FluoroSpot Analysis of TLR-Activated Monocytes Reveals Several Distinct Cytokine-Secreting Subpopulations

Monocytes have long been considered a heterogeneous group of cells both in terms of morphology and function. In humans, three distinct subsets have been described based on their differential expression of the cell surface markers CD14 and CD16. However, the relationship between these subsets and the production of cytokines has for the most part been based on ELISA measurements, making it difficult to draw conclusions as to their functional profile on the cellular level. In this study, we have investigated lipoteichoic acid (LTA)- and lipopolysaccharide (LPS)-induced cytokine secretion by monocytes using the FluoroSpot technique. This method measures the number of cytokine-secreting cells on the single-cell level and uses fluorescent detection, allowing for the simultaneous analysis of two cytokines from the same population of isolated cells. By this approach, human monocytes from healthy volunteers could be divided into several subgroups as IL-1β, IL-6, TNF-α and MIP-1β were secreted by larger populations of responding cells (25.9–39.2%) compared with the smaller populations of GM-CSF (9.1%), IL-10 (1.3%) and IL-12p40 (1.2%). Furthermore, when studying co-secretion in FluoroSpot, an intricate relationship between the monocytes secreting IL-1β and/or IL-6 and those secreting TNF-α, MIP-1β, GM-CSF, IL-10 and IL-12p40 was revealed. In this way, dissecting the secretion pattern of the monocytes in response to TLR-2 or TLR-4 stimulation, several subpopulations with distinct cytokine-secreting profiles could be identified.