Semaphorin 4D (SEMA4D) acts through its receptor Plexin-B1 to regulate axon growth cone guidance, lymphocyte activation, and bone density (Hota & Buck, 2012). It is also overexpressed by some malignancies and plays a role in tumor-induced angiogenesis, similar to vascular endothelial growth factor (VEGF) (Basile et al, 2006). We previously noted that inhibition of VEGF and SEMA4D both restricted tumor vascularity and size, but vessels forming under conditions of VEGF blockade retained association with pericytes while those arising under SEMA4D deficiency did not (Zhou et al, 2012). These findings suggested a novel approach to anti-angiogenic therapy for certain solid tumors, as tumor vessels with pericyte sheaths often exhibit resistance to anti-VEGF therapeutic intervention (Erber et al, 2004). Here we utilize ELISA-based arrays to demonstrate an approximately 2.5-fold increase in production of platelet-derived growth factor-B (PDGF-B), which attracts pericytes to endothelial cells, and angiopoietin-like 4 (ANGPTL4), a protein upregulated in some tumors that induces vascular permeability and metastasis, in human umbilical vein endothelial cells (HUVECs) incubated with SEMA4D but not VEGF (Fig. 1). Confirmation of these results by immunoblot and RT-PCR using the Rho-signaling inhibitor C3 demonstrates that this effect occurs in a Plexin-B1/ RhoA-dependent manner. Upregulation of PDGF-B from HUVECs by soluble SEMA4D leads to differentiation of C3H/10T1/2 mesenchymal stem cells into pericytes (as determined by increased RGS5 expression in an immunoblot), as well as their proliferation, migration, and co-association with HUVEC, while ANGPTL-4 enhanced permeability of a HUVEC monolayer in vivo. Pericyte effects are unlikely to be caused by SEMA4D directly, as an immunoblot analysis of the pericyte cell line hPC-PL does not demonstrate expression of Plexin-B1. We also show that malignancies may use this mechanism to regulate tumor-induced angiogenesis. SEMA4D shed from the head and neck squamous cell carcinoma line HN12 elicits similar responses, not through autocrine or paracrine production of PDGF-B and ANGPTL4 but instead by causing robust upregulation of these proteins in HUVEC, which immunofluorescence analysis shows affects association of endothelial cells with pericytes in the stroma of tumors derived from HN12 grafted subcutaneously into the flanks of athymic Foxn1nu (nude) mice. Taken together, these data show that SEMA4D-mediated induction of PDGF-B and ANGPTL4 from HUVECs influences vessel stability and permeability, possibly in developing malignancies, and suggest that targeting SEMA4D along with VEGF represents a novel anti-angiogenic therapeutic strategy for the treatment of solid tumors.