Clonal expansion of innate and adaptive lymphocytes

One of the hallmarks of the vertebrate adaptive immune system is the prolific expansion of individual cell clones that encounter their cognate antigen. More recently, however, there is growing evidence for the clonal expansion of innate lymphocytes, particularly in the context of pathogen challenge. Clonal expansion not only serves to amplify the number of specific lymphocytes to mount a robust protective response to the pathogen at hand but also results in selection and differentiation of the responding lymphocytes to generate a multitude of cell fates. Here, we summarize the evidence for clonal expansion in innate lymphocytes, which has primarily been observed in natural killer (NK) cells responding to cytomegalovirus infection, and consider the requirements for such a response in NK cells in light of those for T cells. Furthermore, we discuss multiple aspects of heterogeneity that both contribute to and result from the fundamental immunological process of clonal expansion, highlighting the parallels between innate and adaptive lymphocytes, with a particular focus on NK cells and CD8+ T cells.