Accumulating evidence indicates that tumorinfiltrating inflammatory cells contribute significantly to tumor progression by suppressing host immunity, facilitating tumor cell invasion, and participating in the formation of new blood vessels.Many studies have consistently shown that a population of cells with suppressive activity (myeloidderived suppressor cells, MDSCs) contributes to the negative regulation of immune responses during cancer.MDSCs represent a heterogeneous population of myeloid cells in early differential stages that can be identified in mice by the expression of CD 1 lb and Gr-1.