Effector Function-Deficient Memory CD8+ T Cells Clonally Expand in the Liver and Give Rise to Peripheral Memory CD8+ T Cells

Upon adoptive transfer into histocompatible mice, naive CD8(+) T cells stimulated ex vivo by TCR+IL-4 turn into long-lived functional memory cells. The liver contains a large number of so formed memory CD8(+) T cells, referred to as liver memory T cells (T(lm)) in the form of cell clusters. The CD62L(low) expression and nonlymphoid tissue distribution of T(lm) cells are similar to effector memory (T(em)) cells, yet their deficient cytotoxicity and IFN-γ inducibility are unlike T(em) cells. Adoptive transfer of admixtures of TCR+IL-4-activated Vβ8(+) and Vβ5(+) CD8(+) T cells into congenic hosts reveals T(lm) clusters that are composed of all Vβ5(+) or Vβ8(+), not mixed Vβ5(+)/Vβ8(+) cells, indicating that T(lm) clusters are formed by clonal expansion. Clonally expanded CD8(+) T cell clusters are also seen in the liver of Listeria monocytogenes-immune mice. T(lm) clusters closely associate with hepatic stellate cells and their formation is IL-15/IL-15R-dependent. CD62L(low) T(LM) cells can home to the liver and secondary lymphoid tissues, remain CD62L(low), or acquire central memory (T(cm))-characteristic CD62L(hi) expression. Our findings show the liver as a major site of CD8(+) memory T cell growth and that T(lm) cells contribute to the pool of peripheral memory cells. These previously unappreciated T(lm) characteristics indicate the inadequacy of the current T(em)/T(cm) classification scheme and help ongoing efforts aimed at establishing a unifying memory T cell development pathway. Lastly, our finding of T(lm) clusters suggests caution against interpreting focal lymphocyte infiltration in clinical settings as pathology and not normal physiology.