The inhibitory immune receptor programmed cell death-1 (PD-1) is intricately regulated. In T cells, PD-1 is expressed in response to most immune challenges, but it is rapidly downregulated in acute settings, allowing for normal immune responses. On chronically stimulated Ag-specific T cells, PD-1 expression remains high, leading to an impaired response to stimuli. Ab blockade of PD-1 interactions during chronic Ag settings partially restores immune function and is now used clinically to treat a variety of devastating cancers. Understanding the regulation of PD-1 expression may be useful for developing novel immune-based therapies. In this review, the molecular mechanisms that drive dynamic PD-1 expression during acute and chronic antigenic stimuli are discussed. An array of cis-DNA elements, transcription factors, and epigenetic components, including DNA methylation and histone modifications, control PD-1 expression. The interplay between these regulators fine-tunes PD-1 expression in different inflammatory environments and across numerous cell types to modulate immune responses.

The inhibitory receptor programmed cell death-1 (PD-1) is a mediator of central and peripheral immune tolerance and immune exhaustion (16). The immune inhibitory function of PD-1 was demonstrated in PD-1 and PD ligand-1 knockout mice, which presented hyperactive immune phenotypes (4, 7, 8), and mutations in PD-1 have been associated with disease progression in multiple human autoimmune disorders (911). High levels of PD-1 were linked to CD8 T cell exhaustion during chronic viral infections (5, 6, 1218). Exhausted CD8 T cells are unable to secrete normal amounts of cytokines, proliferate, or perform immune functions such as initiating cellular cytotoxicity (12, 1922). Although the PD-1 pathway represents a major contributing factor to cellular exhaustion, other inhibitory receptors also contribute (23, 24). PD-1 expression was also found on poorly functional tumor-infiltrating cells during multiple types of cancers (2529). Thus, the expression of PD-1 is critical to the development and maintenance of a healthy and robust immune response.

PD-1 expression is tightly and dynamically regulated. On resting naive T cells, as well as in certain populations of developing thymocytes, PD-1 is expressed at low basal levels (30, 31). This low level has been linked to immune tolerance (3, 30, 31). Following an initial immune stimulus, PD-1 is transiently expressed on multiple immune cells types, including CD4 and CD8 T cells, B cells, macrophages, and dendritic cells (30, 3238). In the case of acute Ag exposure, PD-1 is downregulated in a time course that is concurrent with, or prior to, Ag clearance. During chronic immune stimulation, PD-1 remains highly expressed, which can lead to functional exhaustion (5, 6, 1218). Functional exhaustion can be partially reversed by blocking PD-1/PD ligand-1 interactions with Abs. These Ab-based immune checkpoint blockade therapies (nivolumab [under the trade name Opdivo] or pembrolizumab [Keytruda]) have been shown to be highly efficacious in reinvigorating the antitumor immune response in patients with advanced cancers, including melanomas, non–small cell lung cancer, colorectal cancer, and renal cell cancer (3945). Additionally, high PD-1 expression is necessary for regulatory T cell development, and follicular helper T (TFH) cells also constitutively express high PD-1 (1, 2, 46, 47), although PD-1 regulation has not been studied in this population.

In this review, the mechanisms that drive the initial PD-1 induction and its continued expression during chronic Ag exposure will be discussed. In addition to providing an understanding of which active immune environments may induce and be affected by PD-1’s function, knowledge of the molecular mechanisms regulating PD-1 may help tailor future immunotherapies for fighting cancer and chronic HIV and hepatitis C virus infections, or for prevention of allergic responses, transplant rejection, and autoimmunity (48), as well as boosting long-lived memory responses to vaccines (49).

Models for studying PD-1 regulation.

The expression and regulation of PD-1 have been studied in a variety of model systems. In culture, PD-1 has been studied using both the EL4 mouse T cell line and isolated mouse and human primary immune cells. A variety of stimuli can activate these cells, including the small molecule drugs PMA and ionomycin (Io), Con A, or anti-CD3/CD28 Abs, which acutely activate immune cells and transiently induce PD-1 expression (30, 5053). In vivo, PD-1 expression is probably most studied and best understood during infection with lymphocytic choriomeningitis virus (LCMV) (12, 19, 20, 5457). The LCMV infection model includes two strains that differ by 2 aa (22). Infection with the Armstrong strain results in an acute infection, correlating with transient upregulation and subsequent loss of PD-1 that coincides with viral clearance. In contrast, establishing a chronic infection with the clone-13 strain results in durable PD-1 expression and concurrent immune cell dysfunction and exhaustion. Side-by-side analysis of these two infections has provided direct comparisons between the transient/acute versus the chronic/prolonged modalities of PD-1 expression in CD8 T cells. Additionally, prolonged PD-1 expression during an antitumor response in mice has been studied using the B16.f10 melanoma cell line or Lewis lung carcinoma cell line, both of which are terminally tumorigenic but nonetheless immunogenic in C57BL/6 mice (25, 58, 59). Although these different models have largely been used to study separate aspects of PD-1 regulation, in vivo infection models have recapitulated findings from in vitro–activated cell lines. Although most of the studies to date have been performed on murine cells and cell lines (36, 5254, 5964), those performed with human cells have yielded similar findings (65, 66). As described below, this is likely due to conservation of important regulatory elements at the PD-1 locus.

Identifying cis-regulatory elements of the Pdcd1 gene.

PD-1 is encoded in the Pdcd1 gene. At least eight cis-elements regulate Pdcd1 expression. Strong mammalian DNA sequence conservation coupled with DNAse I hypersensitivity assays in EL4 cells and CD8 T cells initially identified two conserved regions (CR-B and CR-C) associated with Pdcd1 activation (52). These elements, which are located 100 bp and 1.1 kb upstream of the transcription start site (TSS), contain multiple transcription factor binding sites (Fig. 1A). An AP-1 binding site is encoded within CR-B (59). CR-C contains an IFN-stimulated response element (ISRE) (60), an NFATc1 binding site (52), a FoxO1 binding site (61), and an NF-κB binding site (36). Notably, reporter constructs that use the PD-1 promoter but do not contain the CR-C region failed to induce PD-1 expression in response to a variety of stimuli (36, 52). Collectively, this suggests that CR-C is of critical importance to the observed expression patterns of the gene.

FIGURE 1.

Schematic models of Pdcd1 gene regulation. The Pdcd1 gene is represented in the various activation states (ON, OFF) with (A) major cis-regulatory elements identified relative to the TSS. The positions of two transcriptional insulators (In) that bind CTCF and encompass the locus are represented. Three in vivo transcriptional states representing the activity of the locus in the (A) naive state and then following (B and C) acute (early and late) and chronic (D) LCMV infections are shown. (E) Ex vivo stimulation of CD8 T cells with cytokines IFN-α, IL-6, or IL-12. (F) Ex vivo and/or in vitro stimulation of macrophages with TLR ligands or IFN-α. In each, transcriptional activators/repressors, activating or repressive nucleosomes, and DNA methylation states are depicted as indicated.

FIGURE 1.

Schematic models of Pdcd1 gene regulation. The Pdcd1 gene is represented in the various activation states (ON, OFF) with (A) major cis-regulatory elements identified relative to the TSS. The positions of two transcriptional insulators (In) that bind CTCF and encompass the locus are represented. Three in vivo transcriptional states representing the activity of the locus in the (A) naive state and then following (B and C) acute (early and late) and chronic (D) LCMV infections are shown. (E) Ex vivo stimulation of CD8 T cells with cytokines IFN-α, IL-6, or IL-12. (F) Ex vivo and/or in vitro stimulation of macrophages with TLR ligands or IFN-α. In each, transcriptional activators/repressors, activating or repressive nucleosomes, and DNA methylation states are depicted as indicated.

Close modal

Two conserved DNase I hypersensitive elements located −3.7 and +17.1 kb from the TSS enhanced transcriptional activity following TCR and IL-6 or IL-12 cytokine stimulation (Fig. 1A, 1E). These elements encode STAT binding sites and were found to interact directly with the Pdcd1 promoter following ex vivo cell activation and cytokine treatment (62). Additional elements at −26.7 and +17.5 kb bind the mammalian transcriptional insulator CCCTC-binding factor (CTCF) and form constitutively interacting chromatin loops (Fig. 1) (62). Because transcriptional insulators prevent the actions of distant enhancers from controlling a downstream gene (67), these CTCF sites likely define the extreme ends of the Pdcd1 regulatory locus.

Transcription factors inducing acute PD-1 expression.

PD-1 expression on CD8 T cells correlates with the strength of TCR signaling (12, 54, 68). TCR stimulation initiates a signaling cascade through the calcineurin pathway, resulting in activation and translocation of the transcription factor NFATc1 (also known as NFAT2) (69). NFATc1 binds strongly to the CR-C region (Fig. 1B) and is likely one of the first steps in activating Pdcd1 gene expression (52, 53). Blocking this pathway using the calcineurin inhibitor cyclosporine A or the NFATc1-specific inhibitor peptide VIVIT abrogated PD-1 expression (52). Thus, NFATc1 is necessary for initial activation-induced expression of PD-1 in CD4 and CD8 T cells. PMA/Io-mediated NFATc1 binding and transcriptional activities were also found at the −3.7 and +17.1 sites, suggesting that multiple NFATc1 elements are required for driving Pdcd1 expression or that there is genetic redundancy in the mechanism by which Pdcd1 is induced (62). Intriguingly, NFATc1 expression is repressed at extended time points after induction, and this is likely part of the mechanism for restricting PD-1 expression during acute Ag exposure (53). During chronic LCMV infection, NFATc1 translocation is also impaired (70). These observations suggest that although NFATc1 may be necessary to initially induce PD-1, another mechanism is required to maintain or augment expression during chronic Ag exposure.

AP-1 frequently couples with NFAT activity during T cell activation (71). TCR-mediated stimulation of the protein kinase C/Raf pathway activates the MAPK cascade, ultimately leading to AP-1 activity (71). Notably, PMA/Io stimulation, originally used to identify NFATc1 binding and activity at CR-C, concurrently triggers this same pathway. In a Lewis lung carcinoma tumor model, overexpression of the AP-1 subunit c-Fos was found to inhibit antitumor T cell responses by direct binding to the AP-1 site in the CR-B region of Pdcd1 (Fig. 1B). This resulted in induction of PD-1 expression in CD4 and CD8 T cells (59). A mouse containing a mutation of this AP-1 site had less PD-1 expression on tumor-infiltrating T cells and demonstrated increased antitumor immunity (59). Unlike other NFAT-coupled AP-1 sites, which are typically adjacent (71), the Pdcd1 AP-1 site was located >1 kb away from the NFATc1 binding site (52).

Additionally, the Notch pathway, which regulates T cell effector functions (72, 73), also regulates PD-1 expression in acute Ag settings (64). In an in vitro experiment, blockade of the Notch pathway showed a moderate, dose-dependent reduction of PD-1 expression, without affecting the overall activation of the T cell (64). Corroborating this, the Notch intracellular domain and the recombination signal binding protein for Igκ J (RBPJκ), two critical components of the Notch signaling pathway, were bound to the Pdcd1 locus within 6 h of in vitro peptide–mediated TCR stimulation (Fig. 1B). Similarly, in T cells of patients experiencing sepsis-induced immunosuppression, Notch pathway activation also correlated with PD-1 expression (74). In a model using IL-10 and LPS to cause sepsis-induced immunosuppression in CD4 T cells and macrophages, inhibition of Notch signaling again reduced PD-1 expression. Thus, the Notch pathway appears to directly augment PD-1 expression during T cell activation settings.

Inhibitors of PD-1 expression.

T-box expressed in T cells (T-bet) was the first identified inhibitor of PD-1 (63). T-bet expression influences CD8 T cell differentiation, with high T-bet levels associated with short-lived effector cells and intermediate levels with memory precursor cells (75). Following TCR stimulation or in PD-1lo memory CD8 T cells from acutely infected mice, T-bet binds 500 bp upstream of the Pdcd1 TSS (Fig. 1C) (63). In a chronic LCMV infection, T-bet overexpression (normally only expressed at intermediate levels) was inversely correlated with PD-1 expression, resulting in a PD-1int phenotype (63). Furthermore, PD-1 expression was further increased in this infection model in a T-bet knockout mouse. In acute LCMV infections, CD8 T cells from T-bet knockout mice only showed moderately higher expression of PD-1 compared with controls, rather than the PD-1hi phenotype seen during chronic or peak acute infection. Collectively, these data suggest that whereas T-bet does repress PD-1, T-bet alone is neither sufficient nor necessary to completely downregulate PD-1, and other factors may synergize with T-bet following an acute infection to silence gene expression.

In acute LCMV viral infection, expression of the inhibitory factor B lymphocyte–induced maturation protein-1 (Blimp-1, encoded by Prdm1) is induced by day 8 after T cell activation, and similar to T-bet is necessary for terminal T cell differentiation and functional memory formation (76, 77). At day 8, Blimp-1 was bound between CR-C and CR-B (Fig. 1C) (53). In this system, Blimp-1 binding resulted in the direct loss of NFATc1 occupancy at CR-C. Blimp-1 also directly downregulated PD-1 expression in tissue culture cells. Blimp-1–mediated repression of PD-1 occurred irrespective of whether NFATc1 was overexpressed from an exogenous vector. Moreover, acute LCMV infection of a Blimp-1 knockout mouse resulted in prolonged PD-1 expression on virus-specific CD8 T cells (53). These experiments indicate that Blimp-1 is necessary for normal loss of PD-1 expression following an acute infection. Furthermore, TFH cells, which express high levels of PD-1, are also noted for their lack of Blimp-1 expression (46). In germinal centers, retrovirus-mediated exogenous expression of Blimp-1 resulted in a failure to generate PD-1hi TFH cell subsets, again showing an inverse correlation between Blimp-1 and PD-1 (46). Notably, Bcl6, an antagonist of Blimp-1 (78), was necessary for generation of a PD-1hi TFH population (46). However, a direct role for Bcl6 inducing PD-1 expression has not been reported. Surprisingly, Blimp-1 is expressed in exhausted CD8 T cells, where PD-1 levels are at their highest (79). In exhausted CD8 T cells of Blimp-1 knockout mice, the high levels of PD-1 expression were slightly reduced (79), suggesting a different role for Blimp-1 in regulating Pdcd1 in these cells. The molecular mechanism for how Blimp-1 could function differentially as a repressor or activator of gene expression is unclear. As a repressor, Blimp-1 is known to recruit additional transcriptional repressors that result in silencing the local chromatin environment (8082). In the case where expression is increased in the presence of high levels of Blimp-1 in the cell, several possibilities exist. First, Blimp-1 may not bind to its site in the Pdcd1 gene and PD-1 expression continues to be active. Second, Blimp-1 could bind but fail to recruit the repressive cofactors necessary to silence the gene. This could be due to novel posttranslational modifications that control its function. Third, Blimp-1 could function indirectly by repressing a corepressor necessary for silencing. Lastly, under the exhausted conditions, an additional activating factor may override the Blimp-1 repressor pathway.

Chronic regulators of PD-1 and factors in other cell types.

The transcription factor FoxO1 promotes transcription of a number of genes necessary for homeostatic maintenance of naive and memory T cells (83). FoxO1 antagonizes the transcription factor T-bet (84), and as such is a candidate for preventing PD-1 downregulation. In chronic LCMV infections, FoxO1 protein is highly expressed and retained within the nucleus (61). Despite FoxO1 promoting homeostatic division and relative viral control by supporting high T cell numbers, it was also necessary to generate PD-1hi T cells in the chronic infection model, as FoxO1 knockout mice have lower PD-1 expression. FoxO1 acted directly on the Pdcd1 locus by binding to a region in CR-C, and therein induced promoter activity and PD-1 expression (Fig. 1D). Interestingly, the putative FoxO1 binding site overlaps with one of the NFATc1 binding sites in CR-C. This may indicate a molecular system in which FoxO1 replaces NFATc1 and can override Blimp-1’s repressive activity.

To partially replicate in vitro some of the immune microenvironments seen in chronic infection, CD8 T cells were activated with anti-CD3/CD28 beads and treated with IL-6 or IL-12, resulting in the activation of STAT3 and STAT4 (62). Under these conditions, STAT3 and STAT4 bound the −3.7 and +17.1 regulatory regions and were able to increase PD-1 expression in conjunction with TCR signaling (Fig. 1E). The STAT proteins alone were not sufficient, as cytokine stimulation of naive T cells without concurrent TCR stimulation did not induce PD-1 expression (62). However, this could be due in part to the low levels of IL-12 receptor on naive CD8 T cells (85). Intriguingly, preconditioning CD8 T cells with IL-12 and/or IFN-α (which induces STAT1/STAT2/IFN regulatory factor [IRF]9 activity) resulted in higher expression of the repressor T-bet and subsequently led to lower expression of PD-1 upon later Ag encounter (86). Thus, cytokine exposure prior to Ag-mediated TCR signaling may have profound effects on the level of PD-1 expressed.

Owing to the immune reinvigoration potential related to PD-1 expression on exhausted CD8 T cells, most of the above transcription factors have been studied in CD8 T cells alone or in a combination of CD4 and CD8 T cells. When queried in B cells, a similar NFATc1-based regulatory system was implicated for PMA/Io- or BCR-mediated induction of PD-1 (36). However, in macrophages the calcineurin and MAPK pathways stimulated by PMA/Io did not induce PD-1, and blockade of these pathways had no effect on PD-1 expression induced in macrophages by TLR ligands (36, 87). It was instead found that NF-κB is necessary to induce PD-1 in response to TLR stimulation (36). Inhibition of NF-κB binding to DNA using a small molecule inhibitor resulted in a total loss of PD-1 expression. Furthermore, the NF-κB p65 subunit was bound to a site in CR-C in response to TLR stimulation of macrophages (Fig. 1F), and a mutation of this site abrogated transcriptional activity of the region in reporter assays (36). The ability of NF-κB to induce PD-1 directly in cell types other than macrophages has not been studied.

PD-1 regulation in macrophages in response to cytokine stimuli has also been examined. IFN-α stimulation of macrophages resulted in IFN-stimulated gene factor (ISGF)3 complex binding to the ISRE in the CR-C region (Fig. 1F) and induction of PD-1 expression (60). ISGF3 is a complex composed of STAT1, STAT2, and IRF9 (88), again implicating the STAT family of transcription factors as key members of PD-1 regulation in addition to IRF9. Type I IFN activity was subsequently examined for its effects on cultured T cells (66). In T cells, IFN-α alone had no effect on PD-1 expression. However, although it could not autonomously induce expression, IFN-α administered concurrently with TCR stimulation induced IRF9 binding to the ISRE (Fig. 1E) and enhanced TCR-mediated PD-1 expression (66). Much like the cytokine-induced STAT3 and STAT4 activity, IFN-stimulated STAT1, STAT2, and IRF9 seem to have supplementary roles in increasing PD-1 expression that require additional TCR-mediated signals.

Thus, collectively, to date 10 transcription factor complexes are known to modulate PD-1, including 8 activators (NFATc1, c-fos/AP-1, Notch, FoxO1, STAT3, STAT4, ISGF3, and NF-κB) and 2 inhibitory molecules (Blimp-1 and T-bet), which all interact with the locus in response to different stimuli. The complexity and variability of the Pdcd1 regulome, the entire set of transcription factors and genetic elements that affect this one gene, may contribute not only to a differential expression in response to different inflammatory stimuli, including the difference between acute and chronic infection, but may also account for differential patterns seen across multiple cell types in response to the same infection.

Epigenetic regulation of Pdcd1.

Epigenetics refers to a stable, heritable mechanism by which cells may maintain transcriptional profiles and corresponding states of differentiation across cell generations without modifying the underlying genetic code. Such epigenetic mechanisms include DNA methylation, histone protein modifications, and overall chromatin looping and organization, which individually or together alter the chromatin state/accessibility and control transcriptional activity of a gene (89). These processes are widely used by the immune system to control immune cell differentiation, fate, and gene expression (9093).

Exhausted T cells, even upon total removal of Ag, stably maintain their exhausted phenotype during subsequent divisions (94). This has been demonstrated in exhausted CD8 T cells adoptively transferred from chronic LCMV-infected mice into naive mice, in which the cells proliferated yet remained functionally exhausted and PD-1hi upon rechallenge with an acute virus (LCMV Armstrong), which otherwise would not induce exhaustion (94). Similarly, PD-1 levels remain high on the CD8 T cells of human patients infected with HIV but undergoing antiretroviral therapy, which drastically decreases viral loads (65). These data indicate that a heritable epigenetic mechanism establishes and maintains both the exhausted phenotype and expression of PD-1 itself after the exhaustion-inducing stimulus has been removed.

DNA methylation of Pdcd1.

The DNA modification 5-methylcytosine (5mC) at CpG sites in transcriptional enhancers or at gene promoters is associated with silencing gene expression (95). In mice, two CpG-rich regions upstream of the Pdcd1 TSS (CR-C itself and a regulatory region starting ∼300 bp upstream of the promoter and labeled as CR-B) were dynamically methylated in CD8 T cells responding to an acute LCMV infection (54). Both of these regions were fully methylated in resting, naive CD8 T cells that do not express PD-1. At day 4 following acute LCMV infection, both regions showed a profound loss of methylation that was restored at day 8. DNA methylation was inversely correlated to both PD-1 expression and viral load. In contrast, during chronic LCMV infection, the regions remained unmethylated at day 8 and later time points (Fig. 1B). Similarly, in a model of Ag tolerance induced by peptide immunotherapy, it was found that demethylation of the Pdcd1 locus and corresponding induction of PD-1 was necessary for tolerizing T cells (48). This inverse correlation between DNA methylation and PD-1 expression strongly indicates a role for DNA methylation in regulating Pdcd1 gene expression.

In patients with chronic HIV, the PD-1 regulatory region was similarly demethylated in PD-1hi virus-specific cells, and unmethylated in naive, nonexhausted PD-1lo cells from the same donors (65). Intriguingly, patients who were on antiretroviral therapy at the time showed no remethylation of DNA at the PD-1 locus, despite significantly lower viral loads. This indicates that there is no mechanism to reverse DNA methylation changes at the Pdcd1 locus in exhausted cells, regardless of changes in viral burden. Notably, one instance has been found in which DNA methylation is not linked to PD-1 expression. In macrophages, transient induction of PD-1 through TLR signaling and NF-κB was not accompanied by a corresponding demethylation of the Pdcd1 locus (36). However, TLR-mediated PD-1 induction in macrophages was much shorter in duration and not as robust as that in CD8 T cells during acute infection. Collectively, these data suggest that demethylation may only be necessary for high or prolonged PD-1 expression.

There are two general models for how a gene loses DNA methylation. In the passive model, following DNA replication a locus fails to remethylate hemimethylated CpG-containing DNA, resulting in dilution of the methylation mark during successive divisions. In an active model, methylated CpG loci are targeted by members of the ten-eleven translocase family, resulting in an initial catalytic oxidation of the 5mC to 5-hydroxymethylcytosine (5hmC) as an intermediate (96, 97). Continued oxidation and base removal result in replacement of the cytosine with a nonmethylated cytosine. Note that the data discussed above (54, 65) used bisulfite sequencing, which does not distinguish between 5mC and the 5hmC modification. Intriguingly, 5hmC was enriched at the Pdcd1 locus (albeit at low levels compared with 5mC) in naive mouse CD4 T cells (48). Both 5hmC and 5mC were lost upon CD4 T cell activation and induction of PD-1 (48). This result would argue that demethylation of the Pdcd1 locus upon gene induction occurs through an active and targeted mechanism. The occurrence of this transitional mark in naive cells may indicate that the Pdcd1 locus rests in a state capable of rapid demethylation.

The histone landscape of Pdcd1.

Modifications to histone proteins within a chromatin region can change chromatin accessibility and affect promoter and enhancer activity, leading to changes in gene transcription. On a genomic level, enhancers are marked by histones enriched for histone H3 (H3) lysine 4 monomethylation (H3K4me1), and when considered “active” also contain H3 lysine 27 acetylation (H3K27ac) (98). When PD-1 expression was induced on CD8 T cells in vitro, H3K9ac (another activation-specific mark) and H3K27ac were present at CR-C and the promoter (48, 53). The other cis-elements −3.7 and +17.1 did not contain active histone marks following TCR stimulation alone even though Pdcd1 expression was induced (62), suggesting that these elements do not play a role unless cytokine stimulation occurs. In agreement, treatment of CD8 T cells ex vivo with the cytokines IL-6 or IL-12 alone resulted in the enrichment of the histone mark H3K4me1 at both the −3.7 and +17.1 regulatory sites (62). Cytokine stimulation did not result in the appearance of H3K27ac at these sites or in induction of Pdcd1 expression, suggesting that cytokine treatment was in effect “licensing” the elements for activity. When cytokine treatment and TCR stimulation were combined, both H3K4me1 and H3K27ac were enriched at these elements, indicating the formation of an active chromatin state at these elements that contributed to increased PD-1 expression (62). Thus, distinct cytokine profiles elicited during infections are capable of manipulating the epigenetic program governing Pdcd1 gene expression.

Following resolution of an acute CD8 T cell activation, the CR-C region of Pdcd1 was enriched for the repressive histone modifications H3 lysine 4 trimethylation (H3K9me3), H3K27me3, and H4K20me3, as summarized in Fig. 1 (53). However, H3K9me3 and H3K27me3 were absent from the locus at the same time point in chronic infection or in tolerized cells expressing high levels of PD-1, showing a correlation between appearance of these inhibitory histone profiles and loss of PD-1 expression (48, 54). Exogenous expression of Blimp-1 in acutely activated T cells was capable of inducing these repressive modifications and driving down PD-1 expression (53). Although Blimp-1 itself is not known to directly modify histone proteins, it does recruit other repressive histone-modifying enzymes, including the histone deacetylases HDAC1 and HDAC2, the histone methyltransferase G9a, and the H3K4me2/me1 lysine–specific demethylase LSD1 (81, 82, 99). Importantly, note that both repressive and activating marks are absent from the locus in naive cells (53, 54), indicating that active repression is employed only after PD-1 induction, and that the low PD-1 expression in naive cells may be maintained exclusively by the absence of transcriptional activators and the presence of DNA methylation.

The reviewed data suggest that cell-specific and cytokine microenvironmental regulation of PD-1 allows for multiple expression paradigms. Exhaustion and high levels of PD-1 are correlated with Ag exposure and prolonged TCR stimulation. How prolonged TCR stimulation and cytokine exposure drive the distinct epigenetic states of the locus remains to be discovered. Because these induction pathways are common to other immune system genes, careful consideration of the transcription factors and epigenetic parameters governing Pdcd1 expression is necessary to be able to control the regulation of PD-1 expression in therapeutic settings. Nonetheless, such pathways offer the prospect of novel targets to intervene with or augment PD-1–controlled immune responses.

We thank the members of the Boss laboratory for critical input and feedback for this work.

This work was supported by National Institutes of Health Grant R01 AI113021 (to J.M.B.). A.P.R.B. was supported in part by National Institutes of Health Grant T32AI007610-12.

The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

Abbreviations used in this article:

Blimp-1

B lymphocyte–induced maturation protein-1

CR-B/C

conserved region B/C

CTCF

CCCTC binding factor

H3

histone H3

5hmC

5-hydroxymethylcytosine

Io

ionomycin

IRF

IFN regulatory factor

ISGF

IFN-stimulated gene factor

ISRE

IFN-stimulated response element

K27ac

lysine 27 acetylation

K4me1

lysine 4 monomethylation

K4me3

lysine 4 trimethylation

LCMV

lymphocytic choriomeningitis virus

5mC

5-methylcytosine

PD-1

programmed cell death-1

T-bet

T-box expressed in T cells

TFH

follicular helper T

TSS

transcription start site.

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The authors have no financial conflicts of interest.