CD39 is an immune cell phenotype marker that exhibits ectonucleotidase activity, converting extracellular nucleotides to nucleosides. Although CD39 has been associated with regulatory T cells (Treg), the ectoenzyme is also expressed by a subpopulation of memory T cells (CD4 mem ) with effector functions. We postulate that CD39 imparts plasticity to effector T helper type 17 (Th17) as well as co-ordinating Treg cellular programmes of differentiation. Experiments were performed with peripheral blood mononuclear cells obtained from healthy blood donors. Sorted CD4 + CD45RO + memory cells (CD4 mem ) were exposed to interleukin (IL) 6 plus IL1β plus recombinant transforming growth factor β1 (rTGF) or IL6 plus IL1β plus IL23, or IL6 plus IL1β plus rTGF β1 plus IL23 to induce Th17 polarisation. Cells at Th17 stage were then treated with high-dose IL2 plus anti-CD3/anti-CD28 to favour Treg differentiation and then re-exposed to Th17 differentiating conditions to induce putative reverse or suppressive Th17 (rev/regTh17) cell. Impacts of purinergic mediators on cell effector phenotype and functions were assessed. CD4 mem could be differentiated sequentially to Th17, Treg, and rev/regTh17. In contrast to the inflammatory properties associated with prototypic Th17 cells, rev/regTh17 exhibited a suppressive phenotype (ie, CD39 high , CD73 high , FOXP3 + ) and were able to control CD4 + CD25 − cell proliferation and pro-inflammatory cytokine (IFNγ, IL17) production. rev/regTh17 did not upregulate CD39, CD73, and FOXP3 and did not undergo increase in their suppressive function after culture with adenosine. Differential levels of expression of CD39 designate early Th17 cells from later Treg/revTh17 cell plasticity. The potential for Treg to revert to the inflammatory Th17 phenotype is mitigated by expression of CD39, as indicated by enhancements of suppressive function in vitro. UK Medical Research Council.