Tan, Yuan2025-02-102025-02-102025-02-10http://hdl.handle.net/10393/50171https://doi.org/10.20381/ruor-30921Sepsis is characterized by a systemic dysregulated immune response to severe infection. Research has shown that mesenchymal stromal/stem cells (MSCs) exert immunemodulatory functions in vitro and therapeutic benefits in various animal models of severe immune diseases. These promising results have driven the translation of MSCs as cellular therapy into clinical settings. The host sepsis environment, including the pathogens and the host immune response (cells and soluble factors), may play a crucial role in modifying the therapeutic ePicacy of infused MSCs. Here, we aim to investigate the interplay between MSCs and the host microenvironment in three disease contexts: 1) severe SARS-CoV-2 infection (COVID-19), 2) acute lung injury (ALI) induced by H1N1 influenza A virus (IAV) infection and 3) human sepsis caused by heterogenous organisms. First, we hypothesized that, in severe COVID-19, priming MSCs with a viral mimic would improve their abilities to rebalance the dysregulated immune responses. Transcriptome analysis of Poly(I:C)-primed MSCs (pIC-MSCs) revealed upregulation of pathways involved in antiviral and immunomodulatory responses. Together with increased expression of antiviral proteins, these changes translated into greater MSC ePector functions in regulating monocytes and granulocytes, while enhancing their ability to block SARS-CoV-2 pseudovirus entry into epithelial cells. Importantly, the addition of pIC-MSCs to COVID-19 patient whole blood significantly reduced inflammatory neutrophil populations, increased the proportion of M2 monocytes and enhanced their phagocytic function. In the second study, deep immune profiling was performed on airway and circulating immune cells to examine the ePect of immunomodulation and therapeutic outcomes of MSCs therapy in mice with H1N1-induced ALI. Immune cell populations and phenotypic shifts were mapped in whole blood by mass cytometry, showing altered immune responses in animals receiving MSCs vs vehicle treatment. Compared to sham animals, IAV infection induced a significant increase in BAL total cell counts. MSC administration significantly decreased BAL total cell counts and altered immune infiltrations in IAV-infected mice. Phenotypic immune cell profiling of blood and BAL revealed a significant increase in the monocyte population with M2 phenotype in MSCs-treated animals. However, MSCs treatment did not improve survival of infected mice or reduce viral titres in the lungs of infected mice. Further investigation revealed that MSCs were highly susceptible to H1N1 IAV infection, leading to increased cell death and potentially reduced their ePicacy. Lastly, ex vivo human and in vivo animal models were utilized to investigate neutrophil and monocyte modulation by MSCs in sepsis. Results showed phenotypic and functional improvements of myeloid cells after coculture with MSCs. Interestingly, transcriptomic and cytokine analyses revealed adaptive reprograming of MSCs in response to the sepsis milieu. This activation of caspase-1 pathway of MSCs was subsequently confirmed as an essential molecular mechanism for the immunomodulatory ePects of MSCs on dysfunctional neutrophils and monocytes. Caspase-1 activation in MSCs enhanced their therapeutic benefits in treating sepsis in vivo. Overall, my work suggests that the function and ePicacy of MSCs vary depending on types of infection induced by diPerent pathogens and the host immune microenvironment. These findings highlight the importance of understanding the adaptive responses of MSCs to the host environment in order to better tailor MSC-based therapy for severe viral and bacterial infections.enAttribution-NonCommercial-NoDerivatives 4.0 Internationalhttp://creativecommons.org/licenses/by-nc-nd/4.0/mesenchymal stem cellssepsisimmunomodulationinfectionvirusCOVID-19neutrophilmonocyteacute lung injuryThesis