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Roquin proteins preclude spontaneous T cell activation and aberrant differentiation of T follicular helper (Tfh) or T helper 17 (Th17) cells. Here we showed that deletion of Roquin-encoding alleles specifically in regulatory T (Treg) cells also caused the activation of conventional T cells. Roquin-deficient Treg cells downregulated CD25, acquired a follicular Treg (Tfr) cell phenotype, and suppressed germinal center reactions but could not protect from colitis. Roquin inhibited the PI3K-mTOR signaling pathway by upregulation of Pten through interfering with miR-17 similar to 92 binding to an overlapping cis-element in the Pten 3' UTR, and downregulated the Foxo1-specific E3 ubiquitin ligase Itch. Loss of Roquin enhanced Akt-mTOR signaling and protein synthesis, whereas inhibition of PI3K or mTOR in Roquin-deficient T cells corrected enhanced Tfh and Th17 or reduced iTreg cell differentiation. Thereby, Roquin-mediated control of PI3K-mTOR signaling prevents autoimmunity by restraining activation and differentiation of conventional T cells and specialization of Treg cells.

Prior to invading the nervous system, prions frequently colonize lymphoid organs and sites of inflammatory lymphoneogenesis, where they colocalize with Mfge8+ follicular dendritic cells (FDCs). Here, we report that soft-tissue granulomas, a frequent feature of chronic inflammation, expressed the cellular prion protein (PrPC, encoded by Prnp) and the lymphotoxin receptor (LTbetaR), even though they lacked FDCs and did not display lymphoneogenesis. After intraperitoneal prion inoculation, granulomas of Prnp(+/+) mice, but not Prnp(-/-) granulomas or unaffected Prnp(+/+) skin, accumulated prion infectivity and disease-associated prion protein. Bone-marrow transfers between Prnp(+/+) and Prnp(-/-) mice and administration of lymphotoxin signaling antagonists indicated that prion replication required radioresistant PrPC-expressing cells and LTbetaR signaling. Granulomatous PrPC was mainly expressed by stromal LTbetaR+ mesenchymal cells that were absent from unaffected subcutis. Hence, granulomas can act as clinically silent reservoirs of prion infectivity. Furthermore, lymphotoxin-dependent prion replication can occur in inflammatory stromal cells that are distinct from FDCs.