Lipid A, the active component of lipopolysaccharide (LPS), exists in the outer membrane of Gram-negative bacteria and binds to the Toll-like receptor 4 (TLR4) and MD-2 complex. On the other hand, the synthetic precursor of Escherichia coli lipid A, tetraacylated lipid IVa, is an agonist for TLR4 and MD-2 complex in murine, equine and feline cells but is an antagonist for lipid A in human cells. The aim of the study was to examine the function of canine Toll-like receptor 4 (TLR4) and MD-2 complex on canine blood mononuclear cells (BMC), by analyzing lipid A- or lipid IVa-induction of TNF-α production from these cells in order to understand canine innate immune system. After 5-h culture of canine BMC with lipid A (lipid A culture) or lipid IVa (lipid IVa culture), the TNF-α, as determined by ELISA, had increased in the supernatants of the lipid A cultures in a dose-dependent manner, whereas the TNF-α was undetectable in supernatant of lipid IVa-treated cultures. The TNF-α was statistically significantly different between the lipid A and lipid IVa cultures (100 and 1000 ng/ml). TNF-α production from canine BMC was inhibited, in a lipid IVa-dose-dependent manner, when the BMC were pre-cultured with lipid IVa for 60 min and then cultured with lipid A for 5h, while in control BMC cultures production if TNF-α was unchanged. These results indicate that the TNF-α production stimulated by lipid A was competed out by pre-exposing the BMC to lipid IVa. Thus, lipid A is an agonist for TNF-α production in canine BMC, whereas lipid IVa appears to be an antagonist against this lipid A stimulation of canine BMC.
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