In a newborn pig cystic fibrosis (CF) model, the ability of gland-containing airways to fight infection was affected by at least two major host-defense defects: impaired mucociliary transport and a lower airway surface liquid (ASL) pH. In the gland-containing airways, the ASL pH is balanced by CFTR (CF transmembrane conductance regulator) and ATP12A, which, respectively, control HCO₃⁻ transport and proton secretion. We found that, although porcine small airway tissue expressed lower amounts of ATP12A, the ASL of epithelial cultures from CF distal small airways (diameter < 200 μm) were nevertheless more acidic (compared with non-CF airways). Therefore, we hypothesized that gland-containing airways and small airways control acidification using distinct mechanisms. Our microarray data suggested that small airway epithelia mediate proton secretion via ATP6V0D2, an isoform of the V0 d subunit of the H⁺-translocating plasma membrane V-type ATPase. Immunofluorescence of small airways verified the expression of the V0 d2 subunit isoform at the apical surface of Muc5B⁺ secretory cells, but not ciliated cells. Inhibiting the V-type ATPase with bafilomycin A1 elevated the ASL pH of small airway cultures, in the presence or absence of HCO₃⁻, and decreased ASL viscosity. These data suggest that, unlike large airways, which are acidified by ATP12A activity, small airways are acidified by V-type ATPase, thus identifying V-type ATPase as a novel therapeutic target for small airway diseases.

在新生猪囊性纤维化（CF）模型中，含腺体气道抵抗感染的能力至少受到两个主要宿主防御缺陷的影响：粘液纤毛运输受损和气道表面液体（ASL）pH值较低。在含腺体的气道中，ASL pH 通过 CFTR（CF 跨膜电导调节剂）和 ATP12A 来平衡，它们分别控制 HCO ₃ ^-转运和质子分泌。我们发现，尽管猪小气道组织表达的 ATP12A 量较低，但 CF 远端小气道（直径 < 200 μm）的上皮培养物的 ASL 酸性更高（与非 CF 气道相比）。因此，我们假设含腺体气道和小气道使用不同的机制控制酸化。^{我们的微阵列数据表明，小气道上皮通过 ATP6V0D2 介导质子分泌，ATP6V0D2 是 H +}易位质膜 V 型 ATP酶的 V0 d 亚基的亚型。^{小气道的免疫荧光验证了 Muc5B +}分泌细胞顶端表面 V0 d2 亚基异构体的表达，但纤毛细胞未证实。在存在或不存在 HCO ₃ ^-的情况下，用巴弗洛霉素 A1 抑制 V 型 ATP 酶会升高小气道培养物的 ASL pH 值，并降低 ASL 粘度。这些数据表明，与被 ATP12A 活性酸化的大气道不同，小气道被 V 型 ATP 酶酸化，从而将 V 型 ATP 酶确定为小气道疾病的新治疗靶点。