Abstract
Immunotherapy is one of the most promising approaches to inhibit tumor growth and metastasis by activating host immune functions. However, the arising problems such as low immune response caused by complex tumor microenvironment and extremely systemic immune storm still limit the clinical applications of immunotherapy. Here, we construct Poly I: C-encapsulated poly (lactic-co-glycolic acid) nanoparticles (PLP NPs) with a slow release profile. A biomimetic system (MPLP), which loads PLP NPs on the surface of bone marrow-derived macrophage (BMDM) via the maleimide-thiol conjugation, is synthesized to effectively deliver PLP, control drug release and activate the tumor-specific immune response in situ. The results show that PLP NPs loading does not affect the activity and function of BMDM. Then, BMDM acts as a living cell drug vehicle and promotes the accumulation of PLP NPs in tumors, where Poly I: C is released from PLP NPs and reprograms BMDM into tumoricidal M1 macrophage. Furthermore, MPLP triggers potent antitumor immune responses in vivo and effectively inhibits local and metastatic tumors without causing adverse pathological immune reactions. This study offers an inspiration to facilitate clinical translation through the delivery of drugs by living immune cells for future anticancer therapy.
Introduction
In recent years, immunotherapy has become the most promising strategy in anticancer treatment by training or stimulating the body’s innate immune system to attack the tumor cells [[1], [2], [3], [4], [5], [6], [7]]. Several cancer immunotherapies, including cancer vaccines, checkpoint-blocking therapies, T cell therapies and cytokine therapies, have shown some encouraging clinical results [[8], [9], [10], [11], [12]]. However, so far, immunotherapy still exhibit limitations of efficacy and safety, such as huge individual differences in treatment responses, difficult to work on solid tumors, systemic immune storm and other immunotoxicity [13,14]. Therefore, the development of advanced strategies to activate in situ tumor-specific immune response for the anticancer immunotherapy is highly desirable.
As important immune cells, macrophages (account for a large proportion of tumor mass) can not only directly kill tumor cells, but also present antigens to effector cells as antigen-presenting cells (APCs) [15,16]. What’s more, recent research found that macrophages can also form antigen-specific immune memory, which allowed the body to trigger a stronger immune response when specific antigen appear again [17]. Interestingly, studies showed that macrophage exhibited an excellent tumor tropism through recognizing the cytokines excreted from tumor cells, and actively bound to tumor cells through the interaction of the α4 integrins of macrophages and the vascular cell adhesion molecule-1 (VCAM-1) of tumor cells [[18], [19], [20]]. By virtue of some intriguing instincts, macrophages were used for targeted delivery and enrichment of drugs in tumor. The macrophage membrane was used to promote tumor-targeting of conventional nanomedicines, including gold-based nanoplatforms, upconversion nanoparticles, liposome and mesoporous silica nanoparticles [[21], [22], [23], [24], [25], [26]]. Meanwhile, macrophage was used as living cell carrier by endocytosis to deliver anticancer drugs due to its strong drug tolerance and innate phagocytotic capability [27]. Moreover, Chen and coworkers prepared macrophage loaded immune adjuvant CpG and golden nanorode (GNR)-polyethyleimine (PEI) to suppress tumor by the generation of tumor specific antigens and CpG adjuvant in situ [28]. However, in the previous studies, macrophages were usually used as drug carriers and did not effectively exert their anti-tumor immunity after entering the tumor microenvironment (TME). Therefore, it will be a promising tumor treatment strategy of using natural living macrophages as drug carriers and maintaining the anti-tumor immunity of macrophages.
Polyinosinic-polycytidylic acid (Poly I: C, PIC) is a synthetic double-stranded RNA mimic as an ideal adjuvant agent, which can trigger immune activation and induce apoptosis of tumor cells [[29], [30], [31], [32], [33], [34]]. In the current study, we tried to utilize bone marrow-derived macrophage (BMDM) modified on its surface with PIC-encapsulated poly (lactic-co-glycolic acid) (PLGA) nanoparticles (PLP NPs) to treat metastatic triple-negative breast cancer (mTNBC) (Scheme 1). Our results indicated that PLP NPs exhibited a slow release rate of PIC via biodegradation. Moreover, the sulfhydryl groups on the surface of macrophage were selectively reduced via a mild and safe way, and further conjugated with the maleimide functional groups of PLP NPs. Thus, MPLP were prepared without compromising the cell viabilities. BMDM not only facilitated the entry of PLP NPs into tumors due to its inherent tumor-targeting ability, but also was polarized into tumoricidal M1 macrophage by PIC released in situ. After intravenous injection, PLP NPs-loaded BMDM (MPLP) was effectively enriched in the tumor site due to the good tumor-targeting ability of macrophage. More importantly, in addition to directly inducing apoptosis of tumor cells, the released PIC from MPLP not only polarized exogenous BMDM to tumoricidal M1 macrophage in situ, but also activated endogenous APCs to robustly elicit anti-tumor immune responses. Hence, the biomimetic system (MPLP)-mediated in situ activation of the anti-tumor immune response effectively inhibited local and metastatic tumors in a 4T1 mTNBC murine model.
