# Differentiation in pyroptosis induction by Burkholderia pseudomallei and Burkholderia thailandensis in primary human monocytes, a possible cause of sepsis in acute melioidosis patients

## TL;DR

This study shows that Burkholderia pseudomallei, but not Burkholderia thailandensis, induces pyroptosis in human monocytes, potentially causing sepsis in melioidosis patients.

## Contribution

The study reveals that B. pseudomallei uniquely triggers pyroptosis and specific cytokine release in monocytes, offering insight into sepsis pathogenesis in melioidosis.

## Key findings

- B. pseudomallei, but not B. thailandensis, induces pyroptosis in primary human monocytes and THP-1 cells.
- B. pseudomallei infection increases LDH release, caspase-1, and gasdermin D activation, linked to pyroptosis.
- ROS production is crucial for pyroptosis during B. pseudomallei infection, but not in B. thailandensis.

## Abstract

Melioidosis caused by Burkholderia pseudomallei is an infectious disease with a high mortality rate. In acute melioidosis, sepsis is a major cause of death among patients. Once the bacterium enters the bloodstream, immune system dysregulation ensues, leading to cytokine storms. In contrast to B. pseudomallei, a closely related but non-virulent strain B. thailandensis has rarely been reported to cause cytokine storms or death in patients. However, the mechanisms in which the virulent B. pseudomallei causes sepsis are not fully elucidated. It is well-documented that monocytes play an essential role in cytokine production in the bloodstream. The present study, therefore, determined whether there is a difference in the innate immune response to B. pseudomallei and B. thailandensis during infection of primary human monocytes and THP-1 monocytic cells by investigating pyroptosis, an inflammatory death pathway known to play a pivotal role in sepsis. Our results showed that although both bacterial species exhibited a similar ability to invade human monocytes, only B. pseudomallei can significantly increase the release of cytosolic enzyme lactate dehydrogenase (LDH) as well as the increases in caspase-1 and gasdermin D activations in both cell types. The results were consistent with the significant increase in IL-1β and IL-18 production, key cytokines involved in pyroptosis. Interestingly, there was no significant difference in other cytokine secretion, such as IL-1RA, IL-10, IL-12p70, IL-15, IL-8, and IL-23 in cells infected by both bacterial species. Furthermore, we also demonstrated that ROS production played a crucial role in controlling pyroptosis activation during B. pseudomallei infection in primary human monocytes. These findings suggested that pyroptosis induced by B. pseudomallei in the human monocytes may contribute to the pathogenesis of sepsis in acute melioidosis patients.

Acute melioidosis, caused by B. pseudomallei infection, is considered one of the lethal infectious diseases reported in Southeast Asia and Northern Australia. Infection with this bacterium can lead to a severe systemic inflammatory response or sepsis, which is a major cause of death. The mortality rate of acute melioidosis may reach up to 50%. In contrast, very rare cases were reported from patients infected with B. thailandensis, a non-virulence strain. Although it is well-documented that these two bacteria share genetic similarities and can exhibit similarities in survival and replication inside most murine and human cells, there is very limited information on how only B. pseudomallei can cause sepsis in melioidosis patients, while B. thailandensis dose not. We established an infection model using primary human monocytes to demonstrate that only B. pseudomallei can induce pyroptosis, causing the release of key cytokines that are involved in sepsis. This information may contribute to understanding the mechanisms underlying pathogenesis in acute melioidosis and may guide future treatments.