Background Repeatedly activated effector memory (EM) T helper 1 (Th1) cells in the context of autoimmunity and chronic inflammation exhibit a varity of differences in their cellular programme compared to regular EM Th1 cells. They function independently of activation signals and therefore escape physiological and therapeutical regulation. The acquirement of these properties is probably mediated by reduced expression of the pro-apoptotic protein Bim. Recent results suggest that microRNA (miRNA) mediated regulation of Bim may play an important role for the persistence of EM Th1 cells in autoimmune disease. Therefore, we aimed to identify miRNAs with the ability to suppress the expression of Bim in EM Th1 cells.
Material and Methods Assuming that Th1 cells involved in autoimmune inflammation have a history of repeated restimulation by persistent (auto-) antigens, we use in vitro generated acutely (once) and chronically (four times) activated murine EM Th1 cells. By using high-throughput sequencing of miRNA expression libraries, we have identified miRNAs being differentially expressed between once and repeatedly reactivated Th1 cells. By performing gain or loss of function experiments we examined the functional impact of miR-148a in chronically activated EM Th1 cells.
Results We found that among Th subsets chronically activated EM Th1 cells uniquely express microRNA-148a. MiR-148a regulates expression of the proapoptotic gene Bim leading to a decreased Bim/Bcl2 ratio. When inhibiting miR-148a using antagomirs in Th1 cells Bim expression increases, leading to enhanced apoptosis and reduced expansion of repeatedly reactivated EM Th1 cells. Knockdown of Bim expression by siRNA in miR-148a antagomir treated cells restored viability of the Th1 cells. This clearly proofs that miR-148a controls viability exclusively by regulating Bim expression. T cells isolated from the synovium of arthritic patients exhibit elevated miR-148a expression. Interestingly, Tbet (Th1 master transcription factor) and Twist1 (marker for chronically activated EM Th1 cells) induce expression of miR-148a.
Conclusions Taken together the data imply that Tbet and Twist1, besides controlling pathogenicity of Th1 cells, also regulate the longevity in chronic inflammation via the miR-148a-Bim-axis. MiR-148 plays an important role for the survival of EM Th1 cells in autoimmunity and chronic inflammation, thus, represents a highly potent molecular target for therapeutical treatment.