Selective activation of macrophage innate signaling pathways and inflammatory responses to Orientia tsutsugamushi Karp and Gilliam strains

Orientia tsutsugamushi ( Ot ) is an obligate intracellular bacterium that can cause scrub typhus, an emerging but severely neglected disease with high mortality rates. Ot Karp and Gilliam strains account for most reported cases in Southeast Asia. Our group has reported that Karp-infected outbred and inbred mice exhibit more severe disease outcomes than their Gilliam-infected counterparts, likely due to their excessive inflammation and tissue injury. Macrophages (MΦs) are the main immune cell target for Ot and play a key role in host defense against infection; they are also key players in immune modulation and cytokine/chemokine production. However, it remains unclear as to how MΦ- Ot interactions impact disease outcomes. In this study, we focused on RNA-seq from C57BL/6 mouse-derived MΦs to reveal Ot infection - and strain-related immune signatures. While Ot infection modulated several common canonical pathways/genes, we identified unique pathway/gene signatures that were highly selective for Karp or Gilliam strain, respectively. Karp infection uniquely upregulated proinflammatory signaling and pattern recognition receptors, involving C-type lectin receptors (CLRs), including Mincle/ Clec4e and Dectin-2/ Clec4n . In contrast, Gilliam infection enhanced MΦ proliferation and DNA replication, and Gilliam strain grew better in M0-like MΦs than Karp strain. In IFN𝛾-primed (M1-like) MΦs; however, Karp exhibited significantly greater resistance against host killing than Gilliam, suggesting its superior ability to evade host immune responses. Overall, Karp strain preferentially upregulated CLRs and activated type 1-skewed inflammatory responses, but it is also relatively resistant to IFN𝛾-mediated killing. This study provides new insights into potential mechanisms underlying Ot strain-associated immune responses and disease outcomes.