Single T Cell Potential

T cell immune responses directed against a given epitope originate from a small population of naïve T cells expressing T cell receptors specific to this epitope. After activation, these epitope-specific T cell populations massively expand and undergo phenotypic and functional diversification. Studying T cell immune responses at the population level has shown that both expansion and diversification are robustly regulated, leading to induction of predictable response patterns. Generally, these population level findings were interpreted to directly mirror the response behavior of an individual T cell recruited into an immune response. However, recent studies based on technological advances that have allowed mapping the fate of individual responding T cells and their ensuing progeny in vivo have questioned these assumptions and have shown that despite harboring great developmental potential, single T cells do not realize this potential in a deterministic manner but rather generate response patterns subjected to substantial stochastic variation. These data indicate that robustness is achieved through integrating multiple single T cell-derived responses into a predictable population behavior. Here we review recent findings gathered by single T cell fate mapping in vivo and discuss the implications of these findings for our basic understanding of adaptive T cell immunity.