Abstract
Colorectal cancer (CRC) is one of the most common malignant tumors worldwide, and tumor metastasis is the leading cause of death. Targeting immune inhibitory checkpoint inhibitory pathways has attracted great attention, since the therapeutic efficacy induced by the specific blocking antibodies has been demonstrated even in metastatic CRC patients. However, the clinical outcome is low in many cases, and thus more effective treatments are needed in the clinical settings. A SPARC family member follistatin-like 1 (FSTL1) is known as a key driver of tumor metastasis in various types of cancer. However, the immunological roles of the FSTL1 in the CRC pathogenesis remain to be elucidated. In this study, we investigated the molecular mechanisms underlying the refractory FSTL1+ CRC using murine and human FSTL1-transduced CRC cells. Also, based on the results, we evaluated anti-tumor efficacy induced by agents targeting the identified molecules using murine CRC metastasis models, and validated the clinical relevancy of the basic findings using tumor tissues and peripheral blood obtained from CRC patients. FSTL1 transduction conferred EMT-like properties, such as low proliferative (dormant) and high invasive abilities, on tumor cells. When the transfectants were subcutaneously implanted in mice, CD11b+DIP2A+LAG3+ cells were abundantly expanded locally and systemically in the mice. Simultaneously, apoptotic T cells increased and were lastly excluded from the tumor tissues, allowing tumor aggravation leading to resistance to anti-PD1/PDL1 treatment. Blocking FSTL1 and LAG3, however, significantly suppressed the apoptosis induction, and successfully induced anti-tumor immune responses in the CRC metastasis models. Both treatments synergized in providing better prognosis of the mice. FSTL1 was significantly upregulated in tumor tissues and peripheral blood of CRC patients, and the CD11b+DIP2A+LAG3+ cells were significantly expanded in the PBMCs as compared to those of healthy donors. The expansion level was significantly correlated with decrease of potent Ki67+GZMB+ CTLs. These results suggest that the FSTL1-induced CD11b+DIP2A+LAG3+ cells are a key driver of immune dysfunction in CRC. Targeting the FSTL1-LAG3 axis may be a promising strategy for treating metastatic CRC, and anti-FSTL1/LAG3 combination regimen may be practically useful in the clinical settings.