Graham. 10.1073/pnas.0707221105 |
Fig. 5. Evidence for host-dependent effects of coinfection on cytokines that control helminths (e.g., IL-4) or kill microparasites (e.g., IFN-g). Here, polyclonal (as opposed to antigen-specific) cytokine data suggest that mutual inhibition between these two arms of the immune system during coinfection occurs more strongly in C57BL/6 mice. Overall, IL-4 effect size was predictive of IFN-g effect size (point estimate ± variance shown) for the 13 coinfection studies that measured both cytokines. This pattern may be explained by differences between host genotypes: compared with C57BL/6 mice (filled symbols), there were near-significant trends for BALB/c mice (open symbols) to alter both IL-4 (P = 0.077) and IFN-g (P = 0.055) production less dramatically when coinfected. However, neither host genotype nor polyclonal cytokines predicted microparasite-specific IFN-g.
Table 1. Experiments included only in the metaanalysis of microparasite density according to the bottom-up factor of RBC limitation
Helminth | Microparasite | Host | SI Ref. | RBC limitation? |
Echinostoma caproni | Plasmodium yoelii | BALB/c | 17 | N * |
Echinostoma revolutum | Babesia microti | Albino | 20 | N |
Echinostoma revolutum | Plasmodium yoelii | Albino | 20 | N |
Fasciola hepatica | Babesia microti | Albino | 16 | Y * |
Heligmosomoides polygyrus | Babesia microti | B10G | 25 | Y |
Heligmosomoides polygyrus | Babesia microti | NIH | 26 | Y |
Heligmosomoides polygyrus | Influenza virus | Swiss albino | 27 | N |
Heligmosomoides polygyrus | Influenza virus | Swiss albino | 28 | N |
Heligmosomoides polygyrus | Trypanosoma congolense | TO | 29 | N |
Heligmosomoides polygyrus | Trypanosoma musculi | C3H | 30 | N |
Schistosoma mansoni | Babesia microti | Albino | 14 | Y * |
Schistosoma mansoni | Babesia microti | Albino | 20 | Y |
Schistosoma mansoni | Leishmania major | C57BL/6 | 31 | N |
Schistosoma mansoni | Listeria monocytogenes | CF1 | 32 | N |
Schistosoma mansoni | Murine hepatitis virus | Swiss albino | 33 | N |
Schistosoma mansoni | Murine leukaemia virus | C57BL/6 | 34 | N |
Schistosoma mansoni | Plasmodium berghei | Swiss albino | 18 | Y |
Schistosoma mansoni † | Plasmodium chabaudi | C57BL/6 | 35 | Y |
Schistosoma mansoni | Plasmodium chabaudi | CBA/ca | 11 | Y |
Schistosoma mansoni | Plasmodium yoelii | Swiss TO | 19 | Y * |
Schistosoma mansoni | Plasmodium yoelii | CBA/ca | 11 | Y |
Schistosoma mansoni | Plasmodium yoelii | Albino | 20 | Y |
Schistosoma mansoni | Salmonella enteriditis | CF1 | 32 | N |
Schistosoma mansoni | Salmonella typhi | Albino | 36 | N |
Schistosoma mansoni | Toxoplasma gondii | C57BL/6 | 37 | N |
Schistosoma mansoni | Trypanosoma cruzi | Albino | 38 | N |
Taenia crassiceps | Babesia microti | CF1 | 39 | N |
Trichinella spiralis | Eimeria vermiformis | NIH | 40 | N |
Trichinella spiralis | Giardia lamblia | C57BL/6 | 41 | N |
Trichinella spiralis | Giardia muris | Swiss albino | 42 | N |
Trichinella spiralis | Japanese B encephalitis virus | Swiss albino | 43 | N |
Trichinella spiralis | Leishmania infantum | BALB/c | 44 | N |
Trichinella spiralis | Listeria monocytogenes | Swiss albino | 45 | N |
Trichinella spiralis | Plasmodium berghei | CD1 | 15 | Y * |
Trichinella spiralis | Toxoplasma gondii | NMRI | 46 | N |
Trichinella spiralis | Trypanosoma musculi | C3H | 30 | N |
Potential for RBC limitation was identified as described in Methods. Direct experimental evidence for or against potential RBC limitation was available from coinfection studies for six of the eight helminth genera included in this metaanalysis (see *). Where data on multiple pairwise parasite combinations were presented in one article, data on all combinations were included unless they represented multiple species within a genus that was otherwise absent from the dataset. In that case, the most commonly studied (and thus most comparable with other studies) was included. Data from multiple mouse strains were only included if the authors also presented cytokine data (see † in both Table 1 and Table 2). Y, yes; N, no.
*Indicates that experimental evidence in direct support of the RBC limitation categorization is presented in that study. Categorizations for genera not covered by those citations were based upon published data on single-species infections, as described in Methods.
†
In C57BL/6 mice in this study, only parasitology data were available. A/J mice from the same study are included in the immunoparasitology analysis (Table 2)Table 2. Experiments included in the bottom-up analysis as well as the top-down metaanalysis of microparasite density using immunological predictors (the cytokines IFN-γ and IL-4, both polyclonal and antigen-specific)
Helminth | Microparasite | Host | SI Ref. | RBC limitation? |
Fasciola hepatica | Bordetella pertussis | BALB/c | 47 | N |
Heligmosomoides polygyrus | Citrobacter rodentium | BALB/c | 6 | N |
Heligmosomoides polygyrus | Helicobacter felis | C57BL/6 | 8 | N |
Heligmosomoides polygyrus | Plasmodium chabaudi | C57BL/6 | 10 | Y |
Litomosoides sigmodontis | Leishmania major | C57BL/6 | 48 | N |
Litomosoides sigmodontis | Plasmodium chabaudi | BALB/c | 49 | N * |
Nippostrongylus brasiliensis | Chlamydophila abortus | C57BL/6 | 50 | N |
Nippostrongylus brasiliensis | Mycobacterium bovis | C57BL/6 | 51 | N |
Nippostrongylus brasiliensis | Plasmodium chabaudi | BALB/c | † | Y * |
Nippostrongylus brasiliensis | Toxoplasma gondii | C57BL/6 | 52 | N |
Schistosoma mansoni | Leishmania donovani | C57BL/6 | 9 | N |
Schistosoma mansoni | Lymphocytic choriomeningitis virus | C57BL/6 | 7 | N |
Schistosoma mansoni | Mycobacterium bovis | BALB/c | 53 | N |
Schistosoma mansoni | Plasmodium chabaudi | A/J | 35 | Y |
Schistosoma mansoni | Plasmodium chabaudi | C57BL/6 | 54 | Y |
Schistosoma mansoni | Vaccinia virus | BALB/c | 55 | N |
Taenia crassiceps | Trypanosoma cruzi | BALB/c | 56 | N |
Trichinella spiralis | Influenza virus | NIH | 57 | N |
These experiments were also included in the metaanalysis of the resource-based predictor (RBC limitation). Direct experimental evidence for or against potential RBC limitation was available from coinfection studies for six of the eight helminth genera included in this metaanalysis (see * in both Table 1 and Table 2). Y, yes; N, no.
*Indicates that experimental evidence in direct support of the RBC limitation categorization is presented in that study. Categorizations for genera not covered by those citations were based upon published data on single-species infections, as described in Methods.
†
Hoeve MA, Mylonas KJL, Grocock KJ, Mahajan SM, Allen JE, Graham AL, unpublished data.SI Results and Discussion
Role of Factors Other Than RBC Limitation and Antigen-Specific IFN-g in Determining Microparasite Density.
Analyses of host sex, parasite taxa, infectious dose, and interval between infections revealed no significant effects. Mouse strain, whether based on outbred versus inbred or on comparisons among specific genetic backgrounds (see SI Tables 1 and 2), was not a significant predictor of microparasite density effect size, nor of the effect of IFN-g on microparasites. Host genotype may, however, be predictive of systemic cross-regulation between IFN-g and IL-4-mediated [i.e., microparasite-killing versus helminth-controlling (1)] immune responses, which is suggested to be a key mechanism of within-host interaction during coinfection (2-5). This analysis was only possible for the 12 studies that measured both cytokines and presented variance and for C57BL/6 versus BALB/c mice, the two host genotypes for which multiple studies were published. A negative relationship between polyclonal (nonspecific) IL-4 and IFN-g was apparent (SI Fig. 5; slope = −0.28; Q1,11 = 4.67; P = 0.031), and host strain appeared to explain the pattern: there were trends for C57BL/6 hosts to exhibit more substantial increases in IL-4 and decreases in IFN-g due to coinfection compared with BALB/c mice (Q1,11 = 3.13 and P = 0.077 for IL-4; Q1,11 = 3.68 and P = 0.055 for IFN-g). However, effect sizes for polyclonal IFN-g and IL-4 and for microparasite-specific IL-4 did not predict microparasite density, nor did mouse strain account for the pattern shown in Fig. 3. Because host genotype did not affect microparasite-specific IFN-g nor its relationship with microparasite density effect size, host-dependent cross-regulation by IL-4 does not account for helminth-induced changes in microparasite density. Indeed, this study has not identified the general helminth-induced mechanism that alters microparasite-specific IFN-g (because neither antigen-specific nor polyclonal IL-4 sufficed). There may be too many intermediate steps between helminth-induced immune responses and their bystander effects on microparasite antigens to be detected in such an analysis. Alternatively, other cytokines, such as IL-10, may more directly reduce IFN-g during coinfection (6-10); too few coinfection studies have measured IL-10 to permit metaanalysis now.Relative Importance of RBC Limitation and Antigen-Specific IFN-g.
It would be ideal formally to assess the relative importance of RBC limitation and microparasite-specific IFN-g in predicting peak microparasite density during coinfection. Two-way analysis suggested that IFN-g was the more potent of the two predictor variables: e.g., RBC limitation was not predictive in the cytokine subset of data (F(1,12) = 0.20; P = 0.6643; R2 = 0.02), whereas the direction of the IFN-g effect was consistent and the relationship near-significant in both the RBC limited (F(1,2) = 6.68; P = 0.1227; R2 = 0.77) and nonlimited (F(1,8) = 4.30; P = 0.0717; R2 = 0.35) categories. However, all but four of the immunoparasitology studies used pairs of parasite species that did not pose RBC limitation (see SI Table 2), and the power of the test was weak. A larger sample size of studies, plus quantitative measures of RBC availability, would be necessary for a robust test of the relative importance of bottom-up versus top-down microparasite control.RBC Limitation Versus Reticulocyte Limitation Among Apicomplexan Taxa.
Even if the pattern described in Fig. 2 is strongly influenced by Apicomplexan sensitivity to RBC density, the results suggest testable hypotheses about bottom-up control of microparasites during coinfection. One such hypothesis concerns subpopulations of RBCs preferred by different Apicomplexan taxa. If helminth species that cause anemia limit RBC availability for incoming Apicomplexa, then the effect ought to be strongest for species that are averse to infection of the very young RBCs, or reticulocytes (11), that the host produces in homeostatic response to anemia. Anemia by definition implies decreased RBC density but can actually lead to increased density of immature RBCs (reticulocytes) as the host releases new cells from the bone marrow (12). Apicomplexan species such as Plasmodium berghei or P. yoelii prefer reticulocytes, whereas species such as P. chabaudi or Babesia microti prefer mature RBCs (13). Such niche differences have been previously posited to explain increases in P. yoelii and decreases in P. chabaudi parasitemias during chronic S. mansoni coinfection (11). The studies compiled here suggest that this bottom-up control mechanism may operate more broadly to determine the extent of Apicomplexan replication during helminth coinfection. The kinetics of helminth-induced changes in RBC and reticulocyte densities may be complex (11, 14, 15), such that RBC or reticulocyte-dependent microparasites could be either constrained or enabled by coinfection, depending on when they are introduced. Taking such kinetics into account, effects consistent with (11) that are revealed here include Apicomplexans in combination with fascioliasis (16), trichinosis (15), and echinostomiasis (17), as well as patent (14, 18) and prepatent (19) schistosomiasis. Exceptions are perhaps explained if release of top-down control is cancelled by imposition of RBC limitation in studies of reticulocyte-averse P. chabaudi (20). The present dataset was too small to address the reticulocyte issue quantitatively, but this is a rich vein for future study.Effects of Coinfection on Host Health.
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