TNFSF4 and viral infectious disease: When pDCs are either unstimulated or alternatively activated, they express the context‐dependent expression of indoleamine 2,3‐dioxygenase (IDO), inducible costimulator ligand, OX40 ligand (CD252), PD‐L1 and Granzyme B and induce regulatory T‐cell responses during viral infection, tumor and autoimmune disorders (reviewed by Swiecki and Colonna 2015).46