CXCL12 and neoplasm: Numerous previous studies have indicated that neutrophils accumulate in the tumor microenvironment or metastatic niche to promote tumor metastasis [30,31], and that alveolar ECs are able to recruit neutrophils by secreting CXCL1, CXCL2, CXCL5 and CXCL12 after tumor-derived exosomal RNA stimulation [32].