Group 2 innate lymphoid cells can engulf and destroy bacteria

Group 2 innate lymphoid cells (ILC2s) are now known to play prominent roles in many inflammatory responses involving the production of Th2-type cytokines, which contribute to parasitic infections, tissue repair, and metabolic homeostasis [1]. More recently, it has emerged that ILC2s may act as antigen-presenting cells, although corresponding evidence of phagocytic activity of these cells is currently scarce [2, 3]. Herein, we investigated whether ILC2s can phagocytose bacteria, through in vitro experiments.

To trace the migration of foreign particles within ILC2s, immunofluorescence technology was employed to label the lysosomes. The results showed that fluorescent microspheres engulfed by ILC2s were colocalized with lysosomes (Fig. 1d). To verify that this phenomenon is not restricted to artificial particles, ILC2s were then incubated with viable Enterococcus faecalis for various periods (0, 2, and 4 h) and observed by transmission electron microscopy (TEM) (Fig. 1e). Following 2 h of incubation, we observed engulfment and internalization of E. faecalis bacteria by both RAW264.7 macrophages and ILC2s, which then formed phagosomes (Fig. 1e). Within 4 h of incubation, the phagosomes had disappeared and were replaced by a structure of higher electron density, suggesting fusion of the phagosomes with lysosomes to form phagolysosomes (Fig. 1e). To confirm whether ILC2s possess the propensity to kill phagocytosed bacteria, they were pulsed with live E. faecalis for 30 min and then further incubated for different periods (1, 3, 6, and 16 h) after washing to remove free bacteria. The numbers of viable intracellular bacteria were then determined at each time point, expressed as percentages of bacterial viability at 3, 6, and 16 h after culturing. The data show that the majority of the bacteria phagocytosed by both cell types were killed within 6 h of ingestion, with a significant delay in the ILC2 cells compared with the RAW264.7 cells at the earlier 3 h time point (Figs. 1f and S3). However, Jurkat cells, as a negative control, could not phagocytose/kill bacteria (Fig. S3). These results suggest that ILC2s, similar to RAW264.7 professional phagocytes, are equipped to kill bacteria.