Role of myeloid-derived suppressor cells in tumor recurrence

The establishment of primary tumor cells in distant organs, termed metastasis, is the principal cause of cancer mortality and is a crucial therapeutic target in oncology. Thus, it is critical to establish a better understanding of metastatic progression for the future development of improved therapeutic approaches. Indeed, such development requires insight into the timing of tumor cell dissemination and seeding of distant organs resulting in occult lesions. Following dissemination of tumor cells from the primary tumor, they can reside in niches in distant organs for years or decades, following which they can emerge as an overt metastasis. This timeline of metastatic dormancy is regulated by interactions between the tumor, its microenvironment, angiogenesis, and tumor antigen-specific T-cell responses. An improved understanding of the mechanisms and interactions responsible for immune evasion and tumor cell release from dormancy would help identify and aid in the development of novel targeted therapeutics. One such mediator of dormancy is myeloid derived suppressor cells (MDSC), whose number in the peripheral blood (PB) or infiltrating tumors has been associated with cancer stage, grade, patient survival, and metastasis in a broad range of tumor pathologies. Thus, extensive studies have revealed a role for MDSCs in tumor escape from adoptive and innate immune responses, facilitating tumor progression and metastasis; however, few studies have considered their role in dormancy. We have posited that MDSCs may regulate disseminated tumor cells resulting in resurgence of senescent tumor cells. In this review, we discuss clinical studies that address mechanisms of tumor recurrence including from dormancy, the role of MDSCs in their escape from dormancy during recurrence, the development of occult metastases, and the potential for MDSC inhibition as an approach to prolong the survival of patients with advanced malignancies. We stress that assessing the impact of therapies on MDSCs versus other cellular targets is challenging within the multimodality interventions required clinically.

Bruton's tyrosine kinase: an emerging targeted therapy in myeloid cells within the tumor microenvironment

Bruton’s tyrosine kinase (BTK) is a non-receptor kinase belonging to the Tec family of kinases. The role of BTK in B cell receptor signaling is well defined and is known to play a key role in the proliferation and survival of malignant B cells. Moreover, BTK has been found to be expressed in cells of the myeloid lineage. BTK has been shown to contribute to a variety of cellular pathways in myeloid cells including signaling in the NLRP3 inflammasome, receptor activation of nuclear factor-κβ and inflammation, chemokine receptor activation affecting migration, and phagocytosis. Myeloid cells are crucial components of the tumor microenvironment and suppressive myeloid cells contribute to cancer progression, highlighting a potential role for BTK inhibition in the treatment of malignancy. The increased interest in BTK inhibition in cancer has resulted in many preclinical studies that are testing the efficacy of using single-agent BTK inhibitors. Moreover, the ability of tumor cells to develop resistance to single-agent checkpoint inhibitors has resulted in clinical studies utilizing BTK inhibitors in combination with these agents to improve clinical responses. Furthermore, BTK regulates the immune response in microbial and viral infections through B cells and myeloid cells such as monocytes and macrophages. In this review, we describe the role that BTK plays in supporting suppressive myeloid cells, including myeloid-derived suppressor cells (MDSC) and tumor-associated macrophages (TAM), while also discussing the anticancer effects of BTK inhibition and briefly describe the role of BTK signaling and BTK inhibition in microbial and viral infections.

Role of myeloid-derived suppressor cells in metastasis

The spread of primary tumor cells to distant organs, termed metastasis, is the principal cause of cancer mortality and is a critical therapeutic target in oncology. Thus, a better understanding of metastatic progression is critical for improved therapeutic approaches requiring insight into the timing of tumor cell dissemination and seeding of distant organs, which can lead to the formation of occult lesions. However, due to limitations in imaging techniques, primary tumors can only be detected when they reach a relatively large size (e.g., > 1 cm³), which, based on our understanding of tumor evolution, is 10 to 20 years (30 doubling times) following tumor initiation. Recent insights into the timing of metastasis are based on the genomic profiling of paired primary tumors and metastases, suggesting that tumor cell seeding of secondary sites occurs early during tumor progression and years prior to diagnosis. Following seeding, tumor cells may remain in a dormant state as single cells or micrometastases before emerging as overt lesions. This timeline and the role of metastatic dormancy are regulated by interactions between the tumor, its microenvironment, and tumor-specific T cell responses. An improved understanding of the mechanisms and interactions responsible for immune evasion and tumor cell release from dormancy would support the development of novel targeted therapeutics. We posit herein that the immunosuppressive mechanisms mediated by myeloid-derived suppressor cells (MDSCs) are a major contributor to tumor progression, and that these mechanisms promote tumor cell escape from dormancy. Thus, while extensive studies have demonstrated a role for MDSCs in the escape from adoptive and innate immune responses (T-, natural killer (NK)-, and B cell responses), facilitating tumor progression and metastasis, few studies have considered their role in dormancy. In this review, we discuss the role of MDSC expansion, driven by tumor burden, and its role in escape from dormancy, resulting in occult metastases, and the potential for MDSC inhibition as an approach to prolong the survival of patients with advanced malignancies.