Susceptibility of five strains of mice to Babesia microti of human origin.

One outbred (CF1) and four inbred (BALB/c, C57, CBA and C3H) strains of mice were tested for susceptibility to Babesia microti of human origin. Of these, intact C3H mice developed higher parasitemia than all other intact mice, while BALB/c mice developed the highest parasitemia among splenectomized mice. Susceptibility was not related to H-2 haplotype in any obvious way. Because C3H and BALB/c mice developed relatively high initial peak parasitemias, the parasite was serially passaged in both of these mouse strains in an attempt to increase parasite virulence. After 30 passages in BALB/c and 49 passages in C3H mice over a period of 12 months, maximum parasitemias were 50 times higher than those observed initially. After the peak parasitemias of these two mouse-adapted parasites had stabilized, the relationship between onset and level of maximum parasitemia and number of parasites inoculated was determined. With both C3H- and BALB/c-adapted parasites, as inoculum size increased, the time required to reach maximum parasitemia decreased and the level of maximum parasitemia increased. Studies involving infection of either mouse strain with parasites adapted to the heterologous mouse strain indicated that C3H mice were more susceptible than BALB/c mice to homologous or heterologous parasites. These data suggest that the virulence of B. microti to the mouse can be increased by prolonged passage in this host. Once adaptation to this host species has occurred, virulence appears to be more dependent on the innate susceptibility of the mouse strain than on adaptation of the parasites to a particular strain of mouse.     

Experimental manipulation of population‐level MHC diversity controls pathogen virulence evolution in Mus musculus

The virulence levels attained by serial passage of pathogens through similar host genotypes are much higher than observed in natural systems; however, it is unknown what keeps natural virulence levels below these empirically demonstrated maximum levels. One hypothesis suggests that host diversity impedes pathogen virulence, because adaptation to one host genotype carries trade‐offs in the ability to replicate and cause disease in other host genotypes. To test this hypothesis, with the simplest level of population diversity within the loci of the major histocompatibility complex (MHC), we serially passaged Friend virus complex (FVC) through two rounds, in hosts with either the same MHC genotypes (pure passage) or hosts with different MHC genotypes (alternated passage). Alternated passages showed a significant overall reduction in viral titre (31%) and virulence (54%) when compared to pure passages. Furthermore, a resistant host genotype initially dominated any effects due to MHC diversity; however, when FVC was allowed to adapt to the resistant host genotype, predicted MHC effects emerged; that is, alternated lines show reduced virulence. These data indicate serial exposure to diverse MHC genotypes is an impediment to pathogen adaptation, suggesting genetic variation at MHC loci is important for limiting virulence in a rapidly evolving pathogen and supports negative frequency‐dependent selection as a force maintaining MHC diversity in host populations.     

Quantitative analysis of mouse urine volatiles: in search of MHC-dependent differences

Genes of the major histocompatibility complex (MHC), which play a critical role in immune recognition, influence mating preference and other social behaviors in mice. Training experiments using urine scent from mice differing only in the MHC complex, from MHC class I mutants or from knock-out mice lacking functional MHC class I molecules (beta2m-deficient), suggest that these behavioral effects are mediated by differences in MHC-dependent volatile components. In search for the physical basis of these behavioral studies, we have conducted a comparison of urinary volatiles in three sub-strains of C57BL/6 mice, a beta2m-deficient mutant lacking functional MHC class I expression and two unrelated inbred strains, using the technique of sorptive extraction with polydimethylsiloxan and subsequent analysis by gas chromatography/mass spectrometry. We show (i) that qualitative differences occur between different inbred strains but not in mice with the C57BL/6 background, (ii) that the individual variability in abundance in the same mouse strain is strongly component-dependent, (iii) that C57BL/6 sub-strains obtained from different provenance show a higher fraction of quantitative differences than a sub-strain and its beta2m-mutant obtained from the same source and (iv) that comparison of the spectra of beta2m mice and the corresponding wild type reveals no qualitative differences in close to 200 major and minor components and only minimal differences in a few substances from an ensemble of 69 selected for quantitative analysis. Our data suggest that odor is shaped by ontogenetic, environmental and genetic factors, and the gestalt of this scent may identify a mouse on the individual and population level; but, within the limits of the ensemble of components analysed, the results do not support the notion that functional MHC class I molecules influence the urinary volatile composition.     

Strain-dependent differences in responses to exercise training in inbred and hybrid mice

The aim of this study was to characterize the response to exercise training in several mouse strains and estimate the genetic contribution to phenotypic variation in the responses to exercise training. Male mice from three inbred strains [C57Bl/6J (BL6), FVB/NJ (FVB), and Balb/cJ (Balb/c)] and three hybrid F(1) strains [CB6F1/J (CB6 = female Balb/c x male BL6), B6F F(1) (female BL6 x male FVB), and FB6 F(1) (female FVB x male BL6)] completed an exercise performance test before and after a 4-wk treadmill running program. Distance was used as the primary estimate of endurance exercise performance. FVB mice showed the greatest response to training, with five- to sevenfold greater increases in distance run compared with BL6 and Balb/c strains. Specifically, BL6, FVB, and Balb/c strains increased distance by 33, 172, and 23%, respectively. A similar pattern of changes across strains was observed for run time (17, 87, and 11%) and work (99, 287, and 57%). As a group, F(1) hybrid mice derived from BL6 and FVB strains showed an intermediate response to training (61%). However, further analysis indicated that training responses in FB6 F(1) mice (80%) were approximately 2.5-fold greater than responses in B6F F(1) mice (33%, P = 0.08). A similar pattern of changes between FB6 and B6F F(1) mice was observed for run time (44.5 and 17%) and work (141 and 59%). These data demonstrate that there are large strain-dependent differences in training responses among inbred mouse strains, suggesting that genetic background contributes significantly to adaptation to exercise. Furthermore, the contrasting responses in B6F and FB6 F(1) strains show that a maternal component strongly influences strain-dependent differences in training responses.     

Immune responses to the pertussis toxin in Balb/c and C57B1/6 mice

Abstract Immunization of Balb/c and C57B1/6 mice with the pertussis toxin (Ptx), purified from the culture supernatant of Bordetella pertussis (the whooping cough bacillus) resulted in different immune reactions in these genetically different strains of mice. Antibody responses to Ptx were detected only in Balb/c, whereas both Balb/c and C57B1/6 produced anti-Ptx antibodies when immunized with detoxified Ptx. Also, delayed-type hypersensitivity reactions differ strongly according to the use of Ptx or detoxified Ptx as eliciting antigen.     

Effect of gold(I) compounds on the virulence of an amyocarditic strain of coxsackievirus B3

Coxsackieviruses, especially B strains (CVB), are known etiological agents of myocarditis. Both amyocardititc and myocarditic strains exist and at least one amyocarditic strain, CVB3/0, can convert to virulence when passaged through selenium or vitamin E-deficient mice. Gold(I)-containing compounds, such as aurothiomalate (ATM) and aurothioglucose (ATG), can act as selenium antagonists. In this study, we examined the effect of intraperitoneal administration of equal doses of ATM or ATG on the virulence of CVB3/0. ATM but not ATG increased mortality in CVB3/0-infected mice. CVB3/0-infected mice treated with ATM had total necrosis of the pancreatic exocrine tissue. Heart damage also occurred in ATM-treated mice but did not correlate with mortality. Increased viral titers and persistence were observed in ATM-treated mice and, to a lesser extent, in ATG-treated mice. Thus, under our conditions, only ATM increased the virulence of CVB3/0, whereas ATG did not. On the other hand, both ATG and ATM inhibited thioredoxin reductase activity in heart and pancreas, but neither affected glutathione peroxidase activity. In contrast, dietary selenium deficiency reduces both enzyme activities. Thus, it is unlikely that these compounds affect virulence by acting as selenium antagonists.     

MHC restriction of the antibody repertoire to secretory antigens, and a major allergen, of the nematode parasite Ascaris

Humans vary considerably in the antigen specificity of their immune responses to parasitic nematodes, and in the infection loads of individuals living in the same environment. The possibility that the former has a genetic basis operating through repertoire control of the immune system was investigated using infection of mice with the nematode Ascaris. The specificity of the antibody response was examined using excretory/secretory (ES) materials of the parasite as target Ag. No strain of mouse was found to recognize all of the potentially antigenic components of ES, and the Ag recognition patterns varied considerably from strain to strain. Using H-2 congenic mice on both the BALB and B10 backgrounds, it was established that the antigen recognition patterns were MHC-determined. Focusing on one particular component of ES, of Mr 14,000, only H-2s strains responded in IgG. This MHC restriction of the repertoire was confined to infection, and broke down under adjuvant-assisted immunization with the purified protein. The Mr 14,000 molecule was also found to be a potent allergen in a passive cutaneous anaphylaxis assay, and the IgE response to it was also restricted to H-2s. This haplotype was, however, a low IgE responder on the SJL background. There is, therefore, MHC control of the specificity of the immune response to this molecule, but non-MHC control of the amplitude of the IgE antibody response to it. Hybrids between responder and nonresponder strains (BALB/c x SJL)F1, responded to the Mr 14,000, but their responses to other ES components could not be predicted from the response patterns of parental strains. For example, the BALB/c parent responded to a 118-kDa component, but the SJL parent and the F1 progeny did not. Moreover, the response to a 41-kDa Ag was substantially down-regulated in the F1, whereas both parental strains responded vigorously. This new model system, therefore, has implications for MHC control of responses to the allergens of pathogens, and for the complex immunoregulation in heterozygotes in the context of infection.     

Selection for high and low virulence in the malaria parasite Plasmodium chabaudi.

What stops parasites becoming ever more virulent? Conventional wisdom and most parasite-centred models of the evolution of virulence suppose that risk of host (and, hence, parasite) death imposes selection against more virulent strains. Here we selected for high and low virulence within each of two clones of the rodent malaria parasite Plasmodium chabaudi on the basis of between-host differences in a surrogate measure of virulence--loss of live weight post-infection. Despite imposing strong selection for low virulence which mimicked 50-75% host mortality, the low virulence lines increased in virulence as much as the high virulence lines. Thus, artificial selection on between-host differences in virulence was unable to counteract natural selection for increased virulence caused by within-host selection processes. The parasite''s asexual replication rate and number of sexual transmission forms also increased in all lines, consistent with evolutionary models explaining high virulence. An upper bound to virulence, though not the asexual replication rate, was apparent, but this bound was not imposed by host mortality. Thus, we found evidence of the factors assumed to drive evolution of increased virulence, but not those thought to counter this selection.     

Clinal variation in MHC diversity with temperature: evidence for the role of host-pathogen interaction on local adaptation in Atlantic salmon

In vertebrates, variability at genes of the Major Histocompatibility Complex (MHC) represents an important adaptation for pathogen resistance, whereby high allelic diversity confers resistance to a greater number of pathogens. Pathogens can maintain diversifying selection pressure on their host's immune system that can vary in intensity based on pathogen richness, pathogen virulence, and length of the cohabitation period, which tend to increase with temperature. In this study, we tested the hypothesis that genetic diversity of MHC increases with temperature along a latitudinal gradient in response to pathogen selective pressure in the wild. A total of 1549 Atlantic salmon from 34 rivers were sampled between 46 degrees N and 58 degrees N in Eastern Canada. The results supported our working hypothesis. In contrast to the overall pattern observed at microsatellites, MHC class II allelic diversity increased with temperature, thus creating a latitudinal gradient. The observed temperature gradient was more pronounced for MHC amino acids of the peptide-binding region (PBR), a region that specifically binds to pathogens, than for the non-PBR. For the subset of rivers analyzed for bacterial diversity, MHC amino acid diversity of the PBR also increased significantly with bacterial diversity in each river. A comparison of the relative influence of temperature and bacterial diversity revealed that the latter could have a predominant role on MHC PBR variability. However, temperature was also identified as an important selective agent maintaining MHC diversity in the wild. Based on the bacteria results and given the putative role of temperature in shaping large-scale patterns of pathogen diversity and virulence, bacterial diversity is a plausible selection mechanism explaining the observed association between temperature and MHC variability. Therefore, we propose that genetic diversity at MHC class II represents local adaptation to cope with pathogen diversity in rivers associated with different thermal regimes. This study illuminates the link between selection pressure from the environment, host immune adaptation, and the large-scale genetic population structure for a nonmodel vertebrate in the wild.     

Genetic differences in BCG-induced resistance to Schistosoma mansoni are not controlled by genes within the major histocompatibility complex of the mouse.

Induction of nonspecific resistance to Schistosoma mansoni infection after the i.v. injection of viable BCG was investigated in outbred mice and a panel of inbred and H-2 congenic strains. Significant protection was induced in CF1, A/J, C57BL/6, C57BL/10, DBA/2, C57BR, and SJL mice. BALB/c mice were not protected whereas CBA and C3H mice expressed intermediate degrees of protection. Expression of the protective phenomenon is not controlled by genes within the MHC as shown by the marked differences in response between BALB/c and DBA/2 (H-2d) as well as between C57BR and C3H (H-2k) mice. H-2 congenic strains with C57BL/10 background (B10.A and B10.D2) were high responders. BALB.B10 mice carrying the high responder (B10) MHC on the nonresponder (BALB/c) background were not protected. The degree of splenic hypertrophy did not correlate with the expression of nonspecific resistance. These results demonstrate that, in addition to controlling specific immune responses, genetic differences influence the nonspecific protective phenomena related to BCG administration as well.     

Immunity promotes virulence evolution in a malaria model

Evolutionary models predict that host immunity will shape the evolution of parasite virulence. While some assumptions of these models have been tested, the actual evolutionary outcome of immune selection on virulence has not. Using the mouse malaria model, Plasmodium chabaudi, we experimentally tested whether immune pressure promotes the evolution of more virulent pathogens by evolving parasite lines in immunized and nonimmunized (“naïve”) mice using serial passage. We found that parasite lines evolved in immunized mice became more virulent to both naïve and immune mice than lines evolved in naïve mice. When these evolved lines were transmitted through mosquitoes, there was a general reduction in virulence across all lines. However, the immune-selected lines remained more virulent to naïve mice than the naïve-selected lines, though not to immunized mice. Thus, immune selection accelerated the rate of virulence evolution, rendering parasites more dangerous to naïve hosts. These results argue for further consideration of the evolutionary consequences for pathogen virulence of vaccination.     

The sites of antigen-T cell and antigen-MHC interactions overlap

The immune responses of B10.A and B10.A(3R) strains of mice to a synthetic variant of moth cytochrome c 86-103 were compared, and an immune response difference between the two strains was found. When T cell hybridomas were made from both strains it was found that the responsive T cells showed degenerate MHC restriction. The hybridomas from B10.A(3R) mice consistently showed a specificity pattern, when tested with allogeneic B10.A APC, different from that seen when syngeneic B10.A(3R) APC were used. The difference in the immune responses of the two strains of mice could be attributed to this APC-expressed specificity. The results were analyzed to determine whether the portion of the antigen that effected T cell memory (the epitope) and the portion that effected APC-expressed specificity (the agretope) were independent. It was found that a) these sites overlap and b) the APC-expressed specificity (i.e., the specificity of agretope recognition) was dependent on the T cell clonotype. This implies that the agretope is not an independent parameter in the process of MHC-restricted antigen recognition. Therefore its employment as an explanation for various phenomena will be limited by the likelihood of circularity.     

Vertical Transmission Selects for Reduced Virulence in a Plant Virus and for Increased Resistance in the Host

For the last three decades, evolutionary biologists have sought to understand which factors modulate the evolution of parasite virulence. Although theory has identified several of these modulators, their effect has seldom been analysed experimentally. We investigated the role of two such major factors—the mode of transmission, and host adaptation in response to parasite evolution—in the evolution of virulence of the plant virus Cucumber mosaic virus (CMV) in its natural host Arabidopsis thaliana. To do so, we serially passaged three CMV strains under strict vertical and strict horizontal transmission, alternating both modes of transmission. We quantified seed (vertical) transmission rate, virus accumulation, effect on plant growth and virulence of evolved and non-evolved viruses in the original plants and in plants derived after five passages of vertical transmission. Our results indicated that vertical passaging led to adaptation of the virus to greater vertical transmission, which was associated with reductions of virus accumulation and virulence. On the other hand, horizontal serial passages did not significantly modify virus accumulation and virulence. The observed increases in CMV seed transmission, and reductions in virus accumulation and virulence in vertically passaged viruses were due also to reciprocal host adaptation during vertical passages, which additionally reduced virulence and multiplication of vertically passaged viruses. This result is consistent with plant-virus co-evolution. Host adaptation to vertically passaged viruses was traded-off against reduced resistance to the non-evolved viruses. Thus, we provide evidence of the key role that the interplay between mode of transmission and host-parasite co-evolution has in determining the evolution of virulence.     

Co-administration of immunomodulator tuftsin and liposomised nystatin can combat less susceptible Candida albicans infection in temporarily neutropenic mice

In order to develop a prospective chemotherapeutic agent against opportunistic infections, it is important to know that host factors such as degree of immunological debility as well as recovery of immune functions to normality may contribute significantly to a successful elimination of the pathogens. We demonstrated previously that concomitant delivery of antimicrobial agents and immunomodulators to the pathogen harbouring-host contributes to the complete elimination of the deep-seated fungal infections (aspergillosis and candidiasis) in animals with normal immune status. Considering that neutropenic hosts are the main targets of such infections, it can be argued about the potential of the immunomodulator-based therapy in subjects with non-functional immune system. To resolve the hypothesis, we studied the role of immunomodulator tuftsin against experimental murine candidiasis in temporarily neutropenic Balb/c mice. The neutropenic mice were challenged with an isolate of Candida albicans that was showing less susceptibility to both free and liposomised-amphotericin B. The co-administration of tuftsin increased the efficiency of liposomised-polyene antibiotics (nystatin and amphotericin B) against experimental murine candidiasis in immunocompromised Balb/c mice. Pretreatment with liposomised tuftsin prior to C. albicans infection clearly enhanced protection against candidiasis, suggesting a prophylactic role of tuftsin in normal and temporarily neutropenic animals.     

Bordetella pertussis respiratory infection in C57B1/6 and Balb/c mice: Pathophysiology and immune responses

A virulent clone of Bordetella pertussis, injected intranasally into C57B1/6 or Balb/c mice, induced a respiratory tract infection that mimicked the infectious process of whooping cough. The density of the inoculum influenced the kinetics of in vivo bacterial growth, as well as the associated leucocytosis, which were of equivalent intensity in both strains of mice. Convalescing mice became resistant to re-infection, but not to the effects of the leucocytosis-promoting factor of the pertussis toxin. Prominent immune response, associated with acquired resistance, was a delayed type hypersensitivity (DTH) reaction, the intensity of which depended upon the genotype of the mice when the eliciting antigen contained the pertussis toxin in a biologically active form. Intranasal infection of congenic mice may represent an improved quantitative test for reproducible measurement of virulence and immunogenicity of B. pertussis.     

Alcohol withdrawal-induced changes in brain biogenic amines in mice: Influence of the genotype

Alterations in striatal and hippocampal dopamine (DA) and serotonin (5HT) activities were investigated in two inbred strains of mice (C57B1 and Balb/c) after 3 withdrawal periods following 5 months chronic ethanol administration. Two groups of animals with different levels of ethanol administration (15% and 30%, v/v) were examined. A striking strain dependency has been noted. Striatal dopaminergic mechanisms of the Balb/c strain are profoundly disturbed in both groups. In contrast no changes were noted for either transmitter activities in C57B1 mice at any withdrawal time studied. Strain dependency has also been noted for hippocampal serotonin neurotransmission, since only Balb/c mice showed a progressive decrease in 5HT levels. These impairments observed in striatum and hippocampus could be involved in motor incoordinations and convulsions often associated with the withdrawal syndrome. The differences in withdrawal effects we noted between the two strains may be linked to the specific chemical neuroanatomy of the strains. Such specificities could be implied in the well known variability of withdrawal induced behavior in man.     

Two distinct mechanisms account for the immune response (Ir) gene control of the T cell response to pigeon cytochrome c

Previous experiments have demonstrated that the immune response of MHC congenic mice to pigeon cytochrome c is under Ir gene control. Expression of I-E-encoded gene products influences both the magnitude and fine specificity of the Th cell response to pigeon cytochrome c and phylogenetic derivatives. Results of those experiments implicate both determinant selection and repertoire selection as mechanisms of Ir gene control in this system. In this report we have compared the TCR expressed in pigeon cytochrome c-reactive Th cells from B10.A(I-Ek), B10.A(5R) (I-Eb), and B10.S(9R) (I-Es) mice. The B10.A(5R) strain is a low responder to pigeon cytochrome c, but in response to moth cytochrome c this strain produces T cells which respond to pigeon or moth cytochrome c on B10.A APC. These cells are phenotypically identical to the predominant clonal phenotype seen in the B10.A response to pigeon cytochrome c. In this report, we show that the B10.A and B10.A(5R) pigeon cytochrome c-reactive T cells express essentially identical T cell receptors. These results, coupled with recent studies reporting a relatively low affinity for I-Eb molecules by pigeon cytochrome c peptides compared with moth cytochrome c peptides, strongly argue that the immune response defect in the B10.A(5R) strain is due to a defect in Ag presentation (determinant selection). In contrast, B10.A and B10.S(9R) strains are high responders to pigeon cytochrome c. Both strains produce T cell clones which are capable of responding to cytochrome c presented by either B10.A or B10.S(9R) APC in vitro. We show that, even in T cells with this MHC restriction degeneracy, the TCR expressed in the two strains are different. Because the APC of both strains can clearly present the cytochrome c Ag, we conclude that the differential expression of the TCR in the responses is due to a T cell repertoire selection difference in the two strains. Thus, for the response to one Ag in three MHC congenic strains, there exists evidence that both determinant selection and repertoire selection can be mechanisms of Ir gene control of an immune response.     

The Scent of Genetic Compatibility: Sexual Selection and the Major Histocompatibility Complex

Individuals in some species prefer mates carrying dissimilar genes at the major histocompatibility complex (MHC), which may function to increase the MHC or overall heterozygosity of progeny. Here I review the evidence for MHC-dependent mating preferences from recent studies, including studies on the underlying olfactory mechanisms and evolutionary functions. Many studies indicate that MHC genes influence odour, and some work is beginning to examine the potential role of MHC-linked olfactory receptor genes in mating preferences. MHC-dependent mating preference increases the MHC-heterozygosity of progeny, which is suspected to confer resistance to infectious diseases. In humans, heterozygosity at MHC loci is associated with increased resistance to hepatitis and HIV infections, but experimental evidence for the heterozygote advantage hypothesis has been lacking. Here I re-analyse data from previously published experimental infection studies with mice. I show that although overdominance is rare, resistance is often dominant, suggesting that heterozygotes are often protected. A second (nonmutually exclusive) possibility is that MHC-disassortative mating preferences promotes inbreeding avoidance. This hypothesis is supported by recent evidence that MHC genes play a role in kin recognition, and that mating with close kin has rather deleterious fitness consequences. In conclusion, I discuss other ways that MHC genes might influence sexual selection. The research on MHC-mediated mating preferences is integrating the study of animal behaviour with other seemingly disparate fields, including sensory biology and immunogenetics.