TLR activation on CD11c+ DCs triggers DC maturation, which is critical for T cell activation. Given the expansion of CD11c+ DCs during the progression of atherosclerosis and the key role of T cell activation in atherogenesis, we sought to understand the role of TLR signaling in CD11c+ DCs in atherosclerosis. To this end, we used a mouse model in which a key TLR adaptor involved in DC maturation, MYD88, is deleted in CD11c+ DCs. We transplanted bone marrow containing Myd88-deficient CD11c+ DCs into Western diet-fed LDL receptor knockout mice and found that the transplanted mice had decreased activation of effector T cells in the periphery as well as decreased infiltration of both effector T cells and Tregs in atherosclerotic lesions. Surprisingly, the net effect was an increase in atherosclerotic lesion size due to an increase in the content of myeloid-derived inflammatory cells. The mechanism involves increased lesional monocyte recruitment associated with loss of Treg-mediated suppression of MCP-1. Thus, the dominant effect of MYD88 signaling in CD11c+ DCs in the setting of atherosclerosis is to promote the development of atheroprotective Tregs. In the absence of MYD88 signaling in CD11c+ DCs, the loss of this protective Treg response trumps the loss of proatherogenic T effector cell activation.
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