Researchers have mapped how large groups of virulence proteins from Legionella pneumophila, the bacterium behind Legionnaires' disease, shape the reaction of immune cells to infection. The team built a library of 14 mutant bacterial strains, each missing a different effector-rich stretch of the genome, deleting 85 effector genes in total. These effectors are delivered into host cells by the Dot/Icm type IV secretion system, and more than 330 of them are translocated during infection.

Many carry eukaryotic-like domains that mimic host proteins, but only a minority have been characterized, partly because effectors often perform overlapping functions that complicate the study of single genes. The multi-deletion approach allowed the researchers to test the collective impact of effector groups rather than individual proteins. None of the 14 mutants showed a defect in intracellular replication in THP-1 human macrophage-like cells, yet several were attenuated in amoebae.

One of the attenuated mutants lacked a predicted amino acid transporter. Transcriptional profiling of infected THP-1 cells revealed that wild-type L. pneumophila strain 130b triggers a strong inflammatory and cellular stress response in macrophages. Six of the mutants showed reduced interferon signalling, and one mutant stood out for a marked decrease in the global expression of host heat shock response genes, a transcriptional program controlled by the transcription factor heat shock factor 1 (HSF1).

By generating further sub-deletions within that mutant, the team traced the heat shock effect to a single Dot/Icm effector: the bacterial kinase LegK4. LegK4 was responsible for inducing the host heat shock response via HSF1. The study, conducted in cell culture models, reinforces the value of multi-gene deletions for uncovering effector phenotypes that single-gene knockouts can obscure through redundancy.

It also identifies LegK4 as the bacterial trigger of the host heat shock response in Legionella-infected macrophages.

Subject of Research: Dot/Icm effector deletion mutants of Legionella pneumophila and host transcriptional responses in infected macrophages

Article Title: Multi-Dot/Icm effector mutants reveal distinct host responses in Legionella infected macrophages

Article References: Wibawa, R., Li, P., Gearing, L. J., Pidot, S., Faidiuk, Y., Wong, W., Ng, G. Z., McCaffrey, K., Hertzog, P. J., Pasricha, S., & Hartland, E. L. (2026). Multi-Dot/Icm effector mutants reveal distinct host responses in Legionella infected macrophages. PLOS Pathogens, 22(10), e1014083. https://doi.org/10.1371/journal.ppat.1014083

Image Credits: AI Generated

DOI: 10.1371/journal.ppat.1014083

Keywords: Legionella pneumophila, Dot/Icm, effectors, LegK4, HSF1, heat shock response, macrophages, interferon signalling, THP-1, amoebae, multi-gene deletion, host-pathogen interactions