Figure 6: Differential effect of Spi6 on CTL and MTL survival. Spi6 is required for the clonal burst but not the development of memory cells. In wild-type CTLs Spi6 suppresses the activity of GrB in the cytoplasm and prevents the death of CTLs thus maintaining the size of the CD8 T cell clonal burst (outer circle). IFN-γ produced by the CD8 T cell clonal burst causes death and cell loss during the contraction phase. GrB induces the death of CTLs in Spi6 KO mice reducing the size of the clonal burst but also the production of IFN-γ. This results in negligible cell loss in the contraction phase. Within the clonal burst is a subpopulation of IL-7Rhi memory cells precursors that have low expression of GrB and Spi6 and so their survival is unaffected in Spi6 KO mice. Therefore, the size of the memory-precursor and primary memory pool in Spi6 KO is the same as wild-type mice.