Programmed death ligand 1 (PD-L1,
also known as CD274/B7-H1), a member of B7 family was
identified by searching for molecules that share homology
with the immunoglobulin V and C domains of B7-1 and
B7-2 among the human cDNA expressed sequence tags in
the National Center for Biotechnology Information
database. PD-L1 is a ligand for programmed death 1 (PD-1)
which belongs to the CD28/CTLA4 subfamily. Although in
vitro study indicated that the cross-linking of PD-1 by
PD-L1 leads to down-regulation of T-cell responses, some
studies have shown that T cells stimulated with low levels
of anti-CD3 and immobilized PD-L1-Ig were activated,
proliferation and production of IFN-γ, GM-CSF and IL-10
from the T cells were enhanced.