Macrophages seal them in the cell
GATA6+ macrophages residing in body cavities exhibit phagocytic and repair functions. However, the mechanisms by which these cells can identify and migrate to injury sites remain unclear. Using intravital images of mouse peritoneal cavities, Zindel et al. report that GATA6+ macrophages quickly assemble clot-like structures in a process strongly analogous to thrombosis (see Herrick and Allen’s Perspective). The formation of these aggregates requires the expression of domains of the macrophage-eliminating receptor and acts to cover wounds and promote healing. This pathway can be inadvertently activated during medical procedures, when macrophage aggregates can promote the generation of abdominal scar tissue known as adhesions. Inhibition of macrophage-eliminating receptors can therefore be a useful therapeutic approach after surgeries that cause injuries to body cavities.
Science, this problem p. eabe0595; see also p. 993
Structured Summary
INTRODUCTION
Most multicellular organisms have a main body cavity that houses immune cells. In primordial species such as the purple sea urchin, these cells – called celomocytes – perform dual functions. The sea urchin’s cellomocytes eliminate pathogens from the peritoneal compartment, but they have also been shown to form multicellular aggregates that adhere to the injured tissue and are crucial for repair. In mammals, the peritoneal, pleural and pericardial cavities are filled with a large number of GATA6 residents+ macrophages of the cavity. The role of macrophages in the peritoneal cavity as phagocytes in the elimination of pathogens has been established for decades. Recent evidence suggests that these cells migrate to lesions within the peritoneal cavity, where they have been shown to promote tissue repair.
BACKGROUND
It is still unclear how macrophages in the cavity, which are suspended in the fluid phase (peritoneal fluid), can identify lesions, which can be several thousand micrometers apart, and how they can exhibit chemotaxis at that distance through a compartment full of fluid that is under constant convective flow. In this study, we developed an intravital microscopy (IVM) model to study the dynamics and molecular mechanisms of the resident GATA6+ recruitment of macrophages in the peritoneal cavity after injury.
RESULTS
Using inverted multi-photon IVM with extremely sensitive undiscolored hybrid detectors, we were able to image the peritoneal cavity through the intact abdominal wall in live animals. Traces of the rapidly moving peritoneal macrophages showed that they passively passed through the peritoneal cavity in a seemingly random, breath-dependent pattern. We then use a focused high-powered infrared laser beam to induce focal lesions in the peritoneum and photograph the subsequent immune response. We found that peritoneal macrophages were quickly recruited through a two-step process: (i) an initial attachment of macrophages to the lesion site, followed by (ii) secondary ties that formed an aggregate that resembles a thrombus-like structure in response to the injury. The aggregation of macrophages mirrored and rivaled the speed of platelet aggregation (thrombus formation) in the adjacent vasculature. Upon probing the peritoneal macrophage transcriptome and a series of IVM knockout and inhibition experiments, we found that the aggregation of peritoneal macrophages was independent of canonical mammalian adhesion molecules, such as integrins, selectins and immunoglobulin-like adhesion molecules. Instead, peritoneal macrophage aggregation was dependent on cysteine-rich domains (SRCR) of the primordial scavenger receptor. SRCR domains are highly conserved among species, with many homologues expressed by sea urchin cellomocytes and sea sponges, and some of these proteins have been identified as cell-cell adhesion molecules in these primordial organisms. Macrophage aggregates in the cavity wound physically sealed and promoted rapid repair of focal peritoneal lesions. However, in models of abdominal surgery that reflect iatrogenic surgical situations in which the peritoneal cavity is opened and foreign suture material is introduced, these cavitary macrophages formed extensive aggregates that promoted the growth of intra-abdominal scar tissue called peritoneal adhesions. These peritoneal adhesions cause substantial morbidity for patients and considerable costs for healthcare systems. We show that the number and tenacity of peritoneal adhesions were significantly reduced by the depletion of peritoneal macrophages or by the therapeutic inhibition of their recruitment and aggregation dependent on the necrophagous receptor.
CONCLUSION
Our results reveal an extravascular fluid phase response similar to platelets by macrophages. This rapid response seals peritoneal leaks in minutes and plays an important role in repairing small injuries, such as laser-induced focal or peritoneal thermal injuries. Our hypothesis is that such focal lesions reflect a type of injury for which the immune system has developed a beneficial response. In contrast, iatrogenic procedures, such as abdominal surgery involving the implantation of foreign material, reflect a type of injury that has no evolutionary precedent. In this scenario, peritoneal macrophages can cause harmful scarring, instead of restitution ad integrum, in an attempt to repair the wound. Thus, the aggregation of macrophages and their inhibition by antagonists of the scavenger receptor are of clinical importance and may provide a therapeutic target to prevent the formation of scars after surgery in the peritoneal cavity. In addition, these findings may extend to other cavities, including pleural and pericardial spaces.
(ONE) Human celomic, mouse and sea urchin cavities (example of enlarged mouse) with circulating cellomocytes and macrophages. The lesions were induced by a multiphotonic laser. (B) IVM image immediately after the injury. Scale bar, 50 μm. (Ç) IVM image 30 min after the injury. Scale bar, 50 μm. (D) The aggregation was dependent on eliminating receptors and a polyanionic ligand (still unknown).
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(ONE) Human celomic, mouse and sea urchin cavities (example of enlarged mouse) with circulating cellomocytes and macrophages. The lesions were induced by a multiphotonic laser. (B) IVM image immediately after the injury. Scale bar, 50 μm. (Ç) IVM image 30 min after the injury. Scale bar, 50 μm. (D) Aggregation was dependent on scavenger receptors and a polyanionic ligand (still unknown).
Summary
Most multicellular organisms have a main body cavity that houses cells of the immune system. In primordial species, such as purple sea urchins, these cells perform phagocytic functions, but are also crucial for repairing injuries. In mammals, the peritoneal cavity contains a large number of GATA6 residents+ macrophages, which can work in a similar way. However, it is not clear how the macrophages in the cavity suspended in the fluid phase (peritoneal fluid) identify and migrate towards the lesions. In this study, we used intravital microscopy to show that macrophages in the fluid cavity quickly form thrombus-like structures in response to injury through primordial scan receptor cysteine-rich domains. Macrophage aggregates in the cavity wound physically sealed and promoted rapid repair of focal lesions. In iatrogenic surgical situations, these cavitary macrophages formed extensive aggregates that promoted the growth of intra-abdominal scar tissue known as peritoneal adhesions.