Studies on the Serum Transfer of Immunity to Fasciola hepatica in the Rat

Abstract

Serum from rats infected with Fasciola hepatica was given by intraperitoneal injection to normal recipient rats and its effect on fluke burdens resulting from infection of the recipients assessed. It was confirmed that serum from infected donors (immune serum) resulted in a significant degree of protection compared to normal serum and untreated rats. This immune effect of serum was seen in 2 rat strains, Fisher 344 and Sprague-Dawley. Although serum from donors infected for 7 to 8 weeks (subacute) was effective, serum from rats infected for 25 weeks (chronic) was not protective. When a pool of immune serum was titrated, 5, 2.5, and 1 ml gave a similar protective effect. While immune serum given on the day of infection resulted in significantly fewer flukes, immune serum given 2 or 4 days after infection had only a slight effect, and immune serum given 6 or 8 days after infection had no immune effect. The protective effect of serum was eliminated by heat treatment and by absorption with live or dead F. hepatica. Assessment of the effect of serum transferred from an infected host on an infection of the serum recipient is a basic method of investigating the role of serum factors in host-parasite relationships. Although it is well known that infection with Fasciola hepatica is associated with measurable humoral responses, little work involving serum transfer has been reported. Wickerhauser (1961) injected several guinea pigs with immune or normal bovine serum, but saw no inhibiting effect on an oral infection given 2 to 3 hr later. Immune serum simultaneously injected intraperitoneally with excysted metacercariae may, however, have had some immune effect. Hughes (cited in Dawes and Hughes, 1964) reported that immune rabbit serum given by the intraperitoneal route to mice did not influence infection of the mice. Corba et al. (1971) reported that while immunity to F. hepatica in rats and a pair of calves could be transferred with cells, serum transfer did not have such an effect in either species. More recently, however, Dargie et al. (1973) stated in an abstract that immunity could be transferred with serum in rats, cattle, and sheep. Independently, we observed that protective immunity to F. hepatica could be transferred with serum in rats and a subsequent experiment was reported (Hayes et al., 1974a). The experiments reported herein were performed to further explore the phenomenon of serum transfer of protective immunity to F. hepatica in the rat. Received for publication 24 June 1974. MATERIALS AND METHODS This report contains data from 6 experiments. In all except Experiment II, male inbred Fisher 344 rats [CR:CDF (Fisher 344), Charles River Breeding Laboratories, Inc., Wilmington, Massachusetts] were used as serum donors and recipients. In Experiment II, male outbred Sprague-Dawley rats [CR:CD(SD), Charles River] were used as donors and recipients of serum. In all instances serum donor rats were 4 to 6 weeks old at the time of exposure to 10 metacercariae of F. hepatica. Terminal bleeding of the donors by cardiac puncture was done 7 to 8 weeks after infection (except Group B of Experiment II, 25 weeks) using CO2 and methoxyflurane as anesthetics. Necropsy confirmed the presence (immune serum donors) or absence (normal serum donors) of F. hepatica in all cases. The blood was allowed to clot in glass tubes at 22 or 37 C and the serum recovered by centrifugation. The serum was flash-frozen (ethanol-solid C02) and stored at -20 C. Several serum pools were used in the experiments, the same pool in Experiments IV and V, different pools in the other 4 experiments. Serum was administered by the intraperitoneal route on the day of infection in all cases except in certain groups of Experiment IV as shown in Table IV. Metacercariae of F. hepatica were obtained from aquaria containing Lymnaea tomentosa infected by exposure to eggs of F. hepatica recovered from experimentally infected sheep. Ten selected metacercariae per rat were given by esophageal intubation to all rats except the donors of normal