<i>Atg5</i> Supports <i>Rickettsia australis</i> Infection in Macrophages <i>In Vitro</i> and <i>In Vivo</i>

Abstract

Rickettsiae can cause life-threatening infections in humans. Macrophages are one of the initial targets for rickettsiae after inoculation by ticks. However, it remains poorly understood how rickettsiae remain free in macrophages prior to establishing their infection in microvascular endothelial cells. Here, we demonstrated that the concentration of <i>Rickettsia australis</i> was significantly greater in infected tissues of <i>Atg5^flox/flox</i> mice than in the counterparts of <i>Atg5^flox/flox</i> Lyz-<i>Cre</i> mice, in association with a reduced level of interleukin-1β (IL-1β) in serum. The greater concentration of <i>R. australis</i> in <i>Atg5^flox/flox</i> bone marrow-derived macrophages (BMMs) than in <i>Atg5^flox/flox</i> Lyz-<i>Cre</i> BMMs <i>in vitro</i> was abolished by exogenous treatment with recombinant IL-1β. <i>Rickettsia australis</i> induced significantly increased levels of light chain 3 (LC3) form II (LC3-II) and LC3 puncta in <i>Atg5</i>-competent BMMs but not in <i>Atg5</i>-deficient BMMs, while no p62 turnover was observed. Further analysis found the colocalization of LC3 with a small portion of <i>R. australis</i> and <i>Rickettsia</i>-containing double-membrane-bound vacuoles in the BMMs of B6 mice. Moreover, treatment with rapamycin significantly increased the concentrations of <i>R. australis</i> in B6 BMMs compared to those in the untreated B6 BMM controls. Taken together, our results demonstrate that <i>Atg5</i> favors <i>R. australis</i> infection in mouse macrophages in association with a suppressed level of IL-1β production but not active autophagy flux. These data highlight the contribution of <i>Atg5</i> in macrophages to the pathogenesis of rickettsial diseases.