When intracellular pathogens invade mammalian hosts, naïve CD8+ T cells differentiate into cytotoxic killers, which lyse infected target cells and secrete cytokines that activate intracellular microbicides. We now show that CD8+ T cells deficient in the transcription factors T-bet and eomesodermin (Eomes) fail to differentiate into functional killers required for defense against lymphocytic choriomeningitis virus. Instead, virus-specific CD8+ T cells lacking both T-bet and Eomes differentiate into an IL-17-secreting lineage, reminiscent of the helper T cell fate that has been implicated in autoimmunity and extracellular microbial defense. Upon viral infection, mice with T cells lacking both T-bet and Eomes develop a CD8+ T cell-dependent, progressive inflammatory and wasting syndrome characterized by multi-organ infiltration of neutrophils. T-bet and Eomes, thus, ensure that CD8+ T cells adopt an appropriate course of promoting intracellular rather than extracellular destruction.

Summary Polarized segregation of proteins in T cells is thought to play a role in diverse cellular functions including signal transduction, migration, and directed secretion of cytokines. Persistence of this polarization can result in asymmetric segregation of fate-determining proteins during cell division, which may enable a T cell to generate diverse progeny. Here, we provide evidence that a lineage-determining transcription factor, T-bet, underwent asymmetric organization in activated T cells preparing to divide and that it was unequally partitioned into the two daughter cells. This unequal acquisition of T-bet appeared to result from its asymmetric destruction during mitosis by virtue of concomitant asymmetric segregation of the proteasome. These results suggest a mechanism by which a cell may unequally localize cellular activities during division, thereby imparting disparity in the abundance of cell fate regulators in the daughter cells.