Human Primary Immune Cells Exhibit Distinct Mechanical Properties that Are Modified by Inflammation

Abstract

T lymphocytes are key modulators of the immune response. Their activation requires cell-cell interaction with different myeloid cell populations of the immune system called antigen-presenting cells (APCs). Although T lymphocytes have recently been shown to respond to mechanical cues, in particular to the stiffness of their environment, little is known about the rigidity of APCs. In this study, single-cell microplate assays were performed to measure the viscoelastic moduli of different human myeloid primary APCs, i.e., monocytes (Ms, storage modulus of 520 +90/-80 Pa), dendritic cells (DCs, 440 +110/-90 Pa), and macrophages (MPHs, 900 +110/-100 Pa). Inflammatory conditions modulated these properties, with storage moduli ranging from 190 Pa to 1450 Pa. The effect of inflammation on the mechanical properties was independent of the induction of expression of commonly used APC maturation markers, making myeloid APC rigidity an additional feature of inflammation. In addition, the rigidity of human T lymphocytes was lower than that of all myeloid cells tested and among the lowest reported (Young's modulus of 85 ± 5 Pa). Finally, the viscoelastic properties of myeloid cells were dependent on both their filamentous actin content and myosin IIA activity, although the relative contribution of these parameters varied within cell types. These results indicate that T lymphocytes face different cell rigidities when interacting with myeloid APCs in vivo and that this mechanical landscape changes under inflammation.

Cytoskeletal control of B cell responses to antigens

“…Indeed, B cells achieve best affinity discrimination on stiff, non-deformable membrane substrates containing strongly tethered antigens 122,123 . Notably, APCs differ substantially in their mechanical properties 149 .…”

“…Inflammation enhances the stiffness and contractility of APCs through changes in their actomyosin cytoskeleton 149,151 and the active recycling of antigens by APCs suggests that they may also mechanically respond to B cell contacts 152,153 .…”

Mechanosensing in the immune response

“…Based on the observed stiffness of Jurkat cells, Young’s modulus E ~ 50–100 Pa [82], the expected value of maximum force, F~E × area ~2–5 nN, suggesting that T cells which are softer than adherent cells (1–5 kPa [83]), generate weaker tractions, but these are sufficient to activate individual TCRs.…”

“…Using PA substrates, we showed that traction forces exerted by T cells exhibited sigmoidal behavior as a function of substrate stiffness [19], with the total exerted forces saturating on surfaces of ~5kPa stiffness. Recent studies have found that the stiffness of many immune cells varies in the range of a few kPa, suggesting that T cells need to interact with (or discriminate between) relatively soft surfaces in vivo [82]. Intriguingly, Jurkat cells showed distinct differences in the dynamics of signaling and cell morphology as a function of substrate stiffness [19].…”

“…APCs) whose stiffness is in the low kPa range [82], conventional strategies to expand clonal populations of T cells for adoptive immunotherapies use protein-coated microbeads [93], which are significantly more rigid. Alternate strategies that mimic APCs in vivo may be better in stimulating T cells.…”