Differential TLR signalling

Dendritic cells (DCs) have an important role in determining the immune response to distinct pathogens, and much recent research has focused on the role of Toll-like receptors (TLRs) in sensing the nature of the microbial stimulus. However, the precise molecular mechanisms that influence the outcome are not well understood. Now, Bali Pulendran's group has shown that different TLR ligands can bias DCs to stimulate distinct T helper (T_H)-cell responses by differential modulation of mitogen-activated protein kinases (MAPKs) and the transcription factor c-FOS.

The authors used human monocyte-derived DCs to study the effect of different TLR agonists on DC function. Escherichia coli lipopolysaccharide (EcLPS) signals through TLR4, flagellin through TLR5 and the synthetic agonist Pam3Cys through TLR2, and schistosome egg antigen (SEA) induces T_H2-cell responses through an unknown receptor. All of these stimuli induced the expression of co-stimulatory molecules by the DCs, but there were marked differences in cytokine production. Pam3Cys and SEA induced low levels of the T_H1-cell-inducing cytokine interleukin-12 p70 (IL-12p70), compared with EcLPS and flagellin.

Differentially stimulated DCs were then placed in culture with naive, allogeneic T cells to assess their effect on T-cell responses. DCs stimulated with the various TLR agonists all induced T-cell proliferation, but they observed differences in the T_H-cell cytokines that were produced. DCs stimulated with EcLPS or flagellin produced a typical T_H1-cell response, characterized by high levels of interferon-γ (IFN-γ) and low levels of IL-5 and IL-13, whereas Pam3Cys and SEA favoured a T_H2-cell bias, with higher levels of IL-5 and IL-13.

The authors then examined the signalling mechanisms that govern these differential responses. Differences were observed in the magnitude and duration of MAPK signalling, particularly extracellular signal-related kinase (ERK) signalling, in the DCs. Studies using specific inhibitors for p38, JUN N-terminal kinase 1 (JNK1)/JNK2 and ERK1/ERK2 showed that IL-12p70 is induced in response to TLR4 and TLR5 agonists by a signalling mechanism involving p38 and JNK1/JNK2 phosphorylation, whereas Pam3Cys and SEA enhanced the duration or magnitude of ERK1/ERK2 phosphorylation, leading to the negative regulation of IL-12p70 production. c-FOS, which is stabilized by enhanced ERK signalling, was found to be expressed at high levels by DCs stimulated with Pam3Cys or SEA. Small interfering RNA-mediated knockdown of c-FOS led to the increased production of IL-12p70 by DCs in response to SEA and Pam3Cys, indicating a role for c-FOS in regulating IL-12p70 production by DCs.

These data show that signalling through TLRs can result in qualitative differences in immune responses by a mechanism involving differential ERK signalling and c-FOS.