Myeloid-derived suppressor cells
Myeloid-derived suppressor cells (MDSC) are derived from bone marrow and can support tumor progression by promoting tumor cell survival, angiogenesis, invasion of healthy tissue by tumor cells, and metastases[76]. MDSC consist of two large groups of cells termed granulocytic or polymorphonuclear (PMN-MDSC), which are phenotypically and morphologically similar to neutrophils; and monocytic (M-MDSC)–phenotypically and morphologically similar to monocytes[77]. It was found that a high percentage of MDSC in EGFR mutant lung cancer was correlated with poor prognosis and reducing MDSC can improve the prognosis[78]. In addition, S100A9 positive MDSC counts in EGFR-TKIs resistant lung cancer was higher than healthy donors. Further study found that this kind of MDSC could be transformed into M2 type TAM by activating the RELB gene, thus promoting the proliferation and metastasis of tumors and resulting in EGFR-TKIs resistance[65]. At present, there are few studies on the mechanism of MDSC that participated in EGFR-TKIs resistance. Whether MDSC inhibition can reverse EGFR-TKIs resistance needs further study.