Opinion: Virtual memory CD8 T cells and lymphopenia-induced memory CD8 T cells represent a single subset: Homeostatic memory T cells.

It is well established that lymphopenia induces the formation of the memory-phenotype T cells without the exposure to foreign antigens. More recently, the memory-phenotype antigen-inexperienced memory T cells were described in lymphoreplete mice and called virtual memory T cells. In this review, we compare multiple aspects of the biology of lymphopenia-induced memory T cells and virtual memory T cells, including cytokine requirements, the role of T-cell receptor specificity in the differentiation process, gene expression signature, and the immune response. Based on this comparison, we conclude that lymphopenia-induced memory T cells and virtual memory T cells most likely represent a single T-cell subset, for which we propose a term 'homeostatic memory T cells'.