 Additional transcript variants have been described, but their full-length sequences have not been determined.The binding of CD86 (or the closely related protein CD80) expressed on the surface of an antigen-presenting cell with CD28 on the surface of a mature, naive T-cell, is required for T-cell activation.This protein interaction, along with the primary signal that is the MHC class II with an attached peptide binding to the T-cell receptor (TCR), activates mitogen-activated protein kinase and transcription factor nf-κB in the T-cell.