Electrical Control of Immune Cell Function

Bioelectrical signalling drives behaviour of cells at scales from single bacteria to multicellular tissues. Improved understanding of bioelectrical phenomena, including their influence on immune cell function, will lead to improved electroconductive materials relevant to repair of epithelial and tissue damage.<br/>When an epithelium is wounded an endogenous, steady electric field (EF) of ~100 mV/mm is driven through the neighbouring tissues (cathode at the centre) and the success of wound closure relates to the presence and magnitude of this wound EF. In part this is because the EF steers migration of epithelial cells toward the wound site to close the gap. But Immune cells, including monocytes and macrophages, are also important contributors to successful wound healing. For many years the stimuli driving their function were principally attributed to chemical signalling factors present within the wound microenvironment, with no regard given to bioelectrical signals.<br/>We therefore studied the responses of cultures of primary human monocytes and macrophages exposed to physiologically relevant EFs. Time lapse observation of monocytes, which are macrophage precursors, showed that an EF of 150 mV/mm applied for 2h steered migration toward the cathode and increased the speed of migration compared to controls (no EF). This suggests that these ‘first responders’ would be guided toward the wound centre (cathode) by an endogenous EF.<br/>However, human macrophages responded differently. Macrophages migrated preferentially toward the anode (opposite to monocytes) and their speed of migration was increased compared to controls. Impressively, the macrophages were exquisitely sensitive to the EF stimulus, responding with migration toward the anode at EFs as small as 5 mV/mm, well below the threshold of endogenous wound EFs. This suggests that macrophages arriving, later than the monocytes, migrate to the wound edges (anode) to clear debris and microbes.<br/>Directed migration of macrophages was associated with elongation of the cells parallel to the EF axis and reorganisation of the actin cytoskeleton toward the leading edge. Since a key role of macrophages is phagocytic uptake of microbes and apoptotic cells in the wound environment, we tested whether the EF also enhanced phagocytosis. Macrophages were exposed to an EF for 2h then carboxylate beads, <i>Candida albicans,</i> or apoptotic neutrophils were added to the cultures and left without any EF for a further 2h. Phagocytosis increased significantly in each case, demonstrating that the EF ‘priming’ improved phagocytotic efficiency.<br/>Mechanistically, these EF-induced functional changes are accompanied by clustering of phagocytic receptors toward the anode, enhanced PI3K and ERK activation, and mobilization of intracellular calcium. EFs also modulated cytokine production selectively and can augment some effects of conventional polarizing stimuli on cytokine secretion.<br/>Human T cells, which are present in wounded epithelia (skin or airway), also displayed directed migration toward the cathode and activation upon stimulation with physiological EFs (50 or 150 mV/mm). We showed for the first time that EF stimulation downregulated T cell activation following stimulation with antigen-activated APCs or anti-CD3/CD28 antibodies, as demonstrated by decreased IL-2 secretion and proliferation. Mechanistically, STAT3 modulation by the EF was implicated in the functional changes.<br/>There are increasing efforts to use electrical stimulation to aid wound healing. This is especially important for recalcitrant or chronic wounds prone to infection, such as diabetic foot ulcers. Our work can inform design of electrical wound dressing materials seeking to target immune function and infection.