CSF@E-Hn: A bone marrow-targeted nanosystem for advanced treatment of hematological malignancies

Abstract

In a recent study published in Nature Nanotechnology, the research teams of Professor Siwen Li and Professor Yueqing Gu introduced a bispecific bone marrow-targeted nanosystem, CSF@E-Hn, based on hematopoietic stem cell (HSC) nanovesicles (Hn).¹ The system uses Hn vesicles, decorated with natural killer (NK) cell-activating ligands (aNKG2D) and tumor-targeting antibodies (aPD-L1), encapsulating colony-stimulating factor (CSF) to treat hematological malignancies. Experimental results confirmed the system's therapeutic efficacy in mouse models of acute myelogenous leukemia (AML) and multiple myeloma (MM) and demonstrated its ability to prevent tumor recurrence long-term.

Most malignant hematological tumors arise from uncontrolled clonal expansion of tumor cells within the bone marrow, leading to high mortality and recurrence rates. Although current treatments, including chemotherapy, immunotherapy, and cell therapy, have improved overall survival in patients with hematological malignancies, these strategies still face significant challenges. Targeting bone marrow tumor cells specifically, reducing toxic side effects, and preventing recurrence remain major hurdles due to the lack of effective bone marrow-targeting technologies and the difficulty in reversing the diseased bone marrow microenvironment.^{2, 3} The unique physiological structure of the bone marrow also acts as a formidable barrier, severely limiting the development of in vivo targeting technologies. In this context, Hn shows great potential for overcoming these challenges. As an ideal carrier, Hn has several advantages: it mirrors the characteristics of parent cells, features a drug-loaded bilayer structure, has a small size, and exhibits low immunogenicity.^{4, 5} However, how to translate these significant advantages into clinical treatment remains a critical issue in current research.

To address these challenges, the bone marrow-targeted nanosystem CSF@E-Hn, developed by the team of Professor Li and Professor Gu, innovatively fuses nanotechnology with immunotherapy. This system accomplishes precise bone marrow-targeted treatment by capitalizing on the natural bone marrow hematopoietic stem cells and the generation of memory T cells, thus remodeling the bone marrow microenvironment and augmenting the immune response. Additionally, the system possesses excellent biocompatibility and sustained release properties, guaranteeing favorable safety (Figure 1).

The research team conducted a comprehensive evaluation of CSF@E-Hn's performance and efficacy. In vitro experiments demonstrated that the nanosystem, after antibody modification of HSC cells, maintained stable size distribution in phosphate-buffered saline (PBS) and serum. CSF@E-Hn remained stable for up to 14 days under low-temperature storage and thawing at −80°C. When NK cells were isolated from the bone marrow of C57BL/6J mice, CSF@E-Hn effectively brought NK and C1498 cells together in a mixed cell culture medium. This interaction was verified through Förster resonance energy transfer and transmission electron microscopy, confirming the nanosystem's ability to capture these cells. The study also showed that CSF@E-Hn activated NK cells and inhibited tumor cell proliferation without affecting healthy cells.

Further studies assessed CSF@E-Hn's bone marrow homing ability. Results showed that the system had prolonged blood circulation after injection in mice, with no significant pathological changes observed in hematoxylin-eosin (H&E) staining. While stronger fluorescence signals were noted in the liver and spleen, safety assessments indicated no obvious toxicity. Acute toxicity tests further confirmed its safety profile. In a C1498 tumor-bearing mouse model, CSF@E-Hn treatment significantly eradicated tumor cells, resulting in an 87.5% survival rate after 80 days. Moreover, white blood cell counts and body weight in the CSF@E-Hn treatment group were superior to those of other treatment groups. Normal spleen morphology and a significant increase in hematopoietic cells, dendritic cells, and macrophages indicated that CSF@E-Hn effectively regulated the bone marrow immune environment and restored hematopoietic function. In MM models, CSF@E-Hn reduced osteoclasts and increased osteoblasts, further highlighting its potential in treating hematological malignancies.

These comprehensive evaluation outcomes demonstrate that the CSF@E-Hn nanosystem not only displays good stability and cell-capturing capabilities in vitro but also exhibits remarkable therapeutic effects and satisfactory biosafety in vivo, thereby presenting a promising novel strategy for the treatment of hematological malignancies.

Despite its promising results, several key questions remain regarding CSF@E-Hn's conclusions. While CSF@E-Hn demonstrated efficacy in capturing NK cells and tumor cells, prolonging circulation time, and inhibiting tumor proliferation, its performance in broader clinical applications is still uncertain. Results from animal models are not always predictive of human clinical outcomes, and further investigation is needed to determine CSF@E-Hn's feasibility in clinical trials. Given that the mouse model in the original article is a homologous tumor transplantation model, future studies on human tumor xenograft models or genetically humanized mouse models can be progressively carried out, ultimately leading to the initiation of a small-scale Phase I clinical trial.

Additionally, long-term safety concerns require attention. Although initial assessments showed no obvious toxicity, potential delayed side effects or unexpected biological reactions from prolonged use remain unknown. Biodistribution and tissue accumulation could have unforeseen effects on organ function, which necessitates further exploration. Another question is CSF@E-Hn's versatility in treating different tumors or blood diseases. While current research focuses on AML and MM, its efficacy in other hematological malignancies or solid tumors remains to be evaluated. Moreover, individual patient variability and the system's effectiveness across different demographics need to be confirmed through systematic clinical trials.

Another important consideration is the production and operational cost of CSF@E-Hn. While the system performs well in experimental settings, ensuring its economic feasibility and consistent production will be a key challenge for clinical application. Efficient, safe, and cost-effective manufacturing processes will directly impact the clinical adoption and widespread use of CSF@E-Hn. Although high-purity Hn can be obtained through ultracentrifugation, the equipment is costly and the operation is time-consuming. Consequently, alternative methods that are more cost-effective and efficient, such as ultrafiltration, charge neutralization-based polymer precipitation, size exclusion chromatography, and microfluidics, can be taken into consideration. Furthermore, in the antibody conjugation process, a large quantity of biotinylated antibodies (such as aPD-L1 and aNKG2D) is utilized, resulting in high antibody costs. To curtail costs, antibody fragmentation or site-specific conjugation techniques can be employed to reduce the amount of antibody used while ensuring conjugation efficiency and functionality. Additionally, the ratio of antibodies to nanovesicles can be optimized to minimize unnecessary waste.

In conclusion, CSF@E-Hn shows great promise as a novel therapeutic strategy for hematological malignancies. With further clinical trials and long-term safety evaluations, we anticipate that CSF@E-Hn could become a valuable tool for treating AML and MM. If validated across other tumor types, it has the potential to play a significant role in a wide range of cancer treatments. Continued research and innovation will further reveal the comprehensive value of CSF@E-Hn, leading to more precise and effective therapeutic strategies.

Yinyan Jiang: Conceptualization (lead); visualization (equal); writing–original draft (equal). Jianqiao Shentu: Conceptualization (equal); visualization (equal); writing–original draft (lead). Yalu Chen: Funding acquisition (equal); writing–review and editing (lead). All authors have read and agreed to the published version of the manuscript.

The authors declare no conflicts of interest.

Not applicable.