Neutrophils circulate in our bodies and hunt in infected tissues to ingest, kill, and digest harmful pathogens. To become such effective killers in the very complex situation of an inflamed tissue, they work together as a collective. They release chemical signals to attract other cells to form clusters of cells and attack as a swarm. By studying isolated mouse neutrophils in the culture dish, we identified that the G-protein coupled receptor kinase 2 (GRK2) regulates a process termed G-protein coupled receptor (GPCR) desensitization and thus the responsiveness of neutrophils to swarm-secreted chemoattractants. Intravital imaging of injured skin and infected lymph nodes of mice showed that GRK2 and GPCR desensitization play critical roles at sites where swarming neutrophils accumulate and self-generate local fields of high swarm attractant concentration. Desensitization-resistant neutrophils moved faster and explored larger areas of lymph node tissue infected with the bacterium Pseudomonas aeruginosa. Such behavior suggested more effective bacterial sampling throughout the infected organ. Surprisingly, mice with GRK2-deficient neutrophils showed impaired rather than improved bacterial clearance. This finding could not be explained by altered antibacterial effector functions. In vitro assays for the detailed analysis of swarming behavior and bacterial growth revealed that GPCR desensitization to swarm attractants is required to induce neutrophil arrest for optimal bacterial phagocytosis and containment in swarm clusters. In summary, we identified a navigation strategy, which allows neutrophils to self-limit their swarming dynamics, thus ensuring optimal elimination of bacteria. This work has been published (Kienle et al., Science 2021) and presented on several international conferences. We have also set this work into the context of current knowledge and work of other in two review articles (Glaser et al., Current Opinion in Cell Biology 2021; Mihlan et al., Frontiers in Cell and Developmental Biology, appearance in April/May 2022). Moreover, we compared the GPCR-based navigation strategies of several immune cell types, including neutrophils, in another review article (Lämmermann & Kastenmüller, Immunological Reviews 2019).
Moreover, we also explored the functional role of GRK-controlled GPCR desensitization in immune cells other than neutrophils. We considered this process as a general mechanism that might influence the dynamics of other immune cells in a similar manner to neutrophils. However, we made the unexpected finding that GRK-deficient dendritic cells, T cells and B cells have completely different phenotypes than GRK-deficient neutrophils, and we further delineated distinct functional roles of GRKs in specific immune cell subsets. A research manuscript is currently prepared for the presentation of these findings. Lastly, in our efforts to develop an integrated view of how different immune cell populations influence neutrophil swarming, we made the unexpected observation that mast cells, a tissue-resident myeloid immune cell type, can influence swarming neutrophils. We could identify the signals that mast cells release to initiate the formation of neutrophil swarms. These unexpected findings shed a new light on the role of neutrophils and mast cells during allergic reactions. A research manuscript is currently prepared for the presentation of these findings.
