PD-1^+/+ OT-1 and PD-1^-/- OT-1 mice, expressing a transgenic TCR (TCR^tg) specific for ovalbumin (OVA) peptide residue 257-264:H2K^b and as well as PD-1^-/-, Ly5.1, C57BL/6 and IL-17A- enhanced green fluorescent protein (eGFP) reporter mice were bred under specifically pathogen free conditions following institutional guidance, at the central animal facility of University of Magdeburg Medical School (Germany). PD-1^-/- mice on C57BL/6 background were kindly provided by Prof. Luisa Klotz, Muenster, Germany, and were crossbred with OT-1 mice to generate PD-1^-/- OT-1 mice. IL-17A-eGFP reporter mice were kindly provided by Prof. Anja E. Hauser, Berlin, Germany. All animal experiments were performed in accordance to the institutional and state guidelines. The OVA-transfected B16 tumor cell line (B16-OVA) was kindly provided by Prof. Karl Sebastian Lang, Essen, Germany. Cells were maintained in RPMI 1640, supplemented with 10% heat-inactivated fetal-calf serum (FCS), 25mM HEPES, 1mM Sodium pyruvate (Sigma Aldrich), 50µM 2-mercaptoethanol and 1 mg/ml G418 (Carl Roth).

Naïve CD8⁺ T-cells (CD8⁺ CD62L^high) from spleen, inguinal, axillary and mesenteric lymph nodes of PD-1^+/+ and PD-1^-/- OT-1 mice were isolated by magnetic beads separation using AutoMACSpro (Miltenyi Biotec). Comparable levels of CD25 and CD69 expression were routinely determined using flow cytometry (data not shown). For antigen (Ag)-specific activation, OT-1 CD8⁺ T-cells were stimulated with 0.25 μg/ml of endotoxin-free SIINFEKL (OVA257-264) peptide (Invivogen) and CD90-depleted splenocytes from C57BL/6 mice at a 1:6 ratio. To determine strength of TCR signal in T-cell differentiation, OVA peptide variants of different concentrations and different affinities (SIINFEKL, Invivogen; SIITFEKL, Eurogentec) were used. For antibody specific agonistic stimulation, CD8⁺ T-cells isolated from spleen, inguinal, axillary and mesenteric lymph nodes of C57BL/6 mice were used. The isolated cells were stimulated with antibodies immobilized on microspheres. The total amount of protein was kept at 5µg per 10⁷ microspheres which were immobilized with 1µg anti-CD3 (145-2C11) (20% of total), 1 µg of anti-CD28 (37.51) (Miltenyi Biotec) and 60% of either recombinant PD-L1 (rPD-L1) (R&D systems) or IgG1. All cells were cultured in serum free x-vivo 15 medium (Lonza). For Tc17 differentiation the cells were conditioned with 2 ng/ml TGF-β, 10 ng/ml IL-6 (R&D systems), 10 ng/ml IL-23 (Biolegend) with or without 5 μg/ml anti-IFN-γ XMG.1.2 (Biolegend) or anti-IL-2 (S4B6) (BD Biosciences). To determine Tc17 plasticity, primary anti-CD3, anti-CD28 (coupled to microspheres) stimulated Tc17 cells were re-stimulated with anti-CD3, anti-CD28, and/or rPD-L1 antibodies immobilized to microspheres and were conditioned with 5 ng/ml IL-12 and 5 ng/ml IL-2.

Antibodie (Ab)s used in flow cytometric analysis are listed in Supplementary Table S1. Expression of cell surface molecules CD8α, CD25, CD44, CD45.2, CTLA-4, CD69, IL-6Rα, IL-23R, ICOS, PD-1, PD-L1 etc. was detected on the cells by flow cytometry. For this purpose, the cells were harvested and pelleted by brief centrifugation and stained with specific antibodies listed in Table S1 in phosphate-buffered saline (PBS)/0.2% bovine serum albumin (BSA) for 10 min at 4°C and measured by flow cytometry. Cell surface CD107a was measured using flow cytometry by staining in culture medium with phorbol myristate acetate (PMA), ionomycin and Brefeldin A for 4 h at 37°C. For detection of intracellular cytokines IL-17, IFN-γ, IL-2, IL-21, TNF-α etc. the cells were re-stimulated with PMA, ionomycin and Brefeldin A for 4 h at 37°C. The cells were then harvested and pelleted by brief centrifugation, and stained for surface molecules for 10 min at 4°C followed by fixation with 2% paraformaldehyde (Merck) in PBS for 20 min and permeabilized in 0.5% saponin (Sigma-Aldrich) in PBS/BSA. The cells were then stained for cytokines with the specific antibodies listed in Table S1 in 0.5% saponin (in PBS/BSA) for 20 min at 4°C and measured using flow cytometry in PBS/BSA. In some experiments, cells were labelled with vital dyes carboxyfluorescein succinimidyl ester (CFSE) or cellTrace violet (CTV) (Thermo Fisher Scientific) protected from light and washed twice prior to stimulation. The expression of eomesodermin (Eomes), B-cell lymphoma 6 (BCL6), TCF1, phosphorylated signal transducer and activator of transcription 3 (pSTAT3) (10min pretreatment with IL-6+IL-23 before cell harvesting), RORγt, Granzyme B etc. was also measured by flow cytometry. For this purpose, the cells were harvested and briefly pelleted by centrifugation, and fixed in 4% formaldehyde (Carl Roth) in PBS for 10 min at 37°C, followed by permeabilization in ice-cold 90% methanol for 30 min (Carl Roth). The cells were then stained in PBS/BSA for 60 min at room temperature using antibodies listed in Table S1 and measured using flow cytometry. Variations in flow cytometry (FACS) analyses were normalized. All cytometric measurements were performed using a FACS-Canto II (BD Biosciences) and analyzed with FlowJo software (FlowJo LLC). The gating strategy for flow cytometric analyses is demonstrated in the Supplementary Figure S1.

CD45.1 (Ly5.1) mice were subcutaneously (s.c.) injected with 2x10⁵ B16-OVA melanoma cells. Mice that had developed a palpable tumor received an i.v. injection with either PBS or 1 x 10⁶ in vitro generated CD45.2-expressing PD-1^+/+ or PD-1^-/- OT-1 Tc17 cells (CD8⁺ T-cells stimulated for 3 days under Tc17 condition) on day 8 after the tumor cell injection. Tumor growth was then monitored and upon development of large tumors, the mice were humanely sacrificed, and the experiment was terminated. Following that, adoptively transferred CD8⁺ CD45.2⁺ cells were identified and analyzed ex vivo in single cell suspensions of spleen and tumor draining lymph nodes of tumor bearing mice by flow cytometry.

Using Tc17 stimulated total CD8⁺ T-cells (from C57BL/6 mice) at indicated times, RNA was isolated from pelleted cells with NucleoSpin RNA Isolation Kit (Macherey-Nagel). The cDNA was synthesized using revertAid H minus first strand cDNA synthesis kit (Thermo Fisher Scientific) and stored at –20°C. Primer pairs to investigate the gene expression profile of RORc, BCL6, TCF1 (TCF7), T-box transcription factor (T-bet), hypoxia inducible factor 1 alpha (HIF-1α), interferon regulatory factor 4 (IRF4), IL-23R, IL-21 and IL-17a by real-time PCR (quantative (q) PCR) were purchased from TIB MOLBIOL (primer sequences are shown in Supplemental Table S2). Gene expression was analyzed using Maxima SYBR Green qPCR Master Mix (Thermo Scientific) on a CFX96 Real-Time PCR detection system (Bio-Rad). Fold change in the expression of target genes was normalized to the expression of housekeeping gene GAPDH.

In order to measure antigen-specific CD8⁺ T-cell killing efficiency an in vitro cytotoxicity assay was performed. For this assay, T-cell-depleted splenocytes were used, which were labeled separately, with two different concentrations of vital dye CFSE (5 μM CFSE (CFSE^high cells) or 0.25 μM CFSE (CFSE^low cells)) for 5 min and washed twice with x-vivo 15 medium. CFSE^high cells were pulsed with the antigen OVA_257–264-peptide (1 hr, 37°C) and were considered as target cells for OVA_257–264-specific CD8⁺ T-cells. CFSE^low cells, which were not pulsed with OVA-peptide, were used as controls. OVA_257–264-pulsed (CFSE^high) and -unpulsed (CFSE^low) T-cell depleted splenocytes were mixed at a 1:1 ratio and were cultured together with three days predifferentiated OVA_257–264-specific PD-1^+/+ or PD-1^−/− OT-1 Tc17 cells at 1:6 ratio. After 24 hours, CFSE-labeled cells were detected and quantified using flow cytometry and percent of lysis of target cells was calculated by difference between CFSE^low versus CFSE^high cells.

Data were analyzed by Microsoft Excel 2010 (Microsoft Co., USA) and Graph Pad Prism 8 (Graphpad software Inc, USA). Data are presented as mean ± SD. P-values were computed by using unpaired or Welch’s t-test. Multiple comparisons between more than two independent groups were performed using two-way ANOVA with repeated measures (RM) followed by Holm-Sidak’s multiple comparisons test for each individual time point. Statistical significance is indicated as follows: ****P < 0.0001, ***P < 0.001, **P < 0.01, *P < 0.05, n.s: not significant.

PD-1 signaling subdues IFN‐γ‐production and granzyme B secretion of Tc1 cells (37, 38). In order to determine the PD-1-mediated impact during the differentiation of Tc17 cells, we first applied a model for Tc17 differentiation, therefore, we stimulated naive PD-1^−/− and PD-1^+/+ OT-1 CD8⁺ T-cells with OVA_257–264 peptide and congenic APCs in Tc17‐skewing conditions (IL-6, IL-23, TGFβ, see Materials and methods). Enriched PD-1^−/− and PD-1^+/+ OT-1 CD8⁺ T-cells were routinely controlled having a naïve phenotype using expression of activation‐regulated molecules and effector cytokine expression (data not shown). Strikingly, even under Tc17‐skewing conditions, PD-1 had a suppressive effect on effector cytokine production. PD-1^−/− Tc17 cells showed a significantly high frequency of IL‐17 producers than PD-1^+/+ Tc17 cells three days after beginning of the stimulation (Figure 1A, middle panel). In accordance with other studies (5), under Tc17 conditions without IFN‐γ neutralization, IFN‐γ production was visible at low frequencies (Figure 1A, left panel). Even though IFN‐γ is known to negatively affect differentiation of IL-17-producing cells (39, 40), a marginally enhanced frequency of IFN‐γ producers within PD-1^−/− Tc17 cells had no suppressive effect on enhanced Tc17 differentiation (Figure 1A). To determine the kinetics upon activation of CD8⁺ T-cells in a Tc17 milieu to up-regulate PD-1 at the cell surface, primary stimulation of OT-1 CD8⁺ T-cells under Tc17‐skewing conditions was performed. Within 24hrs of the onset of stimulation, PD-1^+/+ Tc17 cells upregulated PD-1 on their surface with highest frequency of expression occurring on day three (Figure 1B) which explains the strong effect of PD-1 on Tc17 differentiation. Since all T-cells of both PD-1^−/− and PD-1^+/+ Tc17 cell types express the activation-induced surface molecules CD44 and CD25, differences in activation are unlikely to be the cause of differential IL-17 production (Figure 1C). In the same line, expression of the cytokines IL-10, TNF-α (data not shown), IL-2 and IL-4 remained similar in both PD-1^+/+ and PD-1^−/− Tc17 cell types (Figure 1D). In addition, even surface expression of co-receptors cytotoxic T lymphocyte antigen 4 (CTLA-4), inducible T-cell co-stimulator (ICOS) and PD-L1, which are otherwise known to regulate differentiation of IL-17-producing cells (41–44), was observed to be similar in both PD-1^+/+ and PD-1^−/− Tc17 cell types (Figure 1E), as was IL-6R. These results indicated that compared with PD-1 competent CD8⁺ T-cells, despite showing similar upregulation of activation-induced molecules and of co- and inhibitory molecules, PD-1^−/− CD8⁺ T-cells specifically induce higher frequencies of IL‐17⁺ cells under Tc17 conditions.

To exclude the strong inhibitory effects of Type-1 cytokines on PD-1 regulated Tc17 differentiation (39), we compared IL-17 expression by PD-1^+/+ and PD-1^−/− Tc17 population with or without neutralizing antibody against IFN‐γ or IL-2, respectively. However, despite neutralization, PD-1^−/− Tc17 cells still displayed enhanced frequencies of IL-17 producers in comparison to that of PD-1^+/+ (Figure 1F). As strength of TCR signals affects T‐cell differentiation (45) and tumor antigens might often be of low affinity due to their autoantigenic nature (46), low affinity stimulation was initiated using SIITFEKL peptides for OT-1 TCRtg T-cells besides high affinity stimuli (SIINFEKL) with a wide range of concentrations as indicated (Figure 1G). Regardless of the affinity used, genetic inactivation of PD-1 in CD8⁺ T-cells was superior to PD-1-competent CD8⁺ T-cells in mediating increased abundance of IL-17 producers (Figure 1G), which may be a double-edged sword for antitumor responses. On the one hand, Tc17 responses that may not be cytotoxic are enhanced; on the other hand, under many conditions, tumor antigen-specific T-cell responses may even be enhanced against low-affinity tumor antigens under PD-1 blockade.

To better understand the role of PD-1 inactivation in Tc17 responses and to exploit this for tumor therapy, PD-1-mediated effects were next analyzed to determine whether the regulation of Tc17 differentiation was either due to increased differentiation or simply to increased proliferation (47). To determine proliferation-control of CD8⁺ T-cells by PD-1 under Tc17 differentiation, PD-1^+/+ and PD-1^−/− OT-1 CD8⁺ T-cells were labelled with vital fluorescent dye CFSE before primary stimulation under Tc17‐skewing conditions, thus, being able to monitor each single cell cycle generation by fluorescence loss. Our results indicate that in absence of PD-1 signaling Tc17 cells displayed overall enhanced proliferation determined by faster mitotic events and enhanced frequencies of original T-cells (G₀ at beginning of the stimulation) entering cell cycle (Figure 2A). To determine whether soluble factors in the microenvironment or the APCs regulate PD-1-mediated suppression of Tc17 differentiation and proliferation, we performed a co-culture experiment where equal amounts of enriched PD-1^+/+ and PD-1^−/− OT-1 CD8⁺ T-cells were mixed and stimulated together with OVA_257–264 loaded APCs in a Tc17 cytokine milieu (Figure 2B). Since TGFβ a strong survival factor was used in Tc17 differentiation, cell death was negligible (48). To distinguish between the different cell populations, PD-1^+/+ or PD-1^−/− OT-1 CD8⁺ T-cells were stained with vital fluorescent dyes CFSE or CTV or vice versa prior to activation. Our data unambiguously show that PD-1^−/− CD8⁺ T-cells generated higher frequencies of Tc17 cells exceeding that of PD-1^+/+ CD8⁺ T-cells even within the same microenvironment. In addition, frequency of IL-17 expressing cells within fast proliferating cells was also enhanced in PD-1^−/− Tc17 cells in comparison to PD-1^+/+ ones (Figure 2B). Thus, PD-1 controls proliferation of Tc17 cells and their differentiation, both effects by a cell intrinsic mechanism. To strengthen our finding, we used an experimental approach that provided similar amounts of CD3- and CD28-triggering, and excluded further extrinsic signals from B7 ligands on APCs. In this approach, wild type mouse CD8⁺ T-cells were stimulated with microspheres coupled with anti-CD3, anti-CD28 plus either rPD-L1 or IgG control. CD8⁺ T-cells that were engaged with rPD-L1 (anti-CD3, anti-CD28 +rPDL1) displayed a significant reduction in the frequency of IL-17-producing cells compared with Tc17 cells engaging CD3 and CD28 only (Figure 2C). Similarly, enhanced frequency of IL-17 producers was also observed in CD8⁺ T-cells from PD-1^-/- mice that were stimulated by crosslinking with anti-CD3, anti-CD28 and rPD-L1 in comparison to PD-1^+/+ cells (Figure 2D). Together, these data demonstrate that the suppressive effect of PD-1 on Tc17 differentiation is unambiguously cell‐intrinsic and independent of differential APC activation or extrinsic, soluble factors in the microenvironment.

To further investigate the PD-1 regulated intrinsic mechanism underlying the distinct degrees of Tc17 programs, we performed a qPCR analysis to determine mRNA accumulation of Tc17 specific molecules. For this purpose, we used RNA extracted from Tc17 CD8⁺ T-cells that were stimulated with anti-CD3, anti-CD28 with IgG control or with rPD-L1 (Figure 2C). As CD8⁺ T-cells can cycle cytokine production on and off (49), all stimulated CD8⁺ T-cells with or without PD-1 signaling under Tc17 conditions were included in the analysis to determine the effect of PD-1 on mRNA expression. Intriguingly, the Tc17 cells stimulated in the absence of PD-1 signaling demonstrated higher levels of mRNA expression of Tc17-supporting factors RORc, IRF4, HIF-1a, IL-21 and IL-23R at day two after stimulation (Figure 3A) but not of T-bet (Figure 3A). To stress physiological relevance of Tc17-specific accumulation of mRNAs, we also detected increased IL-21, IL-23R and RORγt protein expression by flow cytometry in 3 day stimulated Tc17 cells in the absence of rPD-L1 crosslinking (Figures 3B, C). Next, phosphorylated STAT3, which is known to be a central transcription factor in Tc17 differentiation and is able to enhance IL-17 transcription on its own as well as other Tc17 supporting factors (19), was analyzed. Enhanced frequencies of pSTAT3 expressing Tc17 cells were detected in the absence of PD-1-engagement (Figure 3C). Together, Tc17 cells showed an upregulation of Tc17‐ program supporting molecules in the absence of PD-1 engagement, and this might represent a higher degree of Tc17 differentiation in these cells compared with Tc17 cells getting PD-1 engagement.

To determine if PD-1 transmits an inhibitory signal on Tc17 differentiation only during early activation or also have additional functions on established Tc17 cells, a model including pre-stimulated Tc17 cells generated from CD8⁺ T-cells of IL-17A-eGFP reporter mice was applied. In this model CD8⁺ T-cells from IL-17A-eGFP reporter mice were initially stimulated with anti-CD3, anti-CD28 antibodies immobilized on microspheres in a Tc17-like microenvironment (IL-23, IL-6, TGFβ, see Materials and methods). Two days later, pre stimulated CD8⁺ T-cells were triggered in a recall response with anti-CD3, anti-CD28 with or without rPD-L1 immobilized on microspheres. When analyzing IL-17A-eGFP expressing cells 48h after beginning of the recall response, Tc17 cells in the absence of PD-1 engagement displayed a further enhanced increase in the frequency of IL-17A-eGFP expressing cells compared to stimulations providing PD-1 engagement (Figure 3D). These results indicated that PD-1 signaling provides a strong inhibitory signal on Tc17 differentiation, not only during primary stimulation, but also on Tc17 cells that show an already running Tc17 program.

Tc17 cells are known to be plastic in nature and have ability to convert to Tc1 like cells (18). To determine if PD-1 regulates plasticity of Tc17 cells, a model was applied, in which downstream targets of the Tc1 lineage (IFN‐γ) were analyzed in Tc17 cells re-stimulated in Tc1-type cytokine milieu, with or without rPD-L1 immobilized on microspheres. Intriguingly, re-stimulated Tc17 cells in the absence of rPD-L1 crosslinking displayed enhanced plasticity with increased IFN‐γ and IFN‐γ/IL-17 co-producers expression (Figure 4A). Considering the suppressive effect of rPD-L1 crosslinking on plasticity of re-stimulated Tc17 cells to Tc1 like cells, we next investigated the cytotoxic potential of PD-1^+/+ and PD-1^−/− OT-1 Tc17 cells by performing an in vitro cytotoxicity assay using OVA_257–264 labeled T-cell‐depleted splenocytes (target cells). In support of enhanced Tc17 plasticity in the absence of rPD-L1-crosslinking an increase in target cell‐specific lysis by PD-1^−/− Tc17 cells was observed in comparison to PD-1^+/+ Tc17 cells (Figure 4B). Together these results indicated that PD-1 signaling suppresses Tc17 plasticity and ability to convert toward a Tc1‐like phenotype upon re-stimulation thereby reducing their cytotoxic potential.

To explore the functional mechanism of Tc17 cells with varying lineage plasticity with and without PD-1 signaling, we determined the ability of PD-1^+/+ and PD-1^-/- type 17–skewed cells to promote antitumor immunity (Figure 4C) in vivo. For this we have adoptively transferred equal number of all cultured PD-1^+/+ or PD-1^-/- OT-1 Tc17 cells into mice with pre-established melanoma (by B16 OVA-expressing melanoma cells) in a mouse model. Recipient tumor-bearing mice were used on a Ly5.1 background to differentiate adoptively transferred congenic OT-1 CD45.2 CD8⁺ T-cells. Progression of the established tumor was measured for up to 13 d following adoptive T-cell transfer (Figure 4D). In PBS-treated tumor-bearing mice, tumor outgrowth was progressing uncontrolled dramatically. In tumor-bearing mice adoptively transferred with Tc17 cells, tumor growth measurements clearly showed that despite enhanced IL-17 expression before adoptive transfer, PD-1^−/− Tc17 cells controlled tumor progression, in comparison to that of PD-1^+/+ Tc17 cells (Figures 4E, F). To address PD-1 regulated plasticity in vivo, adoptively transferred Tc17 cells (CD45.2⁺), were identified in the single cell suspensions of spleen and tumor draining lymph nodes (dLN) from tumor bearing mice at the end of the experiment and analyzed ex vivo. Indeed, in contrast to primary stimulation of Tc17 cells before adoptive transfer, they had changed their cytokine profile: Both PD-1^+/+ or PD-1^−/− Tc17 cells lost their IL‐17⁺ phenotype to a large extent and displayed enhanced expression of Tc1-like characteristics. Intriguingly, in comparison to PD-1^+/+, PD-1^−/− Tc17 cells displayed a significant increase in polyfunctional IFN‐γ/TNF‐α co- producing cells ex vivo (Figure 4G). These kind of double producers are well known for anti-tumor immunity (50) and correlate well with reduced melanoma progression in mice adoptively transferred with PD-1^−/− Tc17 cells. Together, these results indicate that absence of PD-1 signals profoundly augments the antitumor activity of Tc17 cells.

Considering the above reported strong anti-tumor immunity by PD-1^−/− Tc17 cells, we hypothesized – that the lack of PD-1 signaling mediates by default conversion of IL-17-producing cells to Tc1-like cells when stimulated in Tc1-type cytokine environment. IL-17A-eGFP expressing CD8⁺ T-cells were used to determine transcriptional plasticity in well-differentiated IL-17 producers (Figure 5A). Flow cytometric analysis showed that upon recall response in Tc1-type cytokine milieu, eGFP+ CD8⁺ T-cells displayed enhanced Tc1 lineage plasticity. However, in the absence of rPD-L1-crosslinking, a further increase in the frequency of IFN‐γ and IL-17/IFN‐γ‐coproducing cells was detected (Figure 5B). Additionally, Tc1 supporting transcription factor Eomes and cytotoxic molecule granzyme B expression were enhanced in the absence of rPD-L1-crosslinking (Figure 5C). In addition, the degranulation marker CD107a expression was also enhanced in the absence of rPD-L1-crosslinking (Figure 5D), further strengthening that PD-1 signaling suppresses Tc17 plasticity towards Tc1 like cells thereby reducing their cytotoxic potential.

Recent research identified TCF1⁺ tumor infiltrating lymphocytes with stem-cell like properties are associated with improved antitumor immunity and response to immunotherapy (51). IL-17-producing T-cells display stem-cell like properties including enhanced persistence, multipotency and self-renewal along with high expression of TCF1 (52). To determine the impact of PD-1 on TCF1 expression in Tc17 cells, we performed a qPCR analysis to determine mRNA accumulation. For this purpose, we used RNA extracted from the whole setting of either CD8⁺ T-cells that were stimulated with anti-CD3, anti-CD28 with rPD-L1 or counterparts stimulated with anti-CD3, anti-CD28 and IgG1 control in the presence of IL-6, IL-23 and TGFβ. (Figure 5E). Intriguingly, in line with enhanced Tc17 differentiation, the control Tc17 cells in the absence of PD-1 signaling demonstrated significantly higher levels of mRNA expression of TCF1 and downstream molecules - surrogate markers for stemness - LEF1 and BCL6 (Figure 5E, data not shown). In support of relevance of mRNA expression, we also detected increased frequencies of TCF1 and BCL6 protein expressing CD8⁺ T-cells stimulated under Tc17 condition in the absence of rPD-L1 crosslinking (Figure 5F). Using iMFI for analysis gave similar results (data not shown). CD27⁺ IL-17-producing T-cells have been shown to exhibit higher expression levels of TCF1, and indeed IL-17-producing T-cells encompassing CD27⁺TCF1^hi subset have been inferred with stemness-associated features (53). In line with enhanced expression of TCF1, CD27 expressing CD8⁺ T-cells stimulated under Tc17 condition were enhanced in the absence of PD-1 engagement (Figure 5F). Together these results further indicated that PD-1 signaling impinges on stemness associated features of Tc17 cells and therby reduce lineage plasticity and cytotoxic potential that supports antitumor immunity.