Low tidal volume ventilation protects the lung in mechanically ventilated patients. The impact of the accompanying permissive hypoxemia and hypercapnia on endothelial cell recovery from injury is poorly understood. CA (carbonic anhydrase) IX is expressed in pulmonary microvascular endothelial cells (PMVECs), where it contributes to CO₂ and pH homeostasis, bioenergetics, and angiogenesis. We hypothesized that CA IX is important for PMVEC survival and that CA IX expression and release from PMVECs are increased during infection. Although the plasma concentration of CA IX was unchanged in human and rat pneumonia, there was a trend toward increasing CA IX in the bronchoalveolar fluid of mechanically ventilated critically ill patients with pneumonia and a significant increase in CA IX in the lung tissue lysates of pneumonia rats. To investigate the functional implications of the lung CA IX increase, we generated PMVEC cell lines harboring domain-specific CA IX mutations. By using these cells, we found that infection promotes intracellular (IC) expression, release, and MMP (metalloproteinase)-mediated extracellular cleavage of CA IX in PMVECs. IC domain deletion uniquely impaired CA IX membrane localization. Loss of the CA IX IC domain promoted cell death after infection, suggesting that the IC domain has an important role in PMVEC survival. We also found that hypoxia improves survival, whereas hypercapnia reverses the protective effect of hypoxia, during infection. Thus, we report 1) that CA IX increases in the lungs of pneumonia rats and 2) that the CA IX IC domain and hypoxia promote PMVEC survival during infection.

低潮气量通气可保护机械通气患者的肺部。伴随的允许性低氧血症和高碳酸血症对内皮细胞从损伤中恢复的影响知之甚少。CA（碳酸酐酶）IX 在肺微血管内皮细胞 (PMVECs) 中表达，它在其中产生 CO ₂pH 稳态、生物能量学和血管生成。我们假设 CA IX 对 PMVEC 的存活很重要，并且 CA IX 的表达和从 PMVEC 的释放在感染期间增加。尽管在人和大鼠肺炎中 CA IX 的血浆浓度没有变化，但在机械通气重症肺炎患者的支气管肺泡液中存在 CA IX 增加的趋势，并且在肺炎大鼠肺组织裂解物中 CA IX 显着增加. 为了研究肺 CA IX 增加的功能影响，我们生成了具有域特异性 CA IX 突变的 PMVEC 细胞系。通过使用这些细胞，我们发现感染促进了 PMVEC 中 CA IX 的细胞内 (IC) 表达、释放和 MMP（金属蛋白酶）介导的细胞外裂解。IC 结构域缺失独特地损害了 CA IX 膜定位。CA IX IC 结构域的缺失促进了感染后的细胞死亡，表明 IC 结构域在 PMVEC 存活中具有重要作用。我们还发现，在感染期间，缺氧可以提高生存率，而高碳酸血症会逆转缺氧的保护作用。因此，我们报告1 ) CA IX 在肺炎大鼠的肺中增加和2 ) CA IX IC 结构域和缺氧在感染期间促进 PMVEC 存活。