T cell activation is regulated by signals derived from the TCR /CD3/CD4 complex and the CD40L and CD28/CTLA-4 co-stimulatory molecules CD4+T Cell Co-stimulatory signals Antigen specific TCR signals () ck CD3 CD28/ a,βTCR CTLA- CD40L Peptide antigen CD40 CD4 MHC class Il signal CD80(B7.1)/ CD86(B7.2) Antigen Presenting Cell (APC) Chess 2002T cell activation is regulated by signals derived from the TCR /CD3/CD4 complex and the CD40L and CD28/CTLA-4 co-stimulatory molecules   e z z CD3 MHC class II , TCR h h Peptide antigen CD4+ T Cell Antigen Presenting Cell (APC) V V C C CD4 Antigen specific TCR signals CD40 CD40L signal CD28/ CTLA- 4 CD80 (B7.1)/ CD86 (B7.2) lck Co-stimulatory signals (- ) / [+] [+] L. Chess 2002