Human CMVpp65-Specific Stem Cell-like Memory T-Cells (T_SCM) Are a Principal Source for Rapid Repopulation of Immunodominant Central Memory (T_CM) and Effector Memory (T_EM) in the Circulation

Latent CMV infection is controlled by a limited repertoire of immunodominant T-cells specific for viral peptides, particularly CMVpp65 and CMV IE-1. The antigen-specific T-cell subsets responsible for maintaining memory T-cells in this repertoire and repopulating them in response to periodic viral reactivations remain unclear. In this study, we generated T-cells specific for CMVpp65 from naïve (T_N), T_SCM, T_CM and T_EM subsets isolated from the blood of HLA-A*0201 normal seropositive donors and then comparatively characterized NLV-HLA-A*0201 Tetramer⁺ T-cells from each of these subsets. Following in vitro sensitization with artificial antigen-presenting cells transduced to express HLA-A*0201 and CMVpp65 and expansion with IL-7, IL-15 and IL-2, Tet⁺ T-cells were regularly generated from CD62L⁺CD45RO^-CD95^- T_N and from CD62L⁺CD45RO^-CD95⁺ T_SCM, as well as T_CM and T_EM. Isolated Tet⁺ T_N, T_SCM, T_CM and T_EM were each able to generate IFN-γ, TNF-α and granzyme B. Each Tet⁺ subset also expressed similar level of PD-1 and KLRG-1. However, Tet⁺ T_N and T_SCM expressed higher levels of CD27 and lower levels of CD57 than T_CM or T_EM. Tet⁺ T_SCM were distinguished from Tet⁺ T_N, T_CM and T_EM by a significantly greater level of proliferation and by their rapid and selective expansion of NLV-specific T-cells bearing TCRs identical by amino acid sequences to those expressed by T_EM and T_CM in the blood. Thus, Tet⁺ T_SCM rather than Tet⁺ T_N constitute the principal repertoire for repopulation of immunodominant memory T-cells sustained in the circulation.