CD4 help regulates expression of crucial genes involved in CD8 T cell memory and sensitivity to regulatory elements.

The role of CD4 help during CD8 memory differentiation has been clearly demonstrated in different experimental models. However, the mechanisms involved to mediate CD4 help and the extent of its effects remain largely unknown. Using gene analysis at a single cell level, which allows the study of gene expression in terms of frequency, intensity and coxpression, we show that unhelped CD8 T cells harbor severe defects in the expression of crucial genes involved in proliferation, survival, and cytotoxic functions, the three main characteristics of CD8 memory differentiation described so far. Importantly, during secondary response, unhelped CD8 T cells exhibit blockade in all cytotoxic pathways (perforin, Fas ligand, IFN-gamma), demonstrating the highly ubiquitous effect of CD4 help. Secondly, resting unhelped CD8 T cells extinguish the majority of their stimulated genes, showing that CD4 help favors the persistence of gene expression. Indeed, during secondary response, unhelped CD8 T cells exhibit a profile very similar to naive T cells, demonstrating that no instructive program has been imprinted in these cells. Finally unhelped CD8 T cells exhibit a higher sensitivity to immunoregulatory genes during secondary immune response. Therefore, these results characterize the multiple effects of CD4 help on CD8 memory differentiation and provide important insights for the understanding of protective memory responses.