Abstract The immune system is stratified into layers of specialized cells with distinct functions. Recently, Lin28b was shown to serve as a master regulator of fetal lymphopoiesis, programming the development of more innate-like lymphocytes in early life. However, it remains unclear whether Lin28b specifies innate functions in more conventional adaptive lymphocytes. In this report, we discovered that Lin28b promotes the development of a more innate-like lineage of CD8+ T cells that is capable of protecting the host against a wide variety of pathogens in the absence of TCR stimulation. Using RNA-seq and ATAC-seq, we found that Lin28b transcriptionally and epigenetically programs CD8+ T cells to be highly responsive to innate cytokines. We also performed scRNAseq and found that the shift from innate-like CD8+ T cells in early life to adaptive CD8+ T cells in adulthood is mediated by changes in the abundance of distinct subsets of cells. Remarkably, the innate CD8+ T cell subset predominates in early life but is also present in adult mice and humans. Collectively, our findings demonstrate that neonatal CD8+ T cells are a distinct lineage of lymphocytes that provide the host with innate defense in early life. One sentence Summary High-dimensional analysis reveals how Lin28b programs neonatal CD8+ T cells for innate defense.

Lin28b specifies an innate-like lineage of CD8+ T cells in early life