Development of functional capacity and bactericidal activity in myeloid cells during GM-CSF driven differentiation

Abstract

Dendritic cells (DC) form an interface between the innate and adaptive immune systems. DC respond to infection or inflammatory signals by orchestrating other immune cells to perform protective functions. These include activation of innate immune cells, like natural killer cells, and induction of an adaptive T cell response. Since the discovery of DC in the early 1970s, a lot of research effort has gone into understanding their uptake mechanisms, the kinetics of proteolysis, and the efficiency through which DC process and present antigens to T cells. While these functional attributes have been well studied in terminally differentiated DC or DC precursors, the stage at which DC develop these functions is yet to be determined. In this study, we examined the ontogeny of antigen uptake, degradative activity, antigen presentation, and bactericidal activity in DC precursor cells during GM-CSF driven differentiation. To determine the functional attributes of DC and DC precursor cells, bone marrow cultured in GM-CSF was isolated into five distinct populations: CMP, GMP, monocyte, moMac/moDP, and moDC that represent the stages of their development. The CMP population demonstrated the lowest uptake capacity, and proteolysis. Also, CMP were unable to process and present antigen to T cells. In addition, CMP were the least efficient at uptake of Listeria monocytogenes and did not support intracellular replication of the bacteria. The GMP and monocyte populations demonstrated the highest uptake capacity of both large and small particles, moderate proteolytic activity, and were able to induce only modest T cell activation. These cell populations showed moderate uptake of L. monocytogenes, yet did not provide a conducive niche for bacteria replication, an observation that was attributed to high levels of reactive oxygen and nitrogen intermediates produced by these cells. Lastly, although the moMac/moDP and moDC populations appeared to be functionally similar, there were notable differences. First, both populations had moderate uptake capacity of bioparticles and dextran, strong proteolytic activity, and were potent inducers of T cell proliferation. Additionally, moMac/moDP and moDC demonstrated the highest level of uptake of L. monocytogenes when compared to other stages. This was attributed to high expression of uptake receptors in these cell populations. The moMac/moDP population supported L. monocytogenes intracellular replication while moDC did not support bacterial growth. In summary, through this project, we have determined the stages at which DC and DC precursor cells acquire different functional capacities. This allows researchers a more targeted approach to isolate their preferred cell types along the developmental spectrum.