Introduction

Most clinical contrast agents are molecular-based gadolinium chelates lacking target specificity with a short in vivo circulation time and a relatively low longitudinal relaxation rate. In contrast, MnO_x is rapidly reduced to O₂ and Mn²⁺ in the tumor microenvironment ^[1]. The released Mn ions show powerful MR imaging effects and can act as a switch for ¹⁹F MRI signals by the paramagnetic relaxation enhancement (PRE) effect ^[2]. The emerging Mn ion immunotherapy offers an alternative way of utilizing the immunomodulatory function of metal ions in tumor treatment. Mn²⁺ is a promising cGAS activator, which can activate the cGAS-STING pathway and heighten cGAS sensitivity to dsDNA ^[3]. The utilization of manganese -based nanoparticles can potentially aid in the diagnosis of tumors and activate the immune system synergistically therapy.

Methods

FS@ICG nanoparticles were synthesized using the CTAB, PFOB and ICG. FMBI nanoparticles were further decorated by a MnO_x shell layer, and BSA was adsorbed onto the outlayer to improve the biocompatibility. The characterization including TEM, EDS and XPS shows the successful synthesis of the nanoparticles. The ¹H/¹⁹F relaxation rates of FMBI NPs were measured in vitro and in vivo. FMBI nanoparticles were evaluated for their immune activation abilities in B16F10 cells using Western blot and immunofluorescence.

Results and Discussion

The FMBI NPs exhibit a core-shell-shell like structure (Fig. 1A) with Mn and F uniformly dispersed within the nanoparticles (Fig. 1B). The X-ray photoelectron spectroscopy results showed that Mn and O elements are present in the FMBI NPs (Fig. 1C). The Mn2p spectrum indicates that the predominant forms are Mn³⁺ and Mn⁴⁺ with relative intensities of 48.47% and 26.98% respectively (Fig. 1D). The ¹⁹F NMR spectrum of FS nanoparticles exhibited the same signal as PFOB, demonstrating the successful encapsulation of PFOB within the FS nanoparticles. Following MnO_x modification, the ¹⁹F NMR signal vanished due to the PRE effect. Subsequently, manganese ions were released upon the addition of glutathione, which turn on the corresponding ¹⁹F NMR signal (Fig. 2A). The phantom experiments demonstrated that the signal intensity of both T₁-weighted and ¹⁹F MRI were dependent on GSH concentration (Fig. 2B and 2C). To evaluate the impact on the cGAS-STING pathway, western blot was employed to detect related protein expression in B16F10 cells (Fig. 2D), which showed that FMBI NPs promoted the upregulation of these proteins. Overall, these findings suggested that FMBI NPs showed potential as a therapeutic approach for cancer treatment. In addition, confocal images demonstrated an increased expression of CRT for immunogenic cell death as well as a decreased expression of HMGB1 (Fig. 2E and 2F). T₁-weighted in vivo MRI signals in the tumor region was enhanced, and the most significant signal enhancement appeared at 8 hours after i.v. injection of FMBI NPs (Fig. 3A). Moreover, ¹⁹F MRI imaging of mice showed a vivid ¹⁹F signal at the tumor site after in situ injection of FMBI NPs (Fig. 3B).

Conclusion

The FMBI NPs were developed that can activate the cGAS-STING pathway for tumor diagnose and therapy. The FMBI NPs can release Mn²⁺ in the tumor microenvironment and enhance the signals of ¹H/¹⁹F MRI in tumor areas, with the cGAS-STING pathway being activated. Combining photosensitizer-induced photothermal and photodynamic therapy with TME activation can convert immunosuppressed TME to immune-activated TME to enhance the anti-tumor efficacy. In summary, utilizing TME-activated manganese-based nanoparticles for MRI can potentially serve as an effective immunostimulatory method for anti-tumor treatment.