Cancer immunotherapy is an efficient strategy to achieve an anti-tumor immune response by activating or reconstructing the body's immune system [[1], [2], [3]]. Due to their pivotal role in preventing tumor metastasis and recurrence, numerous types of immunotherapeutic approaches, including immune checkpoint blockade [[4], [5], [6], [7], [8]], adoptive T-cell therapy [9,10], and cancer vaccines [11,12], have recently been developed and applied, contributing to a paradigm shift in cancer treatment [[13], [14], [15]]. However, most of the more explored tumor therapies are programmed cell death (PCD), characterized by induction of apoptosis, tends to decompose into apoptotic vesicles [[16], [17], [18]], which cannot reverse the intractable immunosuppressive tumor microenvironment (TME), nor can they solve the problems of low immunogenicity of the tumor tissues and insufficient infiltration of T cells, resulting in only a small number of patients responding positively to clinical immunotherapy [[19], [20], [21]]. In addition, immunological medicines are often expensive, have short half-lives, and are associated with adverse effects. [22,23]. Therefore, it is critical to identify techniques that can activate additional unique forms of cell death to break the “immunosilencing” TME and achieve self-sensitizing immunotherapy.

Pyroptosis, an inflammasome-driven distinctive PCD, is characterized by membrane pore formation, cellular swelling and bubbling [[24], [25], [26], [27], [28]]. It is widely regarded as a crucial new weathervane for activating tumor immunotherapy due to its ability to release high quantities of damage-associated molecular patterns (DAMPs) and proinflammatory cytokines to initiate an intensive inflammatory response [29,30]. In the typical pathway, pyroptosis is induced through the cleavage of gasdermin E (GSDME) by caspase-3 to form an N-terminal fragment (GSDME-N) [[31], [32], [33], [34]]. In recent years, several pyroptosis-inducing agents have been reported for pyroptosis-mediated tumor immunotherapy, such as metal oxides [35], metal-organic framework nanoparticles [36], and small molecule reagent drugs [37]. However, these inducers generally trigger pyroptosis in an uncontrolled manner and cannot be metabolized or are metabolized very slowly in vivo, making them highly susceptible to adverse reactions and leading to potential biosafety issues [38], which is a major bottleneck in advancing the biomedical applications of nanomaterials at present. Ultrasound (US) has been recognized as one of the most promising and versatile physical stimuli due to its precise controllability, noninvasiveness and high tissue penetration, allowing deep infiltration into tumor tissues with a dense extracellular matrix to achieve sonodynamic therapy (SDT) [39]. Although the US has unparalleled advantages as a physical means for tumor treatment, the hypoxic environment and high expression of reduced glutathione (GSH) in tumors can substantially reduce the ability of sonosensitizers to generate singlet oxygen (1O2), thereby affecting the therapeutic effect of SDT [40]. Moreover, sonodynamic triggering of enhanced pyroptosis-induced immunotherapy strategies are challenging and rarely reported [41].

Nanozymes are nanoparticles that mimic natural enzymes by acting as catalysts [42,43]. They have been frequently used in tumor catalytic therapy due to their potential to generate reactive oxygen species (ROS) in tumor cells by relying on their multiple enzyme-like capabilities [44]. However, in most of the nanozymes, there are often single or double enzyme-like activities, and there are few studies on triple or even quadruple enzyme-like activities. Studies have shown that the increase of ROS can disrupt the redox homeostasis of tumor cells, thereby activating the occurrence of pyroptosis [45]. Therefore, the use of Nanozymes as “endogenous chemical reactors” to generate ROS is an effective strategy to induce pyroptosis. Copper (Cu) is well known to be an essential trace element for organisms to carry out their life activities [46]. And Cu-based nanomaterials have been reported to possess multiple enzyme-like activity and high Fenton's catalytic efficiency (reaction rate constants of 460 M−1 s−1 for Cu2+ and 1 × 104 M−1 s−1 for Cu+) [47], making them the preferred nanocatalysts for the improvement of TME and the generation of endogenous ROS [[47], [48], [49]]. Encouragingly, natural polyphenolic substances not only have strong coordination ability with most metal ions [32,[50], [51], [52]], but also have the effect of depleting GSH [50,53], which led us to believe that exploiting the assembly properties of natural polyphenols with Cu ions would be promising for obtaining nanomaterials with both multiple enzymatic activity [54,55].

Herein, we designed a novel biodegradable Cu-tannic acid nanoneedle (CuTA) with multiple enzyme-like activity, which was found to be able to act exclusively in the TME and achieve pyroptosis with fewer systemic side effects. Simultaneously, CuTA-Ce6 nanocomposite, obtained by combining CuTA with the sonosensitizer Chlorin e6 (Ce6), as a pyroptosis amplifier, could further amplify the pyroptosis effect under US triggering for sensitizing immunotherapy (Scheme 1). Specifically, CuTA nanoneedles with superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px) and peroxidase (POD)-like activity could be activated to the “on” state in the TME overexpressing GSH and hydrogen peroxide (H2O2). This would destroy the activated antioxidant system of tumor cells while causing self-destructive degradation behaviors, thereby effectively alleviating the hypoxic environment and significantly increasing the ROS levels in tumors. Furthermore, it could initiate an inflammatory response to activate the inflammasome and caspase-3-cleaved GSDME, while releasing DAMPs to further break the “immunosilent” TME, thus realizing pyroptosis-mediated immunotherapy with less adverse systemic consequences. In order to effectively utilize the excellent oxygen-producing ability of CuTA nanoneedles and the unique advantages of US, we combined them with the sonosensitizer Ce6 to further convert the generated O2 into 1O2. It was found that a dual strategy of CuTA-Ce6 nanocomposites-driven cascade reaction and SDT pathway was able to generate a stronger ROS storm, including hydroxyl radicals (·OH) and 1O2, which further enhanced pyroptosis-induced tumor-associated macrophages (TAMs) polarization, dendritic cells (DCs) maturation and T-cell immune response. In conclusion, the present study opened up new avenues for the realization of novel security pyroptosis inducers with simple compositions and triggering pyroptosis-sensitizing immunotherapy.