TGF-β signaling of human T cells is modulated by the ancillary TGF-β receptor endoglin

Transforming growth factor beta (TGF-β) inhibits T cell activation and alters differentiation of naive T cells into effector cells. Although four main cell-surface proteins can interact with TGF-β, only the signaling receptors type I (TGF-βR type I) and type II (TGF-βR type II) have so far been described on T cells. The aim of the present study was to investigate the expression of the ancillary receptor endoglin (CD105) by T cells and its role in TGF-β-mediated signal transduction and function. CD105 expression was analyzed on resting and activated human CD4⁺ T cells by flow cytometry, western blot, immunoprecipitation, proliferation and SMAD-responsive reporter gene assays. CD4⁺ T cells constitutively expressed CD105 in memory T cells and partially also in naive T cells; however, surface expression is regulated and is increased following TCR engagement, which induced serine/threonine phosphorylation of CD105. In contrast to the suppressive signal mediated by the TGF-β, cross-linking of CD105 substantially enhanced T cell proliferation, indicating that CD105 by itself mediates signal transduction. Furthermore, CD105 cross-linking induced SMAD-independent signaling via ERK kinase phosphorylation. The present study demonstrates that CD105 is expressed on the surface by activated CD4⁺ T cells and CD3 regulated by post-translational means. Furthermore, CD105 acts as a regulatory receptor, counteracting TGF-β-mediated suppression.