Functional and Phenotypic Plasticity of CD4+ T Cell Subsets

The remarkable plasticity of CD4(+) T cells allows individuals to respond to environmental stimuli in a context-dependent manner. A balance of CD4(+) T cell subsets is critical to mount responses against pathogen challenges to prevent inappropriate activation, to maintain tolerance, and to participate in antitumor immune responses. Specification of subsets is a process beginning in intrathymic development and continuing within the circulation. It is highly flexible to adapt to differences in nutrient availability and the tissue microenvironment. CD4(+) T cell subsets have significant cross talk, with the ability to "dedifferentiate" given appropriate environmental signals. This ability is dependent on the metabolic status of the cell, with mTOR acting as the rheostat. Autoimmune and antitumor immune responses are regulated by the balance between regulatory T cells and Th17 cells. When a homeostatic balance of subsets is not maintained, immunopathology can result. CD4(+) T cells carry complex roles within tumor microenvironments, with context-dependent immune responses influenced by oncogenic drivers and the presence of inflammation. Here, we examine the signals involved in CD4(+) T cell specification towards each subset, interconnectedness of cytokine networks, impact of mTOR signaling, and cellular metabolism in lineage specification and provide a supplement describing techniques to study these processes.