Sepsis is responsible for 5-6 million deaths per year making it one of the leading causes of preventable death worldwide and its incidence is increasing. Sepsis is caused by a dysregulated immune response to infection; however, our understanding of what causes this dysfunction remains limited. Myeloid-derived suppressor cells (MDSCs), an immature subpopulation of innate immune cells have recently been found in high numbers in septic patients. This population of cells constitute a unique component of the immune system, defined by their ability to suppress the response of T cells, yet the functions of MDSCs in the context of infection, are not well understood. MDSCs are generally detrimental during infection, likely due to their suppression of the adaptive immune response. However, in some patients with chronic inflammation the immunosuppressive functions of MDSCs can be beneficial, suggesting that MDSCs could play a protective role in other infections with hyperinflammation. Controlling the recruitment and function of MDSCs could, therefore, be an innovative strategy for treating infections, including sepsis. Unfortunately, very few studies have investigated MDSCs in the infectious microenvironment due to a) their low abundance in healthy patients, making them challenging to isolate and study, and b) the lack of relevant models of the human infectious microenvironment. In this project, we have overcome both challenges by inducing primary human MDSCs from primary human monocytes and studying their response to infection using our novel “infection-on-a-chip†microfluidic model. This device incorporates key aspects of the human infectious microenvironment including a model endothelial blood vessel, an extracellular matrix, primary human immune cells, and a live pathogen. Using this device, we discovered that the model endothelial blood vessel secretes both IL-6 and GM-CSF in response to the bacterial pathogen
Pseudomonas aeruginosa. Interestingly, IL-6 and GM-CSF are both known to drive MDSC expansion and activation. Therefore, we are also investigating the effect of endothelial activation on MDSC recruitment.
To generate primary human MDSCs, we incubated healthy human monocytes, isolated from peripheral blood, with GM-CSF and IL-6 for 5 days. We then determined their phenotypic identity using flow cytometry to confirm expression of MDSC markers (HLA-DRloCD14+CD33+). Finally, we confirmed their functional identity by evaluating their ability to suppress T cell expansion using a T cell suppression assay. We found a significant reduction in proliferating T cells in the presence of MDSCs compared to stimulated T cells alone (Fig. 1), indicating the presence of functional MDSCs. We are now investigating MDSC recruitment to inflammatory proteins (IL-8) and bacterial pathogens (P. aeruginosa) using our novel infection-on-a-chip device. Finally, we are examining the role of the vascular endothelium in activating MDSC recruitment in the infectious microenvironment using a combination of our device, multiplexed ELISAs, and blocking antibodies to identify key signals in the MDSC response.
