CXCL5 impedes CD8+ T cell immunity by upregulating PD-L1 expression in lung cancer via PXN/AKT signaling phosphorylation and neutrophil chemotaxis.

BACKGROUND: Lung cancer remains one of the most prevalent cancer types worldwide, with a high mortality rate. Upregulation of programmed cell death protein 1 (PD-1) and its ligand (PD-L1) may represent a key mechanism for evading immune surveillance. Immune checkpoint blockade (ICB) antibodies against PD-1 or PD-L1 are therefore widely used to treat patients with lung cancer. However, the mechanisms by which lung cancer and neutrophils in the microenvironment sustain PD-L1 expression and impart stronger inhibition of CD8+ T cell function remain unclear. METHODS: We investigated the role and underlying mechanism by which PD-L1+ lung cancer and PD-L1+ neutrophils impede the function of CD8+ T cells through magnetic bead cell sorting, quantitative real-time polymerase chain reaction (RT-PCR), western blotting, enzyme-linked immunosorbent assays, confocal immunofluorescence, gene silencing, flow cytometry, etc. In vivo efficacy and safety studies were conducted using (Non-obeseDiabetes/severe combined immune deficiency) SCID/NOD mice. Additionally, we collected clinical and prognostic data from 208 patients who underwent curative lung cancer resection between 2017 and 2018. RESULTS: We demonstrated that C-X-C motif chemokine ligand 5 (CXCL5) is markedly overexpressed in lung cancer cells and is positively correlated with a poor prognosis in patients with lung cancer. Mechanistically, CXCL5 activates the phosphorylation of the Paxillin/AKT signaling cascade, leading to upregulation of PD-L1 expression and the formation of a positive feedback loop. Moreover, CXCL5 attracts neutrophils, compromising CD8+ T cell-dependent antitumor immunity. These PD-L1+ neutrophils aggravate CD8+ T cell exhaustion following lung cancer domestication. Combined treatment with anti-CXCL5 and anti-PD-L1 antibodies significantly inhibits tumor growth in vivo. CONCLUSIONS: Our findings collectively demonstrate that CXCL5 promotes immune escape through PD-L1 upregulation in lung cancer and neutrophils chemotaxis through autocrine and paracrine mechanisms. CXCL5 may serve as a potential therapeutic target in synergy with ICBs in lung cancer immunotherapy.

Characterization of TCF-1 and its relationship between CD8+ TIL densities and immune checkpoints and their joint influences on prognoses of lung adenocarcinoma patients.

BACKGROUND: T cell factor-1 (TCF-1) + stem-like tumor-infiltrating lymphocytes (stem-like TILs) are important memory cells in the tumor microenvironment. However, their relationship with clinicopathological features, CD8+ TIL densities, immune checkpoint inhibitors (ICs), and prognostic values remain unknown for lung adenocarcinomas (LUADs). In this study, we aimed to characterize TCF-1+ TILs and their prognostic significance in patients with surgically resected LUADs. METHODS: Expression of TCF-1, CD8, and ICs including programmed death-1 (PD-1), lymphocyte activating-3 (LAG-3), and T cell immunoglobulin and mucin-domain containing-3 (TIM-3) in TILs were estimated using immunohistochemistry of resected LUADs. The association between TCF-1 expressions and clinicopathological characteristics of patient prognoses were analyzed. RESULTS: Positive TCF-1 expression significantly correlated with advanced pathological stage, tumor grade, CD8+ TILs density, TIM-3 expression, LAG-3 expression, and PD-1 expression. TCF-1 positivity was significantly associated with a better recurrence-free survival (RFS), and overall survival (OS). Subgroup analysis revealed that the TCF-1+/CD8+ group had the best RFS and OS, while the TCF-1-/CD8- group had the worst RFS and OS. Similarly, patients with TCF-1 + PD-1- had the best prognoses and patients with TCF-1-PD-1+ had the worst prognoses. CONCLUSION: TCF-1 had relatively high positive expression and special clinicopathological features in patients with LUAD. TCF-1+ TILs were related to CD8 density, TIM-3 expression, LAG-3 expression, and PD-1 expression, and were associated with better prognoses in LUAD patients. A combination of TCF-1 and CD8 densities or PD-1 expression further stratified patients into different groups with distinct prognoses.

Identification of the mitochondrial protein ADCK2 as a therapeutic oncotarget of NSCLC.

The aarF domain containing kinase 2 (ADCK2) is a mitochondria-locating protein, important for fatty acid metabolism and coenzyme Q biosynthesis. The bioinformatics results show that elevated ADCK2 transcripts in NSCLC correlate with poor overall survival and poor anti-PD-1/PD-L1 therapy response. ADCK2 is overexpressed in local human NSCLC tissues and various primary and established NSCLC cells. In NSCLC cells, ADCK2 shRNA or CRISPR/Cas9 knockout remarkably suppressed cell viability, proliferation, cell cycle progression, cell mobility, and provoked cell apoptosis. Moreover, ADCK2 depletion disrupted mitochondrial functions in NSCLC cells, causing cytochrome C release, mitochondrial depolarization, DNA damage and ATP reduction. Contrarily, ectopic ADCK2 overexpression promoted NSCLC cell growth. Further studies revealed that ADCK2 depletion inactivated Akt-mTOR signaling in primary NSCLC cells. NSCLC xenograft growth in nude mice was significantly hindered after ADCK2 silencing or knockout. ADCK2 depletion, apoptosis induction and oxidative injury as well as ATP reduction and Akt-mTOR inactivation were detected in ADCK2-silenced or ADCK2-knockout NSCLC xenograft tissues. Together overexpressed ADCK2 is important for the growth of NSCLC cells, representing an important therapeutic molecular oncotarget.

Targeting sphingosine kinase 1/2 by a novel dual inhibitor SKI-349 suppresses non-small cell lung cancer cell growth.

Sphingosine kinase 1 (SphK1) and sphingosine kinase (SphK2) are both important therapeutic targets of non-small cell lung cancer (NSCLC). SKI-349 is a novel, highly efficient and small molecular SphK1/2 dual inhibitor. Here in primary human NSCLC cells and immortalized cell lines, SKI-349 potently inhibited cell proliferation, cell cycle progression, migration and viability. The dual inhibitor induced mitochondrial depolarization and apoptosis activation in NSCLC cells, but it was non-cytotoxic to human lung epithelial cells. SKI-349 inhibited SphK activity and induced ceramide accumulation in primary NSCLC cells, without affecting SphK1/2 expression. SKI-349-induced NSCLC cell death was attenuated by sphingosine-1-phosphate and by the SphK activator K6PC-5, but was potentiated by the short-chain ceramide C6. Moreover, SKI-349 induced Akt-mTOR inactivation, JNK activation, and oxidative injury in primary NSCLC cells. In addition, SKI-349 decreased bromodomain-containing protein 4 (BRD4) expression and downregulated BRD4-dependent genes (Myc, cyclin D1 and Klf4) in primary NSCLC cells. At last, SKI-349 (10 mg/kg) administration inhibited NSCLC xenograft growth in nude mice. Akt-mTOR inhibition, JNK activation, oxidative injury and BRD4 downregulation were detected in SKI-349-treated NSCLC xenograft tissues. Taken together, targeting SphK1/2 by SKI-349 potently inhibits NSCLC cell growth in vitro and in vivo.

Functional assessment of the cell-autonomous role of NADase CD38 in regulating CD8+ T cell exhaustion.

Exhausted CD8+ T cells with limited effector functions and high expression of multiple co-inhibitory receptors are one of the main barriers hindering antitumor immunity. The NADase CD38 has received considerable attention as a biomarker of CD8+ T cell exhaustion, but it remains unclear whether the increased CD38 directly promotes T cell dysfunctionality. Here, we surprisingly found that although Cd38 deficiency partially reverses NAD+ degradation and T cell dysfunction in vitro, the terminal exhausted differentiation of adoptively transferred CD8+ T cells in tumor is not impacted by either deficiency or overexpression of CD38. Monitoring the dynamic NAD+ levels shows that NAD+ levels are comparable between tumor infiltrated WT and Cd38 -/- OT-1 cells. Therefore, our results suggest that decreased NAD+ are correlated with T cell dysfunction, but deficiency of CD38 is not enough for rescuing NAD+ in tumor infiltrated CD8+ T cells and fails to increase the efficacy of antitumor T cell therapy.

Developing ZNF Gene Signatures Predicting Radiosensitivity of Patients with Breast Cancer.

Adjuvant radiotherapy is one of the main treatment methods for breast cancer, but its clinical benefit depends largely on the characteristics of the patient. This study aimed to explore the relationship between the expression of zinc finger (ZNF) gene family proteins and the radiosensitivity of breast cancer patients. Clinical and gene expression data on a total of 976 breast cancer samples were obtained from The Cancer Genome Atlas (TCGA) database. ZNF gene expression was dichotomized into groups with a higher or lower level than the median level of expression. Univariate and multivariate Cox regression analyses were used to evaluate the relationship between ZNF gene expression levels and radiosensitivity. The Molecular Taxonomy Data of the International Federation of Breast Cancer (METABRIC) database was used for validation. The results revealed that 4 ZNF genes were possible radiosensitivity markers. High expression of ZNF644 and low expression levels of the other 3 genes (ZNF341, ZNF541, and ZNF653) were related to the radiosensitivity of breast cancer. Hierarchical cluster, Cox, and CoxBoost analysis based on these 4 ZNF genes indicated that patients with a favorable 4-gene signature had better overall survival on radiotherapy. Thus, this 4-gene signature may have value for selecting those patients most likely to benefit from radiotherapy. ZNF gene clusters could act as radiosensitivity signatures for breast cancer patients and may be involved in determining the radiosensitivity of cancer.

Characterization of CD66b and its relationship between immune checkpoints and their synergistic impact in the prognosis of surgically resected lung adenocarcinoma.

OBJECTIVES: CD66b positive tumor-infiltrating neutrophils (TINs) are key immunity cells in the tumor microenvironment (TME). However, their relationship with clinicopathological features, immune checkpoints (ICs), and prognostic value remains undetermined in lung adenocarcinoma (LUAD). In this study, we aimed to characterize the infiltration by TINs and the prognostic significance in patients with surgically resected LUAD. MATERIALS AND METHODS: Expression of CD66b and ICs, including PD-L1, PD-1, CTLA4, LAG3, TIM3, TIGIT, VISTA, and BTLA, in both cancer cell and tumor-infiltrating lymphocytes (TILs) were estimated by immunohistochemistry in resected LUAD. The associations between CD66b expression and clinicopathological characteristics in patient prognoses were analyzed. We also verified results in another cohort from 85 patients with untreated LUAD and further analyzed the correlation between CD66b expression and EGFR and KRAS mutation status in addition to the rearrangement of the anaplastic lymphoma receptor tyrosine kinase gene (ALK). RESULTS: A total of 240 patients were included in this study. CD66b expression was observed in 87 (36.2%) samples. ICs including PD-L1, PD-1, CTLA4, LAG3, TIM3, TIGIT, VISTA, and BTLA were observed in percentages that ranged from 23.8% to 59.4%. Positive CD66b expression significantly correlated with smoking history (p = 0.029), pathological stage (p = 0.040), and the positive expression of LAG-3 (p < 0.001), PD-1 (p = 0.008), CTLA-4 (p = 0.013), TIM-3 (p = 0.025), TIGIT (p = 0.002), PD-L1 in TILs (p = 0.015), and PD-L1 in tumor cells (p = 0.010). CD66b positivity was significantly associated with worse recurrence-free survival (RFS) (hazard ratio, HR, 1.687; 95% confidence interval, CI, 1.058-2.690, p = 0.028) and overall survival (OS) (HR, 1.667; 95% CI, 1.097-2.534, p = 0.017). Subgroup analysis revealed that the CD66b+/LAG-3 + group had the worst RFS (5-year rate: 39.5%,) and OS (5-year rate: 53.7%,), while the CD66b-/LAG-3 - group had the best RFS (5-year rate: 65.6%) and OS (5-year rate: 78.8%). The p value in analysis of RFS and OS was 0.005 and 0.008, respectively. In the verification set, high expression of CD66b was also significantly correlated with the positive expression of LAG-3 (p < 0.001), PD-1 (p = 0.002), CTLA-4 (p = 0.034), TIM-3 (p = 0.049), PD-L1 in TILs (p = 0.003), and PD-L1 in tumor cells (p = 0.045). There was no correlation between CD66b expression and positive TIGIT expression (p = 0.077), EGFR mutation (p = 0.223), KRAS mutation (p = 0.151), and ALK fusion (p = 0.310). CONCLUSION: CD66b had a relatively high positive expression rate and special clinicopathological features in patients with LUAD. CD66b + TINs were related to the expression of ICs and associated with poor prognoses in LUAD. A combination of CD66b and ICs, especially LAG-3 could further stratify patients into different groups with distinct prognoses.

[Corrigendum] Increased infiltration of macrophages to radioresistant lung cancer cells contributes to the development of the additional resistance of tumor cells to the cytotoxic effects of NK cells.

Subsequently to the publication of the above paper, the authors have realized that the western blots featured in Fig. 5B were inadvertently copied across from Fig. 4B owing to an error made during the figure compilation process. The corrected version of Fig. 5 is featured on the next page, showing the correct data for the western blot analysis of the programmed death receptor ligand 1 level in radioresistant lung cancer cells under the specified experimental conditions. Note that these changes do not affect the interpretation of the data or the conclusions reported in this paper, and all the authors agree to this correction. The authors apologize to the Editor and to the readership of the Journal for any inconvenience caused. [the original article was published in International Journal of Oncology 53: 317-328, 2018; DOI: 10.3892/ijo.2018.4394].

Correction: MiR-940 inhibits TGF-ß-induced epithelial-mesenchymal transition and cell invasion by targeting Snail in non-small cell lung cancer.

[This corrects the article DOI: 10.7150/jca.31800.].

MiR-940 inhibits TGF-ß-induced epithelial-mesenchymal transition and cell invasion by targeting Snail in non-small cell lung cancer.

Increased evidence reveals that miR-940 inhibits the migration and invasion of cancer cells. Considering transforming growth factor ß (TGF-ß) signaling is crucial to cellular epithelial-mesenchymal transition (EMT) process and metastasis of cancer, it is in urgent to explore whether and how miR-940 plays an essential role in regulating TGF-ß-induced EMT in lung cancer progression. In the present study, we observed a reciprocal expression with down-regulated miR-940 and up-regulated Snail mRNA in non-small-cell lung cancer (NSCLC) tissues. we further found that the expression of miR-940 was decreased in NSCLC tissues with lymph node metastasis, advanced TNM stages and poor cell differentiation, in which, on the contrary, the expression of Snail was increased. Overexpression of miR-940 significantly inhibited Snail mRNA and protein expression in A549 and H226 cells. Mechanistically, Snail mRNA was identified as target of miR-940. In addition, miR-940 repressed TGF-ß-induced EMT and further hampered the cell migration and invasion. Finally, siRNA-mediated knockdown of Snail copied the phenotype of miR-940 overexpression in A549 and H226 cells. Taken together, our study reveals that miR-940 can suppress TGF-ß-induced EMT and cell invasion by targeting Snail 3'-UTR mRNA in NSCLC.

Harnessing the secretome of adipose-derived stem cells in the treatment of ischemic heart diseases.

Adipose-derived stem cells (ASCs) are promising therapeutic cells for ischemic heart diseases, due to the ease and efficiency of acquisition, the potential of myocardial lineage differentiation, and the paracrine effects. Recently, many researchers have claimed that the ASC-based myocardial repair is mainly attributed to its paracrine effects, including the anti-apoptosis, pro-angiogenesis, anti-inflammation effects, and the inhibition of fibrosis, rather than the direct differentiation into cardiovascular lineage cells. However, the usage of ASCs comes with the problems of low cardiac retention and survival after transplantation, like other stem cells, which compromises the effectiveness of the therapy. To overcome these drawbacks, researchers have proposed various strategies for improving survival rate and ensuring sustained paracrine secretion. They also investigated the safety and efficacy of phase I and II clinical trials of ASC-based therapy for cardiovascular diseases. In this review, we will discuss the characterization and paracrine effects of ASCs on myocardial repair, followed by the strategies for stimulating the paracrine secretion of ASCs, and finally their clinical usage.

Inhibition of ATM reverses EMT and decreases metastatic potential of cisplatin-resistant lung cancer cells through JAK/STAT3/PD-L1 pathway.

BACKGROUND: The cisplatin-resistance is still a main course for chemotherapy failure of lung cancer patients. Cisplatin-resistant cancer cells own higher malignance and exhibited increased metastatic ability, but the mechanism is not clear. In this study, we investigated the effects of Ataxia Telangiectasia Mutated (ATM) on lung cancer metastasis. MATERIALS AND METHODS: Cisplatin-resistant A549CisR and H157CisR cell line were generated by long-term treating parental A549 and H157 cells (A549P and H157P) with cisplatin. Cell growth, cell migration and cell invasion were determined. Gene expressions were determined by Western Blot and qPCR. Tumor metastasis was investigated using a xenograft mouse model. RESULTS: The IC50 of the cisplatin-resistant cells (A549CisR and H157CisR cells) to cisplatin was 6-8 higher than parental cells. The A549CisR and H157CisR cells expressed lower level of E-cadherin and higher levels of N-cadherin, Vimentin and Snail compared to the parental A549P and H157P cells, and exhibited stronger capabilities of metastatic potential compared to the parental cells. The ATM expression was upregulated in A549CisR and H157CisR cells and cisplatin treatment also upregulated expression of ATM in parental cells, The inhibition of ATM by using specific ATM inhibitor CP466722 or knock-down ATM by siRNA suppressed Epithelial-to-Mesenchymal transition (EMT) and metastatic potential of A549CisR and H157CisR cells. These data suggest that ATM mediates the cisplatin-resistance in lung cancer cells. Expressions of JAK1,2,、 STAT3 、PD-L1 and ATM were increased in A549CisR and H157CisR cells and could by induced by cisplatin in parental lung cancer cells. Interestedly, ATM upregulated PD-L1 expression via JAK1,2/STAT3 pathway and inhibition of ATM decreased JAK/STAT3 signaling and decreased PD-L1 expression. The treatment of PD-L1 neutralizing Ab reduced EMT and cell invasion. Inhibition of JAK1,2/STAT3 signaling by specific inhibitors suppressed ATM-induced PD-L1 expression, EMT and cell invasion. Importantly, inhibition of ATM suppressed EMT and tumor metastasis in cisplatin-resistant lung cancer cells in an orthotopic xenograft mouse model. CONCLUSIONS: Our results show that ATM regulates PD-L1 expression through activation of JAK/STAT3 signaling in cisplatin-resistant cells. Overexpression of ATM contributes to cisplatin-resistance in lung cancer cells. Inhibition of ATM reversed EMT and inhibited cell invasion and tumor metastasis. Thus, ATM may be a potential target for the treatment of cisplatin-resistant lung cancer.

NFκB and TNFα as individual key molecules associated with the cisplatin-resistance and radioresistance of lung cancer.

Cisplatin-resistant (A549CisR and H292CisR) and radioresistant (A549R26 and H292R22) sub-line non-small cell lung cancer (NSCLC) cells were developed in our lab by long term treatment of parental cells with cisplatin or radiation. Our data showed no cross-resistance between these two sets of cell lines, indicating that molecular mechanisms of developing each resistance may be different. Using these sub-line cells, we sought to reveal the most significantly up-regulated molecules in cisplatin-resistant and radioresistant lung cancer cells, compared with parental cells. In qPCR analyses of screening DNA repair and cell survival-associated molecules, we identified NFκB and TNFα as the most significantly up-regulated molecules in cisplatin-resistant and radioresistant lung cancer cells, respectively, compared with parental cells. Western blot analysis of parental vs. resistant cells and the IHC staining of tumor tissues of A549P, A549CisR, and A549R26 cell-derived xenografts in mice confirmed such results. Next, studies using specific inhibitors of NFκB and TNFα and experiments using NFκB and TNFα-knocked down cells showed that inhibition or knockdown of NFκB overcame cisplatin-resistance, while inhibition or knockdown of TNFα increased radiosensitivity of radioresistant lung cancer cells. Therefore, these two molecules may be used as markers of the prognosis/diagnosis of individual resistance development during lung cancer treatment.

RETRACTED: Neuroendocrine differentiation contributes to radioresistance development and metastatic potential increase in non-small cell lung cancer.

This article has been retracted: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy). This article has been retracted at the request of the authors and their institute. The BBA Editor-in-Chief has agreed to retract the paper. In this paper, there were two errors identified to the journal by the authors: The first error was in Western blot gel band images of Fig. 4A (p-MAPK, MAPK, p-Erk, and Stat3) and the 8 gel band images of Fig. 4G. The second error was in the cell culture images of Figures 3F, 3J, and 4E. The authors state that these errors were due to uploading mistakes in the preparation of the manuscript. The authors apologize for these errors and any inconvenience caused. The Editor-in-Chief initially agreed to retract the paper based on the identification of these two errors. Readers are able to see further discussion of the paper on the PubPeer site here: https://pubpeer.com/publications/569EB2CE7A7335D7F3F8F3FF310936

miR­150 promotes the proliferation and migration of non­small cell lung cancer cells by regulating the SIRT2/JMJD2A signaling pathway.

Lung cancer (LC), as the most common cause of cancer­related mortality worldwide, is characterized by difficulties in early detection, a high degree of malignancy, poor sensitivity to radiotherapy and chemotherapy, and a low 5­year survival rate. MicroRNA (miRNA) is a class of small, non­coding, endogenously expressed RNA that serves vital roles in RNA silencing and post­transcriptional regulation of gene expression. Previous studies have shown that abnormal expression of miRNA is relevant to various malignant tumors, including lung cancer. In the present study, miR­150 was found to be significantly upregulated in non­small cell lung cancer (NSCLC) cells, which also exhibited downregulation of SIRT2. Through downregulation of miR­150 and/or overexpression of SIRT2 in NSCLC cells (A549 and H1299), in vivo assays revealed that the suppression of miR­150 and re­expression of SIRT2 could inhibit NSCLC cell growth. Additionally, the present data demonstrated that miR­150 regulated NSCLC cell viability and mobility through SIRT2/JMJD2A. Finally, it was demonstrated that silencing of miR­150 led to inactivation of the AKT signaling pathway, which eventually inhibited the viability and mobility of NSCLC cells. This inhibitory effect of miR­150 could be exacerbated by upregulation of SIRT2. In conclusion, our results demonstrated that miR­150 plays an important role in the development of lung cancer by serving as an oncogene via the SIRT2/JMJD2A signaling pathway.

Arginine, glycine, aspartic acid peptide-modified paclitaxel and curcumin co-loaded liposome for the treatment of lung cancer: in vitro/vivo evaluation.

PURPOSE: In this study, a novel arginine, glycine, aspartic acid peptide (RGD)-modified paclitaxel and curcumin co-loaded liposomes were developed to evaluate their antitumor activity in vitro and in vivo. MATERIALS AND METHODS: Co-loaded liposomes were prepared using the solvent evaporation method. The particles had spherical shapes under electron microscopy with sizes <130 nm. RESULTS: By comparison with the free drug, RGD-modified paclitaxel and curcumin co-loaded liposomes and paclitaxel and curcumin co-loaded liposomes have sustained-release properties in vitro. In vivo, there was no significant difference in pharmacokinetic parameters between the RGD-modified paclitaxel and curcumin co-loaded liposomes and paclitaxel and curcumin co-loaded liposomes. A strong green fluorescence was observed in the cytoplasmic region after incubation of RGD-modified paclitaxel and curcumin co-loaded liposomes for 2 h. RGD-modified paclitaxel and curcumin co-loaded liposomes showed a superior antiproliferative effect on A549 cells with a possible mechanism that suppressed the multidrug resistance phenomenon and exhibited a clear synergistic effect. CONCLUSION: The results indicate that RGD-modified paclitaxel and curcumin co-loaded liposomes had a better antitumor effect in vivo than the non-modified LPs. These results indicate that RGD-modified co-loaded liposomes are a promising candidate for antitumor drug delivery.

Increased infiltration of macrophages to radioresistant lung cancer cells contributes to the development of the additional resistance of tumor cells to the cytotoxic effects of NK cells.

In this study, in order to investigate the effects of increased macrophage infiltration to radioresistant lung tumors in regulating natural killer (NK) cell-mediated immunity, we examined whether the treatment of radioresistant cells with conditioned medium (CM) from phorbol myristate acetate (PMA)/interleukin (IL)-4 treated THP-1 cells (used as a tumor-associated macrophage source) leads to the development of the additional resistance of tumor cells to NK cell cytotoxicity. We found that the susceptibility of THP-1 CM-treated radioresistant cells to NK cell cytotoxicity was decreased compared to the non-treated cells. In addition, it was found that such a decreased susceptibility was associated with increased programmed death receptor ligand 1 (PD-L1) and decreased natural killer group 2D (NKG2D) ligand levels in tumor cells. We further discovered that the THP-1 cells secreted a high level of IL-6, and that blocking IL-6 action by the addition of a neutralizing antibody (Ab) for IL-6 into the THP-1 CM decreased the resistance of THP-1 CM-treated radioresistant cells to NK cell cytotoxicity. Moreover, we discovered that MEK/Erk was the most critical IL-6 downstream signaling pathway in triggering the THP-1 CM effect; thus, the addition of MEK/Erk inhibitor to THP-1 CM enhanced the susceptibility of the THP-1 CM-treated radioresistant cells to NK cell cytotoxicity. On the whole, the findings of this study suggest the existence of a malignant loop characterized by increased macrophage infiltration into radioresistant cells which, in turn, promotes the development of the additional resistance of these cells to NK cell cytotoxicity.