Articles

Improving effector T-cell functions is highly desirable for preventive or therapeutic interventions of diverse diseases. Signal transducer and activator of transcription 3 (Stat3) in the myeloid compartment constrains Th1-type immunity, dampening natural and induced antitumor immune responses. We have recently developed an in vivo small interfering RNA (siRNA) delivery platform by conjugating a Toll-like receptor 9 agonist with siRNA that efficiently targets myeloid and B cells. Here, we show that either CpG triggering combined with the genetic Stat3 ablation in myeloid/B cell compartments or administration of the CpG-Stat3siRNA drastically augments effector functions of adoptively transferred CD8+ T cells. Specifically, we show that both approaches are capable of increasing dendritic cell and CD8(+) T-cell engagement in tumor-draining lymph nodes. Furthermore, both approaches can significantly activate the transferred CD8(+) T cells in vivo, upregulating effector molecules such as perforin, granzyme B, and IFN-γ. Intravital multiphoton microscopy reveals that Stat3 silencing combined with CpG triggering greatly increases killing activity and tumor infiltration of transferred T cells. These results suggest the use of CpG-Stat3siRNA, and possibly other Stat3 inhibitors, as a potent adjuvant to improve T-cell therapies.

Commensurate with their roles in regulating cytokine-dependent inflammation and immunity, signal transducer and activator of transcription (STAT) proteins are central in determining whether immune responses in the tumour microenvironment promote or inhibit cancer. Persistently activated STAT3 and, to some extent, STAT5 increase tumour cell proliferation, survival and invasion while suppressing anti-tumour immunity. The persistent activation of STAT3 also mediates tumour-promoting inflammation. STAT3 has this dual role in tumour inflammation and immunity by promoting pro-oncogenic inflammatory pathways, including nuclear factor-kappaB (NF-kappaB) and interleukin-6 (IL-6)-GP130-Janus kinase (JAK) pathways, and by opposing STAT1- and NF-kappaB-mediated T helper 1 anti-tumour immune responses. Consequently, STAT3 is a promising target to redirect inflammation for cancer therapy.

Efficient delivery of siRNA to specific cell populations in vivo remains a formidable challenge to its successful therapeutic application. We describe a novel siRNA-based approach – synthetically linking siRNA to an oligonucleotide TLR9 agonist – that targets and silences genes in TLR9⁺ myeloid cells and B cells, both of which are key components of the tumor microenvironment. Because Stat3 in tumor-associated immune cells suppresses antitumor immune responses and hinders TLR9-induced immune stimulation, we tested CpG-Stat3siRNA conjugates for anti-tumor effects. When injected locally at the tumor site or systemically through an intravenous route, the CpG-Stat3siRNA conjugates access tumor-associated dendritic cells, macrophages and B cells, inhibit Stat3 expression, leading to activation of tumor-associated immune cells, and ultimately potent anti-tumor immune responses. Our findings demonstrate the potential of TLR agonist-siRNA conjugates for targeted gene silencing coupled with TLR stimulation and immune activation in the tumor microenvironment.

Although toll-like receptor (TLR) agonists, such as CpG, are used as immunotherapeutic agents in clinical trials for cancer and infectious diseases, their effects are limited and the underlying mechanism(s) that restrains CpG efficacy remains obscure. Here we demonstrate that signal transducer and activator of transcription 3 (Stat3) plays a key role in downmodulating CpG’s immunostimulatory effects. In the absence of IL-6 and IL-10 induction, CpG directly activates Stat3 within minutes through TLR9. Ablating Stat3 in hematopoietic cells results in rapid activation of innate immunity by CpG, with enhanced production of interferon-γ, tumor necrosis factor-α, interleukin-12, and activation of macrophages, neutrophils and natural killer cells marked with Stat1 activation. Innate immune responses induced by CpG in mice with a Stat3-ablated hematopoietic system cause potent antitumor effects, leading to eradication of large (> 1 cm) B16 melanoma tumors within 72h. Moreover, ablating Stat3 in myeloid cells increases CpG-induced dendritic cell maturation, T cell activation, generation of tumor antigen-specific T cells and long-lasting antitumor immunity. A critical role of Stat3 in mediating immunosuppression by certain cytokines and growth factors in the tumor microenvironment has been recently documented. By demonstrating direct and rapid activation of Stat3 by TLR agonists, we identify a second level of Stat3-mediated immunosuppression. Our results further suggest that targeting Stat3 can drastically improve CpG-based immunotherapeutic approaches.

Interactions between tumor and immune cells either enhance or inhibit cancer progression. We show here that Stat3 signaling within the tumor microenvironment induces a procarcinogenic cytokine, IL-23, while inhibiting a central anticarcinogenic cytokine, IL-12, thereby shifting the balance of tumor immunity toward carcinogenesis. Stat3 induces expression of IL-23, which is mainly produced by tumor-associated macrophages, via direct transcriptional activation of the IL-23/p19 gene. Furthermore, Stat3 inhibits NF-κB/c-Rel-dependent IL-12/p35 gene expression in tumor-associated dendritic cells. Tumor-associated regulatory T cells (Tregs) express IL-23 receptor, which activates Stat3 in this cell type, leading to upregulation of the Treg-specific transcription factor Foxp3 and the immunosuppressive cytokine IL-10. These results demonstrate that Stat3 promotes IL-23-mediated procarcinogenic immune responses while inhibiting IL-12-dependent antitumor immunity.

Immune cells in the tumour microenvironment not only fail to mount an effective anti-tumour immune response, but also interact intimately with the transformed cells to promote oncogenesis actively. Signal transducer and activator of transcription 3 (STAT3), which is a point of convergence for numerous oncogenic signalling pathways, is constitutively activated both in tumour cells and in immune cells in the tumour microenvironment. Constitutively activated STAT3 inhibits the expression of mediators necessary for immune activation against tumour cells. Furthermore, STAT3 activity promotes the production of immunosuppressive factors that activate STAT3 in diverse immune-cell subsets, altering gene-expression programmes and, thereby, restraining anti-tumour immune responses. As such, STAT3 propagates several levels of crosstalk between tumour cells and their immunological microenvironment, leading to tumour-induced immunosuppression. Consequently, STAT3 has emerged as a promising target for cancer immunotherapy.