Macrophages seal it in the cell
GATA6+ macrophages found in body cavities exhibit both phagocytic and restorative functions. However, the mechanisms by which these cells can identify and migrate to injury sites have remained unclear. Using intravital imaging of peritoneal cavities of the mouse, Zindel et al. report that GATA6+ macrophages rapidly assemble cohesive structures in a process strongly analogous to thrombosis (see the perspective of Herrick and Allen). The formation of these aggregates requires the expression of macrophage scavenger receptor domains and serves to occlude wounds and promote healing. This pathway can be inadvertently activated during medical procedures, when macrophage aggregates can promote the formation of abdominal scar tissue, known as adhesions. Inhibition of macrophage scavenger receptors may therefore be a useful therapeutic approach after surgeries that injure body cavities.
Science, this issue p. eabe0595; see also p. 993
Structured summary
INTRODUCTION
Most multicellular organisms have a large body cavity that contains immune cells. In primeval species such as the purple hedgehog, these cells – called seloomocytes – perform dual functions. Seachurch coelomocytes remove pathogens from the peritoneal compartment, but have also been shown to form multicellular aggregates that adhere to injured tissue and are essential for repair. In mammals, the peritoneal, pleural and pericardial cavities are filled with a large number of resident GATA6+ cavity macrophage. The role of peritoneal cavity macrophages as phagocytes in cleaning pathogens has been established for decades. Recent evidence suggests that these cells migrate to injuries within the peritoneal cavity, where they have been shown to promote tissue repair.
RATIONALE
It remains unclear how cavity macrophages, which are suspended in the fluid phase (peritoneal fluid), can identify injuries, which can be several thousand micrometers away, and how they can show chemotaxis over the distance through a fluid-filled compartment that is constant convective flow . In this study, we developed an intravital microscopy (IVM) model to study the dynamics and molecular mechanisms of residual GATA6.+ macrophage recruitment into the peritoneal cavity after injury.
RESULTS
Using reverse multiphoton IVM with highly sensitive non-truncated hybrid detectors, we were able to image the peritoneal cavity through the intact abdominal wall in live animals. The traces of the fast-moving peritoneal macrophages showed that they passively traverse the peritoneal cavity in a respiratory-dependent and seemingly random pattern. Next, we used a high-power focused infrared laser beam to cause focus injuries to the peritoneum, and we showed the subsequent immune response. We found that peritoneal macrophages were rapidly recruited through a two-step process: (i) an initial fixation of macrophages at the site of injury, followed by (ii) secondary ters forming a total reminiscent of a thrombus-like structure in response to injury. Macrophage aggregation reflected and matched the rate of platelet aggregation (thrombus formation) in the adjacent vessel. By examining the transcriptome of peritoneal macrophages and a targeted series of knockout and inhibition IVM experiments, we found that peritoneal macrophage aggregation was independent of canonical mammalian adhesion molecules such as integrins, selectins, and immunoglobulin-like adhesion molecules. Instead, peritoneal macrophage aggregation was dependent on original scavenger receptor-cysteine-rich (SRCR) domains. SRCR domains are highly conserved among species, with many homologues expressed by sea urchin selomocytes and sea sponges, and some of these proteins have been identified as cell adhesion molecules in these primordial organisms. Aggregates of cavity macrophages physically sealed injuries and promoted rapid recovery of focal peritoneal lesions. However, in abdominal surgery models that reflect iatrogenic surgical situations in which the abdominal cavity is opened and foreign suture material is introduced, these cavity macrophages have formed extensive aggregates that promote the growth of intra-abdominal scar tissue called peritoneal adhesions. These peritoneal adhesions cause significant disease to patients and significant cost to healthcare systems. We have shown that the number and endurance of peritoneal adhesions are significantly reduced by either depleting peritoneal macrophages or therapeutic inhibition of their scavenger receptor-dependent recruitment and aggregation.
CLOSURE
Our results reveal a platelet-like extravascular fluid-phase response by macrophages. This rapid response seals the peritoneal leaks within minutes and serves an important function in repairing minor injuries, such as focal thermal or laser-induced peritoneal injuries. We assume that such focus injuries reflect a type of injury for which the immune system has developed a beneficial response. In contrast, iatrogenic procedures, such as abdominal surgery involving implantation of foreign material, reflect a type of injury that has no evolutionary precedent. In this scenario, peritoneal macrophages can cause harmful lesions, rather than restitution ad integrum, in an attempt to repair the wound. Thus, macrophage aggregation and its inhibition by scavenger receptor antagonists are of clinical importance and may be a therapeutic target to prevent scarring after surgery in the peritoneal cavity. Furthermore, these findings may extend to other cavities, including pleural and pericardial spaces.
(AHuman, mouse and sea urchin’s selomic cavities (mouse sample enlarged) with circulating selomocytes and macrophages. Injuries were caused by a multiple laser. (B) IVM image immediately after injury. Scale bar, 50 μm. (C) IVM image 30 minutes after injury. Scale bar, 50 μm. (DAggregation was dependent on scavenger receptors and a (as yet unknown) polyanionic ligand.
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(A) Human, mouse and sea urchin’s selomic cavities (mouse sample enlarged) with circulating selomocytes and macrophages. Injuries were caused by a multiple laser. (B) IVM image immediately after injury. Scale bar, 50 μm. (C) IVM image 30 minutes after injury. Scale bar, 50 μm. (DAggregation was dependent on scavenger receptors and a (as yet unknown) polyanionic ligand.
Abstract
Most multicellular organisms have a large body cavity that contains immune cells. In primeval species such as purple sea urchins, these cells perform phagocytic functions, but it is also important to repair injuries. In mammals, the peritoneal cavity contains a large number of inhabitants GATA6+ macrophages, which can function similarly. However, it is unclear how cavity macrophages that are suspended in the fluid phase (peritoneal fluid) identify and migrate to injuries. In this study, we used intravital microscopy to show that cavity macrophages in fluid rapidly form thrombus-like structures in response to injury through the original scavenger-cystine-rich domains. Aggregates of cavity macrophages have physically sealed injuries and promote rapid recovery of focus lesions. In iatrogenic surgical situations, these cavity macrophages have formed extensive aggregates that promote the growth of intra-abdominal scar tissue, known as peritoneal adhesions.