CCL22 and leukemia: Interestingly, these chemokines, abundant in leukemia-conditioned MSC supernatants, were able to potently attract B-ALL cells that, besides CXCR4 [63,71], showed high membrane levels of CCR4 (CCL22 and CCL2 receptor) [62] and variable levels of CXCR1/2 (CXCL1 and CXCL8 receptors) [72].