Role of Toll-like receptor 4 in induction of cell-mediated immunity and resistance to Brucella abortus infection in mice

Initial host defense to bacterial infection is executed by innate immunity, and therefore the main goal of this study was to examine the contribution of Toll-like receptors (TLRs) duringBrucella abortusinfection. CHO reporter cell lines transfected with CD14 and TLRs showed thatB. abortustriggers both TLR2 and TLR4. In contrast, lipopolysaccharide (LPS) and lipid A derived fromBrucellarough (R) and smooth (S) strains activate CHO cells only through TLR4. Consistently, macrophages from C3H/HePas mice exposed to R and S strains and their LPS produced higher levels of tumor necrosis factor alpha (TNF-α) and interleukin-12 compared to C3H/HeJ, a TLR4 mutant mouse. The essential role of TLR4 for induction of proinflammatory cytokines was confirmed with diphosphoryl lipid A fromRhodobacter sphaeroides. Furthermore, to determine the contribution of TLR2 and TLR4 in bacterial clearance, numbers ofBrucellawere monitored in the spleen of C3H/HeJ, C3H/HePas, TLR2 knockout, and wild-type mice at 1, 3, and 6 weeks followingB. abortusinfection. Interestingly, murine brucellosis was markedly exacerbated at weeks 3 and 6 after infection in animals that lacked functional TLR4 (C3H/HeJ) compared to C3H/HePas that paralleled the reduced gamma interferon production by this mouse strain. Finally, by mass spectrometry analysis we found dramatic differences on the lipid A profiles of R and S strains. In fact, S lipid A was shown to be more active to trigger TLR4 than R lipid A in CHO cells and more effective in inducing dendritic cell maturation. In conclusion, these results indicate that TLR4 plays a role in resistance toB. abortusinfection and that S lipid A has potent adjuvant activity.