Clearance of clara cell secretory protein 16 (CC16) and surfactant proteins A and B from blood in acute respiratory failure

Surfactant proteins A and B (SP-A and SP-B) enter the circulation in a manner that acutely reflects changes in pulmonary function in patients with acute respiratory failure (ARF). There is a small but significant gradient in SP-A and SP-B from arterial to mixed venous (A-V) blood, and since we have detected both proteins in urine, the kidney may be a major site of their systemic clearance. Clara cell secretory protein 16 (CC16), which leaks from the respiratory tract, is known to be freely eliminated by the kidney. Lung plasma protein levels will depend on the rates of both protein entry into and clearance from plasma. In order to study the limiting variable determining these levels, we compared plasma CC16, SP-A, and SP-B in matching A-V blood samples from 37 ARF patients with indices of lung dysfunction and glomerular filtration rate (GFR) (of plasma cystatin C and creatinine). Cystatin C, CC16, SP-A, and SP-B were reduced in mixed venous plasma (all p < 0.001) and their A-V gradients were directly related to their arterial levels (all p < 0.03). Whereas CC16, SP-A, and SP-B reflected blood oxygenation (all p < 0.05), only SP-A and SP-B were related to lung injury score (US) (both p < 0.05). In contrast, whereas the clearances of both CC16 and cystatin C were related to that of creatinine (p < 0.02 for both), the clearances of SP-A and SP-B were not. Our study confirms that all three lung proteins are acutely cleared from the circulation of patients with ARF (half-lives < 18 min), and we conclude that whereas the plasma concentration of CC16 depends on GFR, plasma concentrations of SP-A and SP-B reflect lung function independently of this variable.