Heritable changes in division speed accompany the diversification of single T cell fate

Rapid clonal expansion of antigen-specific T cells is a fundamental feature of adaptive immune responses. It enables the outgrowth of an individual T cell into thousands of clonal descendants that diversify into short-lived effectors and long-lived memory cells. Clonal expansion is thought to be programmed upon priming of a single naive T cell and then executed by homogenously fast divi-sions of all of its descendants. However, the actual speed of cell divisions in such an emerging T cell family has never been mea-sured with single-cell resolution. Here, we utilize continuous live -cell imaging in vitro to track the division speed and genealogical connections of all descendants derived from a single naive CD8(+) T cell throughout up to ten divisions of activation-induced prolifera-tion. This comprehensive mapping of T cell family trees identifies a short burst phase, in which division speed is homogenously fast and maintained independent of external cytokine availability or continued T cell receptor stimulation. Thereafter, however, divi-sion speed diversifies, and model-based computational analysis using a Bayesian inference framework for tree-structured data reveals a segregation into heritably fast-and slow-dividing branches. This diversification of division speed is preceded already during the burst phase by variable expression of the interleukin-2 receptor alpha chain. Later it is accompanied by selective expres-sion of memory marker CD62L in slower dividing branches. Taken together, these data demonstrate that T cell clonal expansion is structured into subsequent burst and diversification phases, the latter of which coincides with specification of memory versus effector fate.

Automated summary

This study utilized continuous live-cell imaging to track the division speed and genealogical connections of a single naive CD8(+) T cell and its descendants. The results showed that T cell clonal expansion consists of a short burst phase with homogeneous fast division speed, followed by diversification of division speed and differentiation into memory and effector cells.