[5] 4-1BBL is a type 2 transmembrane glycoprotein receptor that is found on APCs (antigen presenting cells) and binds to 4-1BB (also known as CD137).

[8] The link with 4-1BBL is largely made up of amino acids from the dynamic loops of the CRD2 and the β sheet of CRD3 of 4-1BB, according to a detailed study of the binding between the 4-1BB and 4-1BBL interface.

[9] Studies on the poorly immunogenic Ag104A sarcoma and the extremely tumorigenic P815 mastocytoma provided the first systematic proof that anti-4-1BB antibodies have potent anti-tumor effects.

In the years to come, more studies verified and legitimized the effect of 4-1BB signaling to inhibit tumor growth.

[10] The interaction between 4-1BB and 4-1BBL provide costimulatory signals to a variety of T cells, which can be used to discover cancer immunotherapy.