Adhesion kinetics of human primary monocytes, dendritic cells, and macrophages: Dynamic cell adhesion measurements with a label-free optical biosensor and their comparison with end-point assays.

Monocytes, dendritic cells (DCs), and macrophages (MFs) are closely related immune cells that differ in their main functions. These specific functions are, to a considerable degree, determined by the differences in the adhesion behavior of the cells. To study the inherently and essentially dynamic aspects of the adhesion of monocytes, DCs, and MFs, dynamic cell adhesion assays were performed with a high-throughput label-free optical biosensor [Epic BenchTop (BT)] on surfaces coated with either fibrinogen (Fgn) or the biomimetic copolymer PLL-g-PEG-RGD. Cell adhesion profiles typically reached their maximum at ∼60 min after cell seeding, which was followed by a monotonic signal decrease, indicating gradually weakening cell adhesion. According to the biosensor response, cell types could be ordered by increasing adherence as monocytes, MFs, and DCs. Notably, all three cell types induced a larger biosensor signal on Fgn than on PLL-g-PEG-RGD. To interpret this result, the molecular layers were characterized by further exploiting the potentials of the biosensor: by measuring the adsorption signal induced during the surface coating procedure, the authors could estimate the surface density of adsorbed molecules and, thus, the number of binding sites potentially presented for the adhesion receptors. Surfaces coated with PLL-g-PEG-RGD presented less RGD sites, but was less efficient in promoting cell spreading than those coated with Fgn; hence, other binding sites in Fgn played a more decisive role in determining cell adherence. To support the cell adhesion data obtained with the biosensor, cell adherence on Fgn-coated surfaces 30-60 min after cell seeding was measured with three complementary techniques, i.e., with (1) a fluorescence-based classical adherence assay, (2) a shear flow chamber applying hydrodynamic shear stress to wash cells away, and (3) an automated micropipette using vacuum-generated fluid flow to lift cells up. These techniques confirmed the results obtained with the high-temporal-resolution Epic BT, but could only provide end-point data. In contrast, complex, nonmonotonic cell adhesion kinetics measured by the high-throughput optical biosensor is expected to open a window on the hidden background of the immune cell-extracellular matrix interactions.