Interestingly, a study reported that 4-1BB agonist treatment induces a subset of CD4+Foxp3+ Tregs to eliminate virus-induced tumor cells in mice (51), and the addition of 4-1BB expression in CAR T cells with an intracellular CD28 costimulatory domain improves their proliferation and cytotoxicity (52). Here, CD4 is linked to neoplasm.