Methyl-beta-Cyclodextrin treatment and filipin staining reveal the role of cholesterol in surface membrane antigen sequestration of Schistosoma mansoni and S. haematobium lung-stage larvae

Ex vivo lung-stage larvae of Schistosoma mansoni and S. haematobium do not bind specific antibodies in the indirect membrane immunofluorescence test (IF), probably as a result of confinement of the surface membrane antigens in immobile, lipid-rich sites. Treatment with the membrane-impermeable, cholesterol-extracting drug methyl-beta-cyclodextrin (MBCD) and staining with filipin III (filipin), a fluorescent polyene antibiotic widely used for the detection and quantitation of cholesterol in biomembranes, allowed us to examine the role of cholesterol in surface membrane antigen sequestration of S. mansoni and S. haematobium ex vivo lung-stage larvae. Treatment of S. mansoni larvae with MBCD elicited appreciable cholesterol depletion as judged by filipin-cholesterol fluorescence diminution, which was accompanied by a considerable increase in specific antibody binding in IF, thus suggesting that cholesterol plays a predominant role in sequestration of the surface membrane antigens of S. mansoni lung-stage schistosomula. Despite that, MBCD induced an almost complete depletion of cholesterol from the outer membrane of S. haematobium larvae; no increase in specific antibody binding in IF was evident, implying that cholesterol is not responsible for masking surface membrane antigens of S. haematobium lung-stage larvae.     

Equilibrium in lung schistosomula sphingomyelin breakdown and biosynthesis allows very small molecules, but not antibody, to access proteins at the host-parasite interface

The mechanism by which lung-stage schistosomula expose proteins at the host-parasite interface to nutrient, but not antibody, uptake has been obscure. We have found that Schistosoma mansoni and Schistosoma haematobium larvae emerging from host lung at a pH of around 7.5, and fixed with diluted formaldehyde (HCHO), readily bind specific antibodies in indirect membrane immunofluorescence. Data on inhibitors and activators of parasite tegument-bound, magnesium-dependent, neutral sphingomyelinase (nSMase), and sphingomyelin biosynthesis inhibitors revealed that equilibrium in schistosomular sphingomyelin breakdown and biosynthesis prevents antibody binding, yet permits access of small HO-CH2-OH polymers to interact with and cross-link proteins at the host-parasite interface, allowing for their serological visualization.     

Depletion of Schistosoma mansoni lung-stage schistosomula cholesterol by methyl-beta-cyclodextrin dramatically increases specific antibody binding to surface membrane antigens

Schistosoma mansoni lung-stage larvae appear to not bind antibodies from radiation vaccine or infection sera in the membrane immunofluorescence test. However, treatment of ex vivo lung-stage schistosomula with methyl-beta-cyclodextrin, a hydrophobic oligosaccharide that specifically extracts cholesterol from plasma membranes, induced readily detectable binding of specific antibodies in a concentration- and time-dependent manner. Surface membrane antigen binding of specific antibodies was also conclusively demonstrated by quantitative absorption of anti-schistosome sera with intact ex vivo larvae. These data together suggest that confinement of lung-stage schistosomula surface membrane antigens in cholesterol-rich sites allows only monovalent antibody binding, which can be detected by absorption and not by direct serology.     

Incubation of Schistosoma mansoni lung-stage schistosomula in corn oil exposes their surface membrane antigenic specificities

Several potential cues for increased surface antigenic expression of lung-stage schistosomula, such as lack of glucose and amino acids and extremes of pH or HCO3- concentration, failed to alter the negligible larval reactivity with control, infection, or irradiated cercariae-vaccine serum in indirect membrane immunofluorescence. In contrast, incubation of larvae in 90% corn oil for 6 hr led to surface membrane changes, which allowed specific and strong binding of antibody from antischistosome sera. The data together indicated that the lung-stage worms' confinement of antigenic molecules in lipid-rich sites of the outer membrane could be reversed in vitro after exposure to corn oil, in a concentration- and time-dependent manner.     

Rapid turn-over of plasma membrane sphingomyelin and cholesterol in baby hamster kidney cells after exposure to sphingomyelinase

Plasma membrane sphingomyelin in baby hamster kidney (BHK-21) cells was hydrolyzed with sphingomyelinase (Staphylococcus aureus) and the effects on membrane cholesterol translocation and the properties of membrane bound adenylate cyclase and Na+/K(+)-ATPase were determined. Exposure of confluent BHK-21 cells to 0.1 U/ml of sphingomyelinase led to the degradation (at 37 degrees C) of about 60% of cell sphingomyelin. No simultaneous hydrolysis of phosphatidylcholine occurred. The hydrolysis of sphingomyelin subsequently led to the translocation (within 40 min) of about 50-60% of cell [3H]cholesterol from a cholesterol oxidase susceptible pool to an oxidase resistant compartment. The translocation of [3H]cholesterol from the cell surface to intracellular membranes was accompanied by a paralleled increase in [3H]cholesterol ester formation. When cells were first exposed to sphingomyelinase (to degrade sphingomyelin) and then incubated without the enzyme in serum-free media, the mass of cell sphingomyelin decreased initially (by 60%), but then began to increase and reached control levels within 3-4 h. The rapid re-synthesis of sphingomyelin was accompanied by an equally rapid normalization of cell [3H]cholesterol distribution. The re-formation of cell sphingomyelin also led to a decreased content of cellular [3H]cholesterol esters, indicating that unesterified [3H]cholesterol was pulled out of the cholesterol ester cycle and transported to the cell surface. Exposure of BHK-21 cells to sphingomyelinase further led to a dramatically decreased activity of ouabain-sensitive Na+/K(+)-ATPase, whereas forskolin-stimulated adenylate cyclase activity was not affected. The activity of Na+/K(+)-ATPase returned to normal in parallel with the normalization of cell sphingomyelin mass and cholesterol distribution. We conclude that sphingomyelin has profound effects on the steady-state distribution of cell cholesterol, and that manipulations of cell sphingomyelin levels directly and reversibly affects the apparent distribution of cholesterol. Changes in the lipid composition of the plasma membrane also appears to selectively affect important metabolic reactions in that compartment.     

Evidence for the presence of active cytochrome P450 systems in Schistosoma mansoni and Schistosoma haematobium adult worms

Extracts of the adult worms of both Schistosoma mansoni and Schistosoma haematobium can metabolise some typical P450 substrates but to differing degrees. S. mansoni worm extracts displayed a approximately 12-fold higher specific activity for an aminopyrine substrate than rat liver microsomes. At 4 mM substrate concentration the demethylation reaction with N-nitrosodimethylamine (NDMA) (5 nmol HCHO/mg protein/min) was only half that of rat liver microsomes, whereas in extracts of S. haematobium, no detectable activity was found towards NDMA. Using ethylmorphine as substrate the demethylation activity of S. mansoni extracts (1.82 nmol HCHO/mg protein/min) was 5.5-fold lower than that of rat liver microsomes. Benzphetamine demethylase activity was also readily detectable in S. mansoni worm extracts at 6.79 nmol HCHO/mg protein/min compared with 10.20 nmol HCHO/mg protein/min in the case of rat liver microsomes. When aniline was used as substrate, surprisingly, no activity was found in worm extracts of either S. mansoni or S. haematobium, whereas rat liver microsomes showed high activity towards this amine. The anti-P450 2E1 and 2B1/2 cross-reacted with both worm homogenates and gave a specific band corresponding to a protein of molecular weight of approximately 50.0 kDa. A study with anti-P450 IVA antibody revealed that while this protein was strongly expressed in S. haematobium worm extracts, no immunoreactivity was observed with extracts of S. mansoni. Immunoblotting analyses with anti-P450 IIIA and P450 1A1 did not detect immunoreactive protein in either S. mansoni or S. haematobium.     

Is There a Sphingomyelin-Based Hydrogen Bond Barrier at the Mammalian Host–Schistosome Parasite Interface?

Schistosomes develop, mature, copulate, lay eggs, and live for years in the mammalian host bloodstream, importing nutrients across the tegument, but entirely impervious to the surrounding elements of the immune system. We have hypothesized that sphingomyelin (SM) in the parasite apical lipid bilayer is responsible for these sieving properties via formation of a tight hydrogen bond network with the surrounding water. Here we have used quasi-elastic neutron scattering for characterizing the diffusion of larval and adult Schistosoma mansoni and adult Schistosoma haematobium in the surrounding medium, under various environmental conditions. The results documented the presence of a hydrogen bond barrier around larvae and adult schistosomes. The hydrogen bond network readily collapses if worms are subjected to hypoxic conditions, likely via activation of the parasite tegument-associated neutral sphingomyelinase, and consequent excessive SM hydrolysis. The slower dynamics of lung-stage larvae as compared to adult worms has been related to the existence of hydrogen-bonded networks of different strength and then to their differential resistance to immune attacks.     

Lung-stage expression of a major schistosome surface antigen

The topographical expression of a glycoprotein of 180,000 molecular weight on the surface of lung-stage Schistosoma mansoni schistosomula was determined by immunofluorescence microscopy using a monoclonal antibody. Postfixation treatment with graded ethanols enhanced specific immunofluorescent staining of adult worms, and was required for detection of the antigen on the surfaces of lung-stage schistosomula. The epitope recognized by the monoclonal antibody was also present on the surfaces of adult Schistosoma haematobium, but not on those of Schistosoma japonicum.     

A comparative study of the death of schistosomula of Schistosoma haematobium and Schistosoma mansoni in the skin of mice and hamsters.

This study shows that some cercariae of S. haematobium and S. mansoni die during penetration of mouse or hamster skin. Approximately 30-38% of cercariae of both species die in mouse skin and 14-16% die in hamster skin. The greater number of cercariae which die in the skin of mice seems to account for the higher yield of adult worms recovered in hamsters. Adult worm recoveries from animals infected with S. haematobium were, however, only about half the worm recoveries from hosts infected with S. mansoni.     

Isolation and characterization of surface antigens from Schistosoma mansoni. II. Antigenicity of radiolabeled proteins from adult worms.

Adult Schistosoma mansoni were radiolabeled in vitro with 125I Bolton-Hunter reagent. Surface membrane antigens were solubilized with non-ionic detergent, then reacted with infection or normal serum. The antigen-antibody complexes were then precipitated with staphylococcal protein A immunoadsorbent, eluted with urea and SDS, and fractionated by SDS-PAGE. The results indicated the presence of 6 to 8 tegument antigens, depending on the type of antisera used. Human antisera to S. japonicum and S. haematobium reacted with some but not all of the antigens identified with human S. mansoni infection serum; this implies the presence of species-specific tegument antigens. The molecular weights of the radiolabeled antigens ranged from 10,000 to 100,000. A large (greater than 100,000) molecular weight glycoprotein and an uncharacterized lipid fraction appeared to be precipitated nonspecifically. Immunoprecipitation methods with anti-mouse IgG and anti-mouse whole serum failed to detect the presence of hostlike antigens in the labeled extracts. Several of the labeled proteins from S. mansoni were found to react with serum from patients infected with either S. haematobium or with S. japonicum.     

Effects of sphingomyelin degradation on cholesterol mobilization and efflux to high-density lipoproteins in cultured fibroblasts

The hydrolysis of sphingomyelin from cellular plasma membranes imposes many consequences on cellular cholesterol homeostasis by causing a rapid and dramatic redistribution of plasma membrane cholesterol within the cells (Slotte, J.P. and Bierman, E.L. (1988) Biochem. J. 250, 653-658). The objective of this study was to examine the effects of an extracellular cholesterol acceptor on the directions of the sphingomyelinase-induced cholesterol flow in cultured fibroblasts. We have used HDL3 as a physiological acceptor for cholesterol, and measured the effects of sphingomyelin hydrolysis on efflux and endogenous esterification of cellular [3H]cholesterol. Treatment of cells with sphingomyelinase did induce a dramatically increased esterification of plasma-membrane-derived [3H]cholesterol. The presence of HDL3 in the medium (100 micrograms/ml) did not prevent or reduce the extent of the sphingomyelinase-induced cellular esterification of [3H]cholesterol. Degradation of cellular sphingomyelin (75% hydrolysis) also did not enhance the rate of [3H]cholesterol efflux from the plasma membranes to HDL3. In addition, we also observed that the degradation of sphingomyelin in the HDL3 particles (complete degradation) did not change the apparent rate of [3H]cholesterol transfer from HDL3 to the cells. These findings together indicate that hydrolysis of sphingomyelin did not markedly affect the rates of cholesterol surface transfer between HDL3 and cells. By whatever mechanism cholesterol is forced to be translocated from the plasma membranes subsequent to the degradation of sphingomyelin, it appears that the sterol flow is specifically directed towards the interior of the cells.     

Sphingomyelin/cholesterol ratio: an important determinant of glucose transport mediated by GLUT-1 in 3T3-L1 preadipocytes

Sphingomyelin pathway has been linked with insulin signaling through insulin-dependent GLUT-4 glucose transporter, but a relationship between sphingomyelin and the GLUT-1 transporter responsible for the basal (insulin-independent) glucose transport has not been clearly established. As GLUT-1 is mainly distributed to the cell surface, we explored the effects of changes in membrane sphingomyelin content on glucose transport through GLUT-1. The addition of exogenous sphingomyelin or glutathione (an inhibitor of endogenous sphingomyelinase) to the culture medium increased membrane sphingomyelin and cholesterol contents. Basal glucose uptake was enhanced and positively correlated to sphingomyelin (SM), cholesterol (CL) and SM/CL ratio. The exposure of 3T3-L1 preadipocytes to sphingomyelinase (SMase) significantly increased basal glucose uptake, membrane fluidity and decreased membrane sphingomyelin and cholesterol contents 60 min after SMase addition. There was no significant change in the abundance of GLUT-1 at the cell surface. The membrane sphingomyelin and cholesterol contents, fluidity and basal glucose transport returned to baseline levels within 2 h. The basal glucose uptake was negatively correlated with cholesterol contents and positively with SM/CL ratio. The SM/CL ratio might represent an important parameter controlling basal glucose uptake and a mechanism by which insulin resistance might be induced.     

Cholesterol-sphingomyelin interaction in membrane and apolipoprotein-mediated cellular cholesterol efflux

Helical apolipoproteins interact with cellular surface and generate high density lipoprotein (HDL) by removing phospholipid and cholesterol from cells. We have reported that the HDL is generated by this reaction with the fetal rat astrocytes and meningeal fibroblasts but cholesterol is poorly available to this reaction with the astrocytes (Ito et al. 1999. J. Neurochem. 72: 2362;-2369). Partial digestion of the membrane by extracellular sphingomyelinase increased the incorporation of cholesterol into thus-generated HDL from both types of cell. This increase was diminished by supplement of endogenous or exogenous sphingomyelin to the cells. The sphingomyelinase treatment decreased cholesterol in the membrane mainly in the detergent-resisting domain. The intracellular cholesterol used by acylCoA:cholesterol acyltransferase increased by the sphingomyelinase treatment in the absence of apoA-I, more remarkably in the fibroblast than in the astrocytes. ApoA-I suppressed this increase completely in the astrocytes, but only partially in the fibroblast. The effect of the sphingomyelin digestion was more prominent for the apolipoprotein-mediated reaction than the diffusion-mediated cellular cholesterol efflux. Thus, cholesterol molecules restricted by sphingomyelin in the domain of the plasma membrane appear to be primarily used for the HDL assembly upon the apolipoprotein;-cell interaction.     

Hexadecylphosphocholine alters nonvesicular cholesterol traffic from the plasma membrane to the endoplasmic reticulum and inhibits the synthesis of sphingomyelin in HepG2 cells

The synthetic lipid analogue, hexadecylphosphocholine is an antitumoral and antileishmanial agent that acts on cell membranes and can induce apoptosis. We have previously investigated the effect of hexadecylphosphocholine on the biosynthesis and intracellular transport of cholesterol in the human hepatoma HepG2 cell line. Here we show that the traffic of endocytosed-cholesterol from LDL to the plasma membrane and the transport of newly synthesized cholesterol from the endoplasmic reticulum to the plasma membrane were unaffected by alkylphosphocholine exposure. On the contrary, cholesterol traffic from the plasma membrane to the endoplasmic reticulum was drastically interrupted after 1 h of cell exposition to HePC and, consequently, the intracellular esterification of cholesterol was substantially decreased. Our results also demonstrate that this alkylphosphocholine exclusively affected the nonvesicular, energy-independent cholesterol traffic, without altering the vesicular transport. In addition, hydrolysis of plasma membrane sphingomyelin by exogenously added sphingomyelinase resulted in enhanced plasma-membrane cholesterol esterification, but sphingomyelinase treatment did not prevent the inhibition in cholesteryl ester formation caused by hexadecylphosphocholine. We also found that sphingomyelin synthesis was significantly inhibited in HepG2 cells after exposure to hexadecylphosphocholine. Since sphingomyelin and cholesterol are major lipid constituents of membrane raft microdomains, these results suggest that hexadecylphosphocholine could disturb membrane raft integrity and thence its functionality.     

Utilization of membranous lipid substrates by membranous enzymes. Hydrolysis of sphingomyelin in erythrocyte ''ghosts'' and liposomes by the membranous sphingomyelinase of chicken erythrocyte ''ghosts''.

Incubation at 37 degrees C of haemolysed chicken erythrocytes ('chicken erythrocyte ghosts') resulted in hydrolysis of the membrane sphingomyelin, suggesting an activation of a latent sphingomyelinase during the haemolysis procedure. When this incubation was continued for several hours, the entire sphingomyelin of the erythrocyte 'ghosts' was hydrolysed and membranes were obtained that were devoid of sphingomyelin, but had an active sphingomyelinase. Mixing the latter membranes with human erythrocyte 'ghosts' or positively charged liposomes led to hydrolysis of the sphingomyelin in these two membranes. This suggested that, after haemolysis, the activated sphingomyelinase in the membrane of the chicken erythrocyte 'ghosts' could hydrolyse sphingomyelin in its own membrane ('intramembrane utilization') or adjacent membranes ('intermembrane utilization').     

Sphingomyelin Depletion Impairs Anionic Phospholipid Inward Translocation and Induces Cholesterol Efflux

The phosphatidylserine (PS) floppase activity (outward translocation) of ABCA1 leads to plasma membrane remodeling that plays a role in lipid efflux to apolipoprotein A-I (apoAI) generating nascent high density lipoprotein. The Tangier disease W590S ABCA1 mutation has defective PS floppase activity and diminished cholesterol efflux activity. Here, we report that depletion of sphingomyelin by inhibitors or sphingomyelinase caused plasma membrane remodeling, leading to defective flip (inward translocation) of PS, higher PS exposure, and higher cholesterol efflux from cells by both ABCA1-dependent and ABCA1-independent mechanisms. Mechanistically, sphingomyelin was connected to PS translocation in cell-free liposome studies that showed that sphingomyelin increased the rate of spontaneous PS flipping. Depletion of sphingomyelin in stably transfected HEK293 cells expressing the Tangier disease W590S mutant ABCA1 isoform rescued the defect in PS exposure and restored cholesterol efflux to apoAI. Liposome studies showed that PS directly increased cholesterol accessibility to extraction by cyclodextrin, providing the mechanistic link between cell surface PS and cholesterol efflux. We conclude that altered plasma membrane environment conferred by depleting sphingomyelin impairs PS flip and promotes cholesterol efflux in ABCA1-dependent and -independent manners.     

Schistosoma mansoni polypeptides immunogenic in mice vaccinated with radiation-attenuated cercariae

We compared the humoral immune response of mice protected against Schistosoma mansoni by vaccination with radiation-attenuated cercariae to that of patently infected mice, and we identified antigens that elicit a greater, or unique, immune response in the vaccinated mice. These comparisons were based upon radioimmunoprecipitations and immunodepletion of [35S]methionine-labeled schistosomular and adult worm polypeptides, followed by one- and two-dimensional polyacrylamide gel analyses. The humoral responses of patently infected mice and of mice vaccinated once were remarkably similar and were directed against schistosome glycoproteins ranging in molecular size from greater than 300 to less than 10 kDa. Exposing mice to a second vaccination resulted in a marked change in the immune response, to one predominantly directed toward high molecular size glycoproteins. Sequential immunodepletion techniques identified five schistosomular and seven adult worm antigens that showed a greater or unique immunogenicity in vaccinated mice as compared with patently infected mice. These adult worm antigens were purified by preparative sequential immunoaffinity chromatography and used to prepare a polyclonal antiserum, anti-irradiated vaccine. This antiserum bound to the surface of live newly transformed and lung-stage schistosomula, as assessed by immunofluorescence assays, and was reactive with a number of 125I-labeled schistosomular surface polypeptides, including a doublet of 150 kDa that was also recognized by sera of vaccinated mice but not by sera of patently infected mice.     

Acid sphingomyelinase-deficient macrophages have defective cholesterol trafficking and efflux

Cholesterol efflux from macrophage foam cells, a key step in reverse cholesterol transport, requires trafficking of cholesterol from intracellular sites to the plasma membrane. Sphingomyelin is a cholesterol-binding molecule that transiently exists with cholesterol in endosomes and lysosomes but is rapidly hydrolyzed by lysosomal sphingomyelinase (L-SMase), a product of the acid sphingomyelinase (ASM) gene. We therefore hypothesized that sphingomyelin hydrolysis by L-SMase enables cholesterol efflux by preventing cholesterol sequestration by sphingomyelin. Macrophages from wild-type and ASM knockout mice were incubated with [(3)H]cholesteryl ester-labeled acetyl-LDL and then exposed to apolipoprotein A-I or high density lipoprotein. In both cases, [(3)H]cholesterol efflux was decreased substantially in the ASM knockout macrophages. Similar results were shown for ASM knockout macrophages labeled long-term with [(3)H]cholesterol added directly to medium, but not for those labeled for a short period, suggesting defective efflux from intracellular stores but not from the plasma membrane. Cholesterol trafficking to acyl-coenzyme A:cholesterol acyltransferase (ACAT) was also defective in ASM knockout macrophages. Using filipin to probe cholesterol in macrophages incubated with acetyl-LDL, we found there was modest staining in the plasma membrane of wild-type macrophages but bright, perinuclear fluorescence in ASM knockout macrophages. Last, when wild-type macrophages were incubated with excess sphingomyelin to "saturate" L-SMase, [(3)H]cholesterol efflux was decreased. Thus, sphingomyelin accumulation due to L-SMase deficiency leads to defective cholesterol trafficking and efflux, which we propose is due to sequestration of cholesterol by sphingomyelin and possibly other mechanisms. This model may explain the low plasma high density lipoprotein found in ASM-deficient humans and may implicate L-SMase deficiency and/or sphingomyelin enrichment of lipoproteins as novel atherosclerosis risk factors.     

Interfacial regulation of bacterial sphingomyelinase activity

The objective of this study was to define how the quality of the buffer/membrane interface influences the activity of bacterial sphingomyelinase acting at the interface. The enzyme reaction was carried out in a zero-order trough using a surface barostat. This approach allowed for proper control of the physico-chemical properties of the substrate molecules. Since the molecular area of ceramide is smaller than that of sphingomyelin, the hydrolysis reaction could be followed `on-line' from the monolayer area decrease at constant surface pressure. The hydrolysis reaction could be divided into two separate phases, the first being the lag-phase (time between enzyme addition and commencement of the monolayer area change), and the second phase being the actual hydrolysis reaction (from which a maximal degradation rate could be determined). The activity of sphingomyelinase (Staphylococcus aureus) toward bovine brain sphingomyelin (bb-SM) was markedly enhanced by Mg²⁺ (maximal activation at 5 mM). Mg²⁺ also influenced the lag-phase of the reaction (the lag-time increased markedly when the Mg²⁺ concentration decreased below 1 mM). Saturated sphingomyelins (bb-SM and N-palmitoyl sphingomyelin [N-P-SM]) were more slowly degraded than the mono-unsaturated N-oleoyl sphingomyelin (N-O-SM). Both bb-SM and N-P-SM monolayers underwent a phase-transition at room temperature, whereas the N-O-SM monolayer did not. The phase-transition (liquid-expanded to liquid-condensed) was observed to greatly increase the lag-time of the hydrolysis reaction. The activity of sphingomyelinase was also sensitive to the lateral surface pressure of the monolayer membrane. Maximal degradation rate was achieved at 20 mN/m (with bb-SM, 30°C); above this pressure the lag-time of the reaction increased sharply. The inclusion of 4 mol% of cholesterol into a [³H]sphingomyelin monolayer markedly increased the extent of [³H]sphingomyelin degradation, and shortened the lag-time of the reaction. The inclusion of 10 mol% of zwitterionic or negatively charged phospholipids to the [³H]sphingomyelin monolayer did not affect the sphingomyelinase reaction significantly. In conclusion, this study has demonstrated that the physico-chemical properties of the substrate molecules have a dominating influence on the activity of a bacterial sphingomyelinase acting at the buffer/membrane interface.     

Schistosoma mansoni: exposure in vitro of adult male surface antigens

Incubation of adult male Schistosoma mansoni for 24 hr in medium containing newborn calf serum or normal human plasma resulted in an increase in the amount of parasite antigen exposed at the worm surface. No effect was observed on the amount of host antigen which was present. The increase in the exposure of parasite antigens takes place progressively over 24 hr and is partially dependent on the presence of lipoproteins in the culture medium. The possibility is discussed that the increase is due to environmentally induced changes in surface membrane lipid composition.