The lymphatic endothelium-derived follistatin: activin A axis regulates neutrophil motility in response to Pseudomonas aeruginosa.

The lymphatic system plays an active role during infection, however the role of lymphatic-neutrophil interactions in host-defense responses is not well understood. During infection with pathogens such as Pseudomonas aeruginosa, Staphylococcus aureus and Yersinia pestis, neutrophils traffic from sites of infection through the lymphatic vasculature, to draining lymph nodes to interact with resident lymphocytes. This process is poorly understood, in part, due to the lack of in vitro models of the lymphatic system. Here we use a 3D microscale lymphatic vessel model to examine neutrophil-lymphatic cell interactions during host defense responses to pathogens. In previous work, we have shown that follistatin is secreted at high concentrations by lymphatic endothelial cells during inflammation. Follistatin inhibits activin A, a member of the TGF-β superfamily, and, together, these molecules form a signaling pathway that plays a role in regulating both innate and adaptive immune responses. Although follistatin and activin A are constitutively produced in the pituitary, gonads and skin, their major source in the serum and their effects on neutrophils are poorly understood. Here we report a microfluidic model that includes both blood and lymphatic endothelial vessels, and neutrophils to investigate neutrophil-lymphatic trafficking during infection with P. aeruginosa. We found that lymphatic endothelial cells produce secreted factors that increase neutrophil migration toward P. aeruginosa, and are a significant source of both follistatin and activin A during Pseudomonas infection. We determined that follistatin produced by lymphatic endothelial cells inhibits activin A, resulting in increased neutrophil migration. These data suggest that the follistatin:activin A ratio influences neutrophil trafficking during infection with higher ratios increasing neutrophil migration.