Do Memory CD4 T Cells Keep Their Cell-Type Programming: Plasticity versus Fate Commitment? Complexities of Interpretation due to the Heterogeneity of Memory CD4 T Cells, Including T Follicular Helper Cells

Abstract

Plasticity is the ability of a cell type to convert to another cell type. There are multiple effector CD4 T-cell subtypes, including T_H1, T_H2, T_H17, T_H1*, CD4 CTL, T_H9, and T_FH cells. It is commonly thought that a CD4 T cell can readily show full plasticity-full conversion from one differentiated cell-and this propensity to plasticity is possessed by memory CD4 T cells. However, there remains no direct demonstration of in vivo-generated resting memory CD4 T-cell conversion to a different subtype on secondary antigen challenge in vivo in an intact animal at the single-cell level. What has been clearly shown is that CD4 T cells possess extraordinary capacity for phenotypic heterogeneity, but that is a distinct property from plasticity. Heterogeneity is diversity of the resting memory CD4 T-cell population, not conversion of a single differentiated cell into another subtype. Apparently, plasticity at the population level can be accomplished by either mechanism, as heterogeneity of CD4 T-cell subpopulations could affect large shifts in subtype distribution at the overall population level via differential exponential expansion and death.

Figure 1.

Three models of the development of heterogeneous or plastic CD4 T-cell memory. Each model is discussed in the main text.