Co-stimulatory molecules give critical signals to CD8 T cells during their priming phase to allow the generation of protective cytotoxic T cells (CTLs). In this study, we explored the role of ICOS on CTL differentiation by blocking ICOSL with an antibody in vivo. Using a bacterial and a viral infection models, we showed that despite normal expansion during ICOSL blockade, the generated CD8 T cells had lower granzyme expression, they preferentially differentiated more into memory precursor effector cells (CD127hi KLRG1low) and their cytokine production (IFN-γ, TNF-α, IL-2) was enhanced. Finally, CTLs generated in the absence of ICOS signaling at initial priming accumulated more at memory stage. Interestingly, the observed phenotype is very similar to Blimp-1 deficiency, a transcription factor involved in terminal differentiation, suggesting a link between ICOS and the Blimp-1/Bcl-6 axis. In fact, after ICOSL blockade, Bcl-6 expression was increased in CTLs. Bcl6 and Blimp-1 are known reciprocal and antagonistic regulators, indicating that Blimp-1 might be down regulated following ICOSL blockade. The molecular basis of this regulation is still under investigation. Finally, using mixed bone marrow chimera with cells from WT and ICOS KO mice, we were able to recapitulate the phenotype observed with ICOSL blockade, indicating that the direct triggering of ICOS on CD8 T cells is responsible for tuning the response.