A major histocompatibility complex class I–dependent subset of memory phenotype CD8+ cells

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Abstract
Most memory phenotype (MP) CD44hi CD8+ cells are resting interleukin (IL)-15–dependent cells characterized by high expression of the IL-2/IL-15 receptor β (CD122). However, some MP CD8+ cells have a CD122lo phenotype and are IL-15 independent. Here, evidence is presented that the CD122lo subset of MP CD8+ cells is controlled largely by major histocompatibility complex (MHC) class I molecules. Many of these cells display surface markers typical of recently activated T cells (CD62Llo, CD69hi, CD43hi, and CD127lo) and show a high rate of background proliferation. Cells with this phenotype are highly enriched in common γ chain–deficient mice and absent from MHC-I−/− mice. Unlike CD122hi CD8+ cells, CD122lo MP CD8+ cells survive poorly after transfer to MHC-I−/− hosts and cease to proliferate. Although distinctly different from typical antigen-specific memory cells, CD122lo MP CD8+ cells closely resemble the antigen-dependent memory CD8+ cells found in chronic viral infections.