Characterization of the DMAE-modified juvenile excretory-secretory protein Juv-p120 of Litomosoides sigmodontis.

Juv-p120 is an excretory-secretory 160 kDa glycoprotein of juvenile female Litomosoides sigmodontis and exhibits features typical for mucins. 50% of its molecular mass is attributed to posttranslational modifications with the unusual substituent dimethylaminoethanol (DMAE). By that Juv-p120 corresponds to the surface proteins of the microfilarial sheath, Shp3 and Shp3a. The secreted protein consists of 697 amino acids, organized in two different domains of repeat elements separated by a stretch of polar residues. The N-terminal domain shows fourteen P/S/T/F-rich repeat elements highly modified with phospho-DMAE substituted O-glycans confering a negative charge to the protein. The C-terminal domain is extremely rich in glutamine (35%) and leucine (25%) in less organized repeats and may play a role in oligomerization of Juv-p120 monomers. A protein family with a similar Q/L-rich region and conserved core promoter region was identified in Brugia malayi by homology screening and in Wuchereria bancrofti and Loa loa by database similarity search. One of the Q/L-rich proteins in each genus has an extended S/T-rich region and due to this feature is supposed to be a putative Juv-p120 ortholog. The corresponding modification of Juv-p120 and the microfilarial sheath surface antigens Shp3/3a explains the appearance of anti-sheath antibodies before the release of microfilariae. The function of Juv-p120 is unknown.

Eimeria bovis-induced modulation of the host cell proteome at the meront I stage.

The proteome of Eimeria bovis meront I-carrying host cells was analyzed by two-dimensional gel electrophoresis (2DE) at 14 days p.i. and compared to non-infected control cells. A total of 221 protein spots were modulated in their abundance in E. bovis-infected host cells and were subsequently analyzed by matrix-assisted laser desorption ionization time-of-flight mass spectometry (MALDI-TOF-MS). These analyses identified 104 proteins in total with 25 host cell proteins being up-regulated and 79 proteins being down-regulated in E. bovis-infected host cells. Moreover, 20 newly expressed proteins were identified exclusively in E. bovis-infected host cells and were most likely of parasite origin. Parasite-induced differences in protein abundance concerned distinct functional categories, with most proteins being involved in host cell metabolism, cell structure, protein fate and gene transcription. Some of the modulated molecules also indicated regulatory processes on the level of host cell stress response (HSP70, HSP90), host cell apoptosis (caspase 8) and actin elongation/depolymerization (α-actinin-1, gelsonin, tropomodulin-3, transgelin). Since merozoites I were already released shortly after cell sampling, the current data reflect the situation at the end of first merogony. This is the first proteomic approach on E. bovis-infected host cells that was undertaken to gain a rather broad insight into Eimeria-induced host cell modulation. The data processed in this investigation should provide a useful basis for more detailed analyses concerning Eimeria-host cell interactions.

Microarray-based transcriptional profiling of Eimeria bovis-infected bovine endothelial host cells.

Within its life cycle Eimeria bovis undergoes a long lasting intracellular development into large macromeronts in endothelial cells. Since little is known about the molecular basis of E. bovis-triggered host cell regulation we applied a microarray-based approach to define transcript variation in bovine endothelial cells early after sporozoite invasion (4 h post inoculation (p.i.)), during trophozoite establishment (4 days p.i.), during early parasite proliferation (8 days p.i.) and towards macromeront maturation (14 days p.i.). E. bovis infection led to significant changes in the abundance of many host cell gene transcripts. As infection progressed, the number of regulated genes increased such that 12, 45, 175 and 1184 sequences were modulated at 4 h, 4, 8 and 14 days p.i., respectively. These genes significantly interfered with several host cell functions, networks and canonical pathways, especially those involved in cellular development, cell cycle, cell death, immune response and metabolism. The correlation between stage of infection and the number of regulated genes involved in different aspects of metabolism suggest parasite-derived exploitation of host cell nutrients. The modulation of genes involved in cell cycle arrest and host cell apoptosis corresponds to morphological in vitro findings and underline the importance of these aspects for parasite survival. Nevertheless, the increasing numbers of modulated transcripts associated with immune responses also demonstrate the defensive capacity of the endothelial host cell. Overall, this work reveals a panel of novel candidate genes involved in E. bovis-triggered host cell modulation, providing a valuable tool for future work on this topic.

Development of Eimeria ninakohlyakimovae in vitro in primary and permanent cell lines.

Infections with Eimeria ninakohlyakimovae represent important coccidian diseases of goats severely affecting animal health and profitability of goat industry. For the development of suitable vaccination strategies basic research is needed for which one important prerequisite is the establishment of in vitro cultures guaranteeing the availability of parasitic material. Therefore, primary cell cultures [caprine, bovine and human umbilical vein endothelial cells (CUVEC, BUVEC, HUVEC)] as well as permanent cell lines [bovine foetal gastrointestinal cells (BFGC), bovine colonic epithelial cells (BCEC), African green monkey kidney cells (VERO)] were exposed to vital sporozoites of E. ninakohlyakimovae. The parasites invaded all different cell types used, irrespective of their origin, but further development into macromeronts and subsequent release of viable merozoites I were restricted to ruminant cells. Mature macromeronts developed in both, endothelial (CUVEC, BUVEC) and epithelial cells (BCEC). VERO cells were non-permissive for parasite development, nevertheless sporozoites survived for 21 days p.i. within an enlarged parasitophorous vacuole. Best in vitro development of E. ninakohlyakimovae macromeronts with respect to the production of viable merozoites I was observed in BCEC, followed by BUVEC. However, the largest macromeronts developed in CUVEC. Mature macromeronts were also detected in BFGC, but these cells were less effective concerning infection rates and productivity. The complete life-cycle of E. ninakohlyakimovae leading to oocyst production was not accomplished in any cell type used. In conclusion, we established suitable in vitro systems for the culture of E. ninakohlyakimovae macromeronts, e.g., for the mass production of merozoites I, for basic studies on parasite/host endothelial cell interactions or for pharmaceutical screenings.

T cell reactions of Eimeria bovis primary and challenge-infected calves.

Eimeria bovis infections commonly have clinical impact only on young animals, as homologous reinfections generally are under immunological control. So far, the nature of the immune responses delivering protection to calves has not been investigated. In this study we therefore analysed local and peripheral proliferative T cell activities of primary and challenge-infected calves and investigated the occurrence of T cell phenotypes in the peripheral blood and in mucosal gut segments isolated either by bioptic means or by necropsies.We show that lymphocytes of E. bovis-infected calves exhibit effective, transient antigen-specific proliferative responses in the course of prepatency of primary infection but fail to react after homologous reinfection suggesting early abrogation of parasite development. Whilst in primary infection an expansion of peripheral CD4+ T cells was observed, reinfection had no effect on the proportions of CD4+, CD8+ subsets or gammadeltaTCR+ T cells. In contrast, both E. bovis primary and challenge infections had an impact on local tissue T cell distribution. Primary infection was characterised by a CD4+ T cell infiltration early in prepatency in ileum and later in colon mucosa, whereas CD8+ T cells were only found accumulating in the latter gut segment. Challenge infection led to infiltration of both CD4+ and CD8+ T cells in small intestine and large intestine segments indicating protective functions of both cell types. In contrast, infiltration of ileum and colon mucosa with gammadeltaTCR+ T cells was restricted to primary infection.

Eimeria bovis meront I-carrying host cells express parasite-specific antigens on their surface membrane.

Host immune responses conducted against antigens of Eimeria bovis are key factors for the development of protective immunity against this protozoan disease. In this study we investigated the expression of E. bovis-derived antigens on the host cell surface membrane during E. bovis first merogony in vitro. Host cells carrying E. bovis-meront I stages expressed E. bovis host cell surface antigens (EbHCSAg) on their surface membrane which were recognised by hyperimmune sera of calves and by sera from rats immunized with E. bovis merozoites I, when tested by indirect immune fluorescent antibody test (IIFAT), laser scanning confocal microscopy (LSCM) and immune electron microscopy. Expression of EbHCSAg on permissive host cells was earliest detected 7 days p. i., thus coinciding with the onset of the parasite replication. Membrane-associated EbHCSAg were removed from infected host cells by proteinase K, partially by Triton X-100, Triton X-114 and Triton X-405, but not by 1 M NaCl, CHAPS or phospholipase C treatment. Antibodies, affinity-purified on paraformaldehyde/glutardialdehyde (PAGA)-fixed E. bovis meront I-infected bovine host cells bound to the surface meront I-carrying cells and to merozoites I (IIFAT, LSCM) but, in contrast to untreated sera, not to sporozoites. When tested on methanol-fixed merozoites I and sporozoites by IIFAT, affinity-purified antibodies bound to structures in the apical complex area of merozoites I, but not to sporozoites, whilst untreated sera caused diffuse labelling of internal structures of both parasite stages. Immune electron microscopy demonstrated binding of affinity-purified antibodies to micronemes and dense granules of merozoites I. Although the function of EbHCSAg is still unknown, results of this study might suggest an involvement in the development of protective immunity against E. bovis infections.

Neutrophil extracellular trap formation as innate immune reactions against the apicomplexan parasite Eimeria bovis.

Eimeria bovis infections are under immunological control and recent studies have emphasized the role of early PMN-mediated innate immune responses in infected calves. Neutrophil extracellular traps (NETs) have recently been demonstrated to act as a killing mechanism of PMN against several pathogens. In the present study, the interactions of bovine PMN with sporozoites of E. bovis were investigated in this respect in vitro. For demonstration and quantification of NET formation, extracellular DNA was stained by Sytox Orange. Fluorescence images after Sytox Orange staining as well as scanning electron microscopy (SEM) showed NET formation to occur upon contact with E. bovis sporozoites. Exposure of PMN to viable sporozoites induced stronger NET formation than to dead or homogenized parasites. NET formation was abolished by treatment with DNase and could be reduced by diphenylene iodonium, which is described as a potent inhibitor of NADPH oxidase. After sporozoite and PMN co-culture, extracellular fibres were found attached to sporozoites and seemed to trap them, strongly suggesting that NETs immobilize E. bovis sporozoites and thereby prevent them from infecting host cells. Thus, transfer of sporozoites, previously being confronted with PMN, to adequate host cells resulted in clearly reduced infection rates when compared to PMN-free controls. NET formation by PMN may therefore represent an effector mechanism in early innate immune reactions against E. bovis. This is the first report indicating Eimeria-induced NET formation.

Monocyte- and macrophage-mediated immune reactions against Eimeria bovis.

Innate immune reactions conducted by macrophages may affect the outcome of primary infections and are crucial for the transition to adaptive immune responses. In bovine coccidiosis little is known on early monocyte/macrophage-mediated responses. We therefore investigated in vivo, in vitro and ex vivo reactions of monocytes and macrophages against Eimeria bovis, one of the most pathogenic Eimeria species in cattle. Macrophages significantly infiltrate the gut mucosa of E. bovis-infected calves, particularly after challenge infection. Furthermore, peripheral monocytes of infected animals, as precursor cells of macrophages, exhibited enhanced ex vivo phagocytic and oxidative burst activities. Enhanced levels of both activities were found early after infection and towards the end of first merogony. In vitro exposure of macrophages to sporozoites led to phagocytosis of the pathogen, whilst monocytes failed to do so. Phagocytosis occurred independently of the viability of the sporozoites, indicating that active invasion by the parasites was negligible. Phagocytosis occurred in the absence of immune serum, but could clearly be enhanced by addition of immune serum, suggesting macrophage-derived antibody-dependent cytotoxicity. Furthermore, co-culture of macrophages with sporozoites and stimulation with merozoite I antigen induced distinct levels of cytokine and chemokine gene transcription. Thus, the transcription of genes encoding for IFN-gamma, IL-12, TNF-alpha, IL-6, CXCL1, CXCL8, CXCL10 and COX-2 was upregulated after sporozoite encounter. In contrast, soluble merozoite I antigen only induced the gene transcription of IL-6 and IL-12 and failed to upregulate IFN-gamma and TNF-alpha gene transcripts. In monocytes, IFN-gamma and CXCL10 were found upregulated, all other immunoregulatory molecules tested were not affected. In summary, our results strongly suggest that macrophage-mediated, innate immune reactions play an important role in the early immune response to E. bovis infections in calves.

Inhibition of host cell apoptosis by Eimeria bovis sporozoites.

Sophisticated evasion strategies of obligate intracellular parasites, in particular prevention of host cell apoptosis, are necessary to ensure successful replication. To study the ability of Eimeria bovis in this regard, in vitro experiments were performed applying bovine foetal gastrointestinal cells (BFGC), bovine umbilical vein endothelial cells (BUVEC) and African green monkey kidney cells (VERO) as host cells. BUVEC and BFGC allow maturation of sporozoites to macromeronts, in VERO cells sporozoites survive for weeks without showing further development. In highly infected BUVEC monolayers, infected cells survived until merozoite release whereas uninfected cells underwent apoptosis. Light microscopy and TUNEL assays performed 3-10 days p.i. showed that, within infected BFGC and VERO cell monolayers, uninfected cells underwent programmed cell death after application of various inducers of apoptosis, whereas infected cells survived. Incidentally, the anti-apoptotic efficacies in infected cells were independent of the drugs and the host cell type. We could not demonstrate significant differences between infected and uninfected cells after colchicin treatment in terms of translation of phosphatidylserines to the host cell surface, caspase 3 activity and cytochrome c release, probably since obtainable infection rates were too low. However, we could show by laser scanning confocal microscopy on single cell levels that the expression of the anti-apoptotic factors cellular Flice inhibitory protein (c-FLIP) and cellular inhibition of apoptosis protein 1 (c-IAP1) were enhanced in E. bovis infected cells after application of colchicin, in the latter case also in non-infected cells directly neighbouring infected ones. Our data show that E. bovis protects its host cell from apoptosis by increasing expression of c-IAP1 and c-FLIP.

Antigen-induced cytokine production in lymphocytes of Eimeria bovis primary and challenge infected calves.

Cellular immune responses against Eimeria bovis are highly specific and a key factor for the development of protection against challenge infections. In this study we investigate the cellular immune responses of E. bovis primary and challenge infected calves stimulated in vitro by E. bovis merozoite I-antigen. Primary infection was accompanied by an increase of IFN-gamma and IL-2 gene transcription in whole blood samples, peaking during prepatency (8-12 days p.i.) and declining thereafter, whereas IL-4 gene transcription was induced predominantly in patency. IL-10 mRNA was not influenced by E. bovis infection. Both CD4+ and CD8+ T cells were identified as source of IFN-gamma gene transcripts, whilst IL-2 and IL-4 gene transcription was enhanced mainly in CD4+ T cells. Increased levels of IFN-gamma transcripts and protein were also found in lymphocytes isolated from ileocaecal lymph node biopsy 8 days p.i., and in cell culture supernatants obtained from antigen-stimulated peripheral blood mononuclear cells (PBMC) at days 8 and 12 p.i., respectively. Challenge infections of calves influenced neither IFN-gamma nor IL-2 gene transcription in peripheral blood or in lymph node-derived lymphocytes. In contrast, IL-4 gene transcription was increased in lymphocytes isolated from draining lymph nodes. Besides antigen-specific reactions we also found an infection-triggered induction of the non-specific activation state of PBMC in the course of primary infection as measured by the intracellular IFN-gamma and IL-4 content of phorbol-12-myristate-13-acetate/ionomycin-stimulated PBMC. This may represent a new mechanism of immune cells of E. bovis-infected calves contributing to ongoing immune reactions.

Effect of chemotherapeutic treatment on cytokine (IFN-gamma, IL-2, IL-4, IL-5, IL-10) gene transcription in response to specific antigens in Brugia malayi-infected Mastomys coucha.

Cytokine (interferon (IFN)-gamma, interleukin (IL)-2, IL-4, IL-5, IL-10) gene transcription in response to filarial antigens was determined in peripheral blood mononuclear cells of Brugia malayi-infected Mastomys coucha in the course of untreated and chemotherapeutically abbreviated infections. Transcript levels in infected untreated animals suggest particular time courses for the various cytokines with ongoing parasite development and differing efficacies of female, male, microfilarial, and L3 antigens in inducing cytokine gene transcription. Gene transcription of both of Th1- and Th2-associated cytokines were initiated in the course of the infection in a manner that does not fit in a simple Th1-Th2 paradigm. IFN-gamma and IL-4 gene transcripts prevailed during prepatency. In case of the other cytokine genes considered in the study, transcription in general peaked around beginning of patency. During the phase of increasing microfilaremia (approximately 120-180 days p. i.) cytokine gene transcription was generally decreased. Later on, when the parasitemia had leveled off, except IFN-gamma, transcript levels often tended to increase. In chemotherapeutically treated animals, the outcome varied with the different efficacies of the drugs employed. The highly microfilaricidal cyclodepsipeptide BAY 44-4400 eliminated circulating microfilariae and partially sterilized adult worms without killing them. This kind of treatment hardly affected cytokine responses. In contrast, the therapy with Flubendazole, a selectively macrofilaricidal benzimidazole, and particularly the application of CGP 20376, a highly efficient microfilaricidal and macrofilaricidal benzthiazole, resulted in enhanced transcription of the Th1-associated IFN-gamma and IL-2 genes as well as of the Th2-associated IL-5 gene 2-3 months after treatment. IL-10 gene transcription seemed transiently increased after 1 month. There was no effect of any treatment on the IL-4 gene transcription.

Cytoskeletal changes in Eimeria bovis-infected host endothelial cells during first merogony.

The first merogony of Eimeria bovis takes place in lymphatic endothelial cells of the ileum, resulting in the formation of macromeronts up to 250 microm. In this study, we investigated the host cell cytoskeleton (actin filaments, microtubules, spectrin, vimentin intermediate filaments) associated with parasitic development in vitro by confocal laser scanning microscopy (CLSM) using primary bovine umbilical vein endothelial cells (BUVEC) and bovine spleen lymphatic endothelial cells (BSLEC) as host cells. No prominent changes in the host cell cytoskeleton were detected 1-3 days after E. bovis sporozoite invasion. With ongoing meront maturation a significant increase in microtubules and actin filaments close to the parasitophorous vacuole (PV) was found. Mature macromeronts within the PV were completely enclosed by these cytoskeletal elements. Our findings suggest, that in order to guarantee the survival of the host cell on the enlargement of macromeronts, E. bovis needs not only to augment but also to rearrange its cytoskeletal system.

Fluorescent Eimeria bovis sporozoites and meront stages in vitro: a helpful tool to study parasite-host cell interactions.

A fluorescence-based technique was established to trace intracellular sporozoites of Eimeria bovis for tests on gliding motility, invasion, replication and quantification of infection rates in cultured bovine umbilical vein endothelial cells (BUVEC) by laser scanning confocal microscopy and flow cytometry (FCM) analyses. Employing the fluorescent dye 5(6)-carboxyfluorescein diacetate succinimidyl ester (CFSE), we determined its effects on sporozoites at various concentrations and duration of staining. More than 98% of sporozoites were labelled with the dye at a concentration of 2.5 muM. Staining was predominantly found in refractile bodies and presumptive micronemes. Upon infection of BUVEC, CFSE-labelled sporozoites developed into fluorescent immature macromeronts, which were traceable inside the cells until 22 days postinfection (p. i.). Consistent with a peripheral localisation of the fluorescence signal in macromeronts merozoites released from these lacked detectable fluorescence. As example of use, a multicolour FCM approach for the simultaneous determination of E. bovis infection and host cell surface molecule expression was established. The approach proved suitable to quantify major histocompatibility complex (MHC-I) and MHC-II expression, thereby clearly distinguishing between infected and uninfected BUVEC up to day 14 p. i. In conclusion, CFSE labelling of E. bovis sporozoites facilitates monitoring of intracellular stages in vitro and will be a highly useful tool for studying host cell responses towards parasite invasion.

Studies on synchronous egress of coccidian parasites (Neospora caninum, Toxoplasma gondii, Eimeria bovis) from bovine endothelial host cells mediated by calcium ionophore A23187.

Neospora caninum, Toxoplasma gondii and Eimeria bovis are coccidian parasites of veterinary importance. Tachyzoites of N. caninum and T. gondii and sporozoites of E. bovis are able to invade and replicate in endothelial cells in vivo and in vitro. As it holds true for all eukaryotic cells, the survival of parasitized host cells and the parasites themselves should be dependent on ion balances, especially on extra- and intracellular calcium concentrations. Addition of the calcium ionophore A23187 reliably did release merozoites from mature N. caninum and T. gondii meronts grown in cultured primary bovine umbilical vein endothelial cells (BUVEC). Extent and time course of merozoite release depended on both, maturity of the meronts and concentration of the calcium ionophore. Attempts, however, to achieve synchronous release of merozoites from E. bovis first generation meronts by ionophore treatment failed, suggesting a different biological behaviour of this parasite. According to microscopical observations, the quite variable time of E. bovis macromeront maturation and a hampered merozoite exit owing to dense parasite-induced cytoskeleton elements surrounding the meront may be a reason for the lack of inducible synchronous release.

PMN-mediated immune reactions against Eimeria bovis.

For successful in vivo infection, Eimeria bovis sporozoites have to traverse the mucosal layer of the ileum to infect lymphatic endothelial cells and may, thereby, be exposed to the interstitial fluid and to the lymph representing potential targets for leukocytes. To mimic this situation in vitro, we exposed E. bovis sporozoites to bovine PMN and found enhanced elimination of the parasites. Addition of immune serum clearly increased these reactions, whereas neonatal calf serum had no effect, thus proposing a PMN-derived antibody-dependent cytotoxicity. Scanning and transmission electron microscopy showed PMN engulfing sporozoites or extending filopodia towards them and occasionally incorporating the parasites. PMN reacted with enhanced transcription of IL-6, MCP-1, GROalpha, TNF-alpha, and iNOS genes after exposure to sporozoites while stimulation with merozoite-antigen, in addition, upregulated IL-8, IP-10 and IL-12 gene transcription. Furthermore, enhanced in vitro oxidative burst and phagocytic activities were observed after contact of PMN with viable sporozoites. To verify the potential role of PMN in the in vivo situation, we analysed the general phagocytic and oxidative burst activities of PMN obtained ex vivo from E. bovis experimentally infected calves. Enhanced levels of both activities were found early p.i. (1-5 days) and towards the end of the first schizogony (days 13-22 p.i.) underlining the in vitro data. Our results suggest that PMN-mediated, innate immune reactions play an important role in the early immune response to E. bovis infections in calves.

Eimeria bovis infection enhances adhesion of peripheral blood mononuclear cells to and their transmigration through an infected bovine endothelial cell monolayer in vitro.

The first schizogony of Eimeria bovis takes place in lymphatic endothelial cells of the ileum, resulting in the formation of macroschizonts within 2-3 weeks. In this study, we analyse early cellular immune responses to infected host cells on the basis of peripheral blood mononuclear cell (PBMC) adhesion on and transmigration through infected bovine umbilical vein endothelial cell (BUVEC) monolayers. Adhesion of PBMC was upregulated by an E. bovis infection. Most marked effects were observed 1 day p.i.; thereafter, PBMC adhesion declined reaching control levels from day 8 p.i. onward. CD8(+) T cells adhered more frequently to infected BUVEC (42%) than CD4(+) T cells (25%). About one third of attached PBMC were represented by gammadelta-TCR(+) T cells. Adhesion of T cells was not restricted to parasitised host cells, but occurred almost equally on non-infected BUVEC within the same monolayer. Furthermore, we found moderately enhanced levels of PBMC transmigration through infected BUVEC monolayers, in particular on day 2 p.i. The data presented here suggest that E. bovis infection of BUVEC induces endothelial cell-derived proinflammatory reactions, which appear suitable for the initiation of both adaptive and innate immune responses.

Mapping of quantitative trait loci affecting resistance/susceptibility to Sarcocystis miescheriana in swine.

The outcome of infectious diseases in vertebrates is under genetic control at least to some extent. In swine, e.g., marked differences in resistance/susceptibility to Sarcocystis miescheriana have been shown between Chinese Meishan and European Pietrain pigs, and these differences are associated with high heritabilities. A first step toward the identification of genes and polymorphisms causal for these differences may be the mapping of quantitative trait loci (QTLs). Considering clinical, immunological, and parasitological traits in the above model system, this survey represents the first QTL study on parasite resistance in pigs. QTL mapping was performed in 139 F(2) pigs of a Meishan/Pietrain family infected with S. miescheriana. Fourteen genome-wide significant QTLs were mapped to several chromosomal areas. Among others, major QTLs were identified for bradyzoite numbers in skeletal muscles (F = 17.4; p < 0.001) and for S. miescheriana-specific plasma IgG(2) levels determined 42 days p.i. (F = 20.9; p < 0.001). The QTLs were mapped to different regions of chromosome 7, i.e., to the region of the major histocompatibility complex (bradyzoites) and to an immunoglobulin heavy chain cluster, respectively. These results provide evidence for a direct and causal role for gene variants within these gene clusters (cis-acting) in differences in resistance to S. miescheriana.