PSMalpha Activates Early Growth Response 1 to Recruit Neutrophils to Protect the Host against Staphylococcus aureus pneumonia via Aerosolized Inhalational Infection

Staphylococcus aureus (S. aureus) produces a variety of virulence factors, including alpha-toxin, enterotoxins, leukocidins, and phenol-soluble modulins (PSMs). Among these, PSMalpha is especially significant in the development of diseases associated with S. aureus due to its strong biological activity. Nevertheless, the specific contribution of PSMalpha in S. aureus pneumonia has not been completely clarified. The aim of this study was to examine the effect of PSMalpha on S. aureus pneumonia. A comparative examination of mortality rates and pathological alterations was conducted in mice exposed to an aerosolized inhalational infection with either the wild-type S. aureus strain MW2 (USA400, ST1 lineage) or an isogenic PSMalpha deletion mutant strain. The pneumonia model showed that mice inoculated with S. aureus MW2 demonstrated higher mortality rates and increased neutrophil infiltration in the lungs in comparison with those inoculated with the PSMalpha deletion mutant. Subsequently, a comparison was undertaken of the cDNA profiles of mouse lungs infected with either the S. aureus MW2 or the PSMalpha deletion mutant at 4 hours post-infection (hpi) and 6 hpi. RNA sequencing (RNA-seq) analysis resulted in the identification of 793 differentially expressed genes (DEGs) at 4 hpi and 2461 DEGs at 6 hpi. A total of 229 overlapping DEGs were identified when comparing the DEGs at 4 hpi and 6 hpi. Among these DEGs, the top 20 hub genes were selected (IL6, Csf2, IL17a, Mmp9, Cd80, Cd3e, Cd8a, Cxcl1, Cxcl2, Fos, Thy1, Ccl20, Csf3, Ly6c2, Egr1, Dusp1, Hspa1b, Cd209a, Cd3g, Itgb2l). The results provided evidence that PSMalpha initiates the activation of Early Growth Response 1 (Egr1) during S. aureus pneumonia. Following aerosolized inhalational infection with S. aureus MW2, reduced survival rates, elevated bacterial loads, and decreased numbers of immigrated neutrophils were observed in Egr1 conditional knockout mice compared with control mice. This difference, however, was not seen when the mice were infected with the PSMalpha deletion mutant. In conclusion, our findings suggest that the activation of Egr1 by PSMalpha plays a crucial role in the recruitment of neutrophils to combat S. aureus pneumonia.