Methods: Using an established institutional approved intranasal inhalation exposure model, wild-type (WT), TLR2 knock-out (KO) and TLR4 KO mice were treated daily with swine facility ODE or saline for 3 weeks. Bronchoalveolar lavage fluids, bone marrow cells, and tibias were collected. Osteoclast precursor cell population frequency was analyzed by flow cytometry. Micro-CT systems to quantify 3D changes in bone morphology were conducted.
Results: Similar to prior reports, ODE-induced airway neutrophil influx and cytokine/chemokine release were reduced in TLR2 and TLR4 KO animals as compared to WT mice. In WT animals, exposure to inhalant ODE increased osteoclast precursor cell frequency as compared to saline, an effect that was reduced in TLR4 but not TLR2 KO mice. Trabecular bone micro-CT analysis showed the loss of bone mineral density, volume and deterioration of bone micro-architecture and mechanical strength induced by ODE in WT mice were significantly reduced in TLR4 but not TLR2 KO animals.
Conclusions: TLR2 and TLR4 pathways mediate ODE-induced airway inflammation, but bone deterioration following inhalant ODE treatment is strongly dependent upon TLR4. Inhalant ODE exposures significantly increased the number of bone-resorbing osteoclast precursor cells, which directly depends on the presence of TLR4.