CD4 and infection: In addition, relatively low level of CD8+ response compared with those in natural infection with a non-lethal strain [29], and a low level of CD4+ T response in these ROP17 DNA-immunized mice might be another reason for the low level of protection against acute T. gondii infection in the present study because only the synergy between CD8+ and CD4+ T cells can provide efficient protection against T. gondii [6].