The present invention includes the receptor protein 4-1BB and the cDNA
gene encoding for receptor protein 4-1BB. The nucleotide sequence of the
isolated cDNA is disclosed herein along with the deduced amino acid
sequence. The 4-1BB protein and fragments and derivatives can be used: 1)
as a probe to isolate ligands to receptor protein 4-1BB, 2) to stimulate
proliferation of B-cell's expressing 4-1BB, or 3) to block 4-1BB ligand
binding. A monoclonal antibody against 4-1BB was developed which
specifically recognizes an epitope on the extracellular domain of
receptor protein 4-1BB. The monoclonal antibody can be used enhance
T-cell proliferation and activation by treating T-cells that have
expressed receptor protein 4-1BB with the monoclonal antibody. The
effectiveness of the treatment was enhanced when conducted in the
presence of protein tyrosinase kinase. A fusion protein for detecting
cell membrane ligands to receptor protein 4-1BB was developed. It
comprises the extracellular portion of the receptor protein 4-1BB and a
detection protein bound to the portion of the receptor protein 4-1BB.