Abstract: The present invention generally relates to the field of immunomodulation and provides various costimulatory polypeptides comprising at least one 4-1BBL ectodomain comprising a (core) minimal TNF Homology Domain. Further provided are vectors and oncolytic viruses expressing the 4-1BBL ectodomain and methods for producing the same. Additionally provided are 5 methods and uses for obtaining and optionally purifying the inherently oligomerizing 4-1BBL ectodomain polypeptides and associated oncolytic viruses for highly targeted cancer treatment, including the treatment of tumors. Finally, there are provided kits and uses of the polypeptides and vectors as disclosed.