Primordial GATA6 macrophages function as extravascular platelets in sterile injury

Most multicellular organisms have a major body cavity that harbors immune cells. In primordial species like purple sea urchins, these cells perform phagocytic functions but are also crucial in repairing injuries. In mammals, the peritoneal cavity contains large numbers of resident GATA6+ macrophages, which may play a similar role. It is unclear how cavity macrophages suspended in the fluid phase (peritoneal fluid) identify and migrate towards injuries, however. Here, we show that cavity macrophages in fluid rapidly form thrombus-like structures in response to injury using primordial scavenger receptor (SRCR) domains. Aggregates of cavity macrophages physically sealed injuries and promoted rapid repair of focal lesions. In iatrogenic surgical situations, these cavity macrophages formed extensive aggregates that promoted the growth of intra-abdominal scar tissue termed peritoneal adhesions.