As a kind of typical nanomaterial MnO₂ NPs can be used as carriers to design targeted and controlled delivery systems. MnO₂ NPs with mesoporous shell have been widely proved to show the ability to load chemotherapy drugs for efficient targeted drug delivery with controlled drug release behaviors (21, 22). MnO₂ NPs or MnO₂ nanosheets can also be used as the carrier of the photosensitizer drugs to achieve the controlled intracellular release for enhanced phototherapy efficiency (23). The main role of MnO₂ nanocarriers for drug delivery is as a “gatekeeper” for the encapsulated drugs, which decompositions at the acidic pH after reaching the targeted site to release the encapsulated drugs, thereby improving the bioavailability, efficacy selectivity of drugs for more effective therapy (19).

Except for the drug carrier roles, MnO₂ NPs can also be used as nucleic acid vector for nucleic acid delivery-based therapies. For example, MnO₂ nanomaterials can be used also as siRNA loading system for targeted delivery and controlled release siRNA for more effective gene expression regulation, which therefore allowed tumor-related gene therapy (24, 25). In addition to the ability to load siRNA, MnO₂ nanomaterials can also be used as DNA nanocarrier, which allows the conjugation of DNA aptamers onto the system to achieve the binding of DNA aptamers to their targets for more effective therapy and diagnostic strategy developments (26, 27). Additionally, DNA enzyme can also be adsorbed onto MnO₂ nanomaterials to effectively deliver DNAzyme into cells (23, 28), which enables DNAzyme to catalyze multiple reactions (e.g., RNA or DNA cutting and linking or DNA phosphorylation) for gene therapy (29). Moreover, MnO₂ NPs can also be used to wrap proteins, which can combine the advantages of engineered proteins and nanocarriers to provide an intelligent strategy to efficiently deliver functional proteins in vivo (30). The advanced property of MnO₂ NPs to serve as drug/nucleic acid/protein carriers provide novel possibilities for the development of more effective therapeutics and diagnostics.

MnO₂ NPs can also be used as a T1-weighted magnetic resonance imaging (MRI) probe to significantly enhance MRI signals. The Mn²⁺, which can be generated by MnO₂ NPs decomposition, has been approved by FDA as a clinical T1-weighted magnetic resonance (MR) imaging contrast agent. Mn²⁺ improves the signal intensity and specificity of MRI by shortening T1 relaxation, resulting in efficient positive contrast enhancement and brighter images. MnO₂ NPs have been widely proved to release Mn²⁺ after their entry into cells by reacting with H₂O₂ or GSM. Thus, MnO₂ NPs are also expected to be used as T1-weighted MRI probes due to their ability to release Mn²⁺, which provides new possibility to develop MnO₂ NPs into novel diagnostic and therapeutic agents (5, 31).

Taking the advantages of the outstanding biological activities of Mn, MnO₂ NPs have also been proved to show plenty of biological functions that directly regulate tumor growth. Tumor microenvironment (TME) is typically characterized by its hypoxia and acidic pH, where cancer cells can produce large amounts of H₂O₂ and GSH for their metabolism and resistance to immunological killings. Under the acidic conditions of TME, MnO₂ NPs can effectively catalyze the in situ production of O₂ by reacting with endogenous H₂O₂ to release Mn²⁺ due to its peroxidase-like activity, thus significantly relieving tumor hypoxia ( Figure 1 ).

Similarly, MnO₂ NPs can react with GSH in tumor cells to generate GSSH and Mn²⁺, which would result in the in uncontrolled reactive oxygen species (ROS) levels in tumor cells due to the decreased GSH level ( Figure 2 ). The ability of MnO₂ NPs to regulate TME by catalyzing REDOX activity therefore allows MnO₂ NPs to enhance the sensitivity of anti-tumor therapy (17, 32, 33).

In addition, viral or bacterial infections are often accompanied by inflammation.ROS production increases in tissues under inflammation, and the concentration of ROS such as H₂O₂ can be up to more than 100 times that of normal tissues (34, 35). Besides, hypoxia is a common microenvironmental feature in the inflammatory process related to bacterial infection (36), especially in the site of abscess caused by infection and the bacterial biofilm of chronic wounds, which is characterized by hypoxia, acidic pH and high H₂O₂ content (37, 38). MnO₂ NPs can catalyze H₂O₂ to produce O₂ and release Mn²⁺ under acidic conditions and even physiological conditions (pH=7.4) (39, 40), so MnO₂ NPs can continue to produce oxygen and improve oxidative stress, thus reducing inflammation associated with infection.Therefore, it is believed that MnO₂ NPs can also reshape the environment of the infected site to a certain extent.

Phototherapy, including photodynamic therapy (PDT) and photothermal therapy (PTT), relies on photosensitizers (PS) or photothermal agents (PTA) to convert external light energy into monomorphic oxygen (O₂) and heat for direct tumor cell killings. Phototherapy is heavily dependent on the production of O₂, and the hypoxia of TME will greatly limit the efficacy of phototherapy. The REDOX function of MnO₂ NPs can overcome the hypoxia of TME, thus improving the efficacy of phototherapy against tumor (17, 41). Moreover, phototherapy can also activate acute inflammation or induce immunogenic cell death (ICD) to enhance tumor immunogenicity, which could turn the non-immunogenic tumor into the more sensitive immunogenic tumor for immunotherapy (42, 43). Sonodynamic therapy (SDT), similar to PDT, can activate sound sensitizers to produce ROS and overcome the limitation of light penetration depth in phototherapy. However, SDT is also limited by the hypoxic TME, which can also mediate the resistance of tumor to radiotherapy (RT). Based on the ability to catalyze REDOX activity, MnO₂ NPs can also be applied to enhance the efficacy of SDT and RT for tumor treatment (44–46).

Interestingly, by regulating REDOX, MnO₂ NPs can decompose and release Mn²⁺, which makes MnO₂ NPs can be used as a CDT agent. L/D-MnO₂@Pt NPs could be specifically internalized by tumor cells and efficiently deplete the glutathione (GSH) through redox reaction to release Mn²⁺, while the released Mn²⁺ could exhibit strong chemodynamic effects through Fenton-like reactions for enhanced tumor therapy efficiency (32). Similarly, stimuli-responsive manganese carbonate-indocyanine green complexes (MnCO₃-ICG) can also release “ion reactors” of Mn²⁺ and ICG in acidic tumor environment, which accelerated the generation of hydroxyl radicals for the oxidative stress damage of tumors cells by catalyzing Mn-mediated Fenton-like reaction to suppress tumor growth more effectively (47). These results strongly suggested that MnO₂ NPs based drug delivery system not only introduce the enhanced targeting effects of drugs, but also allowed MnO₂ NPs to be used as a CDT agent to improve the catalytic efficiency of Fenton-like reaction to improve CDT performance (48).

As mentioned above, MnO₂ NPs have many biological properties, which might be very beneficial for the treatment of diseases. The toxicity and biocompatibility of MnO₂ NPs are the most critical issues if MnO₂ NPs are to be effectively and practically used in clinical treatment. Interestingly, the whole-body biocompatibility of MnO₂ NPs has been demonstrated in tumor models (49). Moreover, some literatures have shown that MnO₂ has a crucial advantage -it can be decomposed to release water-soluble Mn²⁺ in the presence of H₂O₂ under acidic conditions, and more importantly, Mn²⁺ can be easily and quickly filtered out of the body by the kidney under physiological conditions (39, 50). Some related works also showed that even when high doses of MnO₂ NPs were injected into healthy mice, their liver function markers, kidney function markers and blood routine indexes were no different from normal values, suggesting that MnO₂ NPs did not cause significant liver and kidney dysfunction in vivo (40). Thus, unlike many other non-biodegradable inorganic nanomaterials, MnO₂ NPs are a kind of biodegradable nanomaterials that can be used in vivo without long-term toxicity concerns. In addition, MnO₂ NPs can also be modified with some good biocompatible and biodegradable materials, such as glycol chitosan (GC) polymer and polyethylene glycol (PEG) (18). Loading MnO₂ NPs with GC or PEG can further enhance the biocompatibility of nanoparticles and improve their colloidal stability (30, 49). In addition, albumin, such as human albumin (HSA) and bovine albumin (BSA), also have excellent biocompatibility and low immunogenicity, which make them very suitable for wrapping MnO₂ NPs (41, 51). Moreover, modification of nanoparticles with hyaluronic acid (HA) or folic acid (FA) can also make MnO₂ NPs more biocompatible in treatment (30, 31).

In addition, MnO₂ NPs also have great potential for immunotherapy or vaccine development due to the key role of Mn²⁺ in triggering innate immunity and enhancing adaptive immunity. In conclusion, MnO₂ NPs can be used as a kind of novel agents to deliver drugs, regulate REDOX, regulate TME, increase MRI signals, enhance the efficiency of photodynamic therapy, photothermal therapy, sonodynamic therapy, radiotherapy and as a CDT agent to improve chemodynamic therapy performance, activate innate and adaptive immunity with high biocompatibility, which therefore allow MnO₂ NPs to be used for novel vaccine, diagnostic, therapeutic, therapeutic vaccine or diagnosis-therapy combined strategy development.

MnO₂ NPs have the ability to regulate ROS levels due to the peroxidase activity of MnO₂ (17), and the released Mn²⁺ from MnO₂ NPs by catalyzing REDOX reactions also has the ability to regulate ROS levels (32). Additionally, MnO₂ can also act as a kind of novel oxidants to consume excess GSH production in tumors to reduce ·OH depletion and release Mn²⁺ (18, 32). The released Mn²⁺ can in turn mediate Fenton-like reactions by catalyzing the generation of highly active ·OH from H₂O₂ (32) to induce high oxidative stress in tumor cells and the production of proinflammatory factors, such as TNF-α (52–55). The high oxidative stress burdens could induce the damage of tumors cells, which is helpful for the tumor antigens release to stimulate anti-tumor immune responses (56–58). And the increased production of proinflammatory factors could induce acute inflammation in tumor, which can stimulate the maturation of DC (7), and thus further enhance the body’s innate anti-tumor immune responses. These effects allow MnO₂ NPs to remodel TME and enhance the immunogenicity of tumor cells for enhanced anti-tumor immunity, which suggested MnO₂ NPs as immunomodulators and immune activators to regulate innate anti-tumor immune response. Moreover, the inflammatory microenvironment caused by infection is also characterized by hypoxia, acidic pH, accumulation of lactic acid and overexpression of ROS. The functions of MnO₂ NPs are also helpful to reverse these microenvironment associated inflammatory responses (59–62). Therefore, MnO₂ NPs are also expected to play important roles in innate immune defense against infections.

It’s well known that tumor cells produce a large amount of lactic acid due to high metabolism and insufficient blood supply in solid tumors, and they also produce large amounts of GSH as antioxidants to cause hypoxia and acidic pH, which are recognized as the characteristics of TME (18, 41, 63). Hypoxia not only causes the accumulation of immunosuppressive metabolites (such as adenosine and lactic acid) in solid tumors, but also restricts the infiltration of immune cells, thus inhibiting the body’s anti-tumor immune responses. For example, the accumulated lactic acid induced by hypoxia could reduce antigen presentation and inhibit NK cell activity by inhibiting DC differentiation, which would further promote the infiltration of M2-TAMs (tumor-associated macrophages) and other immunosuppressive cells into the solid tumors (7).

Hypoxia helps tumor cells recruit macrophages, and the recruitment of tumor-associated macrophages (TAMs) into avascular areas sustains tumor progression (64). Classic M1 phenotype macrophages is a type of proinflammatory cell with anti-proliferative and cytotoxic activity due to its ability to secrete reactive nitrogen and reactive oxygen species (e.g., hydrogen peroxide and NO) (31). Moreover, M1 can directly engulf tumor cells (65) or target tumor cells by releasing pro-inflammatory cytokines (66), and can also act as APC to activate T cells, thus enhancing adaptive anti-tumor immunity (67, 68). However, the continued action of TME can polarize M1 into M2-phenotypic macrophages. In contrast to M1 macrophages, M2 macrophages promote immunosuppression by producing various cytokines and growth factors (69), and can promote tumor cell survival in fragile environments such as TME or infection by limiting alternating activation of interleukin-13 and 4 (70, 71). Interestingly, the alleviating effect of MnO₂ NPs on TME hypoxia can repolarize the M2 phenotype of TAM to the classic M1 phenotype (72, 73), while the released Mn²⁺ from MnO₂ NPs also promotes M2 macrophages to M1 macrophages by activating interferon signaling (67).

DC is the main supervisor of the immune system (74). DC can recognize, capture and treat the antigen of pathogens at the invasion site, convert the antigen into polypeptides and present the antigenic peptide (75) to cytotoxic CD8+T(CTL) cells and pro-inflammatory CD4+T cells (Th1). Meanwhile, DC also secretes IL-12 (76), which plays an important role in the growth and activity of NK cells and T cells. However, the acidic pH of TME and the accumulation of immunosuppressors (e.g., lactic acid) inhibited the immunostimulatory function of DC (77, 78) and limited the differentiation of monocytes into DC (63). MnO₂ NPs can also block the production of lactic acid while increasing the production of oxygen (79), which not only enhances the mature activation of DC (80) but also reverses the inhibitory effect of lactic acid on DC (65). By targeting hypoxia of TME immunosuppression, MnO₂ NPs can exert their peroxide-like activity to react with tumor endogenous H₂O₂, which could generate in situ O₂ to relieve TME hypoxia to remit the immune suppression in solid tumors (33, 81). The remission of hypoxia can remodel TME (82), which can also promote the differentiation of monocytes into DC (67), promote the transformation of M2 into anti-tumor immune type M1 (22, 67, 73), and enhance the anti-tumor activity of NK cells (65, 83).By enhancing the anti-tumor activity of immune cells and promoting the infiltration of immune cells in solid tumors, MnO₂ NPs can significantly enhance the innate immunological responses against tumor ( Figure 3 ).

Tissue hypoxia and lactic acid buildup due to hypoxia are common microenvironmental features of inflammatory processes associated with bacterial infections (84–86). Meanwhile, H₂O₂ is also overexpressed in the inflammatory microenvironment. The peroxidase-like activity of MnO₂ NPs also catalyzes H₂O₂ to relieve tissue hypoxia in inflammatory microenvironments and inhibits lactic acid production by producing in situ oxygen (60–62). MnO₂ NPs can enhance the effects of DC and macrophages by remodeling inflammatory microenvironment, and enhance the innate immunity against bacteria infections ( Figure 3 ). More importantly, some H₂O₂-producing bacteria, such as Streptococcus pneumoniae and Streptococcus oral, release large amounts of H₂O₂ to oxidatively inactivate inflammatosomes in immune cells (87). Moreover, excess ROS production by immune cells is also associated with the decreased activation of inflammasome (88–91). However, inflammasome is a substance that plays a crucial role in the body’s innate immunity against bacterial pathogens (88, 92, 93). Therefore, it is reasonable to speculate that MnO₂ NPs might also inhibit the decreased activation of inflammasome by clearing excessive H₂O₂, and indirectly promote the body’s anti-bacterial innate immune responses. Other studies have also shown (94) that MnO₂ nanosheets can directly contact with the bacterial cell membrane and produce a large amount of ROS(mainly ·O₂-) at the contact site, which would cause damage to the cell membrane, resulting in leakage of electrolytes/intracellular contents and reduction ATPase activity for bacterial death.

The relief of MnO₂ NPs in hypoxic environment enhanced the activity of macrophages, DC and NK cells. Macrophages and DC can secrete IFN-I, and Mn²⁺ released by MnO₂ could promote the production of IFN-I through the cGAS-STING pathway in the presence of viral infection (95). IFN, a group of signaling proteins produced and secreted by host cells in response to pathogen infection (96, 97), has been used in the treatment of hepatitis B virus infection (98–100). IFN-I could induce multiple innate immune responses, such as limiting virus invasion, replication, assembly, and transmission (67). More importantly, the hypoxic state facilitates the transition of Epstein-Barr virus (EBV,HHV-4) and Kaposi Sarcoma-associated herpes virus (KSHV,HHV-8) from latent to dissolved mode (101). And hypoxia could increase the expression of Zta, a protein that mediates the transition between latent and lytic EBV infection, which could in turn increase the amount of viral DNA replication in infected cells (102). Therefore, the ability of MnO₂ NPs to increase IFN-I production and relief the hypoxic environment by oxygen production also provides novel possibilities to enhance the innate immune response against viruses.

The released Mn²⁺ from MnO₂ NPs has also been proved to show critical roles in immune regulations in different diseased conditions. After DNA virus infection, host cells release Mn²⁺ from membrane organelles to cytosol, which is accumulated to activate cGAS-STING signaling pathway to induce IRF3 phosphorylation, activate NF-κB pathway and promote IFN-I production for antiviral effects. The cGAS-STING pathway can also be activated by tumor-derived dsDNA to induce IFN-I, which can promote the maturation/activation of tumor-infiltrating dendritic cell (DCs) or macrophages and enhance the presentation of tumor-specific antigens to activate CD8+T cells and NK cells for tumor immune responses (41, 42). Therefore, Mn²⁺ released by MnO₂ NPs after catalytic REDOX can also trigger innate immunity by activating cGAS-STING signaling pathway. cGAS can be activated by any dsDNA in a sequence independent manner to activate cGAS-STING signaling pathway, which has been reported to be a critical pathway for anti-tumor, antiviral and anti-bacterial defense (53, 103). dsDNA can be detected by the membrane localization protein cGAS (cyclic GMP-AMP(cGAMP) synthetase) (104), leading to the synthesis of the second messenger cGAMP and its binding to STING(stimulator of interferon gene), followed by activation of interferon regulator IRF3 and transcription factor NF-kB. These effects would result in the expression of proinflammatory factors and IFN-I (53, 105), which, in addition to its effects, can promote the maturation and activation of DC and macrophages and enhance NK cell activity (106), thereby further enhancing the innate immune responses ( Figure 4 ). However, Mn²⁺ itself has been reported to be an effective innate immune stimulator by activating cGAS-STING pathway. Even if there is only a low level of dsDNA (less than 10⁴mg/ml) or even no dsDNA in the cytoplasm, Mn²⁺ can activate the cGAS-STING signaling pathway to play the similar anti-tumor, antiviral and anti-bacterial roles (107). Mn²⁺ may promote cGAS-STING pathway activation through two mechanisms. Firstly, Mn²⁺ can directly bind to cGAS and induce the activation of cGAS protein to form a compact conformation that is more easily to be further activated. The direct interactions between Mn²⁺ and cGAS proteins may enhance the sensitivity and enzymatic activity of cGAS to dsDNA, which makes very low concentration of dsDNA can also activate cGAS and initiate noncanonical 2’3’ -cGAMP synthesis as a second messenger (108).The second potential mechanism is the ability of Mn²⁺ to enhance the affinity of cGAMP and STING on the surface of the ER, which could finally enhance the activity of STING (9) and promote the activation of cGAS at the same time (9, 41).

In addition, activation of cGAS by Mn²⁺ can also alarm the immune system to inhibit cancer development by promoting tumor cell senescence and cytokine production (103). And based on these results, cGAS-STING agonists are therefore being developed as a novel cancer therapeutic, and a greater understanding of cGAS-STING pathway regulation is leading to a broadened list of candidate immune regulatory targets (109). Therefore, MnO₂ NPs, with the ability to produce Mn²⁺, can act as novel cGAS-STING agonists to ameliorate innate immune responses.

Some studies have also identified the intrinsic immune regulatory properties of MnO₂ NPs as a unique inducer of nutritionally responsive immunogenic cell death (ICD). While ICDs can directly kill tumor cells, they also highly expose damage-related molecular patterns (DAMPs) on tumor cells (110–112). Lots of evidences support the notion that DAMPs contribute to adaptive immunity in vertebrates (113, 114), and the exposure or release of DAMPs is a key event in the initiation of adaptive immunity (115, 116). DAMPs can not only bind to pattern recognition receptors (PRR) to subsequently promote adaptive immunity by inducing DC activation, but can also enhance adaptive immune responses by affecting the presentation of antigens associated with dying cells, promoting phagocytic action (“ eat-me signaling, “e.g., calreticulin), and facilitating antigen extraction and processing (“ present-me signaling,” e.g., F-actin or HMGB1) (111, 115, 117).

MnO₂ nanoparticles have been widely employed in cancer immunotherapy, playing a subsidiary role in assisting immunostimulatory drugs by improving their pharmacokinetics and/or creating a favorable microenvironment. Moreover, Yang et al. introduced an intrinsic immunomodulatory property of MnO₂ NPs as a unique nutrient-responsive immunogenic cell death (ICD) inducer, capable of directly modulating immunosurveillance toward tumor cells (110). The underlying mechanism of MnO₂ NPs-mediated selective ICD induction might be associated with the concurrently upregulated oxidative stress and autophagy. This starvation- immunotherapy method based on MnO₂ NPs is realized by the cut off intratumoral nutrient supply, eliciting potent efficacy for suppressing local and distant tumors. Their findings open up a new avenue toward biomedical applications of MnO₂ NPs by manipulating ICD, enabling an innovative therapeutics paradigm with great clinical significance.

By activating innate immune responses, MnO₂ NPs can further promote innate immune response and adaptive immunity for disease treatment. It’s widely known that hypoxia helps tumor cells to evade the body’s adaptive immune responses. Under hypoxia, accumulation of hypoxia-inducable factor-1 (HIF-1) activates hypoxia- adapted genes, which induces the expression of the immunosuppressive molecule PD-L1 in tumor cells, thereby increasing the resistance of tumor cells to cytotoxic lymphocyte (CTL)-mediated lysis and inducing CTL apoptosis (118–123). MnO₂ NPs can catalyze tumor-derived H₂O₂ to produce O₂ in situ under the acidic pH environment in TME to alleviate hypoxia (22, 23, 45, 124, 125), which could inhibit immune evasion promoted by hypoxia, and indirectly promote adaptive immunity against tumor cells (67, 72, 126, 127) ( Figure 3 ). By controlling Mn²⁺, MnO₂ NPs can also activate cGAS-STING signaling pathway to trigger adaptive immune responses through the antigen presentation by APCs. It is well known that APCs are the bridge between innate immunity and adaptive immunity, among which DC plays a critical role (74, 80). Mn²⁺ can induce the production of IFN-I by activating cGAS-STING pathway to promote the maturation and activation of tumor-infiltrating macrophages and DCs, and enhance the presentation of tumor-specific antigens (126), which is a key step in activating CD8+T cells and increasing the proportion of memory CD8+T cells (128, 129). IFN-I can also up-regulate costimulatory molecules on the surface of APCs, such as CD80 and CD86 (76, 105). Costimulatory molecules bind to the receptor CD28 protein on the surface of T cells to stimulate the activation, proliferation and differentiation of tumor-specific T cells into memory T cells. This also has significant implications for the treatment of abscesses caused by bacterial infections, given that the immune mimicking effects of Mn²⁺, which can be released by MnO₂ NPs, to induce robust immune memory (38). Similarly, the adaptive immunity triggered by APCs is of great significance to the body’s antibacterial activity. When the host needs to resist pathogen invasion, DC could present antigenic peptides to CD8+T cells and Th1 cells to activate T lymphocytes through direct cell-cell contact (75) ( Figure 3 ). Studies have shown that Mn²⁺ promotes HBs–AG antibody, ALT, and IFN-β production after HBs–AG immunization to regulate antigen presentation and CD4+T cell activation (95). Moreover, IFN-I induced by Mn²⁺ is also involved in the regulation of cellular and humoral immunity mediated by CD8+T cells (130), further suggesting that MnO₂ NPs might be beneficial for the adaptive immunity against viruses by regulating cellular and humoral immunity. In conclusion, MnO₂ NPs can be used as a kind of novel immunological modulators to enhance innate immune responses, increase the efficacy of ICD inducers and improve adaptive immune responses by overcoming hypoxia, regulating ROS levels and controlling cGAS-STING signaling pathway. The ability of MnO₂ NPs to release Mn²⁺ has been regarded as one of the key reasons to activate innate immune responses and trigger adaptive immunity through APCs, which is therefore expected to show promising applications in future vaccine and therapeutic strategy development.