Staphylococcus aureus Research: Humanized Lung Models Unveil Host-Pathogen Dynamics

Staphylococcus aureus remains a formidable human pathogen, being a leading cause of bacterial-induced death, with significant concerns regarding antibiotic resistance and the current lack of an effective vaccine. A major hurdle in combating S. aureus is that it is a human-adapted pathogen, expressing virulence factors specific to human cellular receptors. This specificity limits the utility of traditional mouse models, which often fail to accurately replicate human-specific host responses to infection.

            To address these limitations, researchers have previously developed humanized mouse models incorporating a human immune system. While these models have seen success in studying S. aureus infections at various sites, including pneumonia, they have historically lacked the presence of human airway epithelial cells and tissue. Research by Hui Wang at Columbia University’s Humanized Mouse Core Facility and Dane Parker at Rutgers New Jersey Medical School marks a significant advancement by developing a novel humanized mouse model that integrates autologous fetal lung tissue implants into humanized NOD-scid IL2Rγ-/- (NSG) mice. This innovative model provides an unprecedented platform for studying S. aureus pathogenesis and its adaptation to the human pulmonary environment.

            The new model involves implanting human fetal thymus and lung tissues into NSG mice, followed by injecting autologous fetal liver-derived CD34+ stem cells. This approach successfully generated a model where human lung implants support S. aureus survival and dissemination, with significant bacterial counts recovered from the implants and evidence of spread to the spleen. Infection also led to noticeable hemorrhaging and the formation of S. aureus microcolonies within the implants. Importantly, the implants maintained a high proportion of human-origin cells, and immune cell chimerism remained stable even after infection.

            The study unveiled crucial aspects of the human host response to S. aureus within these humanized lung implants, revealing a more relevant picture compared to prior models:

• Unlike earlier humanized models, this study observed a consistent and significant reduction in resident human immune cell populations following S. aureus infection. Specifically, there were significant decreases in alveolar macrophages (6.4-fold), dendritic cells (3.4-fold), natural killer cells (19-fold), and neutrophils (7.2-fold). Human T lymphocyte populations, including CD4+ and CD8+ T cells, showed even more dramatic reductions (17-fold and 10-fold, respectively). Analysis using the cell viability marker DAPI further confirmed increased cell death within the immune cell compartment.

• While a substantial shift in overall cytokine profiles was detected, the study noted that many quantified cytokines, particularly those involved in chemotaxis, did not show a significant difference after infection. Conversely, key pro-inflammatory cytokines such as Tumor Necrosis Factor (TNF), Interferon-gamma (IFNγ), Interleukin-1 beta (IL-1β), Interleukin-6 (IL-6), Granulocyte-Colony Stimulating Factor (G-CSF), and Interleukin-8 (IL-8) exhibited significant increases. Pathway analysis of these changes indicated enrichment of cytokine production and JAK/STAT (Janus kinase/Signal transducer and activator of transcription) signaling pathways.

• Dual RNA-seq analysis of human transcripts in the lung implants revealed a robust transcriptional response to S. aureus. Major signaling pathways like NF-κB, JAK/STAT, and apoptosis were highlighted as significantly altered, which may be related to the observed cellular death. The most highly induced gene, Cyp24a1, showed a remarkable 54-fold increase, suggesting a potential role for vitamin D metabolism in host defense. While vitamin D deficiency resulted in only a mild, non-significant increase in bacterial counts in the implants, it led to a significant 2.7-fold increase in bacterial dissemination to the spleen.

            The model also provided crucial new insights into how S. aureus adapts to the human lung environment:

• Comparing S. aureus grown within the implants to planktonically grown cells, a significant downregulation of genes associated with major virulence factors was observed. These included the virulence factor regulator Agr, alpha toxin, delta toxin, phenol soluble modulins, LukAB, and Panton Valentine Leukocidin. Genes involved in metabolism, particularly those in the TCA cycle, were also downregulated.

• In contrast, genes involved in protein turnover and chaperones were significantly upregulated. This included the arc genes (arginine deiminase pathway) and heat shock proteins like hrcA, grpE, dnaJ, and dnaK. The arc operon is known to aid survival at low pH and influence antibiotic tolerance.

• Functional studies confirmed the importance of the heat shock response in pathogenesis. An hrcA-deficient S. aureus mutant showed a 58% decrease in bacterial numbers within the lung tissue, highlighting the critical role of this stress response regulator in bacterial survival in the human lung environment.

            This humanized lung implant model represents a substantial improvement in S. aureus research, enabling detailed investigation of both host and pathogen responses in a more physiologically relevant human context. It holds immense promise for identifying novel virulence factors and dissecting the complex host-pathogen interplay, potentially bridging the gap observed between traditional mouse models and the efficacy of vaccine studies. While the model has limitations, such as the implants not undergoing air exchange and the significant time and cost involved in generation, its ability to better mimic human infection makes it a valuable tool for targeted, low-throughput studies aimed at identifying therapeutic targets or testing pathogenic factors.

Wang H, Parker D. Improved humanized mouse model of Staphylococcus aureus infection. Mucosal Immunol. 2025 Aug;18(4):911-917. doi: 10.1016/j.mucimm.2025.05.001. Epub 2025 May 10. PMID: 40354999.

https://www.mucosalimmunology.org/article/S1933-0219(25)00048-0/fulltext