Long Term Duration of the Rapid Rejection Response in Rats Infected with Trichinella spiralis

Abstract

Rapid rejection or the ability of a host to rapidly clear a second inoculum of L1 larvae of Trichinella spiralis has been described from several rat strains (McCoy, 1940, Am. J. Hyg. 32: 105-106; Castro et al., 1976, J. Nutr. 106: 14841491; Bell and McGregor, 1979, Inf. and Immun. 29: 186-193; Russell and Castro, 1979, J. Infect. Dis. 139: 304-312; Love et al., 1976; Immunology 30: 7-15), and at least one mouse strain (Wakelin'and Lloyd, 1976, Parasitology 72:173182). The ability of mice to reject L, larvae wanes within a few weeks following a sensitizing infection (Wakelin and Lloyd, 1976, loc. cit.). In rats, intestinal stimulation by a chemically abbreviated infection primes for a form of rapid rejection. The capacity to express rejection upon subsequent challenge, in this case, lasts only 5 to 6 wk (Bell and McGregor, 1979, loc. cit.). Castro and Harari (1982, Mol. Biochem. Parasit. 6: 191204), reported a strong rapid rejection response in rats immunized by infection 6 mo prior to the administration of a challenge inoculum. These findings in the rat host are compatible with the premise that the presence of encysted larvae may provide antigenic stimulation important in the long term maintenance of the rapid rejection response. The present experiment was carried out to gauge the persistence of the rapid rejection response in rats immunized by a primary infection. Two groups of outbred, male Sprague-Dawley rats (Hilltop Lab Animals, Chatsworth, CA) were studied. At 1 mo of age the first group of rats was immunized by an oral inoculation of 7 x 103 T. spiralis larvae. Larvae were obtained from male CF-1 mice according to published procedures (Castro and Fairbairn, 1969, J. Parasit. 66: 472-477) and administered by gastric intubation in 0.2 ml of saline. A second group of nonimmunized rats was used, also. We tested for the expression of the rapid rejection response of rats immunized 8 to 18 mo previously, using the nonimmunized rats as controls. Immunized rats were 9 to 19 mo of age (mean 13.4 ? 1.1 month, n = 10), and weighed between 536 and 813 gm (mean 677 ?+ 33.1, n = 10) at the time of the secondary infection. Nonimmunized counterparts ranged from 10 to 19 mo of age (mean 16.75 ?+ 1.5 months, n = 8) at the time of infection, which constituted a primary infection. Each immunized and nonimmunized rat was infected with 7 x 103 larvae as described above. Twenty-four hr after receiving the secondary and primary infections, respectively, rats were killed by a blow to the head. The small intestine was removed from each rat and divided into anterior and posterior halves. These were perfused intraluminally with 60 ml of saline (38 C) to remove debris. Worms embedded in the mucosa were collected and counted according to established procedures (Castro and Fairbairn, 1969, loc. cit.). Statistical methods used to evaluate data were Student's t-test (Snedecor and Cochran, 1967, Statistical methods, 6th ed., University Press, Ames, Iowa). The levels of significance determined were presented in the text. The number of larvae recovered from the small intestinal mucosa are presented in Figure 1. The 95% confidence interval (CI) for the number of worms recovered from non-immunized rats is presented. The number of worms recovered from 3 of 10 immune rats fell within the 95% CI (Fig. 1). The number of worms recovered from the remaining 7 rats was below the CI. These results indicated that 2 populations existed within the previously infected (immunized) rats; 1 group of 7 rats exhibiting rapid rejection (responders) and a second group comprised of 3 rats, not exhibiting the response (nonresponders). There was a 90% reduction in the number of worms in the anterior small intestine of responder rats as compared to nonresponders and no difference in the number of worms recovered from the posterior small bowel. The well known affinity of worms for the proximal small intestine following infection (Dick and Silver, 1980, J. Parasit. 66: 472-477) is evident, also, in this report. The strong rapid rejection