Mechanisms underlying lineage commitment and plasticity of human gamma delta T cells

Phenotypic and functional heterogeneity are the hallmarks of effector and memory T cells. Upon antigen stimulation, gamma delta T cells differentiate into two major types of memory T cells: central memory cells, which patrol the blood and secondary lymphoid organs, and effector memory cells, which migrate to peripheral tissues. gamma delta T cells display in vitro a certain degree of plasticity in their function that is reminiscent of that which is observed in conventional CD4 T cells. Similar to CD4 T cells, in which a plethora of specialized subsets affect the host response, gamma delta T cells may readily and rapidly assume distinct Th1-, Th2-, Th17-, Tfh and T regulatory-like effector functions, suggesting that they profoundly influence cell-mediated and humoral immune responses. In addition to differences in cytokine repertoire, gamma delta T cells exhibit diversity in homing, such as migration to lymph node follicles, to help B cells versus migration to inflamed tissues. Here, we review our current understanding of gamma delta T-cell lineage heterogeneity and flexibility, with an emphasis on the human system, and propose a classification of effector gamma delta T cells based on distinct functional phenotypes.