Simkania negevensis is released from infected cells in a caspase-dependent manner

Simkania negevensis (Sne) is an obligate intracellular bacterium related to Chlamydia and associated with community-acquired pneumonia and bronchiolitis in infants. Sne develops within a Sne-containing vacuole, a membranous compartment in close contact with the endoplasmic reticulum and mitochondria. During propagation, bacteria alternate between the intracellular metabolically active reticulate bodies (RB) that re-differentiate into infectious elementary bodies (EB), which are released from infected cells by an unknown mechanism. Bacteria reach a developmental plateau 3 days post-infection, and, unlike Chlamydia, are capable of prolonged infections, lasting up to 15 days, as well as of infecting a wide range of hosts. So far, little is known about the mechanisms that mediate the cellular release of Sne.

In this work, we show that Sne-infected epithelial HeLa, as well as macrophage-like THP-1 cells, reduce in number during infection. At the same time, the infectivity of the cell culture supernatant increases, starting at day 3 for HeLa and day 4 for THP-1 cells and reaching a maximum at day 5 post-infection. This correlates with the ability of Sne to block TNFα-, but not staurosporine-induced cell death up to 3 days post-infection, after which cell death is boosted by the presence of bacteria. Mitochondrial permeabilization through Bax and Bak is not essential for host cell lysis and release of Sne. The inhibition of caspases by Z-VAD-FMK, caspase 1 by Ac-YVAD-CMK, and proteases by an inhibitor cocktail, significantly reduces the number of released infectious particles [1]. In line with the previous results, the specific inhibition of caspase 3, 8, or 9 shows that, unlike the other two, caspase 9 is not required for the exit of Sne from infected cells. The release, however, strongly depends on caspase 8 and seems to be negatively affected by necroptosis, indicating RIPK1/RIPK3/MLKL as an alternative signaling pathway that can be used to limit bacterial propagation.

Koch RD, et al. Modulation of Host Cell Death and Lysis Are Required for the Release of Simkania negevensis. Front Cell Infect Microbiol. 2020; 10:594932.