CTLA4 and infection: Moreover, in Plasmodium yoelii malaria infections, CTLA-4 blockade increased T cell activation and IFN-γ production leading to early resolution of infections with the non-lethal 17X strain, but to increased severity of infections with the highly virulent 17XL strain of the parasite [34], suggesting that enhancing T cell activation can be beneficial in relatively mild infections but can exacerbate virulent infections.