 in γδ T cells. In this study, we investigated the expression and function of PD-1 in human γδ T cells. Expression of PD-1 was rapidly induced in primary γδ T cells following antigenic stimulation, and the PD-1(+) γδ T cells produced IL-2. When PD-1(+) γδ T cells were stimulated with Daudi cells with and without programmed cell death ligand-1 (PD-L1) expression, the levels of IFN-γ production and cytotoxicity in response to PD-L1(+) Daudi cells were diminished compared to the levels seen in response to PD-L1(-) Daudi cells. The attenuated effector functions were reversed by anti-PD-L1 mAb. When PD-1(+) γδ T cells were challenged by PD-L1(+) tumors pretreated with zoledronate (Zol), which induced γδ TCR-mediated signaling, the resulting reduction in cytokine production was only slight to moderate compared to the reduction seen when PD-1(+) γδ T cells were challenged by PD-L1(-) tumors. In addition, cytotoxic activity of PD-1(+) γδ T cells against Zol-treated PD-L1(+) tumors was comparable to that against Zol-treated PD-L1(-) tumors. These results suggest that TCR triggering may partially overcome the inhibitory effect of PD-1 in γδ T cells.     

正常人PBMC中T细胞的PD-1表达及其表型分析

目的观察正常人外周血单个核细胞（PBMC）T细胞的PD-1表达,并对其表型进行分析。方法分离正常成年人PBMCs,利用流式细胞术检测CD4、CD8、CD45RA、CD27和PD-1等表面分子的表达;通过对比分析,了解PD-1＋T细胞表型与记忆T细胞的关系。结果正常成年人周围血中CD4＋PD-1＋T细胞和CD8＋PD-1＋T细胞的平均表达水平为（8.71±3.97）%,CD8＋PD-1＋T细胞的频率为（2.44±0.76）%。正常人外周血CD4＋PD-1＋T细胞可分成3个亚群,即初始T细胞（CD45RA＋CD27＋T细胞）、中央记忆型（CD45RA-CD27＋）T细胞和效应记忆型（CD45RA-CD27-）T细胞,其比例分别为（18.5±8.79）%、（65.0±13.84）%和（15.5±5.12）%,而在CD4＋PD-1-T细胞中3种T细胞的比例分别为（68.25±10.33）%、（27.75±9.04）%和（3.0±1.22）%。结论正常人周围血中存在CD4＋PD-1＋T细胞,并且多为中央记忆T细胞,而CD4＋PD-1-T细胞则多为初始T细胞。     

PD-1 silencing improves anti-tumor activities of human mesothelin-targeted CAR T cells

Chimeric antigen receptor T (CAR T) cell therapy is a new pillar in cancer therapeutics, and has been successfully used for the treatment of cancers, including acute lymphoblastic leukemia and solid cancers. Following immune attack, many tumors upregulate inhibitory ligands which bind to inhibitory receptors on T cells. For example, the interaction between programmed cell death protein 1 (PD-1) on activated T cells and its ligands (widely known as PD-L1) on a target tumor limits the efficacy of CAR T cells therapy against poorly responding tumors. Here, we use mesothelin (MSLN)-expressing human ovarian cancer cells (SKOV3) and human colon cancer cells (HCT116) to investigate whether PD-1–mediated T cell exhaustion affects the anti-tumor activity of MSLN-targeted CAR T cells. We utilized cell-intrinsic PD-1-targeting shRNA overexpression strategy, resulting in a significant PD-1 silencing in CAR T cells. The reduction of PD-1 expression on T cell surface strongly augmented CAR T cell cytokine production and cytotoxicity towards PD-L1-expressing cancer cells in vitro. This study indicates the enhanced anti-tumor efficacy of PD-1-silencing MSLN-targeted CAR T cells against several cancers and suggests the potential of other specific gene silencing on the immune checkpoints to enhance the CAR T cell therapies against human tumors.     

Human CXCR5+PD-1+ CD8 T cells in healthy individuals and patients with hematologic malignancies

Immune checkpoint blockade (ICB) has revolutionized cancer therapy, but varying response rates illustrate the need for biomarkers of response. Studies in mice have identified a subset of CD8 T cells that is essential for response to PD-1 ICB. These CD8 T cells co-express CXCR5, PD-1 and Tcf1, and provide effector T cells upon PD-1 ICB. It is unknown whether similar T cells play a role in PD-1 ICB in humans. We studied human peripheral blood and lymph nodes (LNs) for the frequency, phenotype, and functionality of CXCR5⁺PD-1⁺ CD8 T cells. We find that CXCR5⁺PD-1⁺ CD8 T cells are memory-like cells, express Tcf1, and lack expression of effector molecules. CXCR5⁺PD-1⁺ CD8 T cells produce cytokines upon stimulation, but have limited proliferative capacity. We studied patients with hematologic malignancies with varying response rates to PD-1 ICB. Specifically in chronic lymphocytic leukemia, in which PD-1 ICB does not induce clinical responses, CXCR5⁺PD-1⁺ CD8 T cells show loss of the memory phenotype and increased effector differentiation. In conclusion, we identified CXCR5⁺PD-1⁺ CD8 T cells in human peripheral blood and LN, which could play a similar role during PD-1 ICB. Future studies should analyze CXCR5⁺PD-1⁺ CD8 T cells during PD-1 ICB and their importance for therapeutic response.     

HBcAg induces PD-1 upregulation on CD4+T cells through activation of JNK, ERK and PI3K/AKT pathways in chronic hepatitis-B-infected patients

Hyper-expression of programmed death-1 (PD-1) is a hallmark of exhausted T cells. In chronic hepatitis-B virus (HBV)-infected patients, PD-1 upregulation on T cells was often observed. The mechanism of it has not been fully understood. In this study, we examined the dynamic changes of PD-1 expression on T cells during the natural history of chronic HBV infection and explored the signaling pathway of PD-1 upregulation by the hepatitis-B core antigen (HBcAg). Sixty-seven chronic HBV-infected patients were categorized into an immune tolerance group, an immune clearance group and an inactive virus carrier group, and 20 healthy volunteers were chosen as normal control group. Peripheral blood mononuclear cells from patients and healthy volunteers, and T lymphocytes from healthy volunteers were separated. Results showed that the PD-1 expression level on CD4(+)T cells in every phase of chronic HBV infection was significantly higher than that in healthy volunteers, whereas such effects were not observed on CD8(+)T cells. In the immune clearance phase, a positive correlation was found between serum HBV DNA level and the PD-1 expression level on CD4(+)T cells. In all phases, no correlation was shown between serum alanine amino transferase (ALT) level and PD-1 expression level. Phosphorylation of JNK, ERK and AKT was induced by HBcAg, and inhibitors of JNK, ERK and PI3K/AKT significantly decreased the HBcAg-induced PD-1 upregulation on CD4(+)T cells. In conclusion, the PD-1 expression level on CD4(+)T cells was upregulated in every phase of chronic HBV infection, which was induced by HBcAg through JNK, ERK and PI3K/AKT signaling pathways.     

PD-1 upregulation is associated with HBV-specific T cell dysfunction in chronic hepatitis B patients

Programmed death-1 (PD-1) is demonstrated to have an increased expression on antigen-specific T cells during chronic virus infections, and the blockage of PD-1/PD-ligand (PD-L1) pathway could restore the function of exhausted T cells. We measured the PD-1 expression levels on HBV-specific CD8 T cells and investigated the role of PD-1/PD-L1 pathway in T-cell responses of patients with different HBV infection statuses. Compared to the patients with convalescent acute hepatitis B, PD-1 expression on total CD8 T cells from chronic hepatitis B (CHB) patients was significantly upregulated, especially on the HBV pentamer-positive CD8 T cells. And PD-L1, but not PD-L2, was also significantly upregulated on PBMC from CHB patients. In CHB patients, HBV-specific T cells and cellular proliferation could be observed under the recombinant HBV-Ag stimulation in vitro, and blockade of PD-1 pathway significantly enhanced the IFN-gamma production and cellular proliferation of PBMC. Furthermore, PD-1 expression level on HBV-pentamers positive CD8 T cells was positively associated with plasma viral load in CHB patients. Thus, PD-1 upregulation on HBV-specific CD8 T cells is engaged in the dysfunction of T cells and high viraemia in CHB patients, and the antiviral T-cell responses could be improved by the blockade of this inhibitory PD-1/PD-L1 pathway.     

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.     

Soluble PD-1 is Associated with Aberrant Regulation of T Cells Activation in Aplastic Anemia

Engagement of the membrane program death-1 (PD-1) receptor by its ligands suppresses T cell proliferation and cytokine production. Aberrant over-expression of costimulatory molecules, including PD-1, has been associated with persistent activation of self-reactive T cells in autoimmune diseases. However, the mechanism underlying the dysfunction of PD-1 in the regulation of T-cell activation in such diseases remains unclear. Here, we report the overexpression of CD4⁺ and CD8⁺ T cell PD-1 and elevated serum levels of soluble PD-1 in aplastic anemia (AA) patients. Detailed characterization of soluble PD-1 revealed that it corresponded to an alternative splice variant PD-1Δex3, which lacks the transmembrane domain but has a soluble extracellular domain of the PD-1 molecule. In a further study, PD-1 fusion protein displayed the ability of increasing the proliferation of T cells in vitro, which suggested that soluble PD-1 might serve as an autoimmune antibody to block the function of membrane-bound PD-1 on T cells and lead to aberrant T cell proliferation. Our study revealed a novel pathogenic pathway in which the function of overexpressed PD-1 to restrict over-self-reaction is counteracted by the excessive production of soluble PD-1.     

人PD-1Δex3基因的克隆、表达及其生物学活性的鉴定

目的:构建表达人PD-1Δex3(ΔPD-1)基因的真核表达载体,检测其表达和生物学活性。方法:通过RT-PCR的方法获得人PD-1全长(PD-1)基因,设计特异性引物,PCR方法获得PD-1Δex3基因的2个cDNA片段,通过重组PCR的方法获得ΔPD-1基因,将目的片段双酶切后与pIRES2-EGFP真核表达载体连接,构建重组真核表达载体pIRES2-EGFP/PD-1和pIRES2-EGFP/ΔPD-1并进行鉴定;脂质体转染法将重组载体导入293T细胞;流式细胞术(FCM)检测转染细胞膜表面PD-1的表达;Western blot法检测培养上清中可溶性PD-1(sPD-1)的表达;间接免疫荧光实验分析ΔPD-1蛋白与PD-1两个配体PD-L1和PD-L2的结合。结果:酶切和测序结果均证实插入的基因序列正确,成功构建两个真核表达载体;FCM分析和Western blot检测结果表明,PD-1转染细胞膜表面高表达PD-1蛋白,细胞培养上清中无sPD-1蛋白,而ΔPD-1转染细胞膜表面不表达PD-1蛋白,细胞培养上清中则有大量sPD-1;间接免疫荧光实验结果表明,ΔPD-1蛋白能够与PD-1两种配体产生特异性结合。结论:成功进行PD-1Δex3基因的真核表达,证实PD-1Δex3基因编码sPD-1蛋白,该蛋白具有良好的生物学活性,为进一步研究PD-1Δex3在PD-1/PD-L信号通路中的生物学作用提供了有价值的物质基础。     

Increased B7-H1 Expression on Peripheral Blood T Cells in Oral Lichen Planus Correlated with Disease Severity

Background

Oral lichen planus (OLP) is a chronic and T cell-mediated autoimmune disease whose immunopathogenesis may involve antigen-presentation, T cells activation and migration as well as keratinocytes apoptosis. PD-1/B7-H1 pathway may have a unique function in regulating self-reactive T cells associated with inflammatory response and maintaining tolerance in peripheral tissues. In this study, we aimed to explore the contribution of PD-1/B7-H1 pathway to OLP.

Methods

We determined the expression of PD-1 and B7-H1 on peripheral blood T cells from OLP cases and analyzed their association with disease severity assessed by RAE (reticular, atrophic and erosive lesion) scoring system. In addition, interferon-γ, interleukin (IL)-2, IL-4, IL-10 and soluble PD-1 concentrations in serum were measured using ELISA. Then, we explored the regulation of PD-1/B7-H1 pathway on T cells immune response in OLP by blockade of PD-1 or B7-H1.

Results

We found that PD-1 and B7-H1 were up-regulated on peripheral blood T cells from OLP patients and B7-H1 expression positively correlated with disease severity of OLP. It is suggested that Th1 dominant inflammatory situation might contribute to the high expression of PD-1 and B7-H1 in OLP. Blockade of PD-1/B7-H1 pathway significantly increased the proliferation, and IFN-γ and IL-2 production of T cells.

Conclusions

PD-1/B7-H1 pathway may play an important role in negatively modulating T cell-mediated immune response in OLP, and provide the rationale to employ B7-H1 expression on peripheral blood T cells as a marker of severity of OLP and to develop agonists targeting PD-1/B7-H1 pathway as a promising immunotherapeutic strategy for OLP.     

T cell dysfunction by hepatitis C virus core protein involves PD-1/PDL-1 signaling

Reports have shown that a negative T cell costimulatory pathway mediated by PD-1 (programmed death-1) and PDL-1 (programmed death ligand-1) is associated with T cell exhaustion and persistent viral infection. Persistent hepatitis C virus (HCV) infection in humans is also characterized by impaired T lymphocyte function, but the role of the PD-1 and PDL-1 pathway in HCV infection is unknown. Here we report that T cells isolated from chronically HCV-infected patients express significantly higher levels of PD-1 when compared with healthy donors. In addition, PD-1 and PDL-1 expression is upregulated on healthy donor T cells exposed to HCV core, a nucleocapsid protein that is immunosuppressive; upregulation of PD-1 is mediated through interaction of HCV core with the complement receptor, gC1qR. Importantly, T cell functions that are dysregulated by HCV core, including T cell activation, proliferation, and apoptosis, can be restored by blocking PD-1 and PDL-1 engagement. Our results indicate that HCV core can upregulate a key negative T cell signaling pathway associated with viral persistence and highly expressed on the T cells of persistently infected individuals. This upregulation of the PD-1 and PDL-1 pathway in humans represents a novel and perhaps common mechanism by which a virus usurps host machinery to facilitate persistence.     

Programmed death-1 blockade enhances expansion and functional capacity of human melanoma antigen-specific CTLs

Negative co-stimulatory signaling mediated via cell surface programmed death (PD)-1 expression modulates T and B cell activation and is involved in maintaining peripheral tolerance. In this study, we examined the effects of a fully human PD-1-abrogating antibody on the in vitro expansion and function of human vaccine-induced CD8+ T cells (CTLs) specific for the melanoma-associated antigens glycoprotein 100 (gp100) and melanoma antigen recognized by T cells (MART)-1. PD-1 blockade during peptide stimulation augmented the absolute numbers of CD3+, CD4+, CD8+ and gp100/MART-1 MHC:peptide tetramer+ CTLs. This correlated with increased frequencies of IFN-gamma-secreting antigen-specific cells and augmented lysis of gp100+/MART-1+ melanoma targets. PD-1 blockade also increased the fraction of antigen-specific CTLs that recognized melanoma targets by degranulation, suggesting increased recognition efficiency for cognate peptide. The increased frequencies and absolute numbers of antigen-specific CTLs by PD-1 blockade resulted from augmented proliferation, not decreased apoptosis. Kinetic analysis of cytokine secretion demonstrated that PD-1 blockade increased both type-1 and type-2 cytokine accumulation in culture without any apparent skewing of the cytokine repertoire. These findings have implications for developing new cancer immunotherapy strategies.     

Identification of PD-1 as a unique marker for failing immune reconstitution in HIV-1-infected patients on treatment

PD-1 expression on T cells correlates with T-cell exhaustion and disease progression in HIV-infected patients. Previous studies have shown that combinational antiretroviral therapy induced viral suppression results in immune restoration and reduced PD-1 expression. However, a significant number of patients fail to restore CD4 T cells despite suppression of HIV replication below limit of quantification. In this study, we have analyzed PD-1 expression on CD4 and CD8 T cells in patients with poor immune reconstitution despite successful highly active antiretroviral therapy. We found that T cells of such patients express significantly higher levels of PD-1 than patients who had normal recovery of CD4 cells after treatment. In contrast, failing immune reconstitution was not associated with the expression of activation markers, indicating that PD-1 is a unique marker for failing immune reconstitution despite viral suppression. Furthermore, we show that T cells from patients with poor immune recovery differ from T cells of elderly in respect of their marker profile. PD-1 expression negatively correlated with individual CD4 cell counts, and PD-1 expressing T cells were more prone to programmed death ligand-mediated inhibition of T-cell proliferation, indicating that PD-1-mediated T-cell suppression may have a role in impaired immune reconstitution in HIV patients.     

Chronic virus infection enforces demethylation of the locus that encodes PD-1 in antigen-specific CD8(+) T cells

Functionally exhausted T?cells have high expression of the PD-1 inhibitory receptor, and therapies that block PD-1 signaling show promise for resolving chronic viral infections and cancer. By using human and murine systems of acute and chronic viral infections, we analyzed epigenetic regulation of PD-1 expression during CD8(+) T?cell differentiation. During acute infection, naive to effector CD8(+) T?cell differentiation was accompanied by a transient loss of DNA methylation of the Pdcd1 locus that was directly coupled to the duration and strength of T?cell receptor signaling. Further differentiation into functional memory cells coincided with Pdcd1 remethylation, providing an adapted program for regulation of PD-1 expression. In contrast, the Pdcd1 regulatory region was completely demethylated in exhausted CD8(+) T?cells and remained unmethylated even when virus titers decreased. This lack of DNA remethylation leaves the Pdcd1 locus poised for rapid expression, potentially providing a signal for premature termination of antiviral functions.     

Programmed death-1 upregulation is correlated with dysfunction of tumor-infiltrating CD8+ T lymphocytes in human non-small cell lung cancer

T-cell tolerance is an important mechanism for tumor escape, but the molecular pathways involved in T-cell tolerance remain poorly understood. It remains unknown whether the inhibitory immunoreceptor programmed death-1 (PD-1) plays a role in conditions of human non-small cell lung cancer (NSCLC). In this study, we detected PD-1 expression on CD8⁺ T cells from healthy control peripheral blood mononuclear cells (PBMCs) and the PBMCs of NSCLC patients as well as NSCLC tissues. Results showed that tumor-infiltrating CD8⁺ T cells had increased PD-1 expression and impaired immune function, including reducing cytokine production capability and impairing capacity to proliferate. Blockade of the PD-1/PD-L1 pathway by the PD-L1-specific antibody partially restored cytokine production and cell proliferation. These data provide direct evidence that the PD-1/PD-L1 pathway is involved in CD8⁺ T-cell dysfunction in NSCLC patients. Moreover, blocking this pathway provides a potential therapy target in lung cancer.     

CD8(+) T cells specific for both persistent and non-persistent viruses display distinct differentiation phenotypes but have similar level of PD-1 expression in healthy Chinese individuals

Anti-viral CD8(+) T cell responses involve an initial expansion and effector phase, followed by contraction phase and formation of CD8(+) memory T cells. During this contraction phase, increased surface expression of the negative regulator PD-1 is associated with functional exhaustion of CD8(+) T cells. Although its role in T cell suppression has been established, the importance of PD-1 in the differentiation of CD8(+) T cells remains unclear. In this study, we examine PD-1 expression in relation to viral specificity of CD8(+) T cells against persistent or non-persistent viruses, and further define differentiation phenotypes of CD8(+) T cells by CD27 and CD28 expression. Surprisingly, the inhibitory receptor PD-1 was expressed by Flu-specific CD8(+) T cells in a level comparable to HCMV-and EBV-specific cells. Moreover, in virus-specific CD8(+) T cells, CD127(+)/CD127(-) and CD62L(+)/CD62L(-) cells expressed similar levels of PD-1 molecules. These results suggest that the PD-1/PD-L1 pathway may play a regulatory role in memory T cell subsets in addition to its association with T-cell exhaustion.     

BTLA信号对T细胞活化的起始和早期阶段的调节作用

目的:观察BTLA分子在T细胞上的表达并探讨其在各个阶段不同时相对T细胞活化的抑制。方法:分离人外周血单个核细胞,经阴性选择磁珠分离纯化获得T淋巴细胞。检测T细胞上BTLA、CTLA-4和PD-1的表达;用CD3抗体刺激T细胞活化,比较BTLA、CTLA-4和PD-1在T细胞活化过程中的动态表达。CD3抗体联合CD28抗体活化T细胞,在不同的活化时间,MTT法检测BTLA单抗8H9对T细胞增殖的影响。GM-CSF和IL-4体外诱导单核细胞分化成未成熟DC,CD40抗体刺激DC成熟,流式检测HVEM在DC上的表达。用DC诱导T细胞活化,加入游离8H9或抗HVEM抗体,阻断HVEM和BTLA结合,MTT法检测T细胞增殖。结果:静止T细胞组成性高表达BTLA,不表达CTLA-4和PD-1分子。T细胞活化后,BTLA分子表达有所降低,然后迅速回升至高水平。CTLA-4、PD-1分子在活化后两天几乎不表达,第三天开始表达并逐渐上升。8H9可以抑制CD3和CD28抗体活化的T细胞增殖。CD3和CD28抗体预先活化T细胞24小时或48小时后,再加入8H9仍然具有抑制效应,但不如在T细胞活化之初加入8H9的抑制效应。单核细胞诱导的不成熟DC上高表达HVEM,当DC成熟后,HVEM表达降低。用游离8H9或HVEM抗体阻断DC表面HVEM与T细胞表面BTLA结合,48小时之内均明显增强了DC诱导的T细胞增殖。结论:BTLA信号可以提高T细胞的活化阈值,在T细胞活化的起始和早期阶段发挥重要的负性调控作用。     

Control of pathogenic effector T-cell activities in situ by PD-L1 expression on respiratory inflammatory dendritic cells during respiratory syncytial virus infection

Respiratory syncytial virus (RSV) infection is a leading cause of severe lower respiratory tract illness in young infants, the elderly and immunocompromised individuals. We demonstrate here that the co-inhibitory molecule programmed cell death 1 (PD-1) is selectively upregulated on T cells within the respiratory tract during both murine and human RSV infection. Importantly, the interaction of PD-1 with its ligand PD-L1 is vital to restrict the pro-inflammatory activities of lung effector T cells in situ, thereby inhibiting the development of excessive pulmonary inflammation and injury during RSV infection. We further identify that PD-L1 expression on lung inflammatory dendritic cells is critical to suppress inflammatory T-cell activities, and an interferon–STAT1–IRF1 axis is responsible for increased PD-L1 expression on lung inflammatory dendritic cells. Our findings suggest a potentially critical role of PD-L1 and PD-1 interactions in the lung for controlling host inflammatory responses and disease progression in clinical RSV infection.     

Human immunodeficiency virus (HIV-1) infection selectively downregulates PD-1 expression in infected cells and protects the cells from early apoptosis in vitro and in vivo

Programmed Death-1 (PD-1), a member of T cell costimulatory molecules is expressed in high levels on antigen specific T cells during chronic viral infection, whereas PD-1 expression in the context of HIV-1 infected CD4+ T cells is not known. Here we report that productively infected CD4+ T cells lose PD-1, whereas bystander cells were unaffected. Additionally, p24+/PD-1 negative cells are less susceptible to apoptosis compared to bystander cells in the same infected milieu. Similar results were observed in vivo, as infected T cells isolated from HIV-1+ individuals have significantly low level of PD-1 and the observed loss of PD-1 in vivo is independent of viral load, CD4 count, and/or antiviral treatment. Together these results indicate that productively infected cells are resistant to early apoptosis by downregulating PD-1, whereas PD-1 enhances the susceptibility of effector T cells to apoptosis suggesting a dual role for PD-1 during HIV-1 infection.