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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Cancer Res. Author manuscript; available in PMC 2017 May 1.
Published in final edited form as:
PMCID: PMC4873368
NIHMSID: NIHMS778396

IL-15 AGONISTS OVERCOME THE IMMUNOSUPPRESSIVE EFFECTS OF MEK INHIBITORS

Abstract

Many signal transduction inhibitors being developed for cancer therapy target pathways that are also important for the proper function of anti-tumor lymphocytes, possibly weakening their therapeutic effects. Here we show that most inhibitors targeting multiple signaling pathways have especially strong negative effects on T cell activation at their active doses on cancer cells. In particular, we found that recently approved MEK inhibitors displayed potent suppressive effects on T cells in vitro. However, these effects could be attenuated by certain cytokines that can be administered to cancer patients. Among them, clinically available IL-15 superagonists, which can activate PI3K selectively in T lymphocytes, synergized with MEK inhibitors in vivo to elicit potent and durable antitumor responses, including by a vaccine-like effect that generated resistance to tumor re-challenge. Our work identifies a clinically actionable approach to overcome the T cell-suppressive effects of MEK inhibitors, and illustrates how to reconcile the deficiencies of signal transduction inhibitors which impede desired immunological effects in vivo.

Keywords: Targeted therapy, MEK inhibitor, Trametinib, Cancer Immunotherapy, Tumor Immunology, IL-15

INTRODUCTION

Small molecule inhibitors targeting oncogenic signaling pathways have emerged as a promising new class of drugs in cancer therapy. While these molecules undergo rigorous testing to demonstrate their efficacy in tumor models, their effects on the interplay between leukocytes and tumors have been largely unstudied due to the use of preclinical xenograft models that lack a competent immune system. T cells, crucial for controlling the growth of immunogenic tumors (1), rely upon many of the same signaling pathways targeted by pharmaceutical inhibitors for activation of cytotoxicity against tumor cells. For instance, engagement of the T cell receptor (TCR) and co-stimulatory receptors activates the RAS-MAPK and PI3K-AKT signaling cascades, which are necessary for proliferation and effector function in T cells (2).

The FDA approved small molecule inhibitor of MEK, trametinib, presents an example of seemingly paradoxical interactions with host anti-tumor immunity. Studies have shown that proper T cell activation and proliferation is impaired by pharmacological inhibition of MEK signaling with trametinib (3, 4) and other compounds (5). These data seem to imply that trametinib would impair anti-tumor T cell function in tumor-bearing hosts. However, it was recently found that while trametinib impairs T cell function in vitro, it does not limit the effectiveness of either adoptive cell therapy (6) or checkpoint blockade with antibodies against PD-1, PD-L1, and CTLA-4 (7) in mouse models. A potential explanation is that trametinib in these studies was co-administered with high doses of IL-2 (6) and checkpoint inhibitors (7) that enhance the activation of T cells, thus allowing them to overcome small molecule inhibition of MEK. The possibility that cytokines can rescue some deleterious effects associated with MEK inhibition on T cells has yet to be explored. Additionally, MEK inhibition of tumor cells can lead to increased expression of tumor-specific antigens (5, 8), potentially enhancing recognition of tumors by CD8+ effector T cells, which offers another explanation for the synergy with immunotherapies. At the moment, the mechanisms explaining the paradoxical effects of MEK inhibitors on T cell-mediated anti-tumor immunity remain elusive.

In a rapidly changing clinical scenario, first-line PD-1 inhibitors, alone or in combination with CTLA4 inhibitors, may become a standard of care in the near future against tumors such as melanoma (9). However, the combined effects and optimal sequencing of targeted therapies and immunotherapy remains unknown. Clarifying the immunosuppressive effects of targeted therapies in vivo is crucial for the design of synergistic combinatorial interventions with emerging immunotherapies. Trametinib was the first MEK inhibitor to be approved for clinical use in 2013, and it has demonstrated to improve overall survival in combination with other targeted interventions (10). To elucidate the effects of multiple targeted therapies on the tumor immunoenvironment and, subsequently, anti-tumor immunity, we analyzed a panel of molecules for their inhibitory activity on T cells. Our results indicate that most small molecule inhibitors, and in particular trametinib, exert direct suppressive effects on human T cells in vitro and anti-tumor mouse T cells in vivo in preclinical cancer models. However, the suppressive effects of MEK inhibitors can be overcome by various cytokines. We found that clinically available IL-15 agonists, through a mechanism dependent on the activation of PI3K, were particularly effective at rescuing T cell function.

MATERIALS AND METHODS

Animals, tissues and cell lines

WT C57BL/6 and congenic Ly5.1 female 6–8 week old mice were procured from the National Cancer Institute or Charles River Laboratory. OT-I C57BL/6-Tg (TcraTcrb)1100Mjb/J transgenic mice were obtained from The Jackson Laboratory. Transgenic Krastm4Tyj and Trp53tm1Brn mice (11, 12) were obtained from NCI Mouse Models of Human Cancers Consortium, brought to a full C57BL/6 background (13, 14). All animals were maintained in specific pathogen free barrier facilities and used in accordance with the Institutional Animal Care and Use Committee of the Wistar Institute.

The Brpkp110 primary mammary tumor cell line was generated by culturing a mechanically dissociated C57BL/6 L-Stop-L-KrasG12Dp53flx/flxL-Stop-L-Myristoylated p110αGFPflx/+ primary breast tumor mass as previously described (15). Tumor cells were passaged a total of 10 times and tested for mycoplasma before deriving the Brpkp110 cell line. Tumors were initiated by injecting 5×105 cells into the axillary flanks. Tumor volume was calculated as: 0.5 × (L × W2), where L is the longer of the two measurements.

Peripheral blood lymphocytes were obtained by leukapheresis/elutriation and Miltenyi bead–purified. A2780 cells were obtained from AddexBio Technologies. ID8 cells (16) were provided by K. Roby (Department of Anatomy and Cell Biology, University of Kansas, Kansas City, KS) and retrovirally transduced to express Defb29 and Vegf-a (17) or OVA (18).

T cell stimulation

For human T cell proliferation assays, K562 cells expressing human CD32, termed K32, were generated as described (19), γ-irradiated (100 Gy) and loaded with anti-CD3 (500 ng/ml, clone OKT3; eBioscience) plus anti-CD28 (500 ng/ml, clone 15E8; EMD Millipore) antibodies at room temperature for 10 min (aAPCs). PBMCs were labeled with Cell Trace Violet (Invitrogen) according to the manufacturer’s instructions and co-cultured with loaded aAPCs at a 10:1 PBMC:aAPC ratio or activated with ConA (2 μg/ml, Sigma). Proliferation of T cells was determined 7 days later by FACS and Division Index was calculated using FlowJo software.

For mouse T cell proliferation assays, pan-T cells were negatively purified from spleens with antibodies to B220 (RA3), Mac-1 (M170.13), and MHC-II (M5/114) using magnetic beads. T cells were labeled with Cell Trace Violet (Invitrogen) and stimulated with either agonistic CD3/CD28 beads (Dynabeads, Life Technologies) or tumor-pulsed bone marrow dendritic cells (BMDCs) and analyzed for proliferation by FACS either 3 days (CD3/CD28 beads) or 7 days (BMDCs) later. Day 7 BMDCs were generated as previously described (20) and cultured overnight with double-irradiated (γ-irradiated, 100 Gy; and UV, 30 min) ID8-Defb29/Vegf-a cells. BMDCs were added to cultures of T cells at a 10:1 (T cell:BMDC) ratio. For recall ELISpot assays, mouse T cells were primed with tumor-pulsed BMDCs plus IL-2 (30 U/ml) and IL-7 (5 ng/ml), and restimulated 7 days later with fresh tumor-pulsed BMDCs at a 10:1 ratio in an IFN-γ ELISpot (eBioscience).

Compounds and cytokines

ALT-803 was generously provided by Altor BioScience Corporation and was diluted in sterile PBS for in vitro and in vivo studies. Recombinant human IL-15 (Novoprotein), human IL-2, human IL-21, mouse IL-7 (Peprotech), human IL-27 (eBioscience), and Concanavalin A (Type VI, Sigma-Aldrich) were reconstituted in sterile PBS and stored at −20°C. Trametinib (GSK-1120212) was purchased from LC Laboratories and suspended in vehicle solution of 10% PEG-300 (Sigma Aldrich) and 10% Cremophor EL (EMD Millipore) in sterile dH20 for in vivo oral gavage experiments. For in vitro assays, all inhibitors were dissolved in sterile DMSO and diluted in the assays 1:1000, so that the final concentration of DMSO was 0.1%.

Cell proliferation assays

Compound screening on A2780 cells was performed by adding compounds the morning after plating and measuring proliferation 72 hrs later. Screening on human PBMCs was performed by adding compounds simultaneously with ConA stimulation (2 ug/ml) and measuring proliferation 7 days later. Normalized percent inhibition (NPI) was calculated by measuring resazurin fluorescence with respect to values obtained with DMSO negative control and doxorubicin (5 μM) positive control as NPI=100% * (DMSO–compound) / (DMSO–doxorubicin).

Western blotting

Cells were lysed in RIPA buffer (Thermo Scientific) with Complete Protease Inhibitor Cocktail Tablets (Roche) and phosphatase inhibitors (Halt Phosphatase Inhibitor, Thermo Scientific, and Na3VO4, 1 mM) and cleared by centrifugation. Proteins were quantified by BCA assay (Thermo Scientific), diluted in reducing Lamelli buffer, denatured at 95°C, run on mini Protean TGX Ready Gels (Bio-Rad Laboratories), transferred to a PVDF membrane, blocked, and incubated with primary antibodies for p-ERK1/2 (D13.14.4E), p-AKT (D9E) and beta-tubulin (9F3), all from Cell Signaling; plus beta-actin (Sigma; AC-15). Immunoreactive bands were developed using horseradish peroxidase–conjugated secondary antibodies (Bio-Rad) and ECL substrate (GE Healthcare).

TCR ligation of human CD8 T cells

CD8+ T cells were sorted from PBMCs and rested overnight in R10. T cells (0.5×106 per condition) were stained with OKT3-biotin (BioLegend, 10 μg/ml) for 15 mins on ice, and washed in cold PBS. TCR ligation was performed by adding streptavidin (Promega, 25 μg/ml) and anti-CD28 antibody (Millipore, clone 15E8, 1 μg/ml) in the presence of indicated inhibitors for 10 min at 37°C.

In vivo OT-I proliferation

Congenic Ly5.1 mice were injected with 1.5×106 ID8-OVA cells i.p. (21). Mice were oral-gavaged with trametinib or vehicle on days 9–13. On day 10, mice were injected i.p. with 1.5×106 Cell-Trace Violet labeled, unstimulated OT-I T cells. Mice were administered ALT-803 (0.2 mg/kg on day 10) or IL-2 (50,000 IU/mouse on days 10–12) i.p. On day 14, peritoneal washes were analyzed for proliferating OT-I T cells.

RESULTS

Multiple targeted small-molecule inhibitors suppress human T cell activation in vitro

To determine the sensitivity of human T cells to inhibition of signaling pathways commonly targeted by small molecules in cancer therapy (22), we first designed a high-throughput assay to test a diverse panel of 41 inhibitors over a 6 log concentration range on Concanavalin A (ConA)-induced activation and expansion of human T cells from peripheral blood mononuclear cells (PBMCs). At doses equivalent to or below those required to limit proliferation of A2780 ovarian cancer cells – known to be sensitive to PI3K and MEK inhibitors (23, 24) – a variety of inhibitor classes prevented ConA-driven T cell expansion (Figure 1A). Small molecules targeting PI3K, mTOR, MAPK, and CDK signaling, as well as transcriptional regulators (HDACs) and survival molecules (Bcl-2) were deleterious for T cell expansion. Among these, trametinib, the MEK1/2 inhibitor approved by the FDA for BRAF-mutant melanoma, was particularly potent at inhibiting the in vitro proliferation of human T cells. Overall, the observed EC50 of every tested molecule with some activity on A2780 cells was lower for human T cells than for A2780 cells (Table 1 and Figure S1&2), highlighting the immunosuppressive effects of most small-molecule targeted therapies.

Figure 1
Multiple targeted small molecule inhibitors suppress T cell responses in vitro
Table 1
List of EC50 values of compounds analyzed

We validated our screening approach by focusing on inhibitors of the PI3K and MEK signaling pathways. Abrogation of T cell activation elicited by small molecule inhibitors was not restricted to ConA stimulation, because pan-PI3K (BKM120) and MEK (GDC0973) inhibitors also restricted the proliferation of human T cells in response to artificial antigen presenting cells (aAPC) coated with agonistic CD3 and CD28 antibodies (19) (Figure 1B). Importantly, these effects were consistent among 3 different donors (Figure 1C). Comparable results were obtained with the MEK inhibitor trametinib (Figure 1D). As expected, T cells were more sensitive to kinase inhibitors when they were activated in the absence of costimulation (aAPCs lacking anti-CD28), as could occur within the immunosuppressive microenvironment of tumor-bearing hosts (Figure 2A&B). We noticed only minor and non-significant differences in the repertoire of memory and effector T cell subsets when cultures were activated without anti-CD28 and with lower amounts of anti-CD3, indicating that differences in sensitivity to the kinase inhibitors cannot be attributed to altered T cell differentiation (Figure S3A–D).

Figure 2
Stimulation strength affects T cell sensitivity to PI3K and MEK inhibitors

In order to determine the effects of MEK and pan-PI3K inhibitors on different T cell subsets, we FACS purified human CD8 T cells into naïve, memory, and effector populations based upon CD45RA and CD27 expression and activated them with aAPCs. We found that trametinib equally inhibited proliferation of naïve (CD45RA+CD27+), memory (CD45RA-CD27+), and effector/effector memory (CD45RA-CD27-) cells, although one donor showed a trend that memory and effector/effector memory cells were less sensitive than naïve cells to trametinib (Figure 3A&B). Interestingly, memory and effector/effector memory cells were more sensitive to PI3K inhibition with BKM120 than naïve cells, a result consistent among 3 donors (Figure 3C&D). Differentiated effector cells (CD45RA+CD27-) did not proliferate in response to aAPCs, so we could not conclude their sensitivity to kinase inhibition from this analysis (Figure S4A).

Figure 3
Small-molecule PI3K and MEK inhibitors suppress naïve, memory, and effector T cell responses

We also explored the result of MEK and PI3K inhibition on physiologic activation of T cells with tumor antigens. MEK and PI3K inhibitors completely abrogated the initial priming response of murine T cells activated with tumor lysate-pulsed dendritic cells (DCs) (13, 20) (Figure 3E&F). More importantly, the direct suppressive effects of these inhibitors were not restricted to proliferative responses, because the frequency of tumor-primed T cells secreting IFN-γ in response to re-stimulation with fresh tumor lysate-pulsed DCs was also significantly reduced when either PI3K or MEK were inhibited (Figure 3G&H).

Overall, these data underscore the T cell suppressive effects of most small-molecule targeted therapies clinically approved or in the pipeline of clinical development, at a time when combinatorial immunotherapeutic interventions are being tested against multiple tumors.

IL-15 rescues the suppressive effects of MEK inhibitors through PI3K activation

Despite the strong T cell inhibitory activity of FDA-approved trametinib on both initial priming and recall responses, recent reports suggest that trametinib does not limit the effectiveness of adoptive cell therapy in preclinical tumor models (6). Interestingly, these studies included the administration of high doses of IL-2. We reasoned that cytokines signaling on immune (but not tumor) cells could rescue the deleterious effects of trametinib on T cell activity. We therefore tested a panel of cytokines known to play a role in T cell survival and proliferation for their ability to recover T cell expansion from MEK inhibition. Supporting our hypothesis, IL-2, IL-7, and IL-15 were able to individually rescue the proliferation of human T cells in the presence of trametinib (Figure 4A and S4B). In contrast, IL-21 and IL-27 had no significant rescuing effect (Figure 4A and S4B). Among all cytokines tested, we focused on IL-15 because it provides a strong stimulating signaling to both effector and memory CD8+ T cells without inducing the expansion of Tregs, as compared to IL-2 (25, 26). Indeed, IL-15 was able to dramatically rescue the proliferation of purified CD8 naïve, memory, and effector/effector memory T cells (Figure 4B).

Figure 4
Cytokines can rescue T cell functions from MEK inhibitors

We found that IL-15 can rescue early (within 10 minutes post-activation) TCR-induced MAPK signaling from MEK inhibition, as shown by ERK1/2 phosphorylation (Figure 4C). Mechanistically, this effect depends on activation of PI3K by IL-15 (27), because IL-15 was not able to rescue the defect in ERK1/2 phosphorylation in the presence of a pan-PI3K inhibitor (Figure 4C). Furthermore, activation of protein kinase C (PKC) with phorbol-12-myristate-13-acetate (PMA) completely overcomes the suppressive effect of PI3K inhibition on ERK1/2 phosphorylation without fully restoring PI3K activity as assayed by AKT phosphorylation (Figure 4C). This is consistent with a mechanism of ERK phosphorylation (28) mediated by the activation of PKC isoforms upon stimulation of the PI3K pathway, which is known to result in the production of phosphatidylinositol (3,4,5)-trisphosphate (PIP3) and, subsequently, activation of PDK1 kinase (29). Together, these results show that IL-15 can augment early signaling events downstream of TCR activation to enhance the amplitude of MAPK signaling to overcome MEK1/2 inhibition by trametinib.

The IL-15 superagonist ALT-803 overcomes the suppressive effects of trametinib in vivo in tumor-bearing hosts

To define whether IL-15 signaling can overcome MEK inhibition-induced suppression of T cell activation in the tumor microenvironment in vivo, we investigated the IL-15 superagonist ALT-803 (30) for its ability to rescue T cell functions from MEK inhibition. ALT-803 is a new IL-15 superagonist complex with IL-15N72D bound to the IL-15RαSu/Fc (30). This IL-15N72D:IL-15RαSu/Fc has a significantly longer serum half-life and increased biological activity compared to native IL-15 (30) and is undergoing extensive clinical testing (Supplemental Table 1). In this study, we found that ALT-803 was more effective than IL-7 (Figure 4D) and IL-15 (Figure 5A&B) in rescuing human T cell proliferation in vitro. Human T cell proliferation in the presence of ALT-803 remained unaffected by trametinib, even at the relatively high concentration of 5 μM (Figure 5B). As was the case for IL-15, we found that ALT-803 activity was also dependent on PI3K signaling because T cell expansion could not be fully rescued when pan-PI3K and MEK inhibitors were combined (Figure 5B). We determined that the reduction in proliferation when pan-PI3K and MEK inhibitors were combined was due to a block in cell division rather than an increase in cell death (Figure S5A–E).

Figure 5
IL-15 agonist ALT-803 rescues anti-tumor T cell activity from MEK inhibition in vitro and in vivo

ALT-803 was also able to rescue the proliferation of purified human CD8+ naïve, memory, and effector/effector memory T cells from trametinib, indicating broad activity on a range of T cell subsets (Figure S6A). Intriguingly, ALT-803 (and IL-15 for one donor) induced the proliferation of purified, differentiated effector (CD45RA+CD27-) CD8+ T cells that were otherwise unable to proliferate in response to CD3/CD28 activation (Figure S4A). ALT-803 showed activity on mouse T cells as well, demonstrated by its ability to restore proliferation of bead-activated T cells (Figure 5C) and IFN-γ recall responses of tumor-primed T cells in the presence of trametinib (Figure 5D).

To test the activity of ALT-803 in the tumor microenvironment, we transferred Cell Trace Violet-labelled (Ovalbumin (OVA)-specific) OT-I T cells into mice growing OVA-transduced syngeneic ID8 ovarian tumors, a system that allows the recovery of tumor microenvironment lymphocytes through peritoneal wash (17, 18, 31). After 4 days, we found that in mice treated with trametinib, the OT-I T cells proliferated significantly less than in mice gavaged with vehicle (Figure 5E&F). Importantly, when a single dose of ALT-803 was co-administered with OT-I T cells, proliferation was dramatically enhanced and was not restricted by trametinib (Figure 5E&F). The observation that ALT-803 induces proliferation of some CD44lo cells in addition to CD44hi (antigen-experienced) OT-I T cells suggests that ALT-803 may also result in homeostatic proliferation, as has been reported previously (32). We also found that treatment of mice with a high-dose IL-2 regimen was able to rescue OT-I T cell proliferation (Figure S6B&C). These results indicate, first, that trametinib impairs antigen-specific T cell responses in vivo, although to a lesser degree than in vitro. And secondly, that therapeutic activation of IL-15 or IL-2 signaling can completely overcome trametinib-induced CD8+ T cell suppression in the tumor microenvironment.

Combination of ALT-803 and trametinib induces rejection of KRas-mutated tumors

To investigate the therapeutic potential of combining trametinib and ALT-803 against established KRas-mutated tumors, we utilized a syngeneic tumor model derived from an autochthonous breast cancer initiated in triple transgenic (L-Stop-L-KRasG12Dp53flx/flxL-Stop-L-Myristoylated p110α) mice with adenovirus-Cre (11, 12, 33). We chose this cell line, termed breast-p53-KRas-p110alpha (Brpkp110), to model treatment against tumors that evade single molecule targeting of the MAPK pathway through PI3K activation, as has been commonly reported in human cancer cells (3436). Brpkp110 cells have detectable signaling through MEK that can be inhibited with trametinib (Figure 6A) and generate aggressive tumors when grown subcutaneously in mice.

Figure 6
Combination therapy with trametinib and ALT-803 can drive rejection of Kras-mutated breast tumors

Oral gavage with trametinib significantly reduced the growth of Brpkp110 tumors as a single intervention (Figure 6B), although all mice eventually progressed to terminal disease (Figure 6B–D). In contrast, when ALT-803 was combined with trametinib treatment, Brpkp110 tumors progressed even slower, with 19% of mice remaining tumor free at 50 days, and 15% exhibiting complete regression (Figure 6D). No mice in either single treatment group remained tumor free after 50 days. Most importantly, mice that recovered from tumor challenge with trametinib/ALT-803 combination treatment developed immunological memory against the tumor because these mice were resistant to subsequent re-challenge with Brpkp110 cells in the opposite flank over 30 days after initial tumor rejection, whereas all naïve control mice developed tumors (Figure 6E). These results indicate that a chemo-immunotherapy with ALT-803 and trametinib could provide potent anti-tumor activity against some established and aggressive tumors and elicit protective immunity for tumor recurrences.

DISCUSSION

Immunotherapies are revolutionizing cancer treatments (37). In addition, the effects of existing chemotherapy on immune cells and the tumor microenvironment have recently become better appreciated (38). Here we show that many small molecules used as targeted therapies have significant immunosuppressive effects in vitro by interfering with signaling pathways that are critical for priming the responses of T cells. Nevertheless, MEK inhibitor-induced suppression can be rescued by signaling from some common gamma chain family cytokines, of which the most effective is the IL-15 agonist, ALT-803, as shown in this study.

One of the most immunosuppressive drugs ex vivo in our hands was the FDA-approved MEK inhibitor trametinib. These data are in agreement with previous reports identifying a strong inhibitory activity for this drug in vitro (3, 4). However, recent studies indicate that trametinib does not limit the effectiveness of adoptive cell therapy in vivo (6) and synergizes with PD-1 inhibitors (7). Our study provides a mechanistic rationale to reconcile this paradox by demonstrating that direct T cell inhibition by trametinib, while still detectable in vivo in tumor-bearing hosts, can be effectively overcome by cytokines such as IL-2, IL-7 and IL-15. Therefore, a combination of these cytokines endogenously in tumor-bearing hosts may explain our observed lower suppressive effects of trametinib in vivo as compared to in vitro. Additionally, the study showing that trametinib synergized with an adoptive transfer immunotherapy (ACT) in a mouse BRAF-driven tumor model included high-dose IL-2 treatment (6), a regimen that was capable of rescuing T cell proliferation from trametinib in our in vivo experiments. Our results suggest that potential trametinib-driven T cell inhibition in ACT models may be offset by IL-2 administration. Nevertheless, as MEK inhibitors are being clinically tested against multiple KRas-driven malignancies (3941), our data provide an actionable approach to effectively overcome any direct T cell inhibitory effects in future combinations of trametinib and emerging immune therapies, through the use of T cell-rescuing agonists.

Our study indicates that ALT-803, an IL-15 superagonist complex (30), could be the agent of choice for such a combination therapy. ALT-803 induces memory CD8+ T cells to proliferate, upregulate receptors for innate immunity, secrete interferon-γ and acquire the ability to kill tumor cells (30, 42, 43). Stimulation of NK cells by ALT-803 can also contribute to enhanced anti-tumor immunity. ALT-803 exhibits more potent anti-tumor activities against tumors than recombinant IL-15 in various animal models (30), likely due to its much stronger binding capability to IL2Rβγ displayed on T and NK cells, longer serum half-life, and better biological distribution to and detainment in lymphoid tissues. Currently, ALT-803 is in multiple clinical trials against solid and hematological tumors either as a single agent or in combination with other FDA-approved immuno-stimulators or therapeutic antibodies (Table S1).

As most tumor-reactive T cells in cancer patients are unlikely to be naïve T cells, it is important to understand the outcome of kinase inhibition on memory and effector T cells. Sensitivity to MEK inhibition has been reported to correlate negatively with T cell differentiation, suggesting that naïve T cells require more MEK signaling than memory and effector cells (44). We observed this trend in one of three healthy donors, while two others should similar sensitivities between memory, effector memory, and naïve cells. IL-15 agonists were remarkably successful at reversing the block in proliferation by trametinib on naïve, memory, and effector memory T cells. Additionally, murine tumor-reactive effector T cells were inhibited by trametinib, but ALT-803 was also able to rescue this activity.

Importantly, we found that IL-15 signaling rescues MEK inhibition-induced T cell suppression through the activation of the PI3K pathway. The synergy between PI3K and MEK inhibitors on tumor cells (45) therefore also exists for human T cells. These data suggest that combinatorial therapies in patients may compromise anti-tumor immunity, especially considering that combination treatment prevents the rescue by IL-15 agonists in vitro. Because treatments combining PI3K and MEK inhibitors are being investigated clinically (40), it is important that future studies determine whether this combination negatively impacts the anti-tumor activity of T cells in tumor bearing hosts and thus represents a poor option for chemo-immune therapies.

Overall, our finding in this study further illustrates that a greater understanding of targeted small molecules on the host’s immune system and tumor microenvironment could lead to more effective chemo-immunotherapies against cancer, and provides a novel intervention that should pave the way for combining anti-cancer small molecule targeted and immune therapies.

Supplementary Material

Acknowledgments

FINANCIAL SUPPORT: JRCG: R01CA157664, R01CA124515, R01CA178687, U54CA151662, P30CA10815, The Jayne Koskinas & Ted Giovanis Breast Cancer Research Consortium at Wistar and Ovarian Cancer Research Fund Program Project Development awards. MJA and NS were supported by T32CA009171. APP was supported by the Ann Schreiber Mentored Investigator Award (OCRF).

We thank David Schultz for support with molecular screening; and the Wistar Flow Cytometry and Imaging facilities for technical assistance. This study was supported by R01CA157664, R01CA124515, R01CA178687, U54CA151662, P30CA10815, The Jayne Koskinas&Ted Giovanis Breast Cancer Research Consortium at Wistar, and Ovarian Cancer Research Fund (OCRF) Program Project Development awards. M.J.A. and N.S. were supported by T32CA009171. A.P.P was supported by the Ann Schreiber Award (OCRF). Support for Shared Resources was provided by Cancer Center Support Grant (CCSG) CA010815 to The Wistar Institute.

Footnotes

CONFLICT OF INTEREST: Hing C. Wong and Emily K. Jeng are employees and stockholders of Altor Bioscience. The other authors declare no conflict of interest.

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