Abstract Body (Do not enter title and authors here): Background
Acute kidney injury (AKI) complicated by respiratory failure with hypoxemia is common among critically ill patients and associated with high mortality. Previously, we showed that circulating osteopontin (OPN) released only from the injured kidney during AKI causes remote lung inflammation with hypoxemia (Khamissi Sci Adv 2022). In our latest study, we show that AKI-induces blood flow impeding lung capillary neutrophil retention that is initiated by the neutrophil chemoattractant CXCL2, likely derived from CCR2 monocytes. This perfusion impediment causes a ventilation-perfusion mismatch and hypoxemia (Komaru JCI 2025).
Methods
We used OPN-KI mice carry a point mutation that renders OPN uncleavable by thrombin. Mouse OPN proteins- OPN-FL, OPN-N, OPN-C, and uncleavable OPN-KI were expressed and purified from HEK293T cells. These biologically active proteins were injected in wt mice with mild AKI. CCR2 monocyte-derived peritoneal macrophages were isolated from wt mice, treated with OPN-N+C and analyzed for CXCL2 production.
Results
We now show that OPN targets CCR2 monocytes via its two thrombin cleaved fragments, OPN-N and OPN-C. OPN-KI mice showed kidney injury like wt controls but are protected from remote lung inflammation, showing that OPN acts via its fragments in vivo. Injection of both OPN fragments, OPN-N + OPN-C, into wt mice with mild AKI induced the full severe remote lung inflammation phenotype comparable to OPN-FL injection. OPN-KI injection was ineffective. Consistent with this, only OPN-N+C effectively induced CXCL2 production in CCR2 monocyte-derived peritoneal macrophages in vitro, suggesting that lung CCR2 monocytes are indeed direct target cells of both OPN fragments.
Conclusion
Our findings show that OPN cleavage is necessary, and both OPN fragments are the key mediators of AKI induced remote lung inflammation. These fragments act on CCR2 monocytes inducing CXCL2 production, elicit neutrophil retention and leads to hypoxemia. This implies that lung CCR2 monocytes or OPN fragments could be potential therapeutic targets to prevent AKI induced remote lung inflammation.