Neuropilin-1 is a receptor for transforming growth factor β-1, activates its latent form, and promotes regulatory T cell activity

Neuropilin- 1 ( Nrp1) is a multifunctional protein, identified principally as a receptor for the class 3 semaphorins and members of the vascular endothelial growth factor ( VEGF) family, but it is capable of other interactions. It is a marker of regulatory T cells ( Tr), which often carry Nrp1 and latency- associated peptide ( LAP)- TGF-beta 1 ( the latent form). The signaling TGF-beta 1 receptors bind only active TGF-beta 1, and we hypothesized that Nrp1 binds the latent form. Indeed, we found that Nrp1 is a high- affinity receptor for latent and active TGF- beta 1. Free LAP, LAP- TGF- beta 1, and active TGF-beta 1 all competed with VEGF165 for binding to Nrp1. LAP has a basic, arginine- rich C- terminal motif similar to VEGF and peptides that bind to the b1 domain of Nrp1. A C- terminal LAP peptide ( QSSRHRR) bound to Nrp1 and inhibited the binding of VEGF and LAP- TGF-beta 1. We also analyzed the effects of Nrp1/ LAP- TGF-beta 1 coexpression on T cell function. Compared with Nrp1 - cells, sorted Nrp1 T cells had a much greater capacity to capture LAP- TGF-beta 1. Sorted Nrp1 - T cells captured soluble Nrp1- Fc, and this increased their ability to capture LAP- TGF- beta 1. Conventional CD4 CD25 - Nrp1 - T cells coated with Nrp1- Fc/ LAP- TGF- beta 1 acquired strong Tr activity. Moreover, LAP- TGF- was activated by Nrp1- Fc and also by a peptide of the b2 domain of Nrp1 ( RKFK; similar to a thrombospondin- 1 peptide). Breast cancer cells, which express Nrp1, also captured and activated LAP- TGF-beta 1 in a Nrp1- dependent manner. Thus, Nrp1 is a receptor for TGF-beta 1, activates its latent form, and is relevant to Tr activity and tumor biology.