Memory-phenotype CD8+ T cells can arise even in the absence of overt antigen stimulation. Virtual memory (VM) CD8+ T cells are CD8+ T cells that develop a memory phenotype in the periphery of WT mice in an IL-15-depedent manner. Innate CD8+ T cells, in contrast, are memory-phenotype CD8+ T cells which develop in the thymus in response to elevated thymic IL-4. It is not clear whether VM cells and innate CD8+ T cells represent two independent T cell lineages or whether they arise through similar processes. Here, we use mice deficient in Nedd4-family interacting protein 1 (Ndfip1) to show that overproduction of IL-4 in the periphery leads to an expanded VM population. Ndfip1−/− CD4+ T cells produce large amounts of IL-4 due to a defect in JunB degradation. This IL-4 induces a memory-like phenotype in peripheral CD8+ T cells that includes elevated expression of CD44, CD122 and Eomes, and decreased expression of CD49d. This is the first study to show that excess peripheral IL-4 is sufficient to cause an increase in the VM population. Our results suggest that VM and innate CD8+ T cells may be more similar than previously appreciated.

While the pathways that permit IL-2 production and the full activation of T cells upon antigen encounter are fairly well defined, the negative regulatory circuits that limit these pathways are poorly understood. Here we show that the E3 ubiquitin ligase adaptor Ndfip1 directs one such negative regulatory circuit. T cells lacking Ndfip1 produce IL-2, upregulate IL-2 receptor alpha (IL-2Rα), and proliferate, in the absence of CD28 co-stimulation. Furthermore, T cells in mice lacking both Ndfip1 and CD28 become activated, produce IL-4, and drive inflammation at barrier surfaces. Ndfip1 constrains T cell activation by limiting the duration of IL-2 mRNA expression after TCR stimulation. Ndfip1 and IL-2 have a similar expression pattern and, following TCR stimulation, expression of both Ndfip1 and IL-2 require the activity of NFAT and Erk. Taken together, these data support a negative regulatory circuit in which factors that induce IL-2 expression downstream of TCR engagement also induce the expression of Ndfip1 to limit the extent of IL-2 production and, thus, dampen T cell activation.