B105-14 Airway Lining PH in the Proximal and Distal Airways of the Human Lung

Abstract Background: Decreased airway lining fluid (ALF) pH contributes to thickened airway mucus, impaired ciliary function, viral and bacterial infections, airway injury and bronchoconstriction. ALF pH may be low in several acute conditions, ranging from gastric aspiration to asthma exacerbations. However, the pH of the normal human ALF is unknown. Here, in 39 research bronchoscopies (16 healthy controls HC and 23 with stable asthma SA), we directly measured ALF pH in the 2nd through 4th generation airways; and in nine, we also measured pH in the 1 mm (11th - 12th generation) airway using a wedged pH probe. We also measured CO2, O2 and NO in gas from the wedged bronchoscope. Bronchiolar pH was challenging to measure directly, but was estimated by inhaling alkaline pH buffer: an immediate decrease change in FeNO reflects a low starting pH. We also measured ALF pH in primary human bronchial epithelial cells (HBECs), both from HC and SA subjects, grown at air liquid interface. Finally, we used spatial transcriptomic (ST) analysis of human endobronchial biopsies to analyze epithelial expression of 18 candidate genes encoding known buffering proteins; and we measured the encoded proteins in HBECs. Proximal airway pH values were normally distributed with a mean of 6.85 ±0.65 (SD). Measures did not differ by sex or asthma history. Surprisingly, the pH with the probe wedged was 7.32 +/-0.46 and was unrelated to the high distal airway CO2 tensions (r = 0.18, p = 0.3, n = 7), suggesting epithelial ALF buffering of CO2. FeNO changed by -0.94, -2.56 and -7.17 ppb at 300, 100 and 30 mL/sec flows (p = 0.0045), reflecting a bronchiolar pH 6. In vitro, apical pH was higher in HC HBECs (7.4 ±0.44; n = 18 filters from 6 subjects) than in SA (7.07 ±0.57; n = 30 filters from 10 subjects; p = 0.03). Several pH regulatory genes were expressing in human airway epithelial cells; of these, carbonic anhydrase 12 and lactate dehydrogenase B were lower in HC than in SA (p 0.05) in each case; (Figure, n = 12-18 wells and 4-6 subjects each). pH is pathophysiologically and therapeutically important in gut and renal tubular epithelia, but is poorly understood in the ALF. Here, we provide normative data and technical recommendations for studying human ALF pH. We have also characterized epithelial pH regulatory proteins that could prove to be therapeutic targets. This abstract is funded by: NIH