Nanotechnology

Combination of ferroptosis and pyroptosis dual induction by triptolide nano-MOFs for immunotherapy of Melanoma | Journal of Nanobiotechnology

October 20, 2023

Effects of the combination of Fe³⁺ and TPL on ROS and cytotoxicity

It has been reported that TPL can inhibit the expression of Nrf2, thus inhibiting Nrf2-mediated GSH synthesis and increasing intracellular ROS levels [28]. In addition, the increase in intracellular iron can increase ROS production through the Fenton reaction [23]. Therefore, we considered whether the combination of Fe³⁺ and TPL can synergistically amplify ROS generation. First, intracellular ROS levels were measured using DCFH-DA probe. DCFH-DA itself is not fluorescent, but it can be rapidly oxidized in the presence of ROS to produce fluorescent compound 2′,7′-dichlorofluorescein (DCF), and thus the levels of ROS can be monitored dynamically by measuring the intensity of fluorescence signal. As predicted, there was weak fluorescence in the untreated group, indicating basal levels of intracellular ROS. The fluorescence intensity was enhanced in the Fe³⁺ group compared to the control group, indicating that Fe may increase the production of intracellular ROS. In addition, the fluorescence intensity of the TPL group was also enhanced compared with the control group, indicating that TPL could also induce ROS production. Surprisingly, the fluorescence intensity of the TPL plus Fe³⁺ groups was further enhanced, suggesting that Fe³⁺ could cooperate with TPL to increase ROS production after entering cells (Fig. 1A). For quantitative analysis, we measured the intracellular fluorescence intensity by flow cytometry (Fig. 1B, C), and these data were consistent with the results of fluorescence micrographs. This shows that the combination of Fe and TPL has a significant ability to synergistically amplify ROS production. Then, we initially evaluated the toxicity of Fe³⁺ in combination with TPL to B16F10 cells using CCK8 assays (Fig. 1D). We found that Fe³⁺ alone showed almost no toxicity to B16F10 cells, and although Fe³⁺ was previously shown to promote intracellular ROS levels, it was not sufficient to inhibit cell growth. Notably, Fe³⁺ in combination with TPL exhibited stronger cytotoxicity than free TPL, suggesting that TPL in combination with Fe³⁺ can synergistically amplify ROS and thus exert anti-tumor effect.

Preparation and characterization of TPL@TFBF

TPL is a hydrophobic drug with poor solubility in aqueous solution. Additionally, the ability of free Fe³⁺ to enter the cell is limited. Therefore, it is necessary for us to construct a nanoformulation that can supply Fe³⁺ and load hydrophobic drugs. To construct TPL@TFBF, we prepared pure drug nanonuclear via a drug self-assembly strategy, by simply dissolving the drug in DMSO solution and then adding it dropwise to an aqueous solution under ultrasonication. This was followed by the formation of a MOF shell layer by TA and Fe³⁺ coordination, and finally adding BSA to make the nanoparticles more stable. We first explored the influence of BSA dosing concentration on the particle size and polydispersity index (PDI) (Fig. 2A), and found that the particle size and PDI were minimized when the BSA dosing concentration was 1 mg/mL, therefore, we chose a BSA concentration of 1 mg/mL for subsequent preparations. Then, we optimized the drug delivery concentration of TPL, and the results showed that when the input concentration of TPL was 100 ~ 300 µg/mL, the particle size did not change significantly. However, at 200 µg/mL TPL, the PDI was the smallest and the drug loading (LD%) (36.2%) and encapsulation efficiency (EE%) (81.4%) were the highest (Fig. 2B, C); therefore, we chose a TPL input concentration of 200 µg/mL for the subsequent TPL@TFBF nanoparticle preparation.

To endow the nanostructures with tumor-targeting properties, we modified BSA with FA, a ligand for the folate receptor that is overexpressed on several types of tumor cells, including B16F10 cells [37]. BSA-FA was synthesized by our previous method [38], and its successful coupling was confirmed by UV spectroscopy (Fig. 2E). Using BSA-FA as a stabilizer, the obtained TPL@TFBF had a size of approximately 200 nm and ξ potential of approximately − 17 mV (Fig. 2D, F), both of which were very similar to those of the TPL@TFB. Therefore, the effect of coupling FA onto BSA-modified TPL nanoparticles is small. In addition, we measured the particle size and zeta potential of TPL@TF using a Malvern particle sizer (Fig. 2D, F and Additional file 1: Fig. S1) and found that the hydrodynamic particle size of TPL@TFB after BSA wrapping increased slightly from ~ 160 nm to ~ 200 nm, and the zeta potential decreased from approximately − 29 mV to approximately − 17 mV. Both of these results demonstrate that BSA was effectively modified on the surface of TPL@TF. We used SDS–polyacrylamide gel electrophoresis to characterize the BSA in the sample after TPL@TFBF depolymerization, and the data further demonstrated the successful inclusion of BSA (Additional file 1: Fig. S2). The TEM results showed that the TPL@TFBF exhibited a typical spherical morphology (Fig. 2D) with a size of approximately 100 nm. This is slightly less than that measured by DLS because DLS measures the hydrated particle size, and the expansion of BSA in aqueous solution also increases the particle size of TPL@TFBF. Elemental characterization of the TPL@TFBF was performed using EDS (Fig. 2G, H). The results showed that Fe, a characteristic element in TA-Fe(III) MOFs, and S, a characteristic element in BSA, were present in the NPs. These results showed that the BSA-modified TA-Fe(III) MOFs were successfully prepared. The nanoparticles exhibited excellent colloidal stability as well as long-term storage stability in various biological media due to the surface stabilization provided by BSA (Fig. 2I and Additional file 1: Fig. S3). Finally, the LD% and EE% of Fe in the TPL@TFBF were determined to be 3.28% and 29.52%, respectively, by ICP‒OES.

Tumor targeting in vitro

Next, we studied the cellular uptake of TPL@TFBF by using B16F10 cells as a model. To track intracellular TPL@TFBF, according to our previous method [38], BSA was labelled with green fluorescein isothiocyanate (FITC), and the nucleus was colored blue with DAPI for localization. Compared with the control group, the confocal laser scanning microscopy (CLSM) images showed that cells treated with TPL@TFB showed only weak fluorescence, indicating less uptake of TPL@TFB (Fig. 3A). This may be due to the cell’s rejection of the negatively charged nanoparticles. In contrast, the intracellular fluorescence was enhanced in the TPL@TFBF group. This apparently enhanced cellular uptake may be attributed to folate receptor β (FR-β) overexpression on the membrane of B16F10 cells, promoting the cellular uptake of TPL@TFBF through a specific interaction between FA and FR. To test this, we pretreated cells with free FA to bind to and saturate the folate receptor on the membrane. As expected, this caused the intracellular fluorescence to be significantly reduced, further demonstrating that FA modification can increase the uptake of nano-MOFs. For quantitative examination, we performed flow cytometry experiments (Fig. 3B) and quantified the fluorescence intensity (Fig. 3C). Compared with the TPL@TFB group, the fluorescence intensity in the TPL@TFBF group was significantly enhanced, showing the targeting ability of these NPs. On the other hand, free FA-pretreated cells displayed elimination of this FA-mediated delivery. Overall, the flow cytometry results were in line with the CLSM results.

In vitro analysis of TPL@TFBF

After demonstrating the tumor-targeted delivery, we initially evaluated the in vitro toxicity of various agents to B16F10 cells using the CCK8 assay. After 48 h of treatment with different preparations containing different drug concentrations, cell viability decreased in a dose-dependent manner (Fig. 4A), and at each concentration, the TPL@TFBF exhibited stronger cytotoxicity than free TPL and TPL@TFB, leading to significant decreases in IC50 values (Fig. 4B). This may be because the nanoparticle-mediated delivery of TPL and the cell inhibitory activity of TPL were significantly improved after it was incorporated into nanoparticles. For the TPL@TFBF, FA modification enhanced the anti-tumor effects through targeted delivery. ROS are key factors that promote lipid peroxidation and play an important role in various types of cell death, among which hydroxyl radicals play an important role in anticancer therapy [23]. The Fenton reaction mediated by iron ions can convert endogenous H₂O₂ into hydroxyl radicals (•OH), thereby increasing intracellular ROS levels, especially Fe²⁺, which can trigger a more efficient Fenton reaction [39]. First, we verified whether Fe³⁺ would be reduced to Fe²⁺ by performing a 1,10-O-phenanthroline colorimetric assay before TA-Fe³⁺ MOF application. As shown in Additional file 1: Fig. S4, the color in the TFBF and TPL@TFBF groups did not change significantly after the addition of phenanthroline, and the conversion of Fe²⁺ was determined to be negligible (less than 2%). This result shows that TFBF and the TPL@TFBF have a weak ability to reduce Fe³⁺ to Fe²⁺ before application. Next, we explored the ability of TPL@TFBF to generate Fe²⁺ by TA reduction of Fe³⁺ after cell entry. As shown in Fig. 4C, the conversion to Fe²⁺ was only 6.9% after the addition of TA at pH 7.4, and as the pH of the system decreased, more Fe³⁺ was reduced to Fe²⁺, reaching 40% at pH 5.5. The addition of H₂O₂ to the above system triggered the Fenton reaction mediated by Fe²⁺, and Fe²⁺ was converted to Fe³⁺ again. We further verified the generation of free radicals by using the fading reaction of MB. As shown in Fig. 4D, free MB has strong ultraviolet absorption at 664 nm. The addition of Fe³⁺, TA and H₂O₂ leads to the formation of •OH, which degrades MB and significantly reduces its ultraviolet absorption. In particular, at pH 5.5, MB was completely degraded. The above results indicated that TA could reduce Fe³⁺ to Fe²⁺ under acidic conditions, effectively triggering the Fenton reaction and converting H₂O₂ to •OH, confirming that the TPL@TFBF can induce the Fenton reaction in situ in the acidic tumor microenvironment. In addition, we measured intracellular ROS levels using the DCFH-DA probe. Compared with the control group, the fluorescence intensity of TFBF group was enhanced, further demonstrating that TA reduces Fe³⁺ to Fe²⁺ after TFBF enters tumor cells, triggering the Fenton reaction and increasing ROS production. The fluorescence intensity was also enhanced in the TPL group compared to the control group, implying that TPL could also induce ROS production. Unexpectedly, the fluorescence intensity of the TPL@TFB group was enhanced relative to that of the TFBF and TPL groups, implying that the Fe³⁺ entering the cells could cooperate with TPL to increase the production of ROS. In addition, the strongest fluorescence intensity was observed in the TPL@TFBF group, which may be due to the targeted delivery mediated by FA, further increasing the level of intracellular Fe³⁺ and synergistically amplifying the ROS signal (Fig. 4G). For quantitative analysis, we also measured the intracellular fluorescence intensity by flow cytometry (Fig. 4E, F). The fluorescence intensity of the TPL@TFBF group was 4 times higher than that of the untreated group and significantly different compared with the other groups, which was in accordance with the photographs. This shows that the TPL@TFBF can significantly and synergistically amplify ROS production through the binding of Fe³⁺ and TPL.

Verification of anti-tumor mechanism of action

In summary, there is a synergistic amplification of ROS production by the combination of Fe³⁺ and TPL, which led to a significant enhancement in the efficacy of the TPL@TFBF. We next examined the potential mechanisms. As we all know, Nrf2 has long been considered an important component of the antioxidant system that regulates the expression of antioxidant genes, thereby counteracting oxidative and pro-electrical stress [40]. In addition, a series of redox-related genes targeted by Nrf2 are key mediators of ferroptosis. For example, the inhibition of GPX4-induced ferroptosis could be reversed by overexpressing Nrf2 [41]. However, TPL can directly inhibit the expression of Nrf2 and interfere with de novo synthesis of glutathione [28], thereby increasing ferroptosis sensitivity and producing antitumor effects. On the other hand, the Fe³⁺ mediated Fenton reaction can elevate intracellular ROS levels to induce ferroptosis. In addition, manipulating the content of intracellular iron was found to induce pyroptosis [23]. Therefore, the combination of TPL and Fe³⁺ may enhance the therapeutic effect by synergistically amplifying ROS production, thereby causing simultaneous ferroptosis and pyroptosis. We next examined whether TPL@TFBF could induce both ferroptosis and pyroptosis.

First, B16F10 cells were treated with various ferroptosis inhibitors/promoters in combination with various preparations, and the relative cytotoxicity was calculated by comparison with the group not treated with ferroptosis inhibitors/promoters. As shown in Fig. 5A, the ferroptosis inhibitor Fer-1 alleviated the cytotoxicity of the TPL@TFBF. Similarly, the iron chelator DFO, the ferroptosis inhibitor Cys, and the ferroptosis detoxifier GSH attenuated the cytotoxicity of TPL@TFBF. In contrast, Glu, as a ferroptosis inducer, achieved the opposite effect by inhibiting Cys uptake. Overall, all these results demonstrate that the constructed of TPL@TFBF can effectively promote ferroptosis. To further explore the mechanism by which TPL@TFBF induces ferroptosis, the expression levels of Nrf2 and GPX4 were detected by WB. GPX4 is a marker of ferroptosis. Compared with that in the control and TFBF groups, the expression of Nrf2 and GPX4 was downregulated in B16F10 cells treated with TPL, TPL@TFB and TPL@TFBF, and notably, their expression was the lowest in the TPL@TFBF group (Fig. 5B), suggesting that the codelivery of TPL and Fe³⁺ could significantly reduce the expression of Nrf2 and GPX4. Quantification of the protein levels further confirmed this significant change (Additional file 1: Fig. S5). Correspondingly, intracellular GSH levels showed the same downwards trend (Fig. 5C). Since GPX4 requires the assistance of GSH to participate in lipid peroxidation, the downregulation of GSH induced by TPL can promote the downregulation of GPX4 to induce ferroptosis. By verifying these two key factors required for ferroptosis (GSH reduction and GPX4 downregulation), it was confirmed that the TPL@TFBF could induce ferroptosis.

TPL has been found to induce GSDME-dependent pyroptosis [42], and iron-activated ROS have previously been reported to induce pyroptosis via the Tom20-Bax-caspase-GSDME pathway [24]. Therefore, further research was needed to determine whether GSDME-dependent pyroptosis also contributes to TPL@TFBF-induced cell death. GSDME-dependent pyroptosis is mainly activated by caspase-3. After activation, the GSDME protein hydrolyses and releases N fragments, which assemble to form pores on the cell membrane. The cell swelling and plasma membrane rupture result in the destruction of membrane integrity and the release of cellular contents [43, 44]. To determine whether TPL@TFBF induced pyroptosis via the GSDME pathway, we treated B16F10 cells with DEVD. As shown in Fig. 5D, the cytotoxicities of TPL, TPL@TFB and TPL@TFBF were significantly reduced by the addition of DEVD, and the TPL@TFBF showed a stronger effect, suggesting that TPL@TFBF-induced cell death may depend on the caspase-3-mediated pathway. To further explore the pyroptosis mechanism of TPL@TFBF treatment of B16F10 cells, we examined the levels of pyroptosis-associated protein in B16F10 cells using WB. As shown in Fig. 5E, cleaved caspase-3 and GSDME-N expression were increased in the TPL, TPL@TFB, and TPL@TFBF groups, and GSDME-N was more strongly upregulated in the TPL@TFBF group than in the other groups. These results indicated that free TPL could induce pyroptosis mediated by caspase-3, and the addition of Fe³⁺ could better induce pyroptosis. This phenomenon was further confirmed by semiquantitative analysis (Additional file 1: Fig. S6), which showed that the expression level of GSDME-N in the TPL@TFB group was 1.5 times that in the TPL group and that in the TPL@TFBF group was 1.5 times that in the TPL@TFB group. These results suggest that TPL@TFBF can activate caspase-3 to cleave GSDME into GSDME-N and further induce pyroptosis. Subsequently, we evaluated the release of lactate dehydrogenase (LDH, an indicator of pyroptosis) and IL-1β into the cell supernatant to examine the leakage of intracellular contents. The release of LDH and IL-1β into the supernatant of TPL@TFBF-treated cells was significantly higher than that in the other treatment groups (Fig. 5F and I). To further demonstrate the ability of the TPL@TFBF to induce pyroptosis, we used Annexin V-FITC/PI double staining analysis as reported in the literature [45,46,47]. As shown in Fig. 5G, H and Additional file 1: Fig. S7, B16F10 cells underwent significant pyroptosis after TPL@TFBF treatment. Moreover, there were very few early apoptotic cells, which can rule out the occurrence of apoptosis. Thus, TPL@TFBF enhanced caspase-3 mediated GSDME cleavage and increased the release of IL-1β and LDH, confirming that TPL@TFBF is a good inducer of pyroptosis.

Induction of ICD

Previous studies have shown that both pyroptosis and ferroptosis may lead to immunogenic cell death (ICD), so there is an opportunity to use this combination with immunotherapy for synergistic effects [48,49,50]. To explore this effect, we next studied the DAMPs released from dying cells, including HMGB1, ATP and CRT, which are important features of ICD. We first examined the release of ATP and HMGB1 into the cell supernatants, and extracellular ATP and HMGB1 had been released in the TPL, TPL@TFB and TPL@TFBF groups in comparison with the control and TFBF groups, especially in TPL@TFBF groups (Fig. 5J, K). We also detected intracellular HMGB1 levels by WB. Compared with the control and TFBF groups, TPL, TPL@TFB and TPL@TFBF were all effective in reducing HMGB1 concentration, and the reduction effect of TPL@TFBF was more significant (Fig. 5L). We then examined CRT exposure by flow cytometry, and the results showed that TPL@TFBF significantly increased CRT exposure (Fig. 5M); moreover, quantitative analysis showed that the CRT exposure induced by the TPL@TFBF was significant compared with that in the other treatment groups (Fig. 5N). These results suggest that the ability of TPL@TFBF to induce immunogenicity is enhanced to some extent by FA modification due to tumor targeting activity. Overall, cells experienced ICDs after treatment with TPL@TFBF, which could be attributed to pyroptosis and ferroptosis cytotoxicity induced by nano-MOFs.

In vivo performance of nano-MOFs

As we demonstrated, TPL@TFBF can induce ferroptosis and pyroptosis in vitro. Therefore, we hypothesized that TPL@TFBF can also inhibit tumor growth in vivo. Next, we applied a B16F10 tumor-bearing mouse model to characterize the performance of TPL@TFBF in vivo. Blood compatibility tests were performed before the nanoparticles were injected intravenously. At concentrations as high as 200 µg/ml, hemolysis rates of less than 2% were observed after with TPL@TFBF (Additional file 1: Fig. S8), indicating their high biosafety after intravenous administration. To study the in vivo biodistribution, TPL@TFBF was labelled with Cy5.5. After 24 h of nanoparticle injection, the fluorescence of the nanoparticles was much stronger than that of free Cy5.5, which may be attributed to the longer circulation half-life of the nanosystem. It is worth noting that significant fluorescence from the nanoparticles was observed in tumor tissue, indicating tumor accumulation through the EPR effect, especially for the FA-modified nanoparticles, indicating the active targeting capability of the nanoparticles in vivo (Fig. 6A). In addition, we extracted the major organs and tumor tissues of the mice for ex vivo fluorescence imaging (Fig. 6B) and observed significant fluorescence from [email protected] in the tumor tissues, which was consistent with the in vivo results. Overall, our nano-MOFs can actively target tumors to deliver drugs, which will be beneficial for improving therapeutic efficacy and minimizing nonspecific side effects.

In order to evaluate the anti-tumor effects in vivo, tumor-bearing mice were randomly divided into 6 groups and treated with different formulations. The tumor size was measured to monitor the dynamic efficacy. The tumor volume increased rapidly during treatment in the PBS and TFBF groups, indicating that TFBF had no tumor suppressive effect. Compared with the PBS group, the free TPL group had moderate tumor growth inhibition (~ 30.97%), and the TPL@TFB group showed better tumor control than TPL (~ 41.74%), suggesting a synergistic effect from TPL and Fe³⁺ in vivo. The tumor suppression effect of TPL@TFBF (~ 65.58%) was enhanced to some extent by FA modification, almost reaching that of the positive drug DAC (~ 63.47%), probably due to tumor targeting (Fig. 6C, D). Tumor tissues were extracted after treatment for direct comparison (Fig. 6E, F), and the tumor weights and corresponding tumor images were consistent with the overall trend of tumor volume. We further examined the tumor tissue by H&E staining to evaluate the therapeutic effect. In Fig. 6G, we observed significant cell necrosis and nucleopaenia in the TPL@TFBF and DAC groups. In addition, we constructed survival curves for each group (Fig. 6H), and the survival rate was 16.67% in the free TPL group and 83% in the TPL@TFBF group after 14 days, indicating that the TPL@TFBF significantly prolonged the survival of mice compared with free TPL. Thus, the above results suggest that TPL@TFBF can improve the efficiency of melanoma treatment and alleviate side effects.

To further investigate the tumor suppression mechanism of action, we assessed the expression levels of GPX4, Nrf2, cleaved caspase-3 and GSDME-N in the tumor region (Fig. 6I, J). In line with the results of the in vitro analysis, GPX4 and Nrf2 at the TPL@TFBF-treated mouse tumor sites were significantly downregulated, and cleaved caspase-3 and GSDME-N were significantly upregulated compared with the TPL and TPL@TFB groups, indicating that TPL@TFBF could exert anti-tumor effects by inhibiting Nrf2 expression and elevating intracellular ROS, thereby inducing ferroptosis and pyroptosis.

Moreover, the safety of the nanosystem was evaluated. There was little change in the weights of the mice during treatment (Additional file 1: Fig. S9), indicating no acute toxicity. Analysis of the serum biochemical indices after treatment showed that liver function and renal function were within the normal range (Additional file 1: Fig. S10), indicating that the nano-MOFs had no hepatotoxicity or renal toxicity. Further H&E staining analysis of major organs showed no pathological changes (Additional file 1: Fig. S11). In addition, to determine whether the release of various inflammatory factors from ferroptosis and pyroptosis could cause cytokine storm syndrome, we also measured the changes in serum TNF-α and IL-6 levels, and the changes after various treatments were negligible (Additional file 1: Fig. S12). Overall, our nano-MOF is a platform with high biosafety.

Antitumor immunity induced by TPL@TFBF

To study the in vivo immune effect, we removed tumors after euthanasia of the mice and detected the proportion of immune cells in the tumor tissue by flow cytometry. First, mature dendritic cells (mDCs) and tumor-infiltrating T cells were detected by flow cytometry. The results showed that compared with PBS or TFBF, the proportion of activated DCs (CD86⁺CD11c⁺) in TPL@TFBF tumor tissues increased from 36.6 to 68.85%, indicating an increased level of DCs maturation after TPL@TFBF treatment (Fig. 7A, B and Additional file 1: Fig. S13). This was due to the simultaneous occurrence of ferroptosis and pyroptosis after treatment with the TPL@TFBF, which leads to a stronger immune effect, thus amplifying the maturation of DCs. After DCs maturation, antigens are presented to T cells to initiate T-cell cloning and proliferation. We monitored the ratio of cytotoxic T cells (CD8⁺) and helper T cells (CD4⁺) (Fig. 7A, C and D) and found that tumor infiltration of cytotoxic T lymphocytes (ctls, CD3⁺CD8⁺ T cells) and helper T cells (Ths, CD3⁺CD4⁺ T cells) the TPL@TFBF group was significantly higher than that in other groups. These results suggest that TPL@TFBF treatment can promote T-cell clonal expansion. Then, the expression levels of immune-related proteins in mouse tumor tissues were investigated by WB (Fig. 7E, J). After TPL treatment, the expressions of CD86, CD4 and CD8 in mouse tumor tissues were observably up-regulated. The expression of the above proteins was further up-regulated after treatment with TPL@TFB, and again after treatment with TPL@TFBF, indicating the synergistic immune activation effect of TPL and Fe³⁺. The expression of the above proteins was further investigated by immunofluorescence, and the results were basically consistent with those of WB, indicating that TPL@TFBF treatment initiated an adaptive immune response (Additional file 1: Fig. S14). T cells are activated and secrete cytokines such as tumor necrosis factor α (TNF-α) and interleukin 6 (IL-6), which directly kill tumor cells. The levels of TNF-α and IL-10 in mouse tumor tissues were significantly increased after treatment with TPL@TFBF (Fig. 7K and L), which also indicated the effective activation of anti-tumor immunity. In addition, IL-1β levels of pyroptosis associated inflammasomes were elevated, suggesting pyroptosis (Fig. 7M). Together, these in vivo experimental results show that TPL@TFBF can cause ferroptosis and pyroptosis in cancer cells, enhance immunogenicity, promote DCs activation, and trigger a T-cell-dependent immune response for excellent anti-tumor efficacy and efficient stimulation of adaptive anti-tumor immunity.

TPL@TFBF inhibits tumor metastasis, and its combination with aPD-L1 enhances immunotherapy

The main cause of death in patients with clinical melanoma is due to their susceptibility to lung metastasis, which is also a major challenge in cancer treatment. Fortunately, TPL has been shown to inhibit the transfer of B16F10 cells to the lungs and spleens of mice. In addition, immunotherapy has the potential to inhibit metastasis. As indicated above, our nano-MOFs produce a good immune response. We constructed a lung metastatic tumor model by intravenously injecting B16F10-luc cells and evaluated the ability of TPL@TFBF to inhibit tumor metastasis. After different treatments, in vitro bioluminescence imaging was performed using fluorescein as the substrate (Fig. 8A). The lung tissue from the PBS group showed a strong fluorescence signal, indicating the successful construction of the mouse lung metastasis model. There was no significant change in the fluorescence signal in the TFBF and DAC groups, and the effects of TFBF and DAC on lung metastasis were apparently limited. After treatment with TPL and TPL@TFB, the fluorescence signal in mouse lung tissue were significantly reduced, indicating their inhibition of lung metastasis. After treatment with TPL@TFBF, there was almost no fluorescence signal in the lung tissue, indicating that metastasis activation was completely suppressed, which could be attributed to the strong immune activation produced by the TPL@TFBF (Fig. 8B).

Next, lung tissues were collected for metastatic site analysis. A large number of pulmonary metastatic nodules with melanoma cell aggregates were observed in the PBS group, while no obvious metastatic nodules were observed in the TPL@TFBF group, confirming that TPL@TFBF can effectively inhibit pulmonary metastasis (Fig. 8C). To confirm this, we performed further H&E staining evaluations on each type of lung tissue, and the lung metastatic are circled with red dashed circles in Fig. 8D. Large tumor metastases were visible in the PBS group, TFBF group and DAC group, only a few smaller metastases remained in the TPL group and TPL@TFB group, and no obvious metastases were seen in the TPL@TFBF group, confirming that TPL itself could inhibit lung metastasis, but TPL@TFBF might further inhibit lung metastasis due to activation of the immune response.

We further used the nanosystem in combination with the ICB PD-L1 antibody (aPD-L1) for melanoma for tumor co-immunotherapy. Establishment of the mouse model and the comprehensive treatment protocol are shown in Fig. 8E. Notably, the single agent aPD-L1 showed only marginal tumor suppression. TPL@TFBF was superior to aPD-L1 due to its strong immunomodulatory effect, which were markedly enhanced when used in combination with aPD-L1 (Fig. 8F, K and Additional file 1: Fig. S15). In this combination, TPL@TFBF promotes DCs activation and T-cell infiltration, while aPD-L1 activates T cells to attack tumor cells, producing a synergistic anti-tumor effect.

Effects of the combination of Fe3+ and TPL on ROS and cytotoxicity