Quantitative assessment of the functional plasticity of memory CD8(+) T cells

While the functional plasticity of memory CD4(⁺) T cells has been studied extensively, less is known about this property in memory CD8(⁺) T cells. Here, we report the direct measurement of plasticity by paired daughter analysis of effector and memory OT-I CD8(⁺) T cells primed in vivo with ovalbumin. Naïve, effector, and memory OT-I cells were isolated and activated in single-cell culture; then, after the first division, their daughter cells were transferred to new cultures with and without IL-4; expression of IFN-γ and IL-4 mRNAs was measured 5 days later in the resultant subclones. Approximately 40% of clonogenic memory CD8(⁺) T cells were bipotential in this assay, giving rise to an IL-4(^-) subclone in the absence of IL-4 and an IL-4(⁺) subclone in the presence of IL-4. The frequency of bipotential cells was lower among memory cells than naïve cells but markedly higher than among 8-day effectors. Separation based on high or low expression of CD62L, CD122, CD127, or Ly6C did not identify a phenotypic marker of the bipotential cells. Functional plasticity in memory CD8(⁺) T-cell populations can therefore reflect modulation at the level of a single memory cell and its progeny.