Abstract

Studies of Pneumocystis carinii pneumonia (PCP) suggest an important role for the surfactant system in the pathogenesis of the hypoxemic respiratory insufficiency associated with this infection. We hypothesized that PCP induces selective alterations in alveolar surfactant component expression and resultant biophysical properties. PCP was induced by intratracheal inoculation of 2 x 10(5) P. carinii organisms into C.B-17 scid/scid mice. Six weeks after inoculation, large (LA)- and small (SA)-aggregate surfactant fractions were prepared from bronchoalveolar lavage fluids and analyzed for expression of surfactant components and for biophysical activity. Total phospholipid content was significantly reduced in LA surfactant fractions from mice infected with PCP (53 +/- 15% of uninfected mice; P < 0.05). Quantitation of hydrophobic surfactant protein (SP) content demonstrated significant reductions of alveolar SP-B and SP-C protein levels in mice with PCP compared with those in uninfected mice (46 +/- 7 and 19 +/- 6%, respectively; P < 0.05 for both). The reductions in phospholipid, SP-B, and SP-C in LA fractions measured during PCP were associated with an increase in the minimum surface tension of LAs as measured by pulsating bubble surfactometer (13.1 +/- 1.1 vs. 5.4 +/- 1.8 mN/m; P < 0.05). In contrast to decreases in the hydrophobic SPs, SP-D content in the SA fraction was markedly increased (343 +/- 30% of control value; P < 0. 05) and SP-A levels in LA surfactant were maintained (93 +/- 26% of control value) during P. carinii infection. In all cases, the changes in SP content were reflected by commensurate changes in the levels of mRNA. We conclude that PCP induces selective alterations in surfactant component expression, including profound decreases in hydrophobic protein contents and resultant increases in surface tension. These changes, demonstrated in an immunologically relevant animal model, suggest that alterations in surfactant could contribute to the hypoxemic respiratory insufficiency observed in PCP.