Toward a general model of CD4 + T cell subset specification and memory cell formation.

In the past few decades, a number of transformative discoveries have been made regarding memory CD8 T cell biology; meanwhile, the CD4 T cell field has lagged behind this progress. This perspective focuses on CD4 helper T (Th) cell subset specification and memory cell formation. Here, we argue that the sheer number of Th effector and memory cell subsets and a focus on their differences have been a barrier to a general model of CD4 memory T cell formation that applies to all immune responses. We highlight a bifurcation model that relies on an IL-2 signal-dependent switch as an explanation for the balanced production of diverse Th memory cells that participate in cell-mediated or humoral immunity in most contexts.