Donald L. Wassom

HOST GENETICS: A KEY FACTOR IN REGULATING THE DISTRIBUTION OF PARASITES IN NATURAL HOST POPULATIONS

The immune response that expels the tapeworm Hymenolepis citelli from the small intestine of its host the white-footed deer mouse is genetically controlled. Patent infections with this tapeworm occur only in individuals that are homozygous for a recessive allele expressed at a single gene locus. By studying this natural host-parasite system in the laboratory it was shown that host genetics contributes to parasite overdispersion in a host population in the absence of all other ecological variables. Thus, the substantive influence of the proportions of resistant and susceptible genotypes in the host population must be considered when developing parasite population models of transmission or control measures.

A gene mapping between the S and D regions of the H-2 complex influences resistance to Trichinella spiralis infections of mice

The strains B 10.S(7R), B 10.S(23 R) and B 10.S(24R), all thought to be genetically identical, differ in levels of susceptibility to infection with Trichinella spiralis. In a series of nine independent experiments, B 10.S(7R) was shown to be more susceptible than the other two strains. In another series of seven experiments, the strain B 10.A(18R) was shown to be more susceptible to infection with T. spiralis than the strains B 10.S(21R) or B 10.BAR‐5, all of which were thought to share common H‐2 alleles. These results indicate that a gene mapping between the S and D loci influences susceptibility to infection with T. spiralis. Typing of these strains for Qa and Tl loci rule out the possibility of a double crossover accounting for the differences observed. The new gene is designated Ts‐2. Previously published data have also been reinterpreted and another gene Ts‐1 is shown to be associated with the Aβ locus. When the d allele is expressed at the Ts‐2 locus, strains of mice expressing s, q, f or b alleles at Ts‐1 are rendered more susceptible to infection.

A SURVEY OF SUSCEPTIBILITY TO INFECTION WITH TRICHINELLA SPIRALIS OF INBRED MOUSE STRAINS SHARING COMMON H-2 ALLELES BUT DIFFERENT GENETIC BACKGROUNDS

Twenty-eight different inbred strains of mice representing five different H-2 haplotypes were compared for degree of susceptibility to a primary infection with Trichinella spiralis. Marked differences in susceptibility, measured by the average number of muscle larvae per host, were seen among strains of mice sharing common H-2 alleles. The genes controlling these differences must therefore map at loci outside the major histocompatibility complex. Strains of mice sharing the H-2k haplotype were generally more susceptible than strains expressing other haplotypes and strains expressing H-2q alleles were most resistant. Strains of mice were ranked in order of decreasing susceptibility. Knowledge of these ranking may be of value to researchers wishing to select strains of mice appropriate for studies on T. spiralis.

Trichinella spiralis infections of inbred mice: immunologically specific responses induced by different Trichinella isolates.

The immune response of inbred mice was studied following infection with Trichinella spiralis var. pseudospiralis (TP) or with isolates of T. spiralis derived from a pig or from an arctic fox. Animals given a primary infection with 1 isolate of Trichinella and challenged 21 days later with the same or different isolates responded more quickly by expelling worms from the homologous challenge. In addition, although mesenteric lymph node cells from mice infected with each isolate of Trichinella would proliferate in vitro when cultured with antigen derived from each of the others, the strongest proliferation response always occurred when cells were cultured in the presence of antigen prepared from the specific isolate used to infect the mouse from which the cells were derived. In addition, it was possible to prepare monoclonal antibodies that recognized an antigen expressed by TP which was not shared by T. spiralis isolates and vice versa. Collectively, these data support the conclusion that the differences observed in the kinetics of immune responsiveness to different Trichinella isolates are referable, at least in part, to differences among the isolates in the expression of functionally relevant antigens.

Cloned T cells recognize Trichinella spiralis antigen in association with an Ek ?Ek ? restriction element

A method is described for the production of T-cell lines and clones specific for solubilized Trichinella spiralis antigens. hese T cells are antigen-specific and do not respond to challenge with a third party antigen (lysozyme). The proliferation responses of the cloned T cells are specifically inhibited by anti-I-E but not by anti I-A subregion monoclonal reagents. The inhibition patterns obtained are consistent with cis-gene complementation in B10.K cells involving the Ekβ-chain and the Ek α-chain of the I-E molecule. Inhibition is obtained with an Ekβ-specific monoclonal antibody (H9-14.8) but not with an Akβ-specific monoclonal antibody (10-2.16). Inhibition was also observed with Ia.7-specific (H40-242) or Ia.22-specific (17-3-3) monoclonal antibodies. The inhibition patterns were confirmed by antigen presentation experiments using recombinant inbred mice. Only B 10.K (EkβEkα spleen cells and not B 10.A(5R) (EbβEkα) or B10.S(9R) (EsβEkα) spleen cells could effectively present T. spiralis antigens. The role of “hybrid” Ia molecules in the immune response to T. spiralis is discussed.

Nematospiroides dubius: Two H-2-linked genes influence levels of resistance to infection in mice

Strains of mice sharing common H-2 haplotypes but different genetic backgrounds, and H-2 congenic strains of mice differing only at H-2 genes were studied to assess the role of H-2 and non-H-2 genes in immunity to challenge infections with the nematode parasite Nematospiroides dubius. Strains of mice sharing the H-2k haplotype were uniformly more susceptible to challenge than strains expressing H-2q alleles, regardless of genetic background. However, in some cases strains of mice sharing the k or q haplotypes differed significantly in levels of resistance. Therefore, non-H-2 genes must influence the response observed. H-2 cogenic strains of mice differed markedly in their ability to resist challenge infections. Mice sharing the C57BL/10 background but expressing k alleles were very susceptible to challenge, while the H-2q, H-2f, and H-2s, haplotypes were associated with resistance. Studies of H-2 congenic recombinant strains of mice suggested that two H-2 genes influence the antiparasite response. One of these genes maps to the left of E alpha and the other to the D-end of the H-2 complex. It is concluded also that the unique configuration of H-2 genes in F1 hybrids contributes to increased resistance to challange.

Some Mechanisms of Natural Regulation of Parasitic Helminth Populations

Mechanisms important in population regulation of helminth parasites are discussed as they pertain to natural host-parasite systems in western Utah. Regulation of worm burdens in individual host animals results from a balance established over long periods of symbiosis that in most instances produces a level of parasitemia inimical to the host, the exception consisting of those individuals in the population lack- ing the usual resistance mechanisms. A series of factors are listed, but discussion is limited to those for which new data can be introduced. These are actions of host-immune mechanisms, intermediate host influ- ences, seasonal food selectivity, climatic variability, territoriality and con- trol through the disproportionate distribution of sexes where average wormn burdens are small. Areas for further investigation are suggested.

TRICHINELLA SPIRALIS INFECTIONS OF INBRED MICE: GENETICS OF THE HOST RESPONSE FOLLOWING INFECTION WITH DIFFERENT TRICHINELLA ISOLATES

The immune response of inbred strains of mice was studied following infection with isolates of Trichinella from a pig (P1), an arctic fox (AF1), and T. spiralis var. pseudospiralis (TP). Strains of mice previously characterized as highly resistant to a separate pig isolate of T. spiralis responded to the P1 and AF1 isolates by expelling over 80% of the worms by day 10 postinfection (PI), and by suppressing the in vitro release of newborn larvae by female worms. However, the response induced by AF1 worms was expressed more quickly when compared to responses induced by the P1 and TP isolates. The host response to TP was less as recovery was always higher at day 10 PI and antifecundity effects were not induced in TP worms even in highly resistant strains of mice. Strains of mice previously characterized as susceptible to T. spiralis infection were slow to develop resistance when compared to the resistant mouse strains, but even among the susceptible strains, infection with AF1 induced a more rapid response. The mouse strains used in these experiments allowed us to assess the role of the major histocompatibility complex (MHC) and/or non-MHC genes in influencing the responses observed. As previously reported for a pig isolate of T. spiralis, both MHC and non-MHC genes influenced the rate at which worms were expelled from the gut and the host response that limits the fecundity of adult female worms.

Trichinella spiralis: Role of non-H-2 genes in resistance to primary infection in mice

Two strains of mice which share identical H-2 genes but differ in their genetic backgrounds were compared for their ability to resist infection with Trichinella spiralis. The two strains of mice, C3HeB/FeJ and AKR/J, share the H-2k haplotype which is associated with susceptibility to primary infection with T. spiralis in H-2 congenic strains of mice. AKR/J mice, infected with 150 infective muscle larvae, harbored significantly fewer muscle larvae 30 days postinfection than did mice of the strain C3HeB/FeJ. Approximately equal numbers of worms establish in the small intestine of AKR and C3H mice, but the AKR mice expelled adult worms from the gut more rapidly than did mice of the C3H strain. By Day 9 postinfection, 50% of the worms had been expelled by the AKR mice whereas expulsion of worms from C3H mice was delayed beyond Day 9 and occurred primarily between Days 10 and 12. Over this same experimental period (Days 6-12), fecundity of female worms from AKR mice, measured as the mean newborn larvae/female/hour, was approximately one-half that of worms taken from C3H mice. These results support the conclusion that genes outside of the mouse H-2 complex regulate expulsion of adult worms from the gut. These background genes also markedly influence the fecundity of female worms.

Characterization of an in vitro proliferation response to solubilized Trichinella spiralis antigens: Role of Ia antigens and Ly-1+ T cells

Abstract An antigen extract from Trichinella spiralis muscle larvae was prepared and used to immunize strains of mice which were either relatively resistant (B10.S) or susceptible (B10.BR) to oral infection with T. spiralis larvae. Proliferation of cells from the draining lymph nodes was then measured in vitro after stimulation with the T. spiralis extract as well as appropriate control antigens. Primed cells from resistant B10.S mice responded better to challenge than did cells from the susceptible B10.BR strain. Cell-depletion experiments involving B10.S cells indicated that the in vitro cell proliferation response is dependent upon Ly-1 + T cells. The data were also consistent with a requirement for Ly-1 + , -2 + , and -3 + amplifier cells. Administration of anti-I s serum to the cultures specifically inhibited (nearly 75%) the cell proliferation response. The potential applications of this system as a tool in immunogenetic analyses of immunity to T. spiralis are discussed.