Trichinella spiralis: nonspecific resistance and immunity to newborn larvae in inbred mice

The implantation and development of intravenously injected Trichinella spiralis newborn larvae were examined in different strains of inbred mice by determining muscle larvae burden. This was compared to the numbers of muscle larvae that established after a natural infection during which a quantitative assessment of intestinal newborn larvae production was made. In most inbred strains of mice, newborn larvae do not all successfully implant in muscle. Mice of the DBA/1 strain are the most resistant to successful implantation, and C3H mice are the most permissive. This pattern is evident in the strains studied whether newborn larvae are injected intravenously or are produced by intestinal adults. Thus, after a natural infection, 100% of intestinally produced newborn larvae implanted in C3H mice, whereas in NFR 68% and DBA/1 mice 62% successfully matured in muscle. Immunity to newborn larvae could be demonstrated as early as 10 days after exposure to this stage of the life cycle. This immunity was protective against a complete challenge infection given 9 days after newborn larvae had been injected intravenously. Protection against newborn larvae was identical in male and female mice or in mice from 1 to 9 months of age. We conclude that there are two mechanisms by which mice impair newborn larvae establishment or development in muscle. The first appears to be nonimmunological (non-specific resistance), and the second is immunological. Genetically determined variation in strain-specific expression is apparent with both mechanisms. In strains displaying high intrinsic "resistance" (DBA/1), this process is likely to account for most of the 38% reduction in newborn larvae establishment in a primary infection. However, immunity against newborn larvae develops quickly enough to have a significant effect on migratory larvae in primary infections where adults persist in the intestine (e.g., the B10 congenic mice), or when high adult worm burdens delay adult worm rejection. Muscle larvae burden, therefore, reflects systemic nonspecific resistance to newborn larvae as well as immunological processes that occur in the intestine and systemically.     

Trichinella spiralis: immunity and inflammation in the expulsion of transplanted adult worms from mice.

The technique of implanting adult Trichinella spiralis into the intestines of mice has been used to assess the contributions of direct, anti-worm immunity and of intestinal inflammation to worm expulsion. The survival after transfer of worms exposed to an effective adoptive immunity in donors was no different from that of worms taken from control donors. Worms taken from donors 8 days after infection, i.e., shortly before the onset of expulsion, showed no increased susceptibility to an immunity adoptively transferred to the recipient mice. When worms were implanted into mice responding to a prior, oral infection they were expelled rapidly. This expulsion was independent of the age of the worms transferred and took place at the same time as the expulsion of the existing infection.     

Trichinella spiralis: Characterization and strain distribution of rapid expulsion in inbred mice

Appropriately immunized mice display a response that is biologically equivalent to rat rapid expulsion. Only two inbred strains ($\text{NFR}\text{N}\text{and}\text{NFS}\text{N}$ derived from NIH Swiss mice) have been shown to respond in this manner. Mice of the $\text{Balb}\text{c}$, CBA, $\text{A}\text{He}$, C₃H, SJL, or C₅₇Bl strains are “nonresponders” which require approximately twice as much intestinal exposure (in days) to Trichinella spiralis to elicit a response half as effective. Genetically, the responder is dominant, autosomal, and does not appear to be linked to the MHC. The characteristics of mouse and rat rapid expulsion of T. spiralis are not identical but share these features: initial rejection within 24 hr of challenge; a rejection efficiency >90%, from 1 to 5 weeks after the primary; induction of response does not require exposure to the complete infection; rapid expulsion is immunologically specific for preadults; adult worms are resistant. While a genetic basis for responsiveness exists in mice there is, as yet, no evidence for genetic control in rats. In both mice and rats, rapid expulsion is distinguished from the intestinal hyperreactivity associated with rejection of the primary infection by the kinetics and amplitude of the rejection of transplanted adult worms.     

Trichinella spiralis: mediation of the intestinal component of protective immunity in the rat by multiple, phase-specific, antiparasitic responses.

Rats infected orally with Trichinella spiralis developed an immunity that was induced by and expressed against separate phases of the parasite's enteral life cycle. Infectious muscle larvae generated an immune response (rapid expulsion) that was directed against the very early intestinal infection and resulted in the expulsion of worms within 24 hr. This response eliminated more than 95% of worms in an oral challenge inoculum. Developing larvae (preadults) also induced an immune response that was expressed against adult worms. The effect on adults was dependent upon continuous exposure of worms to the immune environment throughout their enteral larval development. Immunity induced by preadult T. spiralis was not expressed against adult worms transferred from nonimmune rats. While adult worms were resistant to the immunity engendered by preadults they induced an efficient immunity that was autospecific. Both “preadult” and “adult” immunities were expressed in depression of worm fecundity as well as in the expulsion of adults from the gut. However, the two reactions differed in respect to their kinetics and their efficiency against various worm burdens. Preadult immunity was directed mainly against fecundity whereas adult immunity favored worm expulsion. All responses (rapid expulsion, preadult and adult immunity, and antifecundity) acted synergistically to produce sterile immunity against challenge infections of up to 5000 muscle larvae. These findings indicate that the host protective response to T. spiralis is a complex, multifactorial process that operates sequentially and synergistically to protect the host against reinfection.     

Trichinella spiralis: selective intestinal immune deviation in the rat

In rats, infections with 100-2000 Trichinella spiralis muscle larvae lead to a prompt immunity that is expressed in parasite expulsion within 14 days. Rats infected with more than 2000 larvae display impaired immunity with rejection delayed by 50% (7 days) or more. Suppression is selective for expulsive immunity as the antifecundity response of rats is directly proportional to dose and is expressed sooner in heavily infected subjects. Suppression of intestinal expulsive immunity was suggested by the fact that, with low doses (2000 larvae or less), worm rejection was inhibited by cortisone, whereas cortisone inhibited antifecundity but had no discernable effect on worm rejection in high-dose infections. Evidence for local immune deviation as opposed to systemic immunosuppression was obtained in experiments using parabiotic rats. When one partner was infected with 6000 worms and the other with 200, the rat infected with 200 parasites showed earlier rejection than was seen in single controls infected with 200 worms. The prolonged survival of high-dose adults was not accompanied by a change in the site of worm residence in the gut. Immunological parameters such as serum antibody levels, the number of activated cells or specific anti-T. spiralis lymphocytes in thoracic duct lymph were all increased in a dose-dependent manner. These experiments therefore demonstrate a novel autoprotective mechanism by which adult T. spiralis selectively reduce the expression of expulsive immunity in the gut.     

Trichinella spiralis: expression of rapid expulsion in rats exposed to an abbreviated enteral infection.

Rats infected with Trichinella spiralis for the first week of the enteral infectious cycle displayed a strong rapid expulsion reaction during a challenge infection. The response was induced with equal facility in animals given low or high immunizing doses of infectious larvae (500 to 5000 larvae). Large challenge infections resulted in a 10–15% reduction in the efficiency of rejection as assessed 24 hr after challenge. Rats became primed to express rapid expulsion within the first week of primary infection whether the infection remained patent or not. However, maximum effectiveness was not realized until the second week after the initial infection. Once induced, the capacity to express rapid expulsion persisted for 6 weeks after the primary infection. Immunized hosts were capable of resisting two challenge infections spaced by periods of from 12 to 72 hr. This finding suggests that a mediator is not consumed by the initial response.     

Trichinella spiralis: role of different life cycle phases in induction, maintenance, and expression of rapid expulsion in rats.

The capacity of different phases of the life cycle of Trichinella spiralis to induce rapid expulsion was examined. The phases examined included enteral preadults, enteral adults, and parenteral larvae. All had the ability to induce rapid expulsion although there were significant quantitative differences in their inductive capacity and in the kinetics of expression. Immunization with preadults required only a 48-hr enteral exposure to 2000 worms to induce strong rapid expulsion. In contrast rats required a 14-day exposure to adult worms to elicit a comparable response. After immunization with adults the reaction was demonstrable for only 2 weeks. Parenteral larvae produced only a weak rapid expulsion reaction by themselves and this response did not develop until some 8 weeks after challenge. When immunization with the enteral phases (preadult and adult) was combined with exposure to parenteral larvae a strong and enduring rapid expulsion reaction was observed. Phase specificity was also observed in the susceptibility of worms to the rapid expulsion response. The preadult phases, from infectious larvae to worms of up to 2 days of age were highly susceptible. Older worms, from 3 to 4 days old were not susceptible to rapid expulsion and could invade and establish themselves in the primed intestine for at least a 48-hr period without apparent adverse effects.     

Trichinella spiralis: Genetics of worm expulsion in inbred and F1 mice infected with different worm doses

The nematode Trichinella spiralis is rejected from the intestine at a time that is characteristic for each inbred strain of mouse. Previous work (R. G. Bell et al. 1982a) had empirically identified strong, intermediate, and weak phenotypes (NFR, $\text{C}\text{H}\text{He}$, and $\text{C57}\text{10}$ mice, respectively) in mice infected with 400 muscle larvae. It is shown that this classification applies to another eight inbred strains: SWR, $\text{DBA}\text{2}$, $\text{DBA}\text{1}$, LP, $\text{Bub}\text{Bn}$—all intermediate, and $\text{NZB}\text{BIN}$, C57L, A, and Mus molossinus—all weak. This phenotypic classification consistently applies with infections of 400–800 muscle larvae. Below doses of 300 muscle larvae, the strain designation of phenotype does not consistently apply. By this it is meant that the relative rejection rate changes for certain strains so that eventually some strains that were strong (NFR) or intermediate (AKR) responders to 400 muscle larvae become weak responders to 50 muscle larvae. Other strains increase their relative rejection time (B10 · BR, B10 · Q) while many do not change (NFS, $\text{C}\text{3}\text{Heb}\text{Fe}$, $\text{DBA}\text{2}$, $\text{DBA}\text{1}$). The phenomenon is most apparent in inbred parental strains rather than in F₁ crosses, and it represents a phenotypic variation in rejection time that is dependent on dose. It is also demonstrated that time of rejection is directly proportional to dose in all inbred and F₁ mouse strains that we have examined. Analysis of F₁ crosses shows that most have the rejection time of the strongest responding parental line, suggesting simple genetic control of strong, intermediate, and weak responses. Two F₁ crosses invalidated this theory. The $\text{DBA}\text{1}\text{×}\text{C3}\text{H}\text{He}$ (intermediate × intermediate) showed a strong response. The additive effects of parental rejection phenotype indicated that these lines could not be genetically identical for intermediate responsiveness. Similarly, the NFR (strong) × B10 · BR (weak) F₁ showed intermediate rejection, indicating partial dominance of $\text{C}\text{57}\text{B}\text{1}\text{10}$ genes over the strong responder NFR strain. Neither the primary expulsion time phenotype, phenotypic variation to low doses, or the rejection characteristics of F₁ crosses could be ascribed to genes linked to the major histocompatibility complex.     

Trichinella spiralis: correlates in vitro of altered immune responsiveness in mice.

Mixed lymphocyte reactions and in vitro antibody responses to dinitrophenol (DNP) after immunization with DNP-Ficoll were measured in spleen cells from mice following infection with 200 Trichinella spiralis larvae. A depression of the mixed lymphocyte reaction was observed at 14 through 84 days after infection. A reduced response to concanavalin A stimulation was demonstrated over a similar time period, 7 through 63 days of infection. The addition of mitomycin C-treated spleen cells from mice infected with T. spiralis to cultures of normal splenocytes suppressed the mixed lymphocyte reaction by 28% to 65%. The antibody response to DNP-Ficoll immunization was enhanced 20 days after infection, a time when the T-dependent antibody response to sheep erythrocytes was depressed.     

Trichinella spiralis: characterization of phage-displayed specific epitopes and their protective immunity in BALB/c mice

Trichinellosis is a global zoonosis mainly caused by Trichinella spiralis. We have previously reported that a novel Ts87 gene from the cDNA library of adult T. spiralis was cloned and expressed in a prokaryotic expression system. Vaccination with recombinant Ts87 protein (rTs87) induced a muscle larvae burden reduction in BALB/c mice by 29% in response to T. spiralis infection. In the present study, we screened a random phage-displayed peptide library using monoclonal antibody 5A3 which recognized Ts87 protein. Four positive phage clones were selected to subcutaneously immunize BALB/c mice without adjuvant. Two phage clones could effectively stimulate specific antibodies against rTs87. Mice vaccinated with these two combined phage clones showed a 28.7% worm burden reduction as compared to the control group. Therefore, the identified phage clones displayed peptides representing specific epitopes of Ts87 protein and could be considered as potential vaccine candidates for T. spiralis.     

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.     

Heligmosomoides polygyrus: inoculum size and glucose, ion, and gas fluxes in the small intestine of mice

Female CDI mice were inoculated with 10, 50, 100, 250, or 500 larvae of Heligmosomoides polygyrus. At Days 7, 9, and 12 after infection, the anterior third of the small intestine was perfused using an in vivo technique. The distribution of worms in the mouse intestine was determined after 7, 9, and 12 days. All worms that were recovered were from the proximal half of the small intestine. When compared to uninfected controls, there was a significant increase (+56%) in glucose absorption of the small intestine at Day 7 after infection with inocula of 50 and 100 larvae; at Day 9, glucose absorption was significantly increased with a 10-larvae inoculum. A decrease in glucose absorption occurred at Days 7 and 9 after infection with a 500-larvae inoculum. Net water absorption was significantly increased (+183%) with the 50- and 100-larvae inocula at Day 7, but was significantly reduced at Day 9 after infection with the 50-, 100-, 250-, and 500-larvae inocula. Both Cl- and Na+ absorption were significantly increased with the 50-, 100-, and 250-larvae inocula at Day 7 after infection; at 9 and 12 days, there was significant net secretion of both ions. In control mice, there was net secretion of K+, while with the 50-, 100-, and 250-larvae inocula on Day 7 there was significant net absorption of K+ ions.(ABSTRACT TRUNCATED AT 250 WORDS)     

Trichinella spiralis: quantitative relationships between intestinal worm burden, worm rejection, and the measurement of intestinal immunity in inbred mice

The effect of widely different doses of Trichinella spiralis muscle larvae on time to rejection of intestinal adults and on host survival was assessed in mice of the three rejection phenotypes; strong, intermediate, and weak. Rejection is weak with doses of less than 50 larvae per mouse. At these doses all mice rejected worms at a similar rate and no phenotypic variation was evident among strains. In contrast, rejection time was shortest for all strains and phenotypic variation among strains was evident in the range 50-100 muscle larvae/mouse. Above this dose the time taken to rejection increases monotonically with dose for all mouse strains examined. Despite this, the relative strength of rejection (i.e., phenotype) of a given strain of mouse was not changed at higher doses. Based on an end point of 98% rejection of the infective dose, time to rejection was predictable to +/- 1 day for all mouse strains and doses tested over the range 100-1000 worms administered. The principal reason for the increased time to complete rejection with larger worm doses was a delay in the initiation of intestinal rejection. This delay was evident above a dose of 50-100 larvae per mouse and occurred in all strains. Once begun, rejection was faster and eliminated more worms in unit time at higher doses (400-800 more) than at lower doses of worms. This appeared to be due to a stronger immune response of the host at higher doses. However, the increase in the rate of rejection was still not as great as the increase in the dose. We postulate that the delay in rejection with increased dose is due to a requirement for a "critical mass" of effectors/worm required to cause rejection. As dose increases, more time is required to reach the level at which worm rejection commences. Deaths due to higher doses of worms also occurred in a strain-specific manner and were temporally biphasic. The intestinal phase of infection produced mortality from 1 to 5 days after infection and the strongest rejection phenotype (NFS) was also the most resistant to intestinal deaths. Deaths occurring after Day 5 were due to the parenterally migrating newborn larvae. The weakest rejection phenotype, that of the B10 congenics, was also the least resistant to intestinal deaths. An experimental formula describing 98% worm rejection time with different doses was derived from the data.(ABSTRACT TRUNCATED AT 400 WORDS)     

Trichinella spiralis: anaphylactic antibody formation and susceptibility in strains of inbred mice.

The anaphylactic antibody response of various strains of inbred mice of different H-2 specificities was investigated using the passive cutaneous anaphylactic technique (PCA) for the detection of the antibody response. Neither IgC₁ nor reaginic antibody were detected in serum samples obtained at the end of the first week of infection with Trichinella spiralis. Subsequently, all animals had detectable IgG₁ antibodies, although in some strains the titers were very low. Reaginic antibody was detected in relatively high titers in C57L, A, and DBA/1 mice. Two other strains were very poor responders (SJL and AKR). In most strains, reagin and IgG₁ remained detectable for 14 wk or longer. The pattern of response of all strains was very reproducible, indicating genetic control of the anaphylactic antibody production to the infection. In F₁ hybrids obtained from crosses between good and poor anaphylactic antibody responders, intermediate levels of both antibody classes were detected.Adult worm recovery rates were established at various points during the intestinal phase of infection, and no correlation between worm numbers and reaginic antibody titers in the various strains of mice could be demonstrated. There were noticeable differences in larval yields obtained after muscle digestion of mice belonging to the different inbred strains. In fact, we generally observed an inverse relationship between the number of larvae recovered from a given strain and their reaginic antibody titer.The intravenous injection of newborn larvae (NBL), obtained upon in vitro incubation of adult worms, produced detectable antibodies only in mice of the DBA/1 strain. These antibodies were consistently of low titer and became detectable only after the administration of two additional injections of NBL. This contrasted with the results observed after “per os” infection of DBA/1 mice, where high titers of these antibodies were always obtained, in spite of comparable ratios of muscle larval yield.     

Trichinella spiralis: effects on the host-parasite relationship in mice of BCG (attenuated Mycobacterium bovis).

The effects of BCG (Bacillus Calmette-Guérin, i.e., attenuated Mycobacterium bovis) on the host-parasite relationship in murine trichinosis were examined. A total of 2 × 10⁷ colony forming units of BCG given iv 1 week prior to Trichinella spiralis infection delayed the expulsion of adult worms from the gut. The suppression of adult worm elimination was proportional to the dose of BCG given. This finding was associated with a reduction in the degree of partial villous atrophy induced in the small bowel by T. spiralis. Adult female worms were fecund when they were examined 1, 2, and 3 weeks after infection of mice with T. spiralis. Despite the prolongation of fecund adult worms in the gut, there were no significant differences in muscle larval counts 4 and 6 weeks after infection. When newborn larvae were cultivated in vitro and injected iv, there was a significant 25% reduction in larval numbers recovered from the muscles of BCG-treated mice 4 weeks later. The administration of BCG had no effect on the inflammatory reaction around larvae in the muscles 4 and 6 weeks after infection. It is concluded that BCG alters the host-parasite relationship producing retention of adult worms in the gut, reduction in the severity of partial villous atrophy, and increased nonspecific resistance to the systemic larval phase of this parasite.     

Trichinella spiralis: Generation in the presence of rat serum of factors chemotactic for rat cells

Chemotaxis of rat peritoneal cells, of which the eosinophil was the predominant migratory cell type, toward incubates of Trichinella spiralis was studied using a modified Boyden chamber. Excysted muscle larvae, preadults, and adults were incubated in a buffered medium for 20 hr at 37 C. Worms were incubated alone or with serum or spleen cells, or both, from immune and nonimmune rats. Incubates of worm stages alone possessed no chemotactic activity as compared with incubation medium as a negative control and zymosan-activated serum as a positive control. Both normal and immune sera tested alone stimulated cell migration to the same degree. Incubates of spleen cells from either normal or immunized hosts did not show chemotactic activity. Chemotaxis caused by normal and immune sera were not altered by incubation with homologous spleen cells. Addition of larva, preadults, and adult worms to sera, however, enhanced chemotactic activity over sera alone. Chemotaxis caused by larvae plus immune sera was significantly greater than that stimulated by larvae plus normal sera. This difference decreased when preadults were substituted for larvae and was not observed when adult worms were used. Reversal of the chemical gradients showed that active cell migration caused by various incubates was due to Chemotaxis.     

Use of the MHC for mate choice in wild house mice (Mus domesticus)

The mechanisms maintaining natural diversity at the major histocompatibility complex (MHC) are not well understood. To increase knowledge of one potential mechanism, I examined the use of MHC genes for mate choice by wild house mice in a controlled laboratory setting. Three rearing groups of wild test mice were produced: non‐fostered control mice, mice fostered into families of an inbred laboratory mouse strain, and mice fostered into families of a second, MHC‐congenic mouse strain. Mature test mice were given a choice of two opposite‐sex stimulus mice from the two MHC‐congenic strains used for fostering, and were scored for several measures of preference. The results were non‐significant in general, but females of two rearing groups spent significantly more time with mice of one MHC‐type, and in most rearing groups, mice tended to spend more time with this same MHC‐type. Other results showed that male test mice ejaculated indiscriminantly and that female wild mice mated to ejaculation more often in longer length trials, but showed no significant preferences. In this study, fostering seemed to have little or no effect on MHC‐based mate preferences of wild house mice, and wild mice did not appear to be using the MHC to avoid inbreeding. However, some wild female mice used the MHC to choose potential mates. This revised version was published online in July 2006 with corrections to the Cover Date.     

Trypanosoma congolense: inheritance of susceptibility to infection in inbred strains of mice

Inbred strains of mice have shown marked differences in susceptibility to infection with Trypanosoma congolense, as judged by survival and levels of parasitemia. The underlying genetic basis of the susceptibility was examined with F1 hybrids and backcrosses derived from mouse strains of high and low susceptibility. The influence of H-2 haplotype on susceptibility was studied using H-2 congenic resistant strains of mice. F1 hybrids between the most susceptible strain (A/J) and the least susceptible strain (C57Bl/6) showed similar survival to that of the C57Bl/6 parent. This was reflected in a similar undulating pattern of parasitemia, although the level of parasitemia was consistently higher in the F1 hybrids than in the C57Bl/6. Backcrosses of the F1 hybrids with C57Bl/6 also had a similar pattern of parasitemia although there was a greater scatter in survival times so that a few animals survived longer than either of the parental strains. Backcrosses of F1 hybrids with A/J showed a range of survival times; approximately 25% of these animals died during the period when the A/J mice died, approximately 25% had a similar survival to that of C57Bl/6, while the remaining animals showed an intermediate duration of survival. All these backcrosses had a high initial peak of parasitemia; in about 70% of the mice the early parasitemia showed a distinct undulating pattern. F1 hybrids of A/J and C57Bl/6 with C3H/He mice, which are known to be of intermediate susceptibility, were also examined. The degree of dominance for low susceptibility was much less pronounced in these hybrid combinations than in the A/J × C57Bl/6 hybrids. The H-2 congenic resistant strains, all of which were on a C57Bl/10 genetic background, showed a similar pattern of parasitemia and survival. However, although the majority of all these strains survived for more than 100 days, there was a significant difference in survival between the C57Bl/10 mice and the H-2 congenic resistant strains. It was concluded that susceptibility of mice to T. congolense infection is likely to be under complex genetic control and that, at least in C57Bl/mice, H-2 haplotype has little influence on susceptibility.     

Leishmania tropica: pathogenicity and in vitro macrophage function in strains of inbred mice.

Of seven strains of inbred mice and one hybrid that were infected intracutaneously with 5, 10, or 20 × 10⁶ active promastigotes of Leishmania tropica major, two strains (CBA/Ca and C₃H/He) recovered from the infection and their lesions healed within 3 to 5 months. The other strains, with the possible exception of C57B1/6 animals, remained infected, carrying large cutaneous ulcers throughout their lives. These included DBA/2, A/Jax, Balb/c, athymic nude mice of Balb/c origin (nu/nu) and the heterozygote Balb/c (nu⁺). The responses of C57B1/6 animals were of intermediate type with a tendency toward nonhealing at higher doses of the parasite. The cutaneous infection of athymic nude mice invariably gave rise to fulminating visceral infections and death. This condition was never observed in the other strains tested. Concanavalin A (Con A)-stimulated syngeneic or allogeneic lymphocytes of intact mice activated peritoneal macrophages of both healer and nonhealer mice, resulting in complete destruction of phagocytosed L. enriettii within 24 to 48 hr. The destruction of ingested L. tropica was confined to macrophages of healer mice and required 72 to 96 hr to reach completion. However, removal of Con A-stimulated lymphocytes from macrophage cultures and regular pulsing of the cells with a lymphokine-rich supernatant produced a state of sustained activation, resulting in destruction of L. tropica inside macrophages of both healer and nonhealer mice. The ability of Con A-stimulated lymphocytes of nonhealer animals to induce effective levels of activation in healer macrophages on one hand, and eventual destruction of L. tropica in macrophages of nonhealer mice under condition of sustained activation on the other, had indicated that so far as the in vitro situation is concerned, there is no inherent defect in lymphocytes or macrophages of nonhealer animals, although the threshold of activation necessary for killing of the parasite seems to be higher for cells of nonhealer origin.     

The evolution of MHC diversity: evidence of intralocus gene conversion and recombination in a single-locus system

Gene conversion, the unidirectional exchange of genetic material between homologous sequences, is thought to strongly influence patterns of genetic diversity. The high diversity of major histocompatibility complex (MHC) genes in many species is thought to reflect a long history of gene conversion events both within and among loci. Theoretical work suggests that intra- and interlocus gene conversion leave characteristic signatures of nucleotide diversity, but empirical studies of MHC variation have rarely been able to analyze the effects of conversion events in isolation, due to the presence of multiple gene copies in most species. The potbellied seahorse (Hippocampus abdominalis), a species with a single copy of the MH class II beta-chain gene (MHIIb), provides an ideal system in which to explore predictions on the effects of intralocus gene conversion on patterns of genetic diversity. The genetic diversity of the MHIIb peptide binding region (PBR) is high in the seahorse, similar to other vertebrate species. In contrast, the remainder of the gene shows a total absence of synonymous variation and low levels of intronic sequence diversity, concentrated in 3 short repetitive regions and 1-12 SNPs per intron. The distribution of substitutions across the gene results in a patchwork pattern of shared polymorphism between otherwise divergent sequences. The pattern of nucleotide diversity observed in the seahorse MHIIb gene is congruent with theoretical expectations for intralocus gene conversion, indicating that this evolutionary mechanism has played an important role in MHC gene evolution, contributing to both the high diversity in the PBR and the low diversity outside this region. Neutral variation at this locus may be further reduced due to biases in nucleotide composition and functional constraints.