Reevaluation of the origin of CD44(high) "memory phenotype" CD8 T cells: comparison between memory CD8 T cells and thymus-independent CD8 T cells

CD44(high)CD8 T cells in naive mice, which increase with age, and are often referred to as memory CD8 T cells. However, since thymus-independent CD8 T cells have also been shown to be CD44(high), the origin of the CD44(high)C D8 T cells in naive mice remains unclear. In this study, we compared the ch aracteristics of memory CD8 T cells and thymus-independent CD8 T cells in T CR transgenic mice to clarify the origin of the CD44(high)CD8 T cells in na ive normal mice. The memory and thymus-independent CD8 T cells showed diffe rences in surface molecules, spontaneous cell death, cytokine production, a nd response to IL-2R binding of cytokines. Importantly, the "memory phenoty pe" CD8 T cells in naive normal mice showed similar characteristics to the thymus-independent CD8 T cells, but differed greatly from "true" memory CD8 T cells in the TCR transgenic mice. Therefore, we conclude that a signific ant part of the CD44(high) memory phenotype CD8 T cells in naive normal mic e represents thymus-independent CD8 T cells, which may participate in age-r elated changes in immune responses.