The mechanisms underlying tumor metastasis to distant organs remain undetermined.Metastasis has been assumed to originate from the fusion of immune cells with tumor cells.Despite its analytical interpretation, the fusion theory of metastasis remains inconclusive.An alternative explanation involving immune cell-derived microparticles(MPs) is presented in this study.Upon stimulation by PMA or tumor cell-derived supernates, immune cells release membrane-based MPs that are taken up by H22 tumor cells,resulting in tumor cell migration in vitro and metastasis in vivo.The molecular basis was involved in integrin αMβ2 (CD 1 lb/CD18), which could be effectively relayed from stimulated innate immune cells to MPs and then to tumor cells.Blocking either CDllb or CD18 led to significant decreases in MPmediated tumor cell metastasis.This MP-mediated transfer of immune phenotype to tumor cells could also occur in vivo.