Figure 2.

Promotion of CD4⁺ T cell activation by TLRs on dendritic cells. Once TLR2/4 recognize their individual ligands, they can alter the expression of chemokine receptors (CCR2, CCR5, and CCR7), leading to DC migration from the infected tissue to the draining lymph node, where naïve T cells are stimulated. TLR2/4 signaling can promote the antigen process and bind to the major histocompatibility complex II and be presented to the CD4⁺ T cells, thus providing the first signal for activation of the CD4⁺ T cells. In addition, TLR signaling triggers the up-regulation of costimulatory molecules on the cell surface of DCs, which provide the second signal to activate the antigen-specific CD4⁺ T cells. TLR signaling can also induce the production of cytokines such as IL-12, TNF-α in DCs. These cytokines function as “instructive” cytokines and drive the activation and differentiation of CD4⁺ T cells.