library_construction_protocol: Adult or neonatal mouse spleens were isolated in complete media and CD8+ T cells were positively selected for using magnetic microbead (Miltenyi) selection according to manufacturer's instructions. Splenic cells were then prepped for bulk naïve CD8 FACS sorting on the SONY by staining with CD8, CD4 and Viability Dye (~97% purity) and 60K CD8+ cells sorted into PBS + 0.04% BSA. Sorted cells were then counted on a TC20 to capture 8K viable CD8+ T cells. Single cell 3′ RNA sequencing libraries were prepared by the Genomics core at Cornell University using a 10x Genomics Chromium instrument and the Chromium Single Cell 3′ Reagent kit (v3), following the manufacturer's protocol.