, 1997), suggesting that P. carinii uses rapid and robust sterol-scavenging mechanisms. A separate study utilizing in vitro radiolabeling revealed that incorporation of radiolabeled squalene into sterols occurred predominantly in noncholesterol sterol fractions, whereas the relative specific activity of the crude cholesterol fraction was 20-fold less than those of the other sterol fractions, indicating that cholesterol was not synthesized by P. carinii under these conditions (Worsham et al., 2003). The ability of P. carinii to scavenge sterols from alveolar cells was shown using P. carinii attached to A549 alveolar epithelial

cells. In this study, P. carinii-associated fluorescence BI-2536 was observed after an overnight incubation with Bodipy-C12 labeled A549 cells (Furlong et al., 1997), and cellular fluorescence was fivefold higher in P. carinii organisms attached to A549 cells compared with nonadherent P. carinii, suggesting that attachment facilitated lipid transfer (Furlong et al., 1997). In addition to the presence of cholesterol within the membranes of P. carinii, several plant sterols Trichostatin A have been biochemically detected in P. carinii including campesterol, β-sitosterol, brassicasterol and stigmasterol (Giner et al., 2002). It has been proposed that plant sterols were not synthesized by P. carinii, but were originally a part of the host diet

that was incorporated into the lung, and subsequently scavenged by P. carinii and then incorporated into P. carinii cellular membranes (Giner et al., 2002). While cholesterol and plant sterols are incorporated unchanged into P. carinii membranes, experimental data provided by two separate studies suggest that the pathogen can remodel host-derived sterols. An early study looking at the fate of scavenged fluorescent lipids revealed that

although the majority of scavenged Uroporphyrinogen III synthase lipids were incorporated unchanged into P. carinii membranes, detection of the fluorescent label could be found in other lipid classes, including neutral lipids and phospholipids, suggesting the ability of P. carinii to modify scavenged lipids into complex lipid classes (Furlong et al., 1997). An analysis of sterols within P. carinii revealed the presence of sterols that cannot be synthesized de novo by either P. carinii or mammalian cells. Pneumocystis carinii contains a number of Δ5 alkylated C-24 sterols (Giner et al., 2002), but mammals are unable to alkylate the C-24 position of the sterol nucleus, and the lack of triene sterols in P. carinii (Giner et al., 2002) suggests that the organism is not able to destaurate C-5. The lack of the gene encoding C-5 desaturase has led to the belief that these Δ5 alkylated sterols were first scavenged from the host and subsequently modified by P. carinii Erg6 (Giner et al., 2002). The presence of large amounts of cholesterol within the membranes of P. carinii suggests that cholesterol uptake may be a constitutive process in P. carinii.