CTLA4 and arthritic joint disease: CTLA-4 Ig (Abatacept) is a fusion protein of human CTLA-4 and the Fc portion of human IgG1, which binds to B7 ligands expressed by macrophages and dendritic cells to inhibit costimulation of naïve CD4 T cells2 and prevents the development of arthritis in animal models3.