Effects of exogenous interleukin-1β on primary sporocysts of Schistosoma mansoni (Trematoda) incubated with plasma and hemocytes from schistosome-susceptible and resistant Biomphalaria glabrata (Gastropoda)

Abstract. The cytokine interleukin-1β (IL-1β) mediates interactions of immune and inflammatory cells in mammals. Previous reports also have linked plasma (cell-free hemolymph) levels of IL-1β in the snail Biomphalaria glabrata to resistance against Schistosoma mansoni . In the present study, fluorescent probes were used to study larval schistosome and snail hemocyte viability during in vitro encounters. Hemolymph (plasma and hemocytes) from schistosome-susceptible (M-line) and resistant (13–16-R1) B. glabrata was added to sporocysts of S. mansoni and the viability of hemocytes and parasites was assessed. Next, IL-1β was added to sporocyst-hemolymph samples, the viability of sporocysts and hemocytes determined and then compared to control assays. The number of live sporocysts present after incubation for 1 h with hemolymph from M-line snails was significantly greater than the number seen when hemolymph from 13–16-R1 snails was tested. Nearly all sporocysts survived the 1 h incubation with M-line hemolymph, and most of the hemocytes attached to sporocysts were dead. In contrast, nearly all sporocysts were dead when hemolymph from 13–16-R1 snails was tested, and most attached hemocytes were alive. Addition of IL-1β to M-line hemolymph resulted in a dramatic increase in sporocyst death. Addition of IL-1β to 13–16-R1 hemolymph produced a small but significant increase in the rate of sporocyst death. These results indicate that the concentration of IL-1β present in hemolymph from B. glabrata is directly related to the ability of this snail to kill S. mansoni sporocysts in vitro.     

Killing of Schistosoma mansoni sporocysts by hemocytes from resistant Biomphalaria glabrata: role of reactive oxygen species

The fate of Schistosoma mansoni (Trematoda) sporocysts in its molluscan host Biomphalaria glabrata (Gastropoda) is determined by circulating phagocytes (hemocytes). When the parasite invades a resistant snail, it is attacked and destroyed by hemocytes, whereas in a susceptible host it remains unaffected. We used 3 inbred strains of B. glabrata: 13-16-R1 and 10-R2, which are resistant to the PR-1 strain of S. mansoni, and M-line Oregon (MO), which is susceptible to PR-1. In an in vitro killing assay using plasma-free hemocytes from these strains, the rate of parasite killing corresponded closely to the rate by which S. mansoni sporocysts are killed in vivo. Hemocytes from resistant snails killed more than 80% of S. mansoni sporocysts within 48 hr, whereas sporocyst mortality in the presence of hemocytes from susceptible snails was <10%. Using this in vitro assay, we assessed the involvement of reactive oxygen species (ROS) produced by resistant hemocytes, during killing of S. mansoni sporocysts. Inhibition of NADPH oxidase significantly reduced sporocyst killing by 13-16-R1 hemocytes, indicating that ROS play an important role in normal killing. Reduction of hydrogen peroxide (H2O2) by including catalase in the killing assay increased parasite viability. Reduction of superoxide (O2-), however, by addition of superoxide dismutase or scavenging of hydroxyl radicals (*OH) and hypochlorous acid (HOCl) by addition of hypotaurine did not alter the rate of sporocyst killing by resistant hemocytes. We conclude that H2O2 is the ROS mainly responsible for killing.     

Schistosoma mansoni: passive transfer of resistance by serum in the vector snail, Biomphalaria glabrata

Passive transfer of natural resistance to Schistosoma mansoni (PR-1 strain) has been successfully accomplished in the snail intermediate host, Biomphalaria glabrata (PR albino, M-line strain). Injection of serum (cell-free hemolymph) from a naturally schistosome-resistant strain of B. glabrata (10-R2) into PR albino snails induced a complete protection from a primary infection with the parasite in 29 of 48 snails (60.4%). In comparison, inoculation of homologous PR albino serum or heterologous proteins (fetal calf serum) had no effect. Moreover, this protection could be induced 24 hr prior to, or 24 hr after, exposure to the parasite, although heating of 10-R2 serum to 70 C for 30 min destroyed its protective ability. When in vitro transformed sporocysts were preincubated in 10-R2 or PR albino serum and then were injected into susceptible snails, a high level of infection (88.5 and 83.3%, respectively) was produced in both groups. Thus, the 10-R2 serum factor does not appear to be mediating specific parasite recognition by host hemocytes. Alternatively, our results suggest that 10-R2 serum possesses a heat-labile factor which specifically activate B. glabrata hemocytes to encapsulate and destroy sporocysts whereas PR albino serum lacks this factor.     

In vitro effect of larval Schistosoma mansoni excretory-secretory products on phagocytosis-stimulated superoxide production in hemocytes from Biomphalaria glabrata

The in vitro production of the reactive oxygen metabolite superoxide (O2-) was confirmed in hemocytes from the schistosome intermediate host Biomphalaria glabrata. Active forms of the enzyme superoxide dismutase (SOD) inhibited reduction of nitroblue tetrazolium (NBT) to formazan in cells that had phagocytozed zymosan particles, whereas an inactivated form of SOD did not. Moreover, based on the prevalence of O2(-)-positive hemocytes and the relative intensity of NBT staining reactions, hemocytes from the Schistosoma mansoni-resistant 10-R2 strain of B. glabrata possessed an overall greater capacity for generating superoxide than did those from S. mansoni-susceptible M-line snails. Schistosoma mansoni excretory-secretory (E-S) products, released during in vitro transformation of miracidia to sporocysts, inhibited phagocytosis of zymosan particles and superoxide activity in hemocytes from both snail strains, but 10-R2 hemocytes maintained higher levels of phagocytosis and superoxide production than did M-line hemocytes. The dose-dependent decreases in phagocytosis observed in both snail strains in the presence of E-S products could not account fully for the concomitant decrease in superoxide levels detected, indicating that either a single E-S factor differentially affects phagocytosis and superoxide production, or that different E-S factors are involved in the specific interference of each of these hemocyte functions.     

Host reactions in Biomphalaria glabrata to Schistosoma mansoni miracidia, involving variations in parasite strains, numbers and sequence of exposures

M-line Biomphalaria glabrata snails are susceptible to Puerto Rican (PR-1) strain of Schistosoma mansoni, but are resistant to a St. Lucian (LC-1) strain. 10-R2 B. glabrata snails are resistant to both strains of S. mansoni. When 10-R2 snails were exposed repeatedly to PR-1 S. mansoni miracidia for 5 consecutive days, all of the sporocysts were encapsulated and destroyed by the snails. Thirty-four per cent of sporocysts examined in M-line snails with similar exposures were also degraded. In double concurrent infections of M-line B. glabrata with [³H]leucine-labeled and unlabeled PR-1 and Lc-1 S. mansoni, the incompatible Lc-1 miracidia were selectively attacked and destroyed. This destruction occurred irrespective of the sequence of exposure of the 2 strains of miracidia, and whether or not the miracidia were labeled. Successful superinfection of M-line B. glabrata with homologous S. mansoni miracidia was obtained at least 4 days after the primary exposure to the miracidia.     

Lysosomal enzyme activities in susceptible and refractory strains of Biomphalaria glabrata during the course of infection with Schistosoma mansoni

The distribution and abundance of the lysosomal enzyme markers, acid phosphatase (AP), peroxidase (PO), and nonspecific esterase (NE), within circulating blood cells (hemocytes) were examined in a schistosome-susceptible (PR albino M-line) and a resistant (10-R2) strain of Biomphalaria glabrata during the course of infection with Schistosoma mansoni. The dynamics of serum (cell-free hemolymph) AP activities and total hemocyte numbers in infected snails also were investigated. Hemocyte subpopulations, as determined by these enzyme markers, responded differently to parasite infection between snail strains. Generally, the hemocyte subpopulations within PR albino snails remained largely unchanged, whereas the same subpopulations in 10-R2 snails fluctuated considerably. The distribution of AP in the hemocytes of 10-R2 snails decreased by 1 hr postexposure (PE) to the parasite and remained low through 12 hr before increasing to control values at 24 hr and 2 wk PE. In comparison, PO activity increased by 1 hr PE and peaked at 12 hr before dropping to 0 hr values by 2 wk PE. The NE activity exhibited still another pattern with the percentage of NE-positive cells decreasing from 0 to 12 hr PE followed by a recovery to 0-hr values by 24 hr. The abundance of these hemocyte enzymes followed a similar pattern to that of their distribution, although some differences were observed. Serum AP values varied little in PR albino snails except for a significant increase at 2 wk PE, indicating a possible response to tissue damage resulting from migrating daughter sporocysts.(ABSTRACT TRUNCATED AT 250 WORDS)     

Involvement of nitric oxide in killing of Schistosoma mansoni sporocysts by hemocytes from resistant Biomphalaria glabrata

In strains of the snail Biomphalaria glabrata (Gastropoda) that are resistant to the parasite Schistosoma mansoni (Trematoda), hemocytes in the hemolymph are responsible for elimination of S. mansoni sporocysts. The defensive role of reactive nitrogen species was investigated in in vitro interactions between hemocytes derived from the resistant 13-16-R1 strain of B. glabrata and the parasite. The nitric oxide synthase (NOS) inhibitor N(omega)-nitro-L-arginine methylester (L-NAME) and the nitric oxide (NO) scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide reduced cell-mediated killing of S. mansoni sporocysts. To determine if peroxynitrite (ONOO-) is involved in killing, assays were run in the presence of the ONOO- scavengers uric acid and deferoxamine. These did not influence the rate of parasite killing, indicating that NO is directly responsible for mediating cytotoxicity, but ONOO- is not. The combination of the NOS inhibitor L-NAME and catalase, an enzyme that detoxifies hydrogen peroxide (H2O2), reduced average sporocyst mortality to a greater extent than L-NAME alone. Killing of the sporocysts was, however, not totally inhibited. It is suggested that NO and H2O2 are both involved in hemocyte-mediated toxicity of 13-16-R1 B. glabrata against S. mansoni sporocysts.     

Further characterization of passively transferred resistance to Schistosoma mansoni in the snail intermediate host Biomphalaria glabrata.

A heat-labile plasma factor from genetically resistant 10-R2 Biomphalaria glabrata snails confers passively transferred resistance (PTR) to Schistosoma mansoni when injected into susceptible snails within 24-hr of exposure to miracidia. However, no additional details on PTR have emerged since the initial 1984 report, nor has the plasma resistance factor been characterized. In the present study, new information is provided on the occurrence of resistance factor in plasma of additional types of snails, effect of "priming" resistant plasma donors by prior exposure to miracidia, duration of PTR, molecular weight of resistance factor, and fate of sporocysts in snails with PTR. Susceptible NIH albino snails injected 24 hr prior to exposure to miracidia with individual samples of plasma from a different strain (Salvador B. glabrata) or a different species (B. obstructa) of nonsusceptible snail displayed infection prevalences of 49% or 59% of control levels, respectively, whereas injections of homologous plasma had no effect. PTR was not enhanced by prior exposure of resistant Salvador plasma donors to miracidia. Unexpectedly, PTR induced by injections of Salvador plasma persisted for at least 21 days. The molecular weight of the resistance factor(s) was between 10 and 30 kDa, based on results of centrifugal ultrafiltration. A significantly higher proportion of dead sporocysts occurred in histological sections of tentacles from snails injected with Salvador plasma than in tentacles of snails injected with NIH albino plasma at 7 days postexposure to miracidia. Most dead sporocysts in Salvador plasma-injected snails were undergoing gradual degeneration, rather than rapid, hemocyte-mediated destruction, as occurred in Salvador snails.     

Schistosoma mansoni: lectin-dependent cytotoxicity of hemocytes from susceptible host snails, Biomphalaria glabrata

Normally benign hemocytes from a strain (M-line) of the snail, Biomphalaria glabrata, susceptible to Schistosoma mansoni, became cytotoxic toward the sporocyst stage if the parasite was first treated with the lectin, concanavalin A. Concanavalin A binding was inhibitable with alpha-methyl mannoside and killing was dose-dependent. Maximal levels of concanavalin A-induced cytotoxicity were comparable with levels observed when hemocytes from a resistant snail strain (13-16-R1) encountered untreated sporocysts. Induction of the cytotoxic response did not occur if hemocytes alone were pretreated with the lectin. A unique method incorporating ultraviolet microscopy and the vital fluorescent dye, eosin Y, was used for discriminating between live and dead sporocysts. This model may prove useful in understanding mechanisms used by invertebrate effector cells in recognition and killing of invading organisms.     

Echinostoma paraensei: hemocytes of Biomphalaria glabrata as targets of echinostome mediated interference with host snail resistance to Schistosoma mansoni

Earlier in vivo work by Lie et al. (1977) indicated that the innate resistance of the 10R2 strain of Biomphalaria glabrata to PR1 Schistosoma mansoni could be interfered with if the snails were infected previously with another trematode, Echinostoma paraensei. We have studied this interference phenomenon using in vitro methods in an attempt to understand its mechanistic basis. Hemolymph, derived from 10R2 snails infected with E. paraensei for 14-28 days, killed 25% of S. mansoni sporocysts in vitro, significantly less (P less than 0.001) than the 90% killing rate observed with hemolymph from uninfected, control 10R2 snails. Hemolymph from the infected 10R2 snails and from schistosome susceptible M line snails did not differ significantly (P greater than 0.1) in their relative inability to kill S. mansoni sporocysts in vitro. The defect in sporocyst killing exhibited by echinostome infected 10R2 snails was traced to the cellular, rather than the humoral, component of the hemolymph. Preparations containing uninfected 10R2 snail hemolymph and echinostome daughter rediae exhibited significantly less (P less than 0.001) killing of S. mansoni sporocysts than did controls containing only 10R2 hemolymph and S. mansoni sporocysts. Our results suggest that echinostome larvae release factors that interfere with the ability of B. glabrata hemocytes to kill S. mansoni sporocysts.     

Differential binding of hemolymph proteins from schistosome-resistant and -susceptible Biomphalaria glabrata to Schistosoma mansoni sporocysts

Humoral factors have been associated with resistance of Biomphalaria glabrata to infection by Schistosoma mansoni. The goal of this study was to determine which serum (cell-free hemolymph) proteins bind to the surface of S. mansoni sporocysts. For this, 125I-labeled serum from schistosome-resistant (10-R2) and -susceptible (M-line) B. glabrata was incubated with sporocysts, washed, and then subjected to SDS-PAGE and autoradiography. Other samples examined included radiolabeled 10-R2 and M-line serum, sporocysts incubated with unlabeled serum followed by incubation with radiolabeled serum, and radiolabeled sporocysts. Results indicated that many polypeptides in the serum from both strains of B. glabrata were radiolabeled. Dominating both profiles were bands in the 90-210-kDa range. However, some differences between the serum of the 2 snail strains were observed with M-line serum having several radiolabeled polypeptides in the 31-40- and 66-85-kDa range that were absent in serum from 10-R2 B. glabrata. When sporocysts were incubated with radiolabeled serum, 3 polypeptides (116, 180, 210 kDa) from both snail strains bound to the surface of the parasite. Further, a 55-kDa polypeptide bound to sporocysts incubated with 10-R2 serum but did not bind to those parasites incubated with M-line serum. Preincubation of sporocysts with unlabeled serum prior to incubation with radiolabeled serum significantly inhibited the uptake of radiolabeled proteins. This differential binding of serum polypeptides from different strains of B. glabrata may be important in determining resistance or susceptibility of the snail to larval schistosome infection.     

Echinostoma paraensei and Schistosoma mansoni: adherence of unaltered or modified latex beads to hemocytes of the host snail Biomphalaria glabrata.

Hemocytes derived from a strain (13-16-R1) of Biomphalaria glabrata resistant to Schistosoma mansoni were significantly more likely to bind untreated latex beads than hemocytes from the schistosome-susceptible M line strain. Beads preincubated in 13-16-R1 plasma were more readily bound by both 13-16-R1 and M line hemocytes than beads preincubated in M line plasma. Beads preincubated in plasma derived from snails of either strain infected with the trematode Echinostoma paraensei were more readily bound by hemocytes than beads preincubated in plasma from control snails of the corresponding strain. Plasma from snails exposed to S. mansoni did not have a similar effect. Throughout these experiments, beads receiving a particular treatment were consistently bound at higher rates by 13-16-R1 than M line hemocytes. SDS-PAGE of plasma components eluted from beads revealed differences between treatments, particularly in diffuse bands falling into two groups, of 75-130 and 150-220 kDa. The results indicate that both hemocytes and plasma components from the two host strains differ and identify plasma molecules deserving of additional study as possible modulators of hemocyte effector functions. Also, S. mansoni and E. paraensei provoked different responses in the same host snail.     

Differences in the number of hemocytes in the snail host Biomphalaria tenagophila, resistant and susceptible to Schistosoma mansoni infection

The relationships between schistosomiasis and its intermediate host, mollusks of the genus Biomphalaria, have been a concern for decades. It is known that the vector mollusk shows different susceptibility against parasite infection, whose occurrence depends on the interaction between the forms of trematode larvae and the host defense cells. These cells are called amebocytes or hemocytes and are responsible for the recognition of foreign bodies and for phagocytosis and cytotoxic reactions. The defense cells mediate the modulation of the resistant and susceptible phenotypes of the mollusk. Two main types of hemocytes are found in the Biomphalaria hemolymph: the granulocytes and the hyalinocytes. We studied the variation in the number (kinetics) of hemocytes for 24 h after exposing the parasite to genetically selected and non-selected strains of Biomphalaria tenagophila, susceptible or not to infection by Schistosoma mansoni. The differences were analyzed referred to the variations in the number of hemocytes in mollusks susceptible or not to infection by S. mansoni. The hemolymph of the selected and non-selected snails was collected, and hemocytes were counted using a Neubauer chamber at six designated periods: 0 h (control, non-exposed individuals), 2 h, 6 h, 12 h, 18 h and, 24 h after parasite exposure. Samples of hemolymph of five selected mollusks and five non-selected mollusks were separately used at each counting time. There was a significant variation in the number of hemocytes between the strains, which indicates that defense cells have different behaviors in resistant and susceptible mollusks.     

Incompatibility between miracidia of Schistosoma mansoni and Helisoma duryi occurs at two stages: penetration and intramolluscan establishment

Helisoma spp. snails are not susceptible to infection with miracidia of Schistosoma mansoni because the miracidia do not penetrate them. However, in view of the phylogenetic proximity and histocompatibility between Helisoma spp. and the normal intermediate host, Biomphalaria glabrata , schistosome miracidia conceivably could survive if experimentally introduced into the hemocoel of Helisoma spp. To test this hypothesis, schistosome-susceptible NIH albino B. glabrata, schistosome-resistant Salvador B. glabrata, and Helisoma duryi were injected with miracidia of S. mansoni, and the outcome was followed both by monitoring snails for infection for several weeks and by histological examination at 24 and 48 hr post-injection (PI). Patent infections developed in most NIH albino snails but in none of the Salvador B. glabrata or H. duryi individuals. Histological analysis showed a higher proportion of normal sporocysts in various tissues of NIH albino snails at both time periods relative to Salvador snails, which contained mostly sporocysts undergoing hemocytic encapsulation. In H. duryi , nearly all sporocysts were dead by 48 hr PI.     

Rescue of sporocysts of Schistosoma mansoni in nonsusceptible Biomphalaria by head-foot transplantation into susceptible snails

To measure the longevity of sporocysts of Schistosoma mansoni in nonsusceptible snails (13-16-R1 and Salvador strains of Biomphalaria glabrata, and Biomphalaria obstructa), the head-foot (HF) of miracidia-exposed snails was transplanted into the hemocoel of a susceptible NIH albino recipient at 1-36 days postexposure (DPE). Recipient snails which were not exposed to miracidia then were monitored for infection transferred by the implant, and infection prevalences in recipients of HF transplants from nonsusceptible donors were compared to those in snails implanted with an HF from NIH albino donors. Transplants from NIH albino snails between 1 to 15 DPE infected 98% of recipients. Similarly, at 1 DPE, 69-85% of transplants from nonsusceptible snails contained viable sporocysts, as shown by resulting patent infections in the recipients. Recipient infection prevalence, and presumably numbers of transplants containing viable sporocysts, declined as a function of DPE, and by 5-9 DPE this decrease was significant for all 3 types of nonsusceptible donors. However, viable sporocysts still occurred in B. obstructa and 13-16-R1 B. glabrata as late as 19 and 20 DPE, respectively, and in Salvador B. glabrata as late as 33 DPE. Thus, sporocysts persist in nonsusceptible snails considerably longer than suggested by results of previous histological studies.     

Schistosoma mansoni: Cytotoxicity of hemocytes from susceptible snail hosts for sporocysts in plasma from resistant Biomphalaria glabrata

Sporocysts of Schistosoma mansoni (PR1 strain) survive and grow in Biomphalaria glabrata PR albino strain snails, whereas they are encapsulated and die in B. glabrata 10R2 strain snails. These processes also occur in an in vitro system in which the only living cells are those of sporocysts and snail hemolymph. Hemocytes of the susceptible snail are normally not effective in damaging sporocysts. However, when the encounter occurred in the presence of cell-free plasma from resistant snails, previously impotent hemocytes severely damaged sporocysts in 24 hr. The cytotoxic capacity of resistant strain hemocytes was not altered by plasma from susceptible snails. Furthermore, it was retained even when plasma was replaced by culture medium free of snail components. The nature of the plasma factor(s) which facilitated damage by otherwise impotent hemocytes is discussed, and evidence is evaluated for the hypothesis that snail resistance is dependent upon the specificity of cytophilic factors present both in the plasma and on the hemocyte plasma membranes.     

Biomphalaria glabrata: influence of calcium, lectins, and plasma factors on in vitro phagocytic behavior of hemocytes of noninfected or Schistosoma mansoni-infected snails.

In vitro phagocytosis of erythrocytes by hemocytes of B. glabrata, intermediate host of S. mansoni, is strongly influenced by calcium, several lectins, and plasma factors. Our results indicate that two different mechanisms of non-self-recognition in B. glabrata may occur: (1) In the presence of calcium, phagocytosis occurs in noninfected and in infected snails without involvement of any other substances, and hemocytes of schistosome resistant as well as those of susceptible snails are able to recognize and phagocytose the target cells. (2) In the absence of calcium, phagocytosis occurs if bridging molecules (heterologous lectins in our assays) were present for which effector and target cells possess binding sites or if target cells were plasma coated prior to the assays. In suspensions in homologous plasma, hemocytes of both snail strains, infected or noninfected, subsequently showed phagocytic activities of about 70-80%. Preincubation of target cells in homologous plasma resulted in similar high phagocytic activities of hemocytes even in the absence of plasma during the standard assay. In these assays, a significantly higher proportion of hemocytes of resistant snails phagocytosed plasma-opsonized erythrocytes, whereas hemocytes of susceptible snails internalized less erythrocytes per cell and needed 60 min to phagocytose at percentages equivalent to that of resistant hemocytes within 10 min. Preincubation of erythrocytes in resistant plasma significantly increased the subsequent phagocytic activity of susceptible hemocytes, whereas preincubation of erythrocytes in susceptible plasma decreased the phagocytosis level of resistant hemocytes.     

A comparative study of mechanisms underlying digenean-snail specificity: in vitro interactions between hemocytes and digenean larvae

A panel of 4 digenetic trematode species (Echinostoma paraensei, E. trivolvis, Schistosoma mansoni, and Schistosomatium douthitti) and 5 snail species (Biomphalaria glabrata, Helisoma trivolvis, Lymnaea stagnalis, Stagnicola elodes, and Helix aspersa) was examined to determine if known patterns of host specificity could be explained by the tendency of digenean larvae to be bound by snail hemocytes, or by the ability of larvae to influence the spreading behavior of hemocytes. In short-term (1 hr) in vitro adherence assays, there was no overall pattern to suggest that sporocysts were more likely to be bound by hemocytes from incompatible than compatible snails. Compared with the other parasites, sporocysts of E. paraensei were less likely to be bound by hemocytes from any of the snail species tested. All rediae examined, including those of another species Echinoparyphium sp., were also remarkably refractory to binding by hemocytes from any of the snails. Of all the larvae examined, only sporocysts and young daughter rediae of E. paraensei caused hemocytes to round up in their presence. This was true for hemocytes from the compatible species B. glabrata and the incompatible lymnaeid species S. elodes and L. stagnalis. The patterns of host specificity shown by this particular panel of parasites and snails were not predicted by either the extent of hemocyte adherence to digenean larvae or by the ability of larvae to affect hemocyte spreading behavior. The results of this study suggest that a role for hemocytes, although likely, may require different assays, possibly of a more prolonged nature, for its detection. Also, different parasite species (notably E. paraensei) and intramolluscan stages have distinctive interactions with host hemocytes, suggesting that the determinants of specificity vary with the host-parasite combination, and with the parasite life cycle stage.     

Phagocytic activity of hemocytes of M-line Biomphalaria glabrata snails: effect of exposure to the trematode Echinostoma paraensei

The phagocytic activity of hemocytes from 6-8-mm M-line Biomphalaria glabrata snails was studied in an in vitro assay using glutaraldehyde-fixed sheep erythrocytes (SRBC) as target cells. For individual snails, the percentage of hemocytes ingesting SRBC during a 1-hr interval, termed the phagocytic activity index (PAI), was determined. Hemocytes from snails infected for 1 day with Echinostoma paraensei had a slightly elevated PAI, but at both 8 and 30 days postexposure (DPE), hemocytes from infected snails had a significantly lower PAI than controls. Hemocytes taken from snails at 8 DPE also had a low PAI using rabbit erythrocytes and yeast as target cells. The low PAI at 8 DPE is attributed to the presence of large numbers of poorly spreading hemocytes with low phagocytic activity. Hemocytes from snails with 30-day infections were well spread but nonetheless had a low PAI. The presence of plasma from 8-day infected snails did not alter the PAI of hemocytes from control snails, nor was the PAI of hemocytes from infected snails changed by plasma from control snails. SRBC preincubated for 60 min in plasma from various groups of M-line snails did not elicit an increase in PAI when presented to hemocytes from control snails; in some cases, as with plasma from 6-8-mm control snails, such preincubation significantly reduced the PAI below levels obtained using SRBC preincubated in culture medium. As compared to hemocytes from snails with normally developing, 8-day-old intraventricular sporocysts (IS), hemocytes from snails exposed to infection but subsequently lacking IS had a significantly higher PAI.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of miracidial dose on adoptively transferred resistance to Schistosoma mansoni in the snail intermediate host, Biomphalaria glabrata

Adoptively transferred resistance to Schistosoma mansoni in the snail intermediate host Biomphalaria glabrata was measured as a function of miracidial challenge dose. Schistosome-susceptible snails implanted with the amebocyte-producing organ (APO) from resistant donors showed 29 and 39% prevalences of infection after challenge with 5 and 10 miracidia, respectively, but 68-83% prevalences when exposed to 25-200 miracidia. Prevalences in control (untampered) susceptible snails ranged from 97 to 100% at the different miracidial doses. Higher infection prevalences at elevated doses suggest that a range of transferred resistance occurs and possibly that low levels of APO-derived plasma factors or hemocytes in some recipients can be overwhelmed by larger numbers of parasites.