Activin A induces the polarized production of CXCL12 by migrating DCs. (A) Conditioned medium of activin A–stimulated DCs (100 ng/mL for 90 minutes) was used as source of chemotactic factor for DCs. The anti-ActRIIA-blocking antibody (3 μg/mL; anti-RIIA) was used to neutralize the effect of activin A present in the supernatant. (B) SB431542 (5 μM), the inhibitor of signaling transducer chains ALK4, blocked the migration to activin A as well as to TGF-β, but not to CCL3. The inhibitor did not alter the migration to conditioned media. (C) At the end of the migration test, CXCL12 is present in higher concentrations in the lower well of the chemotactic chamber. (D) Evaluation of cell-associated CXCL12 in nonadherent cells and in cells adherent to the upper side or to the lower side (pseudopods + cells) of the filter. This experiment reveals that CXCL12 is selectively released in a polarized manner. *P < .05, **P < .005 by paired Student t test versus respective control groups.