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Thymosin beta-4 (Tβ4) is an actin-sequestering peptide that plays important roles in regeneration and remodeling of injured tissues. However, its function in a naturally occurring pathogenic bacterial infection model has remained elusive. We adopted Tβ4-overexpressing transgenic (Tg) mice to investigate the role of Tβ4 in acute pulmonary infection and systemic sepsis caused by Legionella pneumophila Upon infection, Tβ4-Tg mice demonstrated significantly lower bacterial loads in the lung, less hyaline membranes and necrotic abscess, with lower interstitial infiltration of neutrophils, CD4 + , and CD8 + T cells. Bronchoalveolar lavage fluid of Tβ4-Tg mice possessed higher bactericidal activity against exogenously added L. pneumophila , suggesting that constitutive expression of Tβ4 could efficiently control L. pneumophila Furthermore, qPCR analysis of lung homogenates demonstrated significant reduction of interleukin 1 beta (IL-1β) and tumor necrosis factor alpha (TNF-α), which primarily originate from lung macrophages, in Tβ4-Tg mice after pulmonary infection. Upon L. pneumophila challenge of bone marrow-derived macrophages (BMDM) in vitro , secretion of IL-1β and TNF-α proteins was also reduced in Tβ4-Tg macrophages, without affecting their survival. The anti-inflammatory effects of BMDM in Tβ4-Tg mice on each cytokine were affected when triggering with tlr2, tlr4, tlr5, or tlr9 ligands, suggesting that anti-inflammatory effects of Tβ4 are likely mediated by the reduced activation of Toll-like receptors (TLR). Finally, Tβ4-Tg mice in a systemic sepsis model were protected from L. pneumophila -induced lethality compared to wild-type controls. Therefore, Tβ4 confers effective resistance against L. pneumophila via two pathways, a bactericidal and an anti-inflammatory pathway, which can be harnessed to treat acute pneumonia and septic conditions caused by L. pneumophila in humans., (Copyright © 2021 American Society for Microbiology.)

Legionella pneumophila uses the Icm/Dot type 4B secretion system (T4BSS) to deliver translocated protein substrates to the host cell, promoting replication vacuole formation. The conformational state of the translocated substrates within the bacterial cell is unknown, so we sought to determine if folded substrates could be translocated via this system. Fusions of L. pneumophila Icm/Dot-translocated substrates (IDTS) to dihydrofolate reductase (DHFR) or ubiquitin (Ub), small proteins known to fold rapidly, resulted in proteins with low translocation efficiencies. The folded moieties did not cause increased aggregation of the IDTS and did not impede interaction with the adaptor protein complex IcmS/IcmW, which is thought to form a soluble complex that promotes translocation. The translocation defect was alleviated with a Ub moiety harboring mutations known to destabilize its structure, indicating that unfolded proteins are preferred substrates. Real-time analysis of translocation, following movement during the first 30 min after bacterial contact with host cells, revealed that the folded moiety caused a kinetic defect in IDTS translocation. Expression of an IDTS fused to a folded moiety interfered with the translocation of other IDTS, consistent with it causing a blockage of the translocation channel. Furthermore, the folded protein fusions also interfered with intracellular growth, consistent with inefficient or impaired translocation of proteins critical for L. pneumophila intracellular growth. These studies indicate that substrates of the Icm/Dot T4SS are translocated to the host cytosol in an unfolded conformation and that folded proteins are stalled within the translocation channel, impairing the function of the secretion system.

Legionella pneumophila, the etiological agent for Legionnaires' disease, is ubiquitous in the aqueous environment, where it replicates as an intracellular parasite of free-living protozoa. Our understanding of L. pneumophila pathogenicity is obtained mostly from study of derivatives of several clinical isolates, which employ almost identical virulent determinants to exploit host functions. To determine whether environmental L. pneumophila isolates interact similarly with the model host systems, we analyzed intracellular replication of several recently isolated such strains and found that these strains cannot productively grow in bone marrow-derived macrophages of A/J mice, which are permissive for all examined laboratory strains. By focusing on one strain called LPE509, we found that its deficiency in intracellular replication in primary A/J macrophages is not caused by the lack of important pathogenic determinants because this strain replicates proficiently in two protozoan hosts and the human macrophage U937 cell. We also found that in the early phase of infection, the trafficking of this strain in A/J macrophages is similar to that of JR32, a derivative of strain Philadelphia 1. Furthermore, infection of these cells by LPE509 caused extensive cell death in a process that requires the Dot/Icm type IV secretion system. Finally, we showed that the cell death is caused neither by the activation of the NAIP5/NLRC4 inflammasome nor by the recently described caspase 11-dependent pathway. Our results revealed that some environmental L. pneumophila strains are unable to overcome the defense conferred by primary macrophages from mice known to be permissive for laboratory L. pneumophila strains. These results also suggest the existence of a host immune surveillance mechanism differing from those currently known in responding to L. pneumophila infection.

The unique permissiveness of A/J mouse macrophages for replication of Legionella pneumophila is caused by a deficiency in the Nod-like receptor (NLR) protein and intracellular sensor for L. pneumophila flagellin (Naip5). The signaling pathways and proteins activated by Naip5 sensing in macrophages were investigated. Transcript profiling of macrophages from susceptible A/J mice and from resistant A/J mice harboring a transgenic wild-type copy of Naip5 at 4 h following L. pneumophila infection suggested that two members of the Irf transcriptional regulator family, Irf1 and Irf8, are regulated in response to Naip5 sensing of L. pneumophila. We show that macrophages having defective alleles of either Irf1 (Irf1-/-) or its heterodimerization partner gene Irf8 (Irf8R294C) become permissive for L. pneumophila replication, indicating that both the Irf1 and Irf8 proteins are essential for macrophage defense against L. pneumophila. Moreover, macrophages doubly heterozygous (Naip5AJ/WT Irf8R294C/WT or Nlrc4-/+ Irf8R294C/WT) for combined loss-of-function mutations in Irf8 and in either Naip5 or Nlrc4 are highly susceptible to L. pneumophila, indicating that there is a strong genetic interaction between Irf8 and the NLR protein family in the macrophage response to L. pneumophila. Legionella-containing phagosomes (LCPs) formed in permissive Irf1-/- or Irf8R294C macrophages behave like LCPs formed in Naip5-insufficient and Nlrc4-deficient macrophages which fail to acidify. These results suggest that, in addition to Naip5 and Nlrc4, Irf1 and Irf8 play a critical role in the early response of macrophages to infection with L. pneumophila, including antagonizing the ability of L. pneumophila to block phagosome acidification. They also suggest that flagellin sensing by the NLR proteins Naip5 and Nlrc4 may be coupled to Irf1-Irf8-mediated transcriptional activation of key effector genes essential for macrophage resistance to L. pneumophila infection.

Although Legionella pneumophila can multiply in diverse cell types from a variety of species, macrophages from most inbred mouse strains are nonpermissive for intracellular replication and allow little or no growth of the bacteria. This phenomenon is likely genetically controlled by the mouse naip5 (birc1e) gene located within the Lgn1 locus. In this study, we have investigated the resistance of C57BL/6J macrophages to L. pneumophila infection by examining the fate of both the bacterium and the infected cells compared to that in macrophages from the permissive A/J strain. Our results indicate that although the trafficking of the L. pneumophila-containing vacuole is partially disrupted in C57BL/6J macrophages, this cannot account for the severity of the defect in intracellular growth observed in this strain. Infected macrophages are lost shortly after infection, and at later times a larger fraction of the C57BL/6J macrophages in which L. pneumophila undergoes replication are apoptotic compared to those derived from A/J mice. Finally, a loss of bacterial counts occurs after the first round of growth. Therefore, the resistance mechanism of C57BL/6J macrophages to L. pneumophila infection appears to be multifactorial, and we discuss how early and late responses result in clearing the infection.

The eukaryotic protein synthesis inhibitor cycloheximid has been used by many investigators to selectively radiolabel intracellular bacteria. Although cycloheximide has no direct effect on bacterial gene expression, there are concerns that long-term inhibition of the host cell protein synthesis may have secondary effects on bacterial gene expression. Therefore, prior to further identification and cloning of the macrophage-induced (MI) genes of Legionella pneumophila, the effects of cycloheximide on L. pneumophila-infected U937 cells were evaluated by transmission electron microscopy. Inhibition of protein synthesis of the host cell for 6 h had no major effect on the ultrastructure of the host cell, on the formation of rough endoplasmic reticulum-surrounded replicative phagosome, or on initiation of intracellular bacterial replication. In contrast, by 15 h of cycloheximide treatment, there was profound deterioration in the host cell as well as in the phagosome. To examine protein synthesis by L. pneumophila during the intracellular infection, U937 macrophage-like cells were infected with L. pneumophila, and intracellular bacteria were radiolabeled during a 2-h cycloheximide treatment or following 12 h of cycloheximide treatment. Comparison by two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the protein profile of radiolabeled in vitro-grown L. pneumophila to that of intracellularly radiolabeled bacteria showed that 23 proteins were induced in response to the intracellular environment during 2 h of inhibition of host cell protein biosynthesis. Twelve MI proteins of L. pneumophila were artifactually induced due to prolonged inhibition of the host cell protein synthesis. The gene encoding a 20-kDa MI protein was cloned by a reverse genetics technique. Sequence analysis showed that the cloned gene encoded a protein that was 80% similar to the enzyme inorganic pyrophosphatase. Studies of promoter fusion to a promoterless lacZ gene showed that compared to in vitro-grown bacteria, expression of the pyrophosphatase gene (ppa) was induced fourfold throughout the intracellular infection. There was no detectable induction in transcription of the ppa promoter during exposure to stress stimuli in vitro. The ppa gene of L. pneumophila is the first example of a regulated ppa gene which is selectively induced during intracellular infection and which may reflect enhanced capabilities of macromolecular biosynthesis by intracellular L. pneumophila. The data indicate caution in the long-term use of inhibition of host cell protein synthesis to selectively examine gene expression by intracellular bacteria.