Definition

A recombinant fusion protein composed of the extracellular domain (ECD) of human CD80 (B7.1) fused to a human immunoglobulin G1 (IgG1) Fc fragment, with potential immunostimulatory, immune checkpoint inhibitory and antineoplastic activities. Upon administration of CD80-Fc fusion protein FPT155, the CD80 moiety targets and binds to CD28, which in the presence of antigenic T-cell receptor (TCR) signaling, leads to the co-stimulation of T-cell responses including the activation of naïve and memory T cells. This leads to a cytotoxic T-lymphocyte (CTL)-mediated immune response against cancer cells, thereby killing cancer cells. FPT155 also targets and binds to CTL-associated antigen 4 (CTLA4; CTLA-4), preventing the binding of CTLA-4 to endogenous CD80, thereby enabling CD80-CD28 engagement, CD28 signaling, and T-cell activation in the tumor microenvironment. CD80 is a co-stimulatory molecule expressed on activated antigen presenting cells that plays a key role in T-cell activation upon binding to CD28 on T cells. On the other hand, binding of CD80 to CTLA-4 prevents CD80-CD28 engagement, thereby inhibiting T-cell activity and immune activation. CTLA-4 is a member of the immunoglobulin superfamily (IgSF) and an inhibitory molecule upregulated by T cells following T-cell activation. It plays a key role in the downregulation of the immune system.