The 3' CCACCA sequence of tRNAAla(UGC) is the motif that is important in inducing Th1-like immune response, and this motif can be recognized by Toll-like receptor 3.

In this article, the immunogenicity of tRNA and the recognition of tRNA by Toll-like receptors (TLRs) are analyzed. Analyses of the effects of different tRNA(Ala)(UGC) fragments (tRNA(Ala)1-76 [corresponding to positions 1 through 76], tRNA(Ala)26-76, tRNA(Ala)40-76, tRNA(Ala)62-76, tRNA(Ala)1-70, tRNA(Ala)26-70, tRNA(Ala)40-70, and tRNA(Ala)62-70) on the immune responses of hepatitis B surface antigen (HBsAg) were performed with BALB/c mice. Results show that tRNA(Ala)1-76, tRNA(Ala)26-76, tRNA(Ala)40-76, and tRNA(Ala)62-76 adjuvants not only induced stronger T helper (Th) 1 immune responses but also cytotoxic-T-lymphocyte (CTL) responses relative to tRNA(Ala)1-70, tRNA(Ala)26-70, tRNA(Ala)40-70, and tRNA(Ala)62-70 adjuvants in HBsAg immunization. A deletion of the D loop (tRNA(Ala)26-76), anticodon loop (tRNA(Ala)40-76), or TpsiC (tRNA(Ala)62-76) loop of tRNA(Ala)(UGC) does not significantly decrease the adjuvant characteristic of tRNA(Ala)(UGC). However a deletion of the 3'-end CCACCA sequence (tRNA(Ala)1-70, tRNA(Ala)26-70, tRNA(Ala)40-70, and tRNA(Ala)62-70) of tRNA(Ala)(UGC) significantly decreased the adjuvant characteristic in Th1 and CTL immune responses. Moreover, the recognitions of different tRNA(Ala)(UGC) fragments by TLR3, TLR7, TLR8, and TLR9 were analyzed. Results show that a deletion of the 3' CCACCA sequence of tRNA(Ala)(UGC) significantly decreased the recognition by TLR3. We concluded that the 3' CCACCA sequence of tRNA(Ala)(UGC) is the important motif to induce Th1 and CTL responses and this motif can be effectively recognized by TLR3.