Targeting myeloid villains in the treatment with immune checkpoint inhibitors in gastrointestinal cancer

Abstract

Despite the clinical outcomes being extremely limited, blocking immune inhibitory checkpoint pathways has been in the spotlight as a promising strategy for treating gastrointestinal cancer. However, a distinct strategy for the successful treatment is obviously needed in the clinical settings. Myeloid cells, such as neutrophils, macrophages, dendritic cells, and mast cells, are the majority of cellular components in the human immune system, but have received relatively less attention for the practical implementation than T cells and NK cells in cancer therapy because of concentration of the interest in development of the immune checkpoint blocking antibody inhibitors (ICIs). Abnormality of myeloid cells must impact on the entire host, including immune responses, stromagenesis, and cancer cells, leading to refractory cancer. This implies that elimination and reprogramming of the tumor-supportive myeloid villains may be a breakthrough to efficiently induce potent anti-tumor immunity in cancer patients. In this review, we provide an overview of current situation of the IC-blocking therapy of gastrointestinal cancer, including gastric, colorectal, and esophageal cancers. Also, we highlight the possible oncoimmunological components involved in the mechanisms underlying the resistance to the ICI therapy, particularly focusing on myeloid cells, including unique subsets expressing IC molecules. A deeper understanding of the molecular and cellular determinants may facilitate its practical implementation of targeting myeloid villains, and improve the clinical outcomes in the ICI therapy of gastrointestinal cancer.

Keywords: gastrointestinal cancer; immune checkpoint; immunosuppression; inflammation; metastasis; myeloid cells; treatment resistance.

Figure 1

Myeloid orchestration leading to refractory cancer. Myeloid cells promote tumor progression and metastasis directly and indirectly via providing unbalanced immunity mediated by immunosuppressive and pro-inflammatory molecules to interfere induction and activation of anti-tumor effector cells.

Figure 2

Myeloid subsets expressing immune checkpoint molecules. As well as PDL1, CTLA4 and PD1 are functionally expressed in myeloid cells, including macrophages and dendritic cells, and play key roles in induction of immune suppression and exhaustion in the host.