Activin A induces directional migration of DCs through the activation of ALK4 and ActRIIA receptors. (A) Checkerboard analysis was performed using 100 ng/mL activin A in the lower, upper, or both lower and upper wells. Results show that DCs migrated only in the presence of an activin A–positive (in the lower chamber) gradient. (B,C) DC migration in response to activin A was assessed in the presence of blocking mAbs for ALK4 or ActRIIA. Migration of DCs in response to 100 ng/mL CCL3 was used as positive control. Values represent the mean plus or minus SD of 3 different experiments. (D) Egression of mouse skin DCs in response to 100 ng/mL activin A. DC egression into the medium was evaluated after 48-hour culture; TNF-α (20 ng/mL) was used as a positive control. The results are the mean plus or minus SEM of 3 to 10 independent experiments. PTox: B pertussis toxin. *P < .05 and **P < .005 by paired Student t test versus control group. (E) Real-time DC migration in response to activin A (100 ng/mL) and LTB4 (100 nM). The micropipettes loaded with activin A or LTB₄ were positioned in culture dishes with DCs, and bright-field images were collected using a Nikon TE-300 microscope, as indicated in “Migration assay.”