Variable requirement of dendritic cells for recruitment of NK and T cells to different TLR agonists.

TLRs initiate the host immune response to microbial pathogens by activating cells of the innate immune system. Dendritic cells (DCs) can be categorized into two major groups, conventional DCs (including CD8(+) and CD8(-) DCs) and plasmacytoid DCs. In mice, these subsets of DCs express a variety of TLRs, with conventional DCs responding in vitro to predominantly TLR3, TLR4, TLR5, and TLR9 ligands, and plasmacytoid DCs responding mainly to TLR7 and TLR9 ligands. However, the in vivo requirement of DCs to initiate immune responses to specific TLR agonists is not fully known. Using mice depleted of >90% of CD11c(+) MHC class II(+) DCs, we demonstrate that cellular recruitment, including CD4(+) T cell and CX5(+)DX5(+) NK cell recruitment to draining lymph nodes following the footpad administration of TLR4 and TLR5 agonists, is dramatically decreased upon reduction of DC numbers, but type I IFN production can partially substitute for DCs in response to TLR3 and TLR7 agonists. Interestingly, TLR ligands can activate T cells and NK cells in the draining lymph nodes, even with reduced DC numbers. The findings reveal considerable plasticity in the response to TLR agonists, with TLR4 and TLR5 agonists sharing the requirement of DCs for subsequent lymph node recruitment of NK and T cells.