Novel 4-1BB ligand (4-1BB-L) polypeptides and a human cell surface
receptor designated 4-1BB that binds 4-1BB-L are provided. Isolated
4-1BB-L-encoding and human 4-1BB-encoding DNA sequences, recombinant
expression vectors comprising the isolated DNA sequences, and host cells
transformed with the recombinant expression vectors are disclosed, along
with methods for producing the novel polypeptides by cultivating such
transformed host cells. Soluble forms of the 4-1BB-L or 4-1BB
polypeptides are derived from the extracellular domains thereof.