Targeting myeloid-derived suppressor cells in the treatment of hepatocellular carcinoma: current state and future perspectives

Figures & data

Table 1 Systemic therapy approved or with positive results in phase III trials for advanced hepatocellular carcinoma

Drug and study	Mechanism of action	Trial design	Treatment arms (patient numbers)	Key findings	US-FDA approval
Pivotal trials of sorafenib
Sorafenib, SHARP trial^Citation4	Multikinase inhibitors	Phase III, 1st-line, 1:1 randomization	Sorafenib (299) vs placebo (303)	Median OS: 10.7 m (sorafenib) vs 7.9 m (placebo), HR=0.69 (95% CI, 0.55–0.87), P<0.001	2007
Sorafenib, Asia-Pacific trial^Citation5	Multikinase inhibitors	Phase III, 1st-line, 2:1 randomization	Sorafenib (150) vs placebo (76)	Median OS: 6.5 m (sorafenib) vs 4.2 m (placebo), HR=0.68 (95% CI, 0.50–0.93), P=0.014	2007
Antiangiogenic agents
Lenvatinib, REFLECT trial^Citation7	Multikinase inhibitors	Phase III, 1st-line, 1:1 randomization (noninferiority)	Lenvatinib (478) vs sorafenib (476)	Median OS: 13.6 m (lenvatinib) vs 12.3 m (sorafenib), HR=0.92 (95% CI, 0.79–1.06)	2018
Regorafenib, RESORCE trial^Citation6	Multikinase inhibitors	Phase III, 2nd-line, 2:1 randomization	Regorafenib (379) vs placebo (194)	Median OS: 10.6 m (regorafenib) vs 7.8 m (placebo), HR=0.63 (95% CI, 0.50–0.79), P<0.001	2017
Ramucirumab, REACH II trial^Citation9	Anti-VEGFR mAb	Phase III, 2nd-line, 2:1 randomization (AFP ≥400 ng/mL)	Ramucirumab (197) vs placebo (95)	Median OS: 8.5 m (ramucirumab) vs 7.3 m (placebo), HR=0.71 (95% CI, 0.53–0.95), P=0.0199	Pending
Cabozantinib, CELESTIAL trial^Citation8	Multikinase inhibitors	Phase III, 2nd- or 3rd-line, 2:1 randomization	Cabozantinib (470) vs placebo (237)	Median OS: 10.2 m (cabozantinib) vs 8.0 m (placebo), HR=0.76 (95% CI, 0.63–0.92), P=0.009	2019
Immune checkpoint inhibitors
Nivolumab, CheckMate 040 trial^Citation10	Anti-PD-1 mAb	Phase I/II, multicohort, Both 1st- and 2nd-lines (70% previously treated with sorafenib)	Nivolumab: dose-escalation (48); dose-expansion (214)	ORR: 20% (95% CI, 15–26) in the dose-escalation and 15% (95% CI, 6–28) in the dose-expansion cohorts	2017*
Pembrolizumab, Keynote-224 trial^Citation11	Anti-PD-1 mAb	Phase II, 2nd-line	Pembrolizumab (104)	ORR: 17% (95% CI, 11–26)	2018*

Note: *Accelerated approval.

Abbreviations: US-FDA, Food and Drug Administration of the United States; OS, overall survival; m, months; HR, hazard ratio; VEGFR, vascular endothelial growth factor receptor; mAb, monoclonal antibody; PD-1, programmed cell death protein 1; AFP, α-fetoprotein; ORR, objective response rate.

Table 2 Two major types of myeloid-derived suppressor cells and their immunosuppressive functions

Types	Markers in mice	Markers in human	Main factors mediating immunosuppression	Mechanisms of immunosuppression
M-MDSCs	CD11b^+Ly6G^−Ly6C^high	CD11b^+CD14^+CD15^−HLA-DR^low/−	NO, ARG1, and cytokines such as TGF-β and IL-10	Suppress T-cell responses both in antigen-specific and nonspecific manners; production of NO and cytokines
PMN-MDSCs	CD11b^+Ly6G^highLy6C^low	CD11b^+CD14^−CD15^+HLA-DR^− or CD11b^+CD14^−CD66b^+ or LOX-1^+	ROS, ARG1	Suppressing immune responses primarily in an antigen-specific manner; ROS production

Abbreviations: M-MDSCs, monocytic myeloid-derived suppressor cells; NO, nitric oxide; ARG1, arginase; PMN-MDSCs, polymorphonuclear myeloid-derived suppressor cells.

Table 3 Previous clinical studies on myeloid-derived suppressor cells in hepatocellular carcinoma

Studies	Markers in humans	HCC patient no.	Key findings	Mechanistic insight or translational implication
Hoechest et al Gastroenterology 2008^Citation33	CD14^+HLA-DR^−/low	111	The frequency of MDSCs increased in PBMCs compared with healthy donors or cirrhosis patients.	MDSCs suppress T-cell proliferation and induce Treg.
Hoechst et al Hepatol 2009^Citation38	CD14^+HLA-DR^−/low	30	MDSCs inhibited NK cell cytotoxicity and IFN-γ release in vitro.	Suppression of NK cells by MDSCs was dependent on cell contact but independent of ARG1 or iNOS function. MDSCs inhibited NK cell function via the NKp30 receptor on NK cells.
Arihara et al Cancer Immunol Immunother 2013^Citation34	CD14^+HLA-DR^−/low	123	The frequency of MDSCs in CD14^+ PBMCs was significantly increased in patients with HCC compared with that in non-HCC controls.	The frequency of MDSCs was significantly decreased after RFA (33 patients). Patients with high frequency of MDSCs after RFA had worse RFS than those with low frequency of MDSCs after RFA.
Mizukoshi et al Cancer Immunol Immunother 2016^Citation36	CD14^+HLA-DR^−	36	High frequency of MDSCs in PBMCs was associated with aggressive tumor features such as advanced stage, large tumor size, main PVT, and distant metastasis.	A low frequency of MDSCs was associated with tumor response and longer OS in patients with advanced HCC receiving HAIC.
Gao et al Hepatology Res 2017^Citation35	CD14^+HLA-DR^−/low	183	The frequency of MDSCs increased in PBMCs of HCC patients compared with those of chronic hepatitis and healthy donors.	High MDSCs were associated with early recurrence and poor OS after hepatectomy.
Iwata et al Sci Rep 2016^Citation37	CD33^+HLA-DR^low/−CD11b^+CD14^+	122	PD-L1^+ MDSCs were increased in PBMCs from patients with HCC. TILs contained remarkably higher percentages of PD-L1^+MDSCs than liver-infiltrating lymphocytes and PBMCs (14 patients with HCC).	The percentages of PD-L1^+MDSCs in PB were significantly reduced by curative treatment for HCC (12 patients with HCC). Patients with low PD-L1^+MDSCs in PB before curative treatment had significantly longer DFS than those with high PD-L1^+MDSCs (55 patients with HCC).
Kalathil et al Cancer Res 2013^Citation39	CD14^−HLA-DR^−CD11b^+CD33^+	23	The frequency and absolute number of circulating MDSCs was significantly elevated in patients with HCC.	Depleting Tregs, MDSCs, and PD-1^+ T cells of patients with advanced HCC restored production of granzyme B by CD8^+ T cells in vitro.
Nan et al Immunology 2018^Citation40	LOX-1^+CD15^+	127	MDSCs in PBMCs were significantly elevated in patients with HCC compared with healthy controls.	MDSCs significantly reduced proliferation and IFN-γ production of T cells in vitro through the ROS/ARG1 pathway induced by ER stress.

Abbreviations: HCC, hepatocellular carcinoma; MDSCs, myeloid-derived suppressor cells; PBMCs, peripheral blood mononuclear cells; Treg, regulatory T cells; NK cell, natural killer cell; IFN-γ, interferon-γ; ARG1, arginase; iNOS, inducible nitric oxide synthase; RFA, radiofrequency ablation; RFS, recurrence-free survival; PVT, portal vein thrombosis; OS, overall survival; HAIC, hepatic arterial infusion chemotherapy; PD-L1, programmed death-ligand 1; TILs, tumor-infiltrating leukocytes; PB, peripheral blood; DFS, disease-free survival; PD-1, programmed cell death protein 1; ER, endoplasmic reticulum.

Table 4 Recent preclinical studies of myeloid-derived suppressor cells in experimental hepatocellular carcinoma models

Studies	Preclinical models	Key findings	Mechanistic insight or translational implication
Inducing immunosuppression
Hu et al Scand J Gastroenterol 2011^Citation42	Hepa1-6 subcutaneous mouse liver cancer models	Increased frequency of MDSCs in tumor development was detected in spleen, PB, LN, and tumor, and IL-10 levels were higher in MDSCs derived from tumor-bearing mice than in control.	MDSCs inhibited TLR-ligand-induced IL-12 production of DC through IL-10 production and suppressed T cell stimulatory activity of DC.
Lacotte et al Oncoimmunology 2016^Citation43	RIL-175 orthotopic mouse liver cancer models	Kupffer cells expressed less costimulatory CD86 and MHCII and more coinhibitory CD274 molecules in HCC-bearing livers than in control livers, indicating decreased antigen-presenting activity.	MDSC subsets (Ly6G^high cells, Gr1^high cells, and Ly6C^low cells) were identified and sorted from HCC-bearing mice. Primary isolated Kupffer cells in co-cultured with the three MDSC subsets showed a decrease in CCL2 and IL-18 secretion, increase in IL-10 and IL-1b secretion, and increased expression of CD86, CD274, and MHCII.
Engaging drug efficacy
Chen et al Hepatol 2014^Citation44	HCA-1 orthotopic mouse liver cancer models	Sorafenib induced tumor-infiltration of CD11b^+Gr1^+ MDSCs through SDF1-α/CXCR4 signaling.	CD11b^+Gr1^+ MDSCs mediated the resistance of sorafenib in liver tumors by promoting hepatic stellate cell differentiation and survival and inducing tumor fibrosis. Inhibition of CXCR4 or Gr-1 in combination with sorafenib inhibited HCC growth compared with sorafenib alone.
Chang et al Int J Cancer 2018^Citation45	BNL orthotopic mouse liver cancer models	MDSCs increased in orthotopic liver tumors after sorafenib treatment.	Targeting MDSCs with anti-Ly6G or anti-IL-6 antibodies improved antitumor efficacy of sorafenib.
Chiu et al Nat Commun 2017^Citation46	MHCC97L cells and Hepa1-6 orthotopic mouse liver cancer models	Hypoxia, through stabilization of HIF-1, induced ENTPD2/CD39L1 expression in cancer cells.	Overexpression of ENTPD2 was a poor prognostic factor for patients with HCC. In mouse models, ENTPD2 promoted the maintenance of MDSCs by preventing their differentiation. ENTPD2 inhibition was able to mitigate cancer growth and enhance the efficacy of immune checkpoint inhibitors.
Zhou et al Gut 2018^Citation47	Liver-specific CCRK-inducible transgenic mice and Hepa1-6 orthotopic mouse liver cancer models	Ccrk-IL-6 signaling drove liver tumorigenicity through MDSC immunosuppression.	Targeting tumorous CCRK signaling diminished MDSC-mediated immunosuppression and inhibited tumorigenicity of HCC. Tumorous CCRK depletion upregulated PD-L1 expression and increased intratumoral CD8^+ T cells, thereby enhancing PD-L1 blockade efficacy to eradicate HCC.

Abbreviations: MDSCs, myeloid-derived suppressor cells; PB, peripheral blood; LN, lymph node; HCC, hepatocellular carcinoma; TLR, Toll-like receptor; DC, dendritic cell; MHC, major histocompatibility complex; HIF-1, hypoxia-inducible factor-1; ENTPD2, ectonucleoside triphosphate diphosphohydrolase 2; CCRK, cell cycle-related kinase; PD-L1, programmed death-ligand 1.

Figure 1 Strategies of targeting MDSCs in cancers. In physiological condition, HSCs differentiate into CMPs and GMPs, which subsequently become mature granulocytes or monocytes. In pathological condition such as malignancy, multiple tumor-derived factors affect the differentiation of myeloid cells, leading to the generation M-MDSCs and PMN-MDSCs. Both types of MDSCs migrate to the tumor site through the interaction of chemokine receptors and ligands (CCLs or CXCLs). In TME, MDSCs are activated and can support tumor growth by suppressing antitumor response of T cells through various mechanisms such as ARG1, iNOS, ROS, TGF-β, IL-10, and IDO. MDSCs can also promote macrophage polarization and induce Tregs and tolerogenic DCs. Reversing the protumor effects of MDSCs could be achieved by depleting MDSCs, promoting MDSC differentiation, blocking MDSC trafficking and migration into TME, and inhibiting the immunosuppressive function of MDSCs.

Abbreviations: HSC, hematopoietic stem cells; CMP, common myeloid progenitor; GMP,granulocyte-macrophage progenitor; MDP, macrophage/dendritic cell progenitors; MDSCs,  myeloid-derived suppressor cells; M-MDSCs, monocytic myeloid-derived suppressor cells; PMN-MDSCs, polymorphonuclear myeloid-derived suppressor cells; GM-CSF, granulocyte-macrophage colony-stimulating factor; M-CSF, macrophage colony-stimulating factor; VEGF, vascular endothelial growth factor; G-CSF, granulocyte colony-stimulating factor; SCF, stem cell factor; IFN-γ, interferon-γ; PGE2, prostaglandin E2; ARG1, arginase; IDO, indoleamine 2,3-dioxygenase; NO, nitric oxide; DC, dendritic cell, T, T cell; Treg, regulatory T cell; Mϕ, macrophage; ENTPD2, ectonucleoside triphosphate diphosphohydrolase 2; CCLs, CC chemokine ligands; CXCLs, C-X-C chemokine ligands; TME, tumor microenvironment; iNOS, inducible nitric oxide synthase.

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