TLR4, TRIF, and MyD88 are essential for myelopoiesis and CD11c adipose tissue macrophage production in obese mice.

Obesity-induced chronic inflammation is associated with metabolic disease. Results from mouse models utilizing a high-fat diet (HFD) have indicated that an increase in activated macrophages, including CD11c adipose tissue macrophages (ATMs), contributes to insulin resistance. Obesity primes myeloid cell production from hematopoietic stem cells (HSCs) and Toll-like receptor 4 (TLR4), and the downstream TIR domain-containing adapter protein-inducing interferon-β (TRIF)- and MyD88-mediated pathways regulate production of similar myeloid cells after lipopolysaccharide stimulation. However, the role of these pathways in HFD-induced myelopoiesis is unknown. We hypothesized that saturated fatty acids and HFD alter myelopoiesis by activating TLR4 pathways in HSCs, differentially producing pro-inflammatory CD11c myeloid cells that contribute to obesity-induced metabolic disease. Results from reciprocal bone marrow transplants (BMTs) with and WT mice indicated that TLR4 is required for HFD-induced myelopoiesis and production of CD11c ATMs. Experiments with homozygous knockouts of (encoding a suppressor of MyD88 inactivation) and in competitive BMTs revealed that MyD88 is required for HFD expansion of granulocyte macrophage progenitors and that is required for pregranulocyte macrophage progenitor expansion. A comparison of WT, , , and mice on HFD demonstrated that TLR4 plays a role in the production of CD11c ATMs, and both and mice produced fewer ATMs than WT mice. Moreover, HFD-induced TLR4 activation inhibited macrophage proliferation, leading to greater accumulation of recruited CD11c ATMs. Our results indicate that HFD potentiates TLR4 and both its MyD88- and TRIF-mediated downstream pathways within progenitors and adipose tissue and leads to macrophage polarization.