Background
Immunotherapy with immune checkpoint blockades (ICBs) which target the programmed death-1 (PD-1), or PD-1 ligand (PD-L1), or cytotoxic T-lymphocyte-associated protein-4 (CTLA-4) has revolutionized the management of multiple cancers. Due to non-response or resistance to mono-therapy in some patients, ICB combinations with other anti-cancer therapies are developed, including chemotherapy, radiation therapy, and targeted therapy. While patients benefit from the combined ICB immunotherapies, adverse event rates increased [
1]. The damage can be caused by immunotoxicity, and also by chemotherapeutic regimens or irradiation. Gastrointestinal organs are involved commonly, with high incidence of gastrointestinal toxicities including diarrhea and mucositis [
2]. Generally, interventional treatment was not recommended when low grade of adverse events occurred [
3]. It is uncertain whether ICB-mediated anticancer efficacy is compromised in the patients experiencing acute damage of mucosal barrier, such as acute colitis.
Interleukin 17A (IL-17A) and Th17 cells critically participate in the pathogenesis of gut inflammation [
4,
5]. Generally, IL-17A is expressed in the barrier surface tissues to maintain a healthy microbial population for protection. When the tissue barrier is damaged, abnormally generated IL-17A modulates and amplifies signals locally in a context-dependent manner in the pathogenesis of diseases and promotes disease progression through IL-17A receptor (IL-17R) heterodimers, IL-17RA, and IL-17RC. IL-17R is expressed ubiquitously and non-hematopoietic cells are generally the primary responders to IL-17. IL-17R expression was also identified in the immune cells [
5,
6]. In cancers, abnormally generated IL-17A through the IL-17R signaling in multiple types of cells promotes disease development. It was demonstrated that IL-17A activates certain immune cell types, such as myeloid-derived suppressor cells (MDSCs) and neutrophils to suppress antitumor immunity and promote cancer development [
5,
7‐
10]. IL-17A also promote tumor angiogenesis independent of the conventional vascular endothelial growth factor (VEGF) [
5,
7,
11‐
14]. Elevation of IL-17A level was observed in some ICB-treated patients with immune-related adverse events [
3,
15], and also in the patients treated with irradiation or chemotherapeutic agents [
16,
17]. Compelling evidences demonstrate that the ICB immunotherapeutic efficacy depends significantly on the intra-tumoral, stromal, or invasive marginal CD8
+ cytotoxic T lymphocytes (CTLs) but not the circulating CD8
+ T cells [
18]. IL-17A acts directly on CTLs in tumor microenvironment remains ambiguous.
Both in cancers and in chronic viral infections, two subsets of PD-1
+ CTLs were identified that displayed distinct responses to anti-PD-1/PD-L1 [
19‐
22]. Phenotypically, “terminally exhausted” CTL subsets express high levels of PD-1 and Tim3. The subset of “stem-like exhausted” CTLs express Slamf6 or CXCR5, and PD-1 at an intermediate level, but no Tim3. Functionally, “stem-like exhausted” CTLs expand vigorously upon inhibitory receptor blockade or specific antigen stimulation, exhibiting features of central memory and exhausted T cells. The “stem-like” subset persists long term and can differentiate into the subset of “terminally exhausted” CTLs which are prone to undergo apoptosis. Transcription factor Tcf1, encoded by the
Tcf7 gene, is essential for the stem-like functions of these cells. The “stem-like exhausted” CTL subset confers the capacity for controlling tumor growth in response to ICBs and vaccination [
21,
22]. To kill the tumor cells, CTLs require to transmigrate across the tumor vasculature from bloods. Tumor blood vessels are abnormal, both structurally and functionally relative to those of nonmalignant tissues, restraining the effector T cell infiltration [
23,
24]. Anti-angiogenesis by inhibiting proangiogenic signaling was combined with ICBs for enhancing antitumor immunity [
1,
23,
25]. ICB-immunotherapy efficacy relies on the presence of intra-tumoral “stem-like exhausted” CTLs rather than on the lone reversal of T cell exhaustion programs [
19,
21,
22]. To achieve better ICB-immunotherapeutic efficacy, it is required to understand the impacts of intestinal inflammation-related IL-17A generation on the tumor infiltration of two CTL subsets.
Methods
Cell lines and reagents
Murine melanoma B16F10 and B16-OVA, and human umbilical vein endothelial cells (HUVECs) were purchased from the National Infrastructure of Cell Line Resource (Beijing, China). Murine hepatoma Hepa1-6, colon carcinoma CT26.CL25, endothelial cell C166, and human hepatocellular carcinoma (HCC) cell lines (HepG2, Hep3B) were purchased from the American Type Culture Collection (ATCC). We transfected Hepa1-6 with a pcDNA3.1-OVA [
26] to generate Hepa1-6-OVA. Cells were cultured following the suppliers’ instructions.
The information of used reagents including antibodies and primers is listed in Additional file
1: Table S1, Table S2.
Mice and tumor models
C57BL/6 mice and BALB/c mice were purchased from Beijing HFK Bioscience Company. C57BL/6 background CD45.1 mice, OT-I mice (OVA257–264 peptide-specific CD8 TCR-transgene), and Pmel-1 mice (gp10025–33 peptide-specific CD8 TCR-transgene) were purchased from Aniphe Biolaboratory (Jiangsu, China). Study protocols (NCC2021A037) involving mice were approved by the Institutional Animal Care and Use Committee of the National Cancer Center/Cancer Hospital, Chinese Academy of Medical Sciences (NCC/CH, CAMS).
In each female C57BL/6 mouse, 5 × 10
5 B16-OVA or B16F10, or 3 × 10
6 Hepa1-6-OVA cells in 100-μl PBS were injected subcutaneously. In some experiments, B16-OVA cells in 20-μl PBS were inoculated into a mouse liver. Five days after tumor cell inoculation (D5), each mouse received 1 × 10
6 activated peptide-specific CD8
+ T cells via tail vein injection. Before cell transfer, CD8
+ T cells were purified from splenocytes of OT-I mice or Pmel-1 mice using a mouse CD8
+ T cell Isolation Kit (Miltenyi, Germany) with > 95% purity and stimulated with 200 ng/ml of OVA
257–264 or gp100
25–33 peptides for 4 days. After cell transfer, acute colitis was induced in some mice by providing drinking water containing 2% dextran-sulfate-sodium (DSS, 36–50 kDa) between D5 and D12 as reported [
27]. In some experiments, 4 doses of neutralizing anti-IL-17A antibody (αIL-17A, Clone#17F3) were injected intraperitoneally into the mice that were given DSS drinking water. To test the combined effect of anti-PD-1 (αPD-1, Clone# 29F.1A12) and αIL-17A, BALB/c mice were inoculated with 2.5 × 10
5 CT26.CL25 cells subcutaneously, given 2% DSS drinking water from D5 to D12. One group of the mice received 4 doses of αIL-17A, one group received 4 doses of αPD-1, one group received 4 doses of αIL-17A plus 4 doses of αPD-1, and one group received isotype IgG. Each dose contained 150-μg protein. Tumor growth was measured every 2–3 days and tumor volume was calculated as (width
2 × length × 0.5).
Flow cytometry (FCM)
Tumor tissues were treated as we reported previously [
26]. FCM analysis was performed using standard laboratory protocols. Data were acquired in LSR-II (BD Biosciences, USA) and analyzed using Flowjo software (Tree Star, USA).
CD8+ T cell proliferation, adhesion, and trans-endothelium migration
CD8+ T cells were isolated from OT-I mice, stimulated with 200 ng/ml of OVA257–264 peptides in the presence of 0, 2, and 20 ng/ml of recombinant mouse IL-17A (rmIL-17A, Peprotech, USA). Cell numbers were counted at different time points. In some experiments, the purified CD8+ T cells were labeled with 2.5 μM CFSE (eBioscience, USA) before stimulation.
CD8
+ T cell transmigration across vascular endothelium was assayed according to the literature [
28]. C166 cells were pre-treated with B16-OVA cell medium (B16-OVA/CM) with or without a supplement of 20 ng/ml of rmIL-17A for 48 h. Transwell inserts (5 μm in pore size) were coated with 0.1% gelatin, 7.5 × 10
3 C166 cells were seeded onto the upper surface of each insert and cultured overnight to form confluent monolayers. After removal of the medium, 3 × 10
5 activated CD8
+ T cells were added into each upper chamber and continued to culture for 4 h. The cells migrated into the lower chamber were collected, counted, and analyzed with FCM. In some experiments, 2 μg/ml of neutralizing antibody against IL-17RA, or IL17RC (R&D, USA) was included in the medium.
CD8
+ T cell adhesion to vascular endothelium was analyzed according to the report [
29]. In each well of 6-well plates, 1 × 10
5 C166 cells were seeded and treated with B16-OVA/CM with or without a supplement of 20 ng/ml of rmIL-17A for 48 h. Activated CD8
+ T cells (2 × 10
5 cells/well) were added for another 16 h. Nonattached CD8
+ T cells were washed out using 37ºC pre-warmed PBS three times. All cells were then recovered with trypsin, and CD8
+ T cells adhered to C166 cells were analyzed with FCM.
Multiplex immunohistochemistry (mIHC)
Opal Multiplex IHC assay kit (PerkinElmer, USA) was used for mIHC staining according to the literature [
30]. Slides were scanned with a Pannoramic MIDI slide scanner (3DHISTECH, Hungary) at 20 × magnification and analyzed with HALO image analysis platform (Indica, USA). The relationship of CD8
+ T cells with tumor vasculature was analyzed as reported [
31].
Determination of nitric oxide (NO) and endothelial NO synthase (eNOS)
In 6-well plates, 1.5 × 104 C166 cells/well were treated with B16-OVA/CM with or without a supplement of 20 ng/ml of rmIL-17A. NO assay kit (Beyotime, China) was used to determine NO generation following the manufacturer’s specifications. Immunoblot was performed to determine the expression levels of total eNOS, phosphorylated-eNOS at Ser1177.
Patients
Fifty patients with advanced HCC were treated with anti-PD-1 (sintilimab) plus anti-VEGF (IBI305) every 3 weeks in a phase Ib clinical study conducted at NCC/CH. The therapeutic efficacy was reported previously [
32]. From 33 patients, we obtained the paired serum samples before therapy and 24 h post-therapy, and their demographics and baseline characteristics are listed in Additional file
1: Table S4. ProcartaPlex Human Cytokine/Chemokine/Growth Factor Panel (Affymetrix, USA) was used to determine serum levels of multiple cytokines, including IL-17A.
Statistical analysis
Version 8.0 GraphPad Prism was used for statistical analysis. Continuous variables were compared with unpaired Student’s t-test between two groups, One-way ANOVA between more than two groups. Fisher’s exact test was used to compare categorical variables. Survival curves of treated patients were generated using the Kaplan–Meier method and compared with the log-rank test. P-value of less than 0.05 was considered to be statistically significant.
Discussion
In this study, we uncovered a novel role of abnormally generated IL-17A on tumor infiltration of two exhausted CTL subsets (Additional file
2: Fig. S10). Stem-like CTLs were recognized to express higher IL-17RA and IL-17RC but lower LFA-1, as compared to the terminally exhausted CTLs. In tumor-bearing mice, colitis-related IL-17A significantly suppressed the extravasation and self-renewal of the stem-like subset, dampening CTL-conferred antitumor immunity. Neutralizing IL-17A improved tumor infiltration of “stem-like exhausted” CTLs and enhanced the anti-PD-1-mediated antitumor efficacy. The HCC patients with serum IL-17A elevation 24 h post-therapy displayed resistance to the combined therapy of anti-PD-1 plus anti-VEGF, displaying worse disease progression. ICB-immunotherapy relies on the intra-tumoral stem-like exhausted CTLs rather than the lone reversal of the exhausted program [
21,
22]. Our current study indicated that IL-17A stimulation mainly suppressed the tumor infiltration of stem-like CTLs. Therefore, ICB-based immunotherapeutic efficacy could be upgraded by blocking IL-17A activities when treatment-related colitis occurred, even the adverse events were minor.
IL-17A carry out the function via signals through the IL-17RA and IL-17RC/ IL-17RD receptor subunits and the acts of IL-17A were well recognized in non-hematopoietic cells and innate immune cells [
6,
33]. Recent investigation of IL-17A directly on T cells by using
Il17a−/− mice reported that IL-17A can promote terminal exhaustion of CD8
+ T cells and tumor progression [
38]. However, the exhausted CD8
+ T cell population in the tumor tissues is heterogenous, and the stem-like exhausted CTL subset control tumor growth better than the terminally exhausted CTL subset does [
21,
22]. Restricted expression of IL-17RC/IL-17RD in non-hematopoietic cells limits IL-17A signaling downstream [
6,
33]. In this study, we detected higher surface expression of both IL-17RA and IL-17RC in the “stem-like exhausted” CTLs than that in the “terminally exhausted” CTLs, and validated by two public data GSE84105 and GSE123235 which quantified various gene transcription levels of in two exhausted CTL subsets [
19,
22]. Our results indicated that the “stem-like exhausted” CTLs are more susceptible to IL-17A stimulation. Indeed, the tumor infiltration of the stem-like subset, not the terminal subset, reduced significantly in the colitis mice with abnormal IL-17A generation. Nevertheless, downstream molecular events after IL-17R heterodimer signaling in the specified CTLs need to be identified.
LFA-1 interaction with ICAM-1 on vascular endothelium is essential for effector T cell extravasation into peripheral tissues [
24]. In the tumors of colitis mice, ICAM-I expression on the tumor blood vessels decreased, and perivascular CD8
+ T cell density reduced, particularly those within 25 μm of the vasculature. In the cell culture system, IL-17A addition profoundly inhibited ICAM-I expression on endothelial cells and suppressed the transmigration of CTLs, particularly the stem-like subset, across the tumor vascular endothelium. On the stem-like CTLs, we detected lower levels of LFA-1 expression, and the surface LFA-1 expression was further reduced by colitis-induced IL-17A. While IL-17A showed minor effects on LFA-1 surface expression on the “terminally exhausted” CTLs. Tumor-associated high endothelial venules (TA-HEVs) were recognized as major sites of lymphocyte extravasation into tumors both at baseline and upon ICB treatment. MECA-79
+ TA-HEVs display a unique phenotype, expressing certain adhesion molecules to facilitate the entry of some types of T cells via interacting with the counterpart molecules on the T cells [
39]. Two exhausted CTL subsets express distinct cytokine receptors and adhesion molecules [
19,
21,
22], implying that the two CTL subsets might have different entry gates into tumors. Our current results indicated that the cell-surface molecule LFA-1 is an important molecule regulated by IL-17A. The entry of stem-like CTLs into tumors was particularly restrained in response to the instigated cytokine IL-17A from the inflamed intestine. In addition to LFA-1, integrin VLA-4 (α4β1 integrin) expression of T cells, which binds to vascular cell adhesion molecule-1 (VCAM1) on endothelial cells, also controls effector T cell tumor infiltration [
24]. It was reported that LFA-1 activation by a small-molecule activator of LFA-1 and VLA-4 improved ICB therapeutic efficacy by promoting T cell infiltration into the tumor microenvironment [
40]. We observed no difference in α4 subunit transcription between two CTL subsets, but higher β1 subunit transcription in the stem-like subset. The impacts of IL-17A on VLA-4 in CD8
+ T cell tumor infiltration require our further investigation.
To improve ICB-immunotherapy, targeting angiogenesis such as anti-VEGF, which was considered to inhibit the sprouting of new vessels and also “normalize” the tumor vasculature to improve immune cell infiltration, was combined with anti-PD-1/PD-L1 for the cancer treatment and obtained efficacious effects in HCC treatment [
23,
25,
41]. Nevertheless, the effect of blocking VEGF alone displayed less efficacious than double-blocking VEGF and ANG2 for improving T cell tumor infiltration [
31]. IL-17A is able to activate multiple IL-17R-expressing cell types to repress CD8
+T cells mediated antitumor immunity [
5,
7‐
14,
42]. IL-17A could activate myeloid-derived suppressor cells, recruit neutrophils, and induce the nuclear translocation of hypoxia-inducible factor-1α in cancer-associated fibroblasts to exclude CD8 + T cell infiltration [
5,
7,
9,
42,
43]. IL-17A has also been demonstrated to promote tumor angiogenesis through promoting the generation of conventional VEGF or directly acting on the endothelial cells [
7,
11‐
14,
23]. IL-17 was found directly acts on endothelial cells of the vasculature to decrease the NO production by inducing the phosphorylation of eNOS at the inhibitory site threonine 495 [
44] and to participate in hypertension [
45]. In our cell culture system, we detected that the production of stimulatory eNOS at serine 1177 from endothelial cells was reduced after IL-17A stimulation, resulting in decreased NO generation. The transmigration of stem-like CD8
+ T cells was significantly inhibited. Results from our current study indicated that IL-17A stimulation exaggerates tumor vasculature dysfunction to restrain the extraversion of CTLs, particularly the stem-like CTLs, leading to uncontrol of tumor growth. Therefore, neutralizing IL-17A could recover the interaction of effector CD8
+ T cells and tumor vascular endothelium via the LFA-1/ICAM-1, and also restore the NO production from endothelial cells, facilitating the infiltration of CD8
+ T cells, particularly the stem-like subset, into the tumor bed. Local delivery of NO proves to improve the ICB-immunotherapy efficacy [
35‐
37]. Clinical studies have reported that the use of angiotensin system inhibitors could improve immunotherapy [
46].
Clinical studies pointed out the association between the onset of immune-related adverse events and better overall survival and overall response to ICB immunotherapy [
47]. The hypothesis is being challenged. A study reported recently that colitis tissues of ICB-treated patients with immune-related enterocolitis (irEC) displayed significantly higher Th17 cell gene expression scores, compared to the paired normal intestinal tissues. Transcription of IL-6 cytokine increased by 11.7-folds, and IL-17A increased by 6.3-folds in the colitis tissues of ICB-treated patients with irEC. Their tumor-bearing mice models showed that IL-6 blockade improves ICB-induced antitumor efficacy. Clinical data indicated that targeting IL-6 can mitigate the immune-related adverse event without compromising the overall tumor response to ICB [
15]. IL-6 is required for the retaining transcriptional and functional identity of Th17 cells [
48]. In addition, IL-17A elevation also occurs in patients receiving chemotherapy of irradiation [
16,
17]. Our current study indicated that abnormally generated IL-17A conferred more suppressive effects on the extravasation of ″stem-like'' CTLs to dampen the CD8
+ T cell-mediated antitumor activity. Neutralizing IL-17A could restore the tumor infiltration of ''stem-like exhausted'' CTLs and enhance the anti-PD-1-mediated antitumor efficacy. We provided an option by neutralizing IL-17A to improve ICB-based therapeutic efficacy when abnormal IL-17A generation in the setting, such as colitis, occurred during cancer treatment.
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