Abstract Rationale Pneumonia, caused by Streptococcus pneumoniae, accounts for approximately two-thirds of bacterial pneumonia cases worldwide. Infections caused by S. pneumoniae serotype 3 have been associated with substantial mortality rates in humans. Numerous studies have demonstrated that the 13-valent pneumococcal conjugate vaccine is somewhat ineffective in reducing the risk or severity of disease caused by this serotype, highlighting the urgent need for novel therapeutic strategies. Methods We first assessed the extracellular bacterial killing abilities of Sca-1+ lung-derived mesenchymal stem cells (LMSCs), LMSC-derived supernatants, and bone marrow-derived neutrophils (BMDNs) against S. pneumoniae. We then evaluated the phagocytic capacity and intracellular bacterial killing abilities of LMSCs and bone marrow derived macrophages (BMDMs) using opsonized S. pneumoniae. To determine whether LMSC treatment attenuates pneumococcal pneumonia, female C57BL/6J mice were infected oropharyngeally with S. pneumoniae (5x104 cfu/75 µl PBS/mouse) and treated 4 hours post-infection with LMSCs or PBS. The mice were sacrificed 48 hours post-infection, and bronchoalveolar lavage fluid (BALF) was collected for immune cell phenotyping and cytokine/chemokine quantification. Lung pathology was evaluated in H&E-stained lung sections, and neutrophil localization was assessed via immunofluorescence labeling using anti-mouse GR-1 antibodies. Bacterial burden was quantified in the lungs and extrapulmonary organs (liver and spleen). Paraffin-embedded lung sections were also stained with Masson Trichrome and Periodic acid-Schiff’s (PAS) reagents. For survival studies, mice were infected oropharyngeally with a higher dose of S. pneumoniae (1x105 cfu/75 µl PBS/mouse) and treated with either 3T3 mouse embryonic fibroblasts (MEFs-control) or LMSCs at 4 and 24 hours post-infection. Results Both LMSCs and LMSC-derived supernatants more effectively suppressed the growth of S. pneumoniae in vitro than BMDNs. Additionally, LMSCs demonstrated a greater capacity for bacterial internalization and intracellular killing compared with BMDMs. Adoptive transfer of LMSCs into S. pneumoniae-infected mice improved survival, reduced inflammatory cell infiltration (predominantly neutrophils and macrophages) and cytokine/chemokine levels (TNFα, IL-1β, G-CSF, IL-6, and CXCL-5) in the BALF, and diminished neutrophil accumulation within lung tissues. LMSC treatment also mitigated lung injury and enhanced bacterial clearance in pulmonary and extrapulmonary organs. Furthermore, LMSCs attenuated pleural thickness, pulmonary fibrosis, and reduced the number of PAS-positive cells in the large airways. In contrast, treatment with 3T3 MEFs had no significant effect on mortality caused by S. pneumoniae. Conclusions Collectively, these findings indicate LMSCs contribute to a protective response against pneumococcal pneumonia, highlighting their therapeutic potential to control the infection and undesirable fibrosis caused by the highly virulent S. pneumoniae. This abstract is funded by: 5R01AI180123-02, 1RO1AI157353-05, and 5P20GM130555-07
Rangasamy et al. (Fri,) studied this question.