CTLA4 and rheumatoid arthritis: As soluble CTLA-4 fusion protein competed with p75NTR for binding to hB7-1 (Figure 2D), we examined whether abatacept, an FDA-approved drug for the treatment of rheumatoid arthritis composed of the extracellular domain of human CTLA-4 and the Fc domain of hIgG1 (34), could interfere with the biological actions of hB7-1 on neurons.