Th17-Derived Memory Cells Are Long-Lived and Retain a Stem-Like Molecular Signature

Abstract 1018

Th17 cells are thought to be meta-stable, short-lived end-effector T lymphocytes and purportedly have a limited ability to form memory. Paradoxically, we have recently reported that tumor-specific Th17-polarized CD4⁺ T cells eradicate established tumor more efficiently than their Th1 counterparts did. Mounting evidence suggests that the advanced differentiation stage of the effectors impairs their self-renewal ability and negatively correlates with the anti-tumor functionality of T cells upon adoptive transfer. We hypothesized that in vivo advantage of Th17-polarized cells over Th1 cells might result from more naïve, less senescent properties preserved during the initial in vitro programming. Using a TCR gene engineered Th17 cells we showed that IL-17A or CCR6 signaling contributed only modestly to tumor rejection, and that Th17-polarized cells had to activate type 1 defining transcription factor tbx21 (t-bet) and secrete IFN-γ in vivo, underscoring the less mature, dualistic nature of type 17 responses. Although we confirmed that Th17-polarized cells acquired an end-effector-like phenotype (CD27^low/CD62L^low), we found that they also exhibited lower levels of inhibitory receptors and multiple markers of T cell terminal differentiation. While previous work used IL-17 to trace the fate of Th17 cells, in a congenic system we observed that most type 17 cells lost the capacity to produce IL-17A during in vivo expansion, although they efficiently persisted and formed memory. Serial global gene expression profiling demonstrated the loss of IL-17A expression and the acquisition of a phenotype with Th1-like features, nevertheless the molecular signature of Th17-derived cells remained significantly distinct from that of Th1-derived memory cells. Using gene set enrichment analysis (GSEA) we observed a highly significant non-random distribution of genes associated with less mature memory T cell subset in Th17-derived population. Ex vivo recovered Th17-derived memory cells expressed high levels of genes related to improved survival, functionality and self-renewal, including wnt/β-catenin target tcf-7, thus closely resembling the pattern previously described by us in a self-renewing memory stem-like CD8⁺ subset generated pharmacologically. We confirmed the activity of this stem-related pathway during the initial Th17 polarization, as evidenced by the striking accumulation of β-catenin and progressive induction of tcf-7 to a level above one found in naïve cells. In summary, we report that Th17-derived cells expressed more “stem-like” characteristics distinct from Th1-derived effectors as evidenced by their molecular signature and dramatically enhanced ability to survive in vivo and reject tumor. This suggests that Th17-polarizing conditions trigger a distinctive developmental program that dissociates proliferation from maturation.