Toll-like receptor 9 signaling is associated with immune responses to Trypanosoma brucei infection

•A mutation at the 359th amino acid of TLR9 inhibits the expansion of splenic T cells during Trypanosoma brucei infection.•The MAPK signaling pathway was suppressed and cytokine production reduced in Tlr9M7Btlr mice during trypanosome infection.•Trypanosome DNA activates the MAPK signaling pathways through TLR9. Trypanosoma brucei, a causative agent of human and animal trypanosomiasis, regularly switches its major surface antigen to avoid elimination by the immune system. Toll-like receptor 9 (TLR9) is a key modulator for resistance to host-infective trypanosomes; however, the underlying molecular mechanism remains indistinct. Thus, we first approached the issue using Tlr9-mutant mice that render them non-responsive to TLR9 agonists. After infection, T cells in the spleens of Tlr9-mutant mice were analyzed by flow cytometry and a reduction in CD8+, CD4+ T, and NKT cells was observed in Tlr9-mutant mice compared to WT mice. We further found that the responses of inflammatory cytokines in the sera were reduced in Tlr9-mutant mice after T. brucei infection. The underlying molecular mechanism was that T. b. brucei DNA activated TLR9, which consequently upregulated the expression of p38 and ERK/MAPK, resulting in host resistance to trypanosome infection. In conclusion, these findings provide novel insights into the TLR9-mediated host responses to trypanosome infection.