Acute lower respiratory tract infections are a leading cause of morbidity and mortality world-wide. Streptococcus pneumoniae (pneumococcus) is the most common bacterial cause of community-acquired pneumonia. Recovery from pneumococcal pneumonia results in the formation of resident memory CD4+ T cells, which act on lung epithelial cells to accelerate immune responses. Alveolar macrophages (AMs) are tissue-resident macrophages localized in the air spaces, where they orchestrate the lung anti-microbial responses. We hypothesized that recovery from pneumococcal pneumonia results in remodeling of the pool of alveolar macrophages, which act in concordance with other immune cells to protect the lungs from future infections. Although AM numbers were unchanged in experienced lungs, their surface phenotype showed significant changes, most prominently an increased MHC-II and a decreased SiglecF. This experienced AM phenotype was regionally-localized and long-lasting. Experienced AMs also exhibited extensive remodeling on the metabolomics and transcriptional level. Experienced AMs demonstrated significant increases in phosphocreatine and its metabolite precursors. The transcriptional analyses also revealed extensive changes. At baseline, experienced AMs exhibited a significant reduction in cell cycle activity and mRNA processing compared to naïve mice. During acute pneumonia, experienced AMs exhibited significant increases in immune signaling and energy metabolism. Moreover, transcriptional data also revealed strong but imperfect enrichment of a signature previously associated with IFN𝛾 signaling and marrow-derived AMs. IFN𝛾 gain and loss of functions experiments corroborated transcriptional data and revealed an essential role for IFN𝛾 in directly driving the AM MHC-II remodeling. Several immune cells produced IFN𝛾, with neutrophils being the most prominent source after the 1st pneumococcal challenge but other cells predominating after the 2nd pneumococcal challenge. CD4+ T cell depletion studies demonstrated that AMs' experienced phenotype was independent of CD4+ T cells. In contrast to naïve mice, lineage-tracing studies demonstrated that marrow-derived AMs predominately constitute the experienced AM pool. Upon experience, both embryonic AMs and marrow-derived AMs demonstrated similar remodeling for both SiglecF and MHC-II on their surfaces. While all AM similarly remodeled independent of their origin, marrow-derived AMs in experienced lungs displayed some differences from their embryonic counterparts, being less phagocytic.
In conclusion, recovery from pneumococcal pneumonia remodels the pool of alveolar macrophages to acquire adaptive characteristics. This remodeling involves a combination of recruitment of new cells and trained immunity via IFN𝛾 signaling.
| Identifer | oai:union.ndltd.org:bu.edu/oai:open.bu.edu:2144/42694 |
| Date | 16 June 2021 |
| Creators | Arafa, Emad I. |
| Contributors | Mizgerd, Joseph |
| Source Sets | Boston University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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