Genetics of resistance to African trypanosomes: role of the H-2 locus in determining resistance to infection with Trypanosoma rhodesiense

Influence of internalin A murinisation on host resistance to orally acquired listeriosis in mice

Background

The bacterial surface protein internalin (InlA) is a major virulence factor of the food-born pathogen Listeria monocytogenes. It plays a critical role in the bacteria crossing the host intestinal barrier by a species-specific interaction with the cell adhesion molecule E-cadherin. In mice, the interaction of InlA with murine E-cadherin is impaired due to sequence-specific binding incompatibilities. We have previously used the approach of ‘murinisation’ to establish an oral listeriosis infection model in mice by exchanging two amino acid residues in InlA. This dramatically increases binding to mouse E-cadherin. In the present study, we have used bioluminescent murinised and non-murinised Listeria strains to examine the spatiotemporal dissemination of Listeria in four diverse mouse genetic backgrounds after oral inoculation.

Results

The murinised Listeria monocytogenes strain showed enhanced invasiveness and induced more severe infections in all four investigated mouse inbred strains compared to the non-murinised Listeria strain. We identified C57BL/6J mice as being most resistant to orally acquired listeriosis whereas C3HeB/FeJ, A/J and BALB/cJ mice were found to be most susceptible to infection. This was reflected in faster kinetics of Listeria dissemination, higher bacterial loads in internal organs, and elevated serum levels of IL-6, IFN-γ, TNF-α and CCL2 in the susceptible strains as compared to the resistant C57BL/6J strain. Importantly, murinisation of InlA did not cause enhanced invasion of Listeria monocytogenes into the brain.

Conclusion

Murinised Listeria are able to efficiently cross the intestinal barrier in mice from diverse genetic backgrounds. However, expression of murinized InlA does not enhance listerial brain invasion suggesting that crossing of the blood brain barrier and crossing of the intestinal epithelium are achieved by Listeria monocytogenes through different molecular mechanisms.

Modulation of innate immunity by African trypanosomes

The experimental studies of Brucei group trypanosomes presented here demonstrate that the balance of host and parasite factors, especially IFN-γ GPI-sVSG respectively, and the timing of cellular exposure to them, dictate the predominant MP and DC activation profiles present at any given time during infection and within specific tissues. The timing of changes in innate immune cell functions following infection consistently support the conclusion that the key events controlling host resistance occur within a short time following initial exposure to the parasite GPI substituents. Once the changes in MP and DC activities are initiated, there appears little that the host can do to reverse these changes and alter the final outcome of these regulatory events. Instead, despite the availability of multiple innate and adaptive immune mechanisms that can control parasites, there is an inability to control trypanosome numbers sufficiently to prevent the emergence and establishment of virulent trypanosomes that eventually kill the host. Overall it appears that trypanosomes have carefully orchestrated the host innate and adaptive immune response so that parasite survival and transmission, and alterations of host immunity, are to its ultimate benefit.

Trypanosoma brucei: immunisation with plasmid DNA encoding invariant surface glycoprotein gene is able to induce partial protection in experimental African trypanosomiasis

Trypanosoma brucei is the etiological agent responsible for African trypanosomiasis, an infectious pathology which represents a serious problem of public health and economic losses in Sub-Saharan Africa. As one of the foremost neglected illnesses, few resources have been available for the development of vaccines or new drugs, in spite of the current therapeutical drugs showing little efficiency and high toxicity. Hence, it is obviously important to widen effective therapeutics and preventive strategies against African trypanosomiasis. In this work, we use the DNA vaccine model to evaluate immunisation effectiveness in mice challenged with Trypanosoma brucei brucei. We demonstrate that Balb/C mice immunised intramuscularly with a single dose of a DNA plasmid encoding a bloodstream-stage specific invariant surface glycoprotein (ISG) are partially protected from a lethal dose of T. b. brucei. Interestingly, the surviving animals show high levels of IgG2a anti-trypanosoma antibodies, suggesting that the Th1 response profile seems important for the induced mechanisms of immune protection.

Biological variation among african trypanosomes: I. Clonal expression of virulence is not linked to the variant surface glycoprotein or the variant surface glycoprotein gene telomeric expression site

The potential association of variant surface glycoprotein (VSG) gene expression with clonal expression of virulence in African trypanosomes was addressed. Two populations of clonally related trypanosomes, which differ dramatically in virulence for the infected host, but display the same apparent VSG surface coat phenotype, were characterized with respect to the VSG genes expressed as well as the chromosome telomeric expression sites (ES) utilized for VSG gene transcription. The VSG gene sequences expressed by clones LouTat 1 and LouTat 1A of Trypanosoma brucei rhodesiense were identical, and gene expression in both clones occurred precisely by the same gene conversion events (duplication and transposition), which generated an expression-linked copy (ELC) of the VSG gene. The ELC was present on the same genomic restriction fragments in both populations and resided in the telomere of a 330-kb chromosome; a single basic copy of the LouTat 1/1A VSG gene, present in all variants of the LouTat 1 serodeme, was located at an internal site of a 1.5-Mb chromosome. Restriction endonuclease mapping of the ES telomere revealed that the VSG ELC of clones LouTat 1 and 1A resides in the same site. Therefore, these findings provide evidence that the VSG gene ES and, potentially, any cotranscribed ES-associated genes do not play a role in the clonal regulation of virulence because trypanosome clones LouTat 1 and 1A, which differ markedly in their virulence properties, both express identical VSG genes from the same chromosome telomeric ES.

Contributions of experimental mouse models to the understanding of African trypanosomiasis

African trypanosomiasis is the collective name for a wide variety of trypanosome infections that affect humans and livestock. In recent years, experimental mice infection models have provided new insights into both human and animal trypanosomiasis. Mouse models seem to be a valuable and versatile tool in trypanosomiasis-associated pathology and immunology research and highlight the variety shown by African trypanosomiases. Indeed, inbred mouse strains have enabled the study of genetic determinants of susceptibility and of the roles of anti-parasite antibodies, inflammatory mediators and anti-inflammatory mediators for each trypanosome species. Remarkable advances relating to the encephalitic stage of sleeping sickness have also been achieved thanks to murine models. The different contributions of murine models to the African trypanosomiases knowledge are presented here. Future search directions are finally proposed, with respect to mouse model opportunities and limitations.

Host genetics in African trypanosomiasis

In Africa, the protozoan parasite of the genus Trypanosoma causes animal (AAT) and human African trypanosomiasis (HAT). These diseases are responsible for considerable mortality and economic losses, and until now the drugs commonly used have often been very toxic and expensive, with no vaccine available. A range of clinical presentations, from chronic to acute symptoms, is observed in both AAT and HAT. Host, parasite, and environmental factors are likely to be involved in this clinical variability. In AAT, some West African cattle (N'Dama, Bos taurus) have the ability to better control the disease development (and therefore to remain productive) than other taurine breeds (Zebu, Bos indicus). This phenomenon is called trypanotolerance and seems to have major genetic components. In humans, tolerance/resistance to the disease is suspected, however, this needs confirmation. This review focuses on recent advances made in the field of host genetics in African trypanosomiasis in animals (mouse and bovine) and humans. The perspectives for the development of new control strategies and their applications as well as a better understanding of the physiopathology of the disease are discussed.

CpG oligodeoxynucleotide treatment enhances innate resistance and acquired immunity to African trypanosomes

Relative resistance to African trypanosomiasis is based on the development of a type I cytokine response, which is partially dependent on innate immune responses generated through MyD88 and Toll-like receptor 9 (TLR9). Therefore, we asked whether enhancement of the immune response by artificial stimulation with CpG oligodeoxynucleotide (ODN), a TLR9 agonist, would result in enhanced protection against trypanosomes. In susceptible BALB/c mice, relative resistance to infection was significantly enhanced by CpG ODN treatment and was associated with decreased parasite burden, increased cytokine production, and elevated parasite-specific B- and T-cell responses. In relatively resistant C57BL/6 mice, survival was not enhanced but early parasitemia levels were reduced 100-fold and the majority of the parasites were nondividing, short stumpy (SS) forms. CpG ODN treatment of lymphocyte-deficient C57BL/6-scid and BALB/cByJ-scid mice also enhanced survival and reduced parasitemia, indicating that innate resistance to trypanosome infection can be enhanced. In C57BL/6-scid and BALB/cByJ-scid mice, the parasites were also predominantly SS forms during the outgrowth of parasitemia. However, the effect of CpG ODN treatment on parasite morphology was not as marked in gamma interferon (IFN-gamma)-knockout mice, suggesting that downstream effects of IFN-gamma production may play a discrete role in parasite cell differentiation. Overall, these studies provide the first evidence that enhancement of resistance to African trypanosomes can be induced in susceptible animals in a TLR9-dependent manner and that CpG ODN treatment may influence the developmental life cycle of the parasites.

Regulation of innate and acquired immunity in African trypanosomiasis

African trypanosomes are well known for their ability to avoid immune elimination by switching the immunodominant variant surface glycoprotein (VSG) coat during infection. However, antigenic variation is only one of several means by which trypanosomes manipulate the immune system of their hosts. In this article, the role of parasite factors such as GPI anchor residues of the shed VSG molecule and the release of CpG DNA, in addition to host factors such as IFN-gamma, in regulating key aspects of innate and acquired immunity during infection is examined. The biological relevance of these immunoregulatory events is discussed in the context of host and parasite survival.

Genetics and trypanotolerance

Genetic resistance to disease and its use in domestic livestock usually ranks last, if at all, amongst preferred disease control measures - usually preceded by measures such as chemotherapy, vector control and vaccination. Thus, interest in genetic resistance is often a reflection of dissatisfaction with other control strategies, and the current emphasis on trypanotolerant cattle in Africa is just such a case. Eighty years of tsetse fly eradication programmes have had little impact on tsetse distribution, although recent research with odour baited targets impregnated with insecticide brings hope for the future. The search for a vaccine has proved more arduous than anticipated and the number of drugs available for therapy and prophylaxis is limited. In the search for alternative solutions to the problem of African trypanosomiasis, attention has recently focused on genetic resistance - a subject normally covered by immunologists or veterinarians(3-7). In this article, Rosemary Dolan discusses the concept from the geneticist's viewpoint.

Innate resistance to experimental African trypanosomiasis: differences in cytokine (TNF-alpha, IL-6, IL-10 and IL-12) production by bone marrow-derived macrophages from resistant and susceptible mice

Resistance to African trypanosomiasis is under multigenic control. BALB/c mice are highly susceptible while C57Bl/6 mice are relatively resistant. Macrophages eliminate opsonized trypanosomes from the bloodstream and are involved in immunosuppression. We therefore investigated the production of a number of cytokines (IL-10, IL-6, TNF-alpha and IL-12) by bone marrow-derived macrophages (BMDM) from C57Bl/6 and BALB/c mice following challenge with either Trypanosoma congolense or Trypanosoma brucei. BMDM from C57Bl/6 mice, upon challenge with whole cell extracts (WCE) of T. congolense or T. brucei, produced significantly more TNF-alpha and IL-12 than those from BALB/c mice. The production of these cytokines was significantly enhanced by pretreatment of the cells with IFN-gamma. BMDM from BALB/c mice, however, produced significantly more IL-6 and IL-10 than those from C57Bl/6 mice. In contrast to LPS stimulation, simultaneous treatment of cells with WCE and IFN-gamma enhanced IL-10 synthesis by BMDM from BALB/c mice. These results indicate that cytokine genes are differentially regulated in macrophages from trypanosome-susceptible and -resistant mice and are consistent with our previous findings wherein retrovirus-immortalized macrophage cell lines from BALB/c and C57Bl/6 mice produce differential amounts of cytokines after phagocytosis of trypanosomes.

Innate resistance to Trypanosoma congolense infections: differential production of nitric oxide by macrophages from susceptible BALB/c and resistant C57Bl/6 mice

BALB/c and C57B1/6 mice differ in resistance to Trypanosoma congolense infections. Evidence suggests that macrophages play a central role in the resistance to trypanosomiasis. Nitric oxide (NO) produced by macrophages in response to various stimuli or pathogens is one of the important arms of nonspecific immunity. We investigated the production of NO by the peritoneal macrophages and bone marrow-derived macrophages (BMDM) from trypanosome-resistant C57B1/6 and -susceptible BALB/c mice following stimulation with T. congolense whole cell extract (WCE) or following phagocytosis of T. congolense mediated by anti-variant surface glycoprotein (VSG) antibodies of IgM or IgG2a isotype. C57B1/6 peritoneal macrophages as well as BMDM produced significantly more NO than similar BALB/c macrophages in response to T. congolense lysate and IFN-gamma. In both BALB/c and C57B1/6 BMDM cultures, phagocytosis of T. congolense mediated by anti-VSG antibodies of IgG2a isotype in the presence of IFNgamma induced two- to ninefold more NO than phagocytosis mediated by IgM antibodies and C57B1/6 BMDM produced significantly higher amounts of NO than BALB/c BMDM under these conditions. NO produced by BMDM was found to exert trypanostatic effect on T. congolense in vitro, but was not found to influence the in vivo infectivity of these treated parasites under the conditions used in this study.

Innate resistance to experimental Trypanosoma congolense infection: differences in IL-10 synthesis by macrophage cell lines from resistant and susceptible inbred mice

BALB/c and C57Bl/6 mice differ in resistance to T. congolense infections. We investigated the production of various cytokines (IL-10, IL-6, TNF-alpha and TGF-beta) by macrophages from these mice. Macrophage cell lines (BALB.BM cells) of BALB/c mice but not (ANA-I cells)of C57BL/6 mice constitutively produced IL-10. Challenge of these cells with trypanosomes induced the production of 50-100 times more IL-10 in BALB.BM cells than in ANA-1 cells. Pre-incubation of the cell lines with IFN-gamma. prior to the trypanosome challenge, further upregulated this IL-10 production in BALB.BM but not in ANA-1 cells. Primary cultures of bone marrow-derived macrophages (BMDM) from BALB/c mice also produced more IL-10 following challenge with IFN-gamma and opsonized trypanosomes than did the C57Bl/6 BMDM. Similarly after challenge with trypanosomes, BALB.BM and BALB/c BMDM produced significantly more IL-6 than did the analogous cells from the C57Bl/6 mice following such challenges. Higher steady state levels of TNF-alpha mRNA accumulated in ANA-1 cells than in BALB.BM cells following challenge with IFN-gamma and opsonized trypanosomes. Findings of this study for the first time indicate a differential regulation of cytokines (IL-10, IL-6 and TNF-alpha) in macrophages of mice that significantly differ in their susceptibility to infections with T. congolense.

Resistance to the African Trypanosomes Is IFN-γ Dependent

The role of variant surface glycoprotein (VSG)-specific Th cell responses in determining resistance to the African trypanosomes was examined by comparing Th cell responses in relatively resistant and susceptible mice as well as in cytokine gene knockout mice infected with Trypanosoma brucei rhodesiense. Resistant B10.BR and C57BL/6 mice expressed Th1 cell cytokine responses to VSG stimulation during infection, while susceptible C3H mice produced weak or no Th1 cell cytokine responses. Neither resistant B10.BR and C57BL/6 mice nor susceptible C3H mice made detectable Th2 cell cytokine responses to parasite Ag. To more closely examine the potential role of IFN-γ and other cytokines in host resistance, we determined the resistance phenotypes and Th cell responses of IFN-γ and IL-4 knockout mice. Infected C57BL/6-IFN-γ knockout mice were as susceptible as C57BL/6-scid mice and made an IL-2, but not an IL-4, cytokine response to VSG, while C57BL/6-IL-4 knockout mice were as resistant as the wild-type strain and exhibited both IL-2 and IFN-γ cytokine responses. Passive transfer of spleen cells from wild-type mice to IFN-γ knockout mice resulted in enhanced survival. Both wild-type and IFN-γ knockout mice controlled parasitemia with VSG-specific Ab responses, although parasitemias were higher in the IFN-γ knockout mice. Overall, this study demonstrates for the first time that relative resistance to African trypanosomes is associated with a strong Th1 cell response to parasite Ags, that IFN-γ, but not IL-4, is linked to host resistance, and that susceptible animals do not make compensatory Th2 cell responses in the absence of Th1 cell cytokine responses.

Genetic Basis of Trypanotolerance in Cattle and Mice

Under most circumstances, certain breeds of domestic ruminants show a remarkable resistance to the effects of African trypanosomiasis: they can tolerate the presence of parasites while apparently controlling levels of parasitaemia and, crucially, not showing the severe anaemia and production loss that are characteristic of infection in susceptible hosts. As discussed here by Stephen Kemp and Alan Teale, the genetic control of this phenomenon might finally be yielding to gene mapping studies. Genetic regions determining susceptibility to trypanosomiasis in mice have been identified and parallel studies are well advanced in cattle. There is growing evidence that only modest numbers of genes are involved in determining the difference between a susceptible and a resistant animal. These observations raise a new series of important questions concerning the possible exploitation of major trypanotolerance genes and the way that they might function in different genetic and physical environments.

Interleukin-4-dependent immunoglobulin G1 isotype switch in the presence of a polarized antigen-specific Th1-cell response to the trypanosome variant surface glycoprotein

This study examines B-cell immunoglobulin (Ig) class-switching events in the context of parasite antigen-specific Th-cell responses in experimental African trypanosomiasis. Inbred mice were infected with Trypanosoma brucei rhodesiense, and the coordinate stimulation of Th-cell cytokine responses and B-cell responses to the trypanosome variant surface glycoprotein (VSG) was measured. The cytokines produced by T cells in response to VSG, at both the transcript and protein levels, were gamma interferon and interleukin-2 (IL-2) but not IL-4 or IL-5. Isotype profiles of antibodies specific for VSG showed that IgG1, IgG2a, and IgG3 switch responses predominated; no VSG-specific IgE responses were detected. To determine whether cryptic IL-4 responses played a role in promoting the unexpected IgG1 switch response, IL-4 knockout mice were infected; the cytokine responses and Ig isotype profiles of IL-4 knockout mice were identical to those of the wild-type control mice except for dramatically reduced IgG1 levels in response to VSG. Thus, these results revealed an IL-4-dependent component of the VSG-driven B-cell Cmu-to-Cgamma1 switch. We speculate that an IL-4 response is mediated primarily by cells other than T lymphocytes since IL-4-secreting but parasite antigen-unresponsive, "background" cells were detected in all infected mice and since infected nude mice also displayed a detectable IgG1 switch response. Overall, our results suggest that B-cell clonal stimulation, maturation, and Ig class switching in African trypanosomiasis may be partially regulated by unusual mechanisms that do not include antigen-specific Th1 or Th2 cells.

Localization of genes controlling resistance to trypanosomiasis in mice

Tsetse fly-transmitted trypanosomes (Trypanosoma spp.) cause "sleeping sickness' in man and have a serious impact on livestock-based agriculture in large areas of Africa. Multigene control of variation in susceptibility to trypanosomiasis is known to occur in mice, where the C57BI/6 (B6) strain is relatively resistant and the A/J (A) and Balb/c (B) strains are susceptible. Such resistance is also well described among several types of west African cattle. We report here the results of genome-wide scans for genes controlling this trait in the B6 mouse using crosses with two different susceptible strains. Regions on mouse chromosomes 5 and 17 were found to be important in determining resistance in both crosses while an additional region on chromosome 1 showed evidence of involvement in only one cross. We confirmed the size of the effect due to chromosome 17 in F3 intercross populations fixed for alternative parental chromosomes. The three loci are of large effect and account for most of the genetic variation in both F2 populations. We propose that they be designated Tir1, Tir2 and Tir3.

Replication, differentiation, growth and the virulence of Trypanosoma brucei infections

This study had 2 objectives: first, to investigate how the processes of slender form replication, of differentiation from dividing slender to non-dividing stumpy forms, and of stumpy mortality, combine to determine the initial (acute-phase) growth rate of Trypanosoma brucei populations; second, to determine how acute-phase growth rates influence parasite densities during the subsequent, chronic phase of infection. During the acute phase, slender and stumpy populations both grew approximately exponentially, the latter more slowly than the former. Mathematical models showed how this difference in slender and stumpy growth rates can be explained in terms of heterogeneous replication and differentiation rates. Stumpy life-expectancy was determined for one stock and found to be age-dependent with a half-life of 48-72 h, much larger than observed population doubling times of 5-10 h. A comparison of cloned stocks showed that the highest parasite densities during the chronic phase were associated with the highest acute-phase growth rates of both the whole parasite population and of the subpopulation of slender forms. By contrast, high chronic-phase parasitaemias artificially produced following rapid syringe passage were associated with low acute-phase growth rates of slender forms. Syringe-passaging is a laboratory procedure which selects for virulent parasites, but these parasites behave differently from naturally virulent stocks.

Specificity of ethanolamine transport and its further metabolism in Trypanosoma brucei

Ethanolamine is found in trypanosomes as an integral component of the variant surface glycoprotein (VSG) and the membrane phospholipid phosphatidylethanolamine (PE). Steps in the utilization of ethanolamine could represent novel targets for the development of chemotherapeutic drugs and were therefore investigated in detail. Transport of [3H]ethanolamine was studied using structural analogs of ethanolamine. Compounds with substitutions in the amino group or of one of the methylene hydrogens of ethanolamine were the most effective inhibitors. Those analogs studied in detail with respect to their kinetic properties were all found to be competitive inhibitors of ethanolamine transport. Following uptake, ethanolamine is rapidly phosphorylated by an ethanolamine-specific kinase to form phosphoethanolamine. Other acid-soluble intermediates identified by thin layer chromatography were CDP-ethanolamine, dCDP-ethanolamine, and glycerophosphorylethanolamine. The relative amounts of these metabolites varied between slender (dividing) and stumpy (non-dividing) trypanosomes and may reflect special biosynthetic needs of the different morphological forms. Pulse-chase experiments indicated that the acid-soluble metabolites served as precursors for chloroform/methanol-soluble lipids. Radioactive lipids included PE, mono-methyl and dimethyl PE, and lysoPE. Further methylation of dimethylPE to phosphatidylcholine was not observed under the experimental conditions described. These results are consistent with the conclusion that trypanosomes are able to synthesize phospholipids via the Kennedy pathway.

Mechanism of long slender (LS) to short stumpy (SS) transformation in the African trypanosomes

The transformation of the long slender to the short stumpy stages of the African trypanosomes is an essential part of the trypanosome life cycle. Four possible mechanisms which could control this event have been investigated. It has been shown that (a) the dividing long slender to non-dividing short stumpy transition is not a programmed event in the trypanosome life cycle; nor (b) would it appear to be initiated by some form of cell to cell contact inhibition of growth. In addition, evidence is presented which would suggest that (c) the transition is not started by the depletion of a critical growth nutrient from the environment during the growth of the trypanosomes. The last possibility (d) considered is that during trypanosome growth, a growth inhibitor-short stumpy inducer accumulates in the trypanosomes' environment. Evidence is presented which shows that plasma from infected animals can inhibit the incorporation of thymidine by the trypanosomes. These data are consistent with the suggestion of an exogenous growth inhibitor accumulating during the infection.

African trypanosomes: inheritance of factors involved in resistance

The C3HeB/FeJ mouse strain has a shorter survival time and is therefore more susceptible to a Trypanosoma brucei rhodesiense infection than the B10.BR/SgSnJ strain. The work reported here demonstrated that survival time is inherited as a recessive trait, whereas the ability to produce antibody to the first variant antigen population is inherited as a dominant trait. It was therefore not possible to correlate survival time with the ability to produce antibody in the F-1 and F-2 offspring. Both characteristics appeared to be multigenic. In addition, it was not possible to link the ability of an animal to control its early parasitemia, or its change in hematocrit, with either antibody production or survival time. The work strongly suggests that the increased survival time of the B10.BR/SgSnJ mouse is due at least partially to nonspecific but unidentified factors which do not segregate with VSG-specific immune responses. These nonspecific factors could include differences in susceptibility to toxic trypanosome catabolites.

[Lymphocyte system and relative resistance of inbred mice to Trypanosoma brucei brucei]

The distribution of B-(Ig+) and T-(Thy1.2+, Lyt1+, Lyt2+) lymphocyte subsets in murine lymphoid organs was analysed by immunofluorescence (FACS) on cell suspensions throughout a primary infection of C3H/He (susceptible) and CBA/Ca (subtolerant) inbred mice with metacyclic Trypanosoma brucei brucei EATRO 1125. A direct correlation was observed between (1) the level of first-peak parasitaemia, (2) its subsequent control by the host and (3) survival time. In the course of the infection, the overall population of spleen and lymph node lymphocytes was subject to polyclonal activation whilst the proportion of differentiated B and T subsets decreased accordingly. No correlation was found between modulation of the lymphocyte system and susceptibility to trypanosomiasis.

13C-NMR analysis of alanine metabolism by isolated perfused livers from C3HeB/FeJ mice infected with African trypanosomes

1. Isolated perfused livers from mice infected with Trypanosoma brucei rhodesiense formed substantially more [3-13C]-lactate from [3-13C]-alanine than livers from uninfected mice. Quantities formed by infected livers increased as infection progressed. 2. Infected livers produced more 13C-labeled glutamate and glutamine, with label scrambled between C-2 and C-3. Scrambling also produced [2,3-13C]-aspartate, [2-13C]-alanine and [2-13C]-lactate. Delayed appearance of label in C-4 of glutamate/glutamine in infected livers reflects significant endogenous stores of unlabeled acetyl CoA. 3. Although differences do exist in catabolism of [3-13C]-alanine by perfused livers from infected and control mice, trypanosomiasis does not cause permanent breakdown or blockage of hepatic alanine metabolism.

Trypanosoma brucei rhodesiense infection in mice prevents virus-induced diabetes: possible role of interferon and immunological mechanisms

B10.BR, DBA/2, and BALB/c by J mice were infected with Trypanosoma brucei rhodesiense (Lou Tat clone 1). Subsequent infection with the D variant of encephalomyocarditis virus (EMC-D) resulted in no diabetes or encephalitis, even in the susceptible DBA/2 and BALB/c by J strains. Low levels of circulating interferon (IFN) were detected in trypanosome-infected mice at the time of EMC-D infection. All strains were severely immunosuppressed as a result of trypanosome infection, as evidenced by decreased virus-specific neutralizing antibody titers, compared to virus-infected controls. We attempted to simulate some aspects of T.b. rhodesiense infection in B10.BR mice by pretreating mice with cyclophosphamide and IFN prior to EMC-D infection. Immunosuppression by cyclophosphamide greatly enhanced the pathogenesis of EMC-D, while IFN protected against the diabetogenic effect of this virus. Our results indicate that: (i) T.b. rhodesiense infection inhibited EMC-D-induced diabetes, (ii) this inhibition was not due solely to the immunosuppression generated by the trypanosome infection, and (iii) IFN generated by the trypanosome infection could play some protective role in the inhibition of EMC-D-induced diabetes by trypanosome infection.

Trypanosoma rhodesiense: variable effects of cyclophosphamide on antibody production, survival, and parasitemia in infected mice

Mice of the CBA/CaJ strain, infected with Trypanosoma rhodesiense, were injected with a single high dose (approximately 200 mg/kg) of the immunosuppressive drug cyclophosphamide to determine if an induced, transient inability to make antibody affected survival or parasitemia. When given on the day of infection, the drug had no significant effect on survival. It delayed, but did not prevent, the appearance of specific antibodies and the clearance of the infecting trypanosome variants. When cyclophosphamide was injected 1 week after infection, survival mass significantly decreased. Antibody production to specific variant antigens and to common trypanosome antigens were terminated, but the mice were able to eliminate the infecting trypanosomes. These findings suggest that a temporary inability to make antibody to trypanosomes does not result in more rapid death when only the infecting trypanosome variant is present. However, immunosuppression may accelerate death if it occurs when there are many different types of trypanosomes present.

Natural resistance to African trypanosomiasis

Glossina infected with African trypanosomes infest 10(7) Km2 of intertropical Africa. Ten thousand cases of human sleeping sickness are officially recorded each year and 35 X 10(6) human beings are at risk. Animal trypanosomiasis impedes the use of 7 X 10(6) Km2 of land adequate for cattle raising and constitutes a major constraint to increasing protein production in Africa. Several approaches are used to combat trypanosomiasis (a) vector eradication has been successful in certain defined situations but cannot be realistically extended to the whole area at risk, (b) prophylactic drugs, which are too toxic for humans, are widely used to protect cattle. This practice has led to field resistance to many drugs used in the past (reviewed in Holmes & Scott 1982) and the appearance of resistance to the last available prophylactic drug, isometamidium, is being reported (Bourn & Scott 1978, Küpper & Wolters 1983, Pinder & Authié 1984), (c) the hope for a vaccine has been largely abandoned, in the present state of knowledge, due to the considerable extent of antigenic diversity in trypanosomes (reviewed in Doyle 1977, Turner 1982, Roelants & Pinder 1984), (d) consequently, the possible use of certain West African breeds of cattle, which appear resistant to trypanosomiasis, has been emphasized as a solution to this problem in domestic animals. The analysis of this natural resistance is the subject of the present essay.

Trypanosoma brucei: infection in murine diabetes

The course of infection due to Trypanosoma brucei infection was observed in genetically diabetic and streptozotocin-induced diabetic mice. A strain of T. brucei, TREU 667, was used which produces a chronic infection in C57BL/6(B6) mice lasting greater than 60 days. Genetic diabetic mice (+db/+db) are obese, and have elevated blood glucose levels, normal levels of insulin, and impaired cell-mediated immunity. Their littermates (m+/m+, m+/+db) are of normal weight, and are normoglycemic and immunocompetent. The infected +db/+db mice lived significantly longer than the nondiabetic littermates. In contrast to this finding, streptozotocin-induced diabetic B6 mice developed higher parasitemia and had shorter survival times than control B6 mice. Continuous treatment with insulin of these streptozotocin-induced diabetic mice led to normalization of blood glucose and a significant reduction of parasitemia. While hyperglycemia may be associated with higher parasitemia and death in streptozotozin-induced diabetes, genetic factors may play an additional role in the genetic models.

Mice varying in resistance to African trypanosomiasis respond differently to treatments with variant surface glycoprotein

A comparative analysis of responses between resistant and susceptible hosts revealed that DBA/2 mice, after treatment with variant surface coat glycoprotein (VSG) from virulent or avirulent African trypanosomes, developed splenomegaly as the result of a near-doubling of the splenic cell population, had less polyclonal activation of B cells and were protected upon challenge with homologous trypanosomes. The susceptible C3H/Anf and C3H/HeJ mice on the other hand increased their splenic cell population by only 12%, had about twice the production of unelicited antibodies and were not immunized by the VSG treatments. This indicated that (a) proliferation of spleen cells during African trypanosomiasis may reflect an attempt to generate a specific and protective immune response and is not merely the result of polyclonal activation of lymphocytes; (b) production of unelicited antibodies is not merely a "bystander reaction" to the generation of antigen-specific responses; and (c) polyclonal antibody production in response to VSG is not linked to the LPS gene. Nonspecific immunosuppression as measured in mitogen assays was not elicited by VSG in either resistant or susceptible mice, indicating that polyclonal lymphocyte activation and nonspecific immunosuppression are unlinked phenomena. Mice injected with VSG from either virulent or avirulent isolates at levels normally encountered by hosts during severe, acute infection developed the same degree of splenomegaly and production of unelicited (polyclonal) antibodies. Therefore, any differences in polyclonal activation of lymphocytes measured between mice with acute vs. chronic African trypanosomiasis can be attributed to quantitative and not qualitative differences in VSG.

The role of humoral immune responses in determining susceptibility of A/J and C57BL/6 mice to infection with Trypanosoma congolense

The relative resistance of C57BL/6 mice to infection with Trypanosoma congolense as compared to A/J mice was found to be independent of the infective dose of trypanosomes and required an intact immune system, as sublethal levels of gamma irradiation abolished the differences in susceptibility between the two strains. C57BL/6 mice produced earlier and quantitatively superior antibody responses both to the variable surface glycoprotein and to common membrane antigens on the trypanosome than A/J mice. No difference was observed in the class of antibody produced. In parallel with the specific response, C57BL/6 mice also generated higher levels of antibody to an unrelated antigen (TNP) and developed higher levels of total serum IgM. However, despite the low levels of both specific antibody and antibody to TNP in A/J mice, these animals developed massive increases in total serum IgG2a. The role of this selective activation of IgG2a producing cells in the susceptibility of the A/J mice was unclear. Although susceptibility was closely correlated with specific antibody responses during infection, the two strains of mice showed a similar capacity to respond to fixed doses of irradiated trypanosomes. This indicates that an inherent difference in immune responsiveness to the trypanosomal antigens is not the major factor determining susceptibility. Moreover, the finding that a proportion of A/J mice which received infective and irradiated trypanosomes simultaneously showed depressed antibody responses to the trypanosome, suggests that active infection of A/J mice with T. congolense impairs their ability to initiate an appropriate immune response to the trypanosome.

Lymphocyte function in experimental African trypanosomiasis. VIII. Loss of suppressor T cell function in lymph nodes

The immunosuppression that occurs in mice experimentally infected with African trypanosomiasis has been examined further. In the present study we have examined lymph node cells from Trypanosoma rhodesiense-infected C57Bl/6J mice for the ability to produce mitogen induced antigen-nonspecific suppressor T cells (Ts). Inguinal, mesenteric, and brachial lymph node cells were harvested from uninfected control mice and from mice at different periods of infection. These cells were cultured with or without concanavalin A (Con A) for 48 hr to induce Ts activity. After stimulation, the control and infected lymph node cells were passed over Sephadex G-10 columns to remove suppressor macrophages that arise during the infection from Con A-induced Ts. The column passed cells were then added to normal mouse responder spleen cells in a primary in vitro antibody response culture system with sheep erythrocytes (SRBC) as antigen. The resultant plaque-forming cell responses to SRBC indicated that Ts function was not induced in infected lymph node cell populations. However, early in the infection, a stimulatory signal was provided by both the untreated and Con A-treated infected lymph node cells, which was lost in the terminal stage. Determinations of T cell subpopulations revealed that the infected Lyt 2.2-bearing subpopulation was not significantly altered from normal controls. We conclude that T. rhodesense infected mice fail to mount normal lymph node cell antigen nonspecific Ts responses and that this loss of activity may be due to an intrinsic dysfunction in the suppressor T cell population.

Lymphocyte function in experimental African trypanosomiasis. VII. Loss of antigen-nonspecific suppressor-T-cell activity

The extent of immunosuppression occurring in mice infected with the pathogenic African trypanosomes was studied. Spleen cells from Trypanosoma rhodesiense-infected C57BL/6J mice were tested for antigen-nonspecific suppressor-T-cell (Ts) activity after concanavalin A (Con A) treatment in vitro. After exposure to Con A, control and infected mouse spleen cells were added to responder spleen cell cultures stimulated with sheep erythrocytes (SRBC). Assays for the resultant plaque-forming cell responses to SRBC revealed that antigen-nonspecific Ts activity was lost during the first week of infection. Changes in infected mouse T-cell subpopulations, including a terminal loss of Lyt 2.2+ cells, accompanied but did not precede the demonstrable loss of Ts function. Splenic suppressor macrophages which arise during infections with T. rhodesiense also did not seem to be associated with the loss of antigen-nonspecific Ts activity. It is concluded that the generalized immunosuppression associated with experimental African trypanosomiasis extends to the mitogen-induced Ts population.

Trypanosoma musculi with Trichinella spiralis or Heligmosomoides polygyrus: concomitant infections in the mouse

Inbred mice infected with Trypanosoma musculi displayed wide variations in peak blood parasitemia. The most susceptible mice were C3H and A strain, while Balb/c, C57B1/6, and the related congenic B10 strains were the most resistant. The effect of an intestinal infection with either Trichinella spiralis or Heligmosomoides polygyrus on proliferation of T. musculi was investigated. T. spiralis infections given at the same time or up to 45 days before a T. musculi infection always caused an increase in blood parasitemia in C3H mice. Maximum increases were observed when T. spiralis infections preceded T. musculi by 5-10 days. In all mouse strains examined, dual infections increased maximum parasitemia by two- to four-fold, regardless of the degree of resistance of that mouse strain to either T. musculi or T. spiralis. This suggested that the immunological "cost" of a T. spiralis infection was the same for strains that were strong or weak responders to a primary infection with T. spiralis. In contrast, infection with H. polygyrus did not promote T. musculi parasitemia over the level of a single infection. The increase in blood parasitemia in T. spiralis-infected mice was largely due to the intestinal adult worm, but migratory larvae and mature muscle larvae also stimulated increased parasitemias. The increase in parasitemia was proportionate to the dose of T. spiralis, and the sex of the host did not affect the blood trypanosome level.

Genetics of resistance to the African trypanosomes. IV. Resistance of radiation chimeras to Trypanosoma rhodesiense infection

The cellular bases of resistance to the African trypanosomes were examined in inbred mice. As part of these studies, reciprocal bone marrow cell transplants were performed between H-2 compatible mice which differ in relative resistance to Trypanosoma brucei rhodesiense infection. Survival times, parasitemias and IgM antibody responses to the surface antigen of the infecting variant type were measured in these semiallogeneic bone marrow chimeras. Relatively resistant C57BL/10 mice, intermediate A.By mice, and least resistant C3H.SW mice that were reconstituted after lethal irradiation with syngeneic bone marrow cells displayed resistance and immunity characteristic of the homologous donor strain. When C57BL/10 mice were reconstituted with C3H.SW mouse bone marrow cells they retained the ability to produce antibodies to trypanosome surface antigen but the antibody titers were significantly reduced. Control of parasitemia and mean survival time were reduced in these chimeras, but differed significantly from C3H.SW mice. A.By mice that received cells from C57BL/10 donors exhibited antibody responses and survival times similar to the C57BL/10 mice. Survival times of A.By mice given syngeneic cells or C3H.SW cells were the same, but the antibody responses of A.By mice given C3H.SW cells were lower than those of A.By mice given syngeneic cells. C3H.SW mice reconstituted with C57BL/10 bone marrow cells were capable of making antibodies and controlling parasitemia, in marked contrast to the absence of such responses in C3H.SW mice reconstituted with syngeneic cells. Survival times, however, were indistinguishable from those of C3H.SW mice given syngeneic cells. Thus, resistance to T. b. rhodesiense was shown for the first time to depend on donor bone marrow derived cells as well as upon radiation-resistant cells/factors associated with host genetic background. Also, parasite-specific IgM antibody responses seem to be regulated by a mechanism which does not depend on bone marrow derived cells alone, and the presence of such immune responses is not linked to survival time.

Genetic control of susceptibility of mice to infection with E. histolytica

Genetic susceptibility to Entamoeba histolytica infection in nine inbred strains and one outbred strain of mice was studied. The number of E. histolytica trophozoites in the ceca of the mice was examined 5 days after intracecal inoculation of axenic amoebae. C3H/HeCr, BALB/c, NZB/BIN, B10.A, DBA/2 and C57BL/6 were susceptible whereas A/J, CE, DBA/1 and CD-1 mouse strains were relatively resistant. Examination of F1 hybrid animals derived from susceptible B10.A and resistant A/J strains of mice showed that susceptibility was dominant over resistance. Segregation analysis of backcross and F2 progeny derived from the same progenitor strains is compatible with the hypothesis that susceptibility to E. histolytica infection in mice is controlled by a single, dominant gene which has been designated Enh. No association was found between the H-2 haplotype and the trait of susceptibility to amoebiasis, indicating that the major histocompatibility complex does not play a major role in regulating the early phase of the response to infection with E. histolytica.

Trypanosoma congolense: genetic control of resistance to infection in mice

Primary isolates of Trypanosoma congolense show a range of virulence in NMRI mice. Stabiliates derived from an isolate (Dinderesso/80/CRTA/3) which showed moderate virulence in most NMRI mice (moderate parasitemia and survival) were used in inbred mice. C57Bl/6 were resistant with low parasitemia and survival. Parasitemias were higher in males than females. BALB/c were the most sensitive of the strains tested and died with fulminating parasitemia. Inheritance of resistance, defined as low parasitemia, was studied using these two strains. Male F1 showed high parasitemia; the backcrosses of F1 to the resistant parent had a ratio of one susceptible to one resistant product; the product of F1 to susceptible parent were all susceptible; and the F2 crosses showed a ratio of three susceptible to one resistant product. The results obtained with female F1, backcrosses, and F2 mice showed similar segregation to that found using males, but the range of parasitemia was always 1-2 log10 lower, except for the F1 backcrossed to BALB/c, where female and male parasitemia were undistinguishable. The segregation ratios were identical whether resistant females were crossed with sensitive males or vice-versa. The results obtained are compatible with resistance being a recessive trait controlled by a single autosomal gene (or gene cluster). In addition, sex-associated factors appear to confer higher resistance in females.

Trypanosoma rhodesiense: analysis of the genetic control of resistance among mice

Inbred mouse strains differ in their resistance to infection with the human pathogen Trypanosoma rhodesiense. Of the strains tested, C57BL/6 (B6) mice were the most resistant, and BALB/c (C) mice were among the most susceptible. The genetic basis underlying the different susceptibility of these two strains was analyzed. (CXB6)F1 progeny of either sex were more resistant than the BALB/c parent. Also, the backcross of F1 mice to the susceptible male or female BALB/c parent resulted in 52.0% susceptible (i.e., death on or before day 24) progeny, as compared with only 0.64% susceptible F1 progeny. The data suggested that resistance was the dominant phenotype and that the resistant allele was carried by the B6 parent. The presence of another locus regulating resistance to death was suggested by the facts that only a small percentage of F2 mice were susceptible and that a number of F1 and F2 mice were more resistant than their B6 parent. The locus responsible for these phenomena was presumably hypostatic in nature and carried by BALB/c mice, and its effects were only evident in the presence of other resistance genes. In addition, the observation that many of the susceptible individuals among F2 and backcross mice were more resistant than the BALB/c mice suggested that other minor genes also modulated the response of mice to infection. A set of CXB recombinant inbred mice was tested as well, and the individual strains within this set could also be placed into four groups: susceptible, intermediate, resistant, or hyperresistant. These findings are compatible with the multigenic model suggested by the Mendelian analyses.

Trypanosoma rhodesiense infection in mice: sex dependence of resistance

There is large variation in the survival of inbred mouse strains infected with Trypanosoma rhodesiense (EATRO 1886). Of those strains that survived for at least 22 days postinfection, female mice were markedly more resistant than male mice. The longer a strain survived, the greater was the difference in survival between male and female mice. Parasite counts were higher in male mice than in females, suggesting that the decreased resistance of males was due to their relative inability to control parasite growth. To determine the possible role of an X-linked resistance gene, resistant (C57BL/6) and susceptible (BALB/c) mice were mated, and their F1 progeny were infected with T. rhodesiense. There was no difference in the resistance between reciprocal F1 male mice (C57BL/6 X BALB/c versus BALB/c X C57BL/6), indicating that an X-linked gene does not account for the difference in resistance between susceptible and resistant mice.

Trypanosomiasis in mice with naturally occurring immunodeficiencies

By using mice with naturally occurring defects, we have shown that an intact macrophage system is crucial to survival with the pathogenic protozoan Trypanosoma rhodesiense, since a defect in these cells decreased survival by half. Deficiencies in natural killer cell function or complement levels had no effect on survival. However, the capacity to survive trypanosomiasis was not related to the levels of parasitemia achieved during infection.

Effect of splenectomy on Trypanosoma lewisi infection in young rats

The effect of splenectomy on animals infected with Trypanosoma lewisi is unclear, and previous reports are inconclusive or conflictive. We splenectomized rats of different ages after they had been infected with T. lewisi. Female Sprague-Dawley rats, Hemobartonella-free, were assigned to four groups according to weight: 80, 108, 140, and 170 g. Each group had splenectomized, sham-operated, and nonoperated control subgroups, all infected with T. lewisi (0.5 ml of 10(6) parasites per ml) 90 h before surgery. Before surgery, parasite levels in host blood were similar. At 24 h after splenectomy in all groups, regardless of weight, blood parasite levels were much higher than they were in sham-operated or control animals (P less than 0.001 to P less than 0.0001; analysis of variance). Younger rats (80 and 108 g) had a higher mortality rate after splenectomy than sham-operated and control animals. Older rats (150 and 170 g) had no mortality. These results show the impact of age and the importance of the spleen on parasite-host interactions in rats infected with T. lewisi.

Antibody responses induced by immunization of inbred mice susceptible and resistant to African trypanosomes

We tested the ability of inbred mice that were either susceptible (strain A/J) or resistant (strain C57BL/6 and A/J X C57BL/6 hybrids) to African trypanosomes to produce specific antibodies to trypanosome antigens in the absence of living parasites. This experiment was carried out to eliminate the influence of trypanosome growth or metabolism on immune responsiveness. Mice were immunized with keyhole limpet hemocyanin or solubilized Trypanosoma brucei gambiense, and serum antibodies were measured in solid-phase radioimmunometric assays after primary and challenge injections. Both susceptible and resistant mice showed increases in keyhole limpet hemocyanin-specific or trypanosome-specific immunoglobulin M and immunoglobulin G after immunization. When immunized with trypanosome antigens, resistant mice made qualitatively and quantitatively superior specific immunoglobulin M responses, particularly to the trypanosome major variable surface glycoprotein. Susceptible A/J mice produced good specific antibody responses, although these were predominantly of the immunoglobulin G isotypes. These results show that A/J and C57BL/6 mice respond differentially in terms of immunoglobulin isotype and repertoire in response to injected antigens. The possibility that this differential antibody response influences susceptibility to African trypanosomes is discussed.

Cell-mediated killing of protozoa

Cell-mediated immunity represents an important host defence mechanism against protozoal infections. The effector cells directly involved are neutrophils, macrophages and, ultimately, activated macrophages. Within this simple scheme there are, however, considerable variations in activity. Effector cells from different animal species, and even from different strains of the same species, may be more or less effective in controlling a certain protozoal infection. Different protozoa differ in their susceptibility to cell-mediated killing according to genus, species, strain and morphological form. The most susceptible morphological form is that which occurs in the insect vector, and which has not yet adapted to protect itself from the vertebrate host. Epimastigotes of Trypanosoma and promastigotes of Leishmania are readily killed by phagocytic cells, while the corresponding trypomastigote and amastigote forms are considerably more resistant. Protozoa which live in macrophages, such as amastigotes of Leishmania, endozoites (tachyzoites) of Toxoplasma and amastigotes of reticulotropic strains of T. cruzi, have developed a remarkable resistance to the microbicidal activity of the host cell. Conversely, amastigotes of myotropic strains of T. cruzi, which live in muscle cells, have not developed this resistance to cell-mediated killing by macrophages. Readily accessible protozoa, such as T. brucei trypomastigotes and Plasmodium merozoites in the bloodstream, while they lack the marked resistance developed by reticulotropic protozoa, have a partial protection since they are attacked by phagocytic cells only when specific antibody is present. Granulocyte-mediated killing can be largely attributed to neutrophils. Eosinophils appear to play only a minor role and compete ineffectually when neutrophils are also present. The only group of protozoal species which may be significantly controlled by eosinophils are the stercorarian species of Trypanosoma. In vitro experiments show that antibody-coated trypomastigotes of T. cruzi can be killed by eosinophils, although there is little evidence that this occurs in vivo. Interestingly, this is the only species that has been reported to be susceptible to the major basic protein of eosinophils, a toxic component of the lysosomal granules which is very active against helminths. Neutrophils are not very active against endozoites of Toxoplasma gondii, Trypanosoma, trypomastigotes of salivarian Trypanosoma, free merozoites of Plasmodium, and promastigotes and amastigotes of Leishmania.(ABSTRACT TRUNCATED AT 400 WORDS)

Immune responses to Candida albicans in genetically distinct mice

Mice from six genetically distinct strains were examined for their immune responses to Candida albicans in in vitro and in vivo assays, and naive mice and mice immunized with the fungus were challenged intravenously with three different doses of C. albicans to determine differences in susceptibility. Naive mice from the six groups showed substantial differences in resistance to challenge based on mortalities and quantitative cultures of kidneys, with mice from strains C57BL/6J and BALB/cByJ showing the most resistance; mice from strains A/J, C3H/HeJ, and CBA/J showing moderate susceptibility; and mice from strain DBA/2J showing the highest degree of susceptibility to challenge. Unimmunized mice from strains C57BL/6J and BALB/cByJ did not produce detectable levels of Candida-specific antibody by the end of the 28-day observation period when challenged intravenously, but the other strains did. Immunized mice showed a degree of protection to challenge, with all groups except mice from strain BALB/cByJ showing a reduction of two to three log units in the level of colonization in their kidneys and all strains producing significant levels of antibody. Additionally, the immunized mice of all strains developed substantial levels of delayed-type hypersensitivity and demonstrated nearly identical lymphocyte proliferative responses to Candida antigens. The results indicate that resistance to systemic candidiasis is dependent upon a combination of innate factors, predominately an intact complement system, and the acquisition of an immune response, most likely of a cell-mediated type. Additionally, the findings suggest that genetic control of acquired resistance to C. albicans may not be associated with the H-2 complex.