Full Paper

Genes and Immunity (2005) 6, 672–682. doi:10.1038/sj.gene.6364254; published online 4 August 2005

Genetic control of suceptibility to Candida albicans in susceptible A/J and resistant C57BL/6J mice

A Tuite1, M Elias2, S Picard2, A Mullick2 and P Gros1

  1. 1Department of Biochemistry, Center for the Study of Host Resistance, McGill University, Montreal, Quebec, Canada
  2. 2Biotechnology Research Institute, Montreal, Quebec, Canada

Correspondence: Dr P Gros, Department of Biochemistry, 3655 Promenade Sir William Osler, Room 907, Montreal, Quebec, Canada H3G 1Y6. E-mail: philippe.gros@mcgill.ca

Received 9 May 2005; Revised 20 June 2005; Accepted 6 July 2005; Published online 4 August 2005.

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Abstract

The importance of host factors in determining susceptibility to systemic Candida albicans infections is evident in both humans and mice. We have used a mouse model to study the genetic basis of susceptibility, using the inbred strains A/J and C57BL/6J, which are susceptible and resistant, respectively, based on different parameters of the response to infection. To identify genes responsible for this differential host response, brain and kidney fungal load were measured in 128 [A/J times C57BL/6J] F2 mice 48 h after infection with 5 times 104 C. albicans blastospores. Segregation analysis in this informative population identified complement component 5 (C5/Hc) as the major gene responsible for this differential susceptibility (LOD of 22.7 for kidney, 19.0 for brain), with a naturally occurring mutation that causes C5 deficiency leading to enhanced susceptibility. C5 was also found to control heart fungal load, survival time, and serum TNF-alpha levels during infection. Investigation of the response to C. albicans challenge in a series of AcB/BcA recombinant congenic strains validated the importance of C5 in determining the host response. However, the strains BcA67 and BcA72 showed discordant phenotypes with respect to their C5 status, suggesting additional complexity in the genetic control of the inter-strain difference in susceptibility observed in A/J and C57BL/6J following systemic infection with C. albicans.

Keywords:

C. albicans, genetics, infection, C5

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Introduction

Candida albicans normally exists as a relatively benign commensal in the gastrointestinal and genitourinary tracts of healthy humans,1 although in certain immunocompromised hosts it has the ability to enter into the bloodstream and colonize various organs, including the kidney, digestive tract, lung, and brain.2 This opportunistic fungal pathogen is a major causative agent of nosocomial infections, accounting for up to 15% of hospital-acquired infections in intensive care units.3 With an overall prognosis comparable to septic shock with multiple organ failure, systemic candidiasis is a life-threatening infection.3 Host genetic factors have been proposed to influence infection establishment, progression, and ultimate outcome. For chronic disseminated candidiasis (CDC), a form of invasive infection that occurs primarily in acute leukemia patients undergoing intensive therapy,4 a genetic contribution to susceptibility has been observed.5 Additionally, there is some evidence that there is a genetic component affecting susceptibility to systemic candidiasis in humans, as illustrated by the elevated incidence of candidiasis in patients with congenital defects affecting phagocyte function, such as myeloperoxidase deficiency.6

Mouse models of disseminated candidiasis represent a valuable tool to study the genetic basis of susceptibility to fungal infections. Intravenous (i.v.) and intraperitoneal (i.p.) infection models have been established and the ensuing pathology in susceptible strains mimics the human infection, with the major targets for colonization being the kidney, brain, and heart. Inbred mouse strains display a spectrum of responses to systemic C. albicans infection, ranging from highly susceptible to resistant, as assessed by fungal burden in affected organs, type and extent of cytokine response elicited and overall survival.7, 8

Studies in animal models have helped to clarify the aspects of innate and adaptive immunity that are required to protect the host against invasive infections with C. albicans. Neutrophils are rapidly recruited to sites of infection, where they can kill both the yeast and hyphal forms of C. albicans.9, 10 Components of the neutrophil microbicidal system, including NADPH oxidase and myeloperoxidase protect against infection,11, 12 while members of the Toll-like receptor (TLR) family appear to have disparate effects on infection outcome.13, 14, 15, 16 A T helper (Th)1-type response has been shown to be protective against reinfection with C. albicans, while a Th2 response is more frequently observed in susceptible strains.17 Additionally, C. albicans has been shown to activate the complement system through the alternative pathway,18 with the chemotactic and inflammatory activities of complement apparently the crucial components for mounting an effective host defence. In particular, a relationship between a naturally occurring deficiency in complement component 5 (C5) and enhanced susceptibility to systemic candidiasis is evident.7 However, to date, a genetic analysis of the overall contribution of this locus in determining the differential response to infection in a C5-deficient and a C5-sufficient strain has not been undertaken, making it difficult to draw any conclusions about the overall importance of C5 in the host defence against systemic candidiasis.

Ashman and his co-workers19 have used a set of recombinant inbred strains to identify a putative gene effect on chromosome 14 that controls tissue damage following i.v. infection, designated C. albicans resistance gene 1 (Carg1). An additional gene effect, named Carg2, was also identified, although it was not localized to any region within the genome.20 Characterization of the genes underlying these loci awaits further mapping and cloning.

A detailed analysis of the pathophysiology and host response to acute C. albicans infection demonstrated that A/J mice are extremely sensitive to infection, whereas C57BL/6J (B6) are relatively resistant to this infectious challenge.8 The dramatically different response elicited in A/J and B6 mice following systemic challenge with C. albicans prompted us to investigate whether these phenotypic differences have an underlying genetic basis. To do this, we analyzed infection progression and outcome in A/J and B6 mice following i.v. challenge with C. albicans and performed a genome-wide scan and linkage analysis in an informative [A/J times B6] F2 population. We further attempted to characterize the genetic basis of the differential response to infection in these strains by surveying the response to infection of a series of AcB/BcA recombinant congenic strains (RCS).21

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Results

The inbred strains A/J and B6 show discrete differences in their response to i.v. infection with C. albicans, particularly in terms of tissue damage, fungal replication, and overall survival.22 C. albicans replication was assessed in different organs from three A/J and B6 mice 48 h after i.v. injection with a low dose of C. albicans (Figure 1a). In both strains, C. albicans was cleared rapidly from the bloodstream, with levels below detection by 1 h postinjection (data not shown). Levels of fungal replication in the liver, lung, and spleen of A/J and B6 mice were low and there was no significant difference between the two strains. However, fungal load in the brain (P<0.05), kidney (P<0.01), and heart (P<0.001) of A/J mice was found to be significantly and reproducibly higher than that observed in B6. The highest fungal loads in both strains of mice were detected in the kidney, with the kidneys of A/J mice containing an approximately 100-fold greater number of fungi at this point in the infection process. The levels of several cytokines that had previously been reported to be elevated during systemic C. albicans infection8, 23, 24, 25, 26 were measured in A/J and B6. Cytokine levels were measured by ELISA in serum isolated from A/J and B6 mice 48 h after low dose injection with C. albicans blastospores. Cytokine levels were comparable between strains prior to infection (Figure 1b). No detectable differences in serum IL-12 or IFN-gamma levels were observed postinfection between the two strains. However, A/J was found to have elevated levels of IL-6 and TNF-alpha at the 48 h time point compared to B6. Thus, in this model of infection, susceptible A/J mice display high levels of fungal growth in the kidney, heart, and brain, and a corresponding elevation of serum levels of IL-6 and TNF-alpha.

Figure 1.
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Differential susceptibility of A/J and B6 mice to systemic candidiasis. A/J and B6 mice were infected i.v. with a low dose (5 times 104) of C. albicans blastospores. (a) The number of colony-forming units (CFU) was determined in the brain, heart, kidney, liver, lung, and spleen of three mice of each strain 48 h postinjection. Fungal loads in the brain, heart, and kidney of A/J mice were significantly higher than in B6 at this time point. (b) Serum cytokine levels in 4–6 mice of each strain were also measured prior to infection and at the 48 h time point, as described in Materials and methods. TNF-alpha and IL-6 levels were significantly higher in A/J mice compared to B6. The standard error of the mean (s.e.m.) is indicated for each group. Statistically significant differences are indicated on the graphs (*P<0.05; **P<0.01; ***P<0.001).

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The clear phenotypic differences between A/J and B6 confirmed that these inbred strains do indeed provide a good model system for studying the genetic basis of differential susceptibility to systemic candidiasis. We used an [A/J times B6] F2 intercross population to determine the mode of inheritance of susceptibility to candidiasis, as measured by fungal load in the kidney and brain. These mice, in addition to A/J and B6 controls, were infected i.v. with 5 times 104 C. albicans blastospores and the extent of fungal replication in the kidney and brain was assessed 48 h later. [A/J times B6] F1 generation mice were found to be as resistant to infection as the B6 parental controls (data not shown), while the bimodal distribution pattern of 128 F2 mice was indicative of susceptibility acting as an autosomal recessive trait under simple genetic control (Figure 2). The segregation pattern of the F2 population for these traits approximated that expected for a Mendelian trait if susceptibility to infection is recessive, with an approximately 3 : 1 ratio of resistant to susceptible mice. The mean log10 CFU counts in the F2 population were at 4.1 (kidney) and 2.5 (brain), values closer to the resistant B6 controls (3.8 for kidney, 2.3 for brain) than the susceptible A/J controls (5.9, 4.2), suggesting that susceptibility segregates as a recessive trait in this cross. We observed that the levels of fungal load in the brain were significantly correlated with kidney fungal load in individual mice (P<0.0001), indicating that these two traits are likely under the same genetic control. Finally, male mice displayed higher kidney CFU counts than female mice and this was observed for all experimental groups tested.

Figure 2.
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Kidney and brain fungal load in [A/J times B6] F2 mice infected with C. albicans. 128 [A/J times B6] F2 mice were infected i.v. with a low dose of C. albicans. Fungal load in the (a) kidney and (b) brain was measured 48 h after infection. F2 mice were separated by gender, with males displaying an overall enhanced susceptibility to infection as measured by kidney (P<0.01) but not brain CFU. Each data point represents an individual mouse, and horizontal bars represent the mean CFU counts for each group. CFU values for parental A/J and B6 mice are shown for comparison.

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The phenotypic data for this F2 cross behave as a quantitative trait that is amenable to study by linkage analysis. To identify the genetic locus or loci responsible for control of fungal load, we performed a genome-wide scan on the DNA from these F2 mice. A total of 138 polymorphic dinucleotide repeat markers informative for A/J and B6 (Table 1) provided an average coverage of 10 centimorgan (cM) along each chromosome. The largest gap was estimated at approximately 29 cM for the proximal portion of chromosome X. Marker mapping and assignment, as well as genome-wide multipoint linkage analysis, was performed using Mapmaker/EXP version 3.0 and Mapmaker/QTL 1.1. The results from this analysis are shown as a multiple point LOD score trace (Figure 3), with numerical data for the significant interval shown in Table 2.

Figure 3.
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Linkage analysis by whole-genome scan of susceptibility to intravenous infection with C. albicans. LOD score traces for chromosome 2, for which a highly significant linkage was detected that controls fungal replication in both the brain and kidney after i.v. infection with a low dose of C. albicans. Separate analyses using brain (dashed line) and kidney (solid line) fungal load as phenotypic traits were conducted.

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Using either kidney or brain fungal load CFU data as a quantitative trait, a statistically significant linkage was identified on the proximal portion of chromosome 2, encompassing a region of greater than 25 cM. Maximum linkage was found to the marker D2Mit295 (chi2=79.4 (kidney) and 73.2 (brain); LOD=17.2 (kidney) and 15.9 (brain)), which maps to 17 cM on chromosome 2 (Table 2). Since the gene for complement component 5 (C5/Hc) mapped less than 7 cM away from this marker27 and had been previously implicated for its role in controlling response to candidiasis in inbred mouse strains,28, 29, 30 we looked directly at this gene to determine if it was responsible for the linkage identified on this chromosome. A/J mice are deficient in C5 production, whereas B6 produce normal levels.31 We used a unique BsgI restriction site that is introduced by the 2-bp deletion in C5-deficient mice to determine the C5 status of the F2 mice. C5 status did in fact show a highly significant linkage with C. albicans susceptibility (chi2=104.6 (kidney) and 87.3 (brain); LOD=22.7 (kidney) and 19.0 (brain)), accounting for 60 and 50% of the phenotypic variance for kidney and brain, respectively. No other significant linkages were identified in this cross, likely due to the strong effect of C5 in determining infection outcome. Although the initial linkage analysis was conducted using a free genetic model, dominant, recessive, and additive models were subsequently tested for the chromosome 2 interval. The strongest evidence was for a dominant-acting locus (chi2=103.7 (kidney) and 86.9 (brain); LOD=22.5 (kidney) and 18.9 (brain)), consistent with the wild-type C5 conferring protection against candidiasis, even in the heterozygous state.

To further investigate the role of C5 in regulating the host response to systemic candidiasis, we used additional F2 mice to assess other phenotypic measures that differ between resistant and susceptible mice during candidiasis. Specifically, we looked at host response parameters that showed clear differences between A/J and B6. Heart fungal load was measured 48 h after i.v. injection with a low dose of C. albicans in 64 F2 mice. Survival was followed for up to 28 days after i.v. infection with a high dose of C. albicans in 122 F2 mice. In a separate group of 56 F2 mice, serum TNF-alpha levels were measured 48 h after low dose inoculation. The C5 status of each of these F2 mice was established by genotyping and there was a significant correlation between C5-deficiency and increased susceptibility to systemic candidiasis, as measured by each of the different phenotypes (P<0.001 for all phenotypes) (Figure 4). MapManager QT was used to perform tests for single-locus association between the genotype at C5 and each of these phenotypes, using a log10 transform for all of the data to reduce skewness. C5 status explained greater than 70% of the phenotypic variance observed for survival (chi2=148, LOD=32.1) and circulating TNF-alpha levels (chi2=76.0, LOD=16.5). The weakest evidence of linkage with C5 was observed using the heart fungal load data (chi2=26.3, LOD=5.7), although the LOD score was significant and the 32% phenotypic variance described by this trait was well above the 20% cutoff that describes a strong, and likely Mendelian QTL.31

Figure 4.
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Effect of C5 status on different measures of susceptibility to candidiasis. (a) Heart fungal load was measured in 64 F2 mice 48 h after low dose infection with C. albicans. (b) Survival was monitored in 122 F2 mice for up to 28 days after infection with a high dose of C. albicans. (c) Serum TNF-alpha levels were measured 48 h after infection with a low dose of C. albicans in 56 F2 mice. For all phenotypes there was a significant linkage between C5-deficiency and an increased susceptibility to infection (***P<0.001). The response of A/J and B6 parents are included for comparison. C5 alleles are identified as 'a' and 'b', for A/J and B6, respectively, and were identified by genotyping (see Materials and methods). Horizontal bars indicate the mean value for each of the traits analyzed for each group.

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To assess the possible involvement of additional genetic determinants (alone or in combination with C5) in the A/J vs B6 inter-strain difference in susceptibility to systemic candidiasis, we investigated the response of a series of AcB/BcA RCS derived by systematic inbreeding from a double backcross (N3) between A/J and B6 parents.21 In this breeding scheme, each of these strains derives 12.5% of its genome from either A/J or B6, fixed as a set of discrete congenic segments on the background of the other parental strain (87.5%). We measured fungal load in the kidney (Table 3 and Figure 5) and heart (data not shown) for these strains. The C5 status of each of these strains was also determined by genotyping and is shown in Table 3. For analysis, mice were separated according to their C5 status. A/J and B6 controls were included in each experimental group of mice and controls from within each experiment were used to determine statistical significance, in order to account for variations in dose between experiments. In general, we observed that the C5 status of the strains was a strong predictor of their response to C. albicans infection, with a C5 deficiency causing increased susceptibility to infection, even when present on a resistant B6 background, as seen in the strains BcA70 and BcA83. Likewise, the presence of wild-type alleles at C5 on an otherwise susceptible A/J background was associated with resistance to infection in the strains AcB55 and AcB63 (Table 3).

Figure 5.
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Phenotypic response of discordant AcB/BcA recombinant congenic strains. For each strain, a minimum of five mice were tested for C. albicans replication in the kidney 48 h after infection with a low dose of C. albicans. Horizontal bars represent the mean CFU values for each strain. Males and females were analyzed separately. Mean CFU counts for each strain were compared to the contemporaneous A/J (C5-deficient) or B6 (C5-sufficient) control of the corresponding gender. The s.e.m. is indicated for each group and strains that differed significantly from the controls are indicated on the graph (*P<0.05; **P<0.01, ***P<0.001).

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Among the C5-deficient strains, mean CFU counts were compared to the appropriate sex-matched C5-deficient A/J controls to identify strains that displayed enhanced resistance to systemic C. albicans infection in the absence of functional C5. This analysis identified the strain BcA72 as displaying a discordant phenotype. In BcA72, there was a strong gender effect on resistance, with male BcA72 mice showing CFU counts 40- to 50-fold greater than those seen in female BcA72. When compared to gender-matched controls, BcA72 were significantly more resistant to infection than their respective A/J counterparts (Figure 5) and this by a factor of 10- to 100-fold in females and males, respectively. A similar analysis of C5-sufficient strains identified the strain BcA67 as having a significantly higher kidney fungal burden than the B6 controls. These results indicate that additional susceptibility and resistance genetic factors, distinct from the major effect of C5, have become fixed in the strains BcA67 and BcA72.

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Discussion

The onset, response to, and outcome of systemic infection with C. albicans varies greatly among inbred strains. Infection manifestation in susceptible strains resembles human candidiasis, in terms of the organs targeted for colonization and active replication, as well as the ensuing pathology.2 We focused on two strains that display disparate responses to infectious challenge: A/J and C57BL/6J. A/J is extremely sensitive to infection, with overwhelming fungal replication occurring primarily in the kidney, and to a lesser extent, the brain and heart. Elevated levels of the proinflammatory cytokines TNF-alpha and IL-6 are observed early in infection, suggesting that these mice undergo a heightened, and possibly unregulated, inflammatory response after challenge with C. albicans.8 In contrast, B6 is relatively resistant to systemic candidiasis, with an approximately 100-fold lower fungal load in the kidney and a muted production of IL-6 and TNF-alpha compared to A/J.

A genome-wide scan was conducted in an informative F2 population derived from A/J and B6, in order to study the genetic control of susceptibility to C. albicans. Linkage analysis using kidney or brain fungal load as a phenotypic marker of susceptibility identified an interval on chromosome 2 with a significant linkage, with the highest LOD score associated with the gene for complement component 5. Heterozygotes and mice homozygous for wild-type C5 displayed a degree of resistance comparable to B6 controls, whereas C5-deficient mice showed an enhanced susceptibility to candidiasis. No additional gene effects were detected in this cross. Single-marker linkage analysis identified C5 as an important regulator of heart fungal load, survival, and serum TNF-alpha levels in additional F2 mice. The association of C5 with these independent measures of the host response provides strong evidence that a deficiency in C5 is indeed responsible for enhancing susceptibility to infection.

A previous study identified a putative gene effect on chromosome 14 (Carg1) that controlled overall susceptibility to tissue damage.19 We detected no such effect on chromosome 14. Although the infection conditions were similar, a number of variables differed between these studies, including the strains of mice used and the phenotypes assessed, which may account for the lack of correlation. Additionally, it has been proposed that both A/J and B6 possess resistance alleles at Carg1, based on a phenotypic evaluation of the patterns of tissue damage observed in these strains.32 Although this hypothesis could not been tested in the current study, since we did not use tissue damage as a phenotypic measure of susceptibility, a lack of polymorphism between A/J and B6 at this locus would explain the absence of this gene effect in the present cross. Interestingly, the strains used to identify Carg1, namely AKR and C57/L, differ in their C5 status in a manner similar to A/J and B6, with AKR being C5 deficient and C57/L being C5 sufficient. A comparison of the C5 status of the AKXL strains used in this study33 indicates that C5 is in fact a strong predictor of the response to systemic candidiasis as measured by tissue damage. For 80% of the strains tested, their response to infection followed their C5 status. The existence of three discordant strains could indicate either the presence of additional gene effects or a difficultly in ascertaining the tissue damage phenotype using a bimodal distribution.

The finding that C5 is the major gene responsible for the differential susceptibility to systemic candidiasis observed between A/J and B6 is not unexpected, given that A/J has a known C5 deficiency. This C5 deficiency is caused by a 2-bp deletion in exon 6 of the C5 gene, which leads to the introduction of a premature stop codon and the production of a nonfunctional, truncated polypeptide that is not secreted.31 This deletion occurs in up to 40% of the commonly used inbred strains.34 A correlation between the absence of functional C5 and increased susceptibility to candidiasis among inbred strains has been noted,7, 22 although this study represents the first formal demonstration of this relationship, using linkage mapping in informative crosses.

Complement component 5 is a member of the complement system, a series of plasma proteins and soluble and membrane-bound receptors that react with one another to opsonize invading pathogens and induce various inflammatory responses to prevent infection.35 Following activation of the alternative pathway by C. albicans, a complement cascade is initiated, leading to C5 cleavage and the generation of C5a and C5b. C5b is a component of the membrane attack complex (MAC), which joins with additional terminal components of complement to form a pore in the cell membrane of the pathogen, leading ultimately to cell lysis.35 The relevance of the MAC in the clearance of C. albicans has not been appreciably demonstrated and direct lysis of C. albicans in the blood has not been observed.36

It is much more likely that a C5 deficiency has its effect at the level of C5a, a chemotactic agent and anaphylatoxin.37 The most well-defined role for C5a during invasive candidiasis is as a chemotactic agent for neutrophils. In the absence of C5a, mice have a reduced recruitment of effector cells to the site of infection.8 The G-protein coupled C5a receptor (C5aR/CD88) is expressed on neutrophils, monocytes, macrophages, dendritic cells, basophils, eosinophils, and mast cells, among other cell types.38 Neutrophils express high levels of this receptor,39 explaining their rapid infiltration to sites of infection and the ensuing phagocytosis of C. albicans in response to C5. C5a signalling pathways in neutrophils lead to the assembly of catalytically active NADPH oxidase at the cell membrane, which has an established role in the killing of phagocytosed C. albicans. Neutropenia is one of the major risk factors for developing systemic candidiasis in humans,1 and the lack of functional C5 may mirror this process in mice, with the absence of C5 effectively causing a localized neutrophil-poor response during infection. At 2 h after i.v. infection, C. albicans deposition per milligram of organ weight is highest in the lungs, followed by intermediate levels in the liver, spleen, and kidneys, and low levels in the brain and heart.40, 41 It is therefore notable that the major sites of C. albicans replication in A/J at the 48 h time point are the kidney, brain, and heart. The ability of the liver, spleen, and lungs to control fungal replication despite relatively high levels of initial infection may reflect the presence of resident macrophages in these organs that are able to contain and control infection in the absence of a C5-induced recruitment of neutrophils.

While the results of the whole-genome scan demonstrate the major effect that C5 has in controlling the response to C. albicans infection, it is expected that additional genes are also integral to the host response to systemic challenge. As an alternate approach to both validate the importance of C5 in infection and identify any additional gene effects, we have used a set of RCS.21 These mice have the advantage of being genetically well-characterized, as they have been genotyped at a high density across the entire genome. The strain distribution patterns can be used to localize any detected gene effects through a comparison of the regions of the genome that are in common among strains that display the phenotype of interest. The mosaic nature of the genomes of these lines of mice makes them useful tools for mapping, since they contain a mixture of the genomes of the two progenitor strains of interest.

In this study, the C5 status of all of the strains was known and each RCS was tested for its response to systemic challenge in the context of the absence or presence of C5. C5 status is a very strong predictor of susceptibility to candidiasis, as demonstrated by the resistance of the C5-sufficient strain AcB55, despite being on a susceptible background. However, the identification of discordant strains indicates that additional mechanisms may be involved in the host response to systemic candidiasis. In particular, the strain BcA72 displays an enhanced resistance to infection, despite its C5-deficiency. This resistance could be caused by a number of different events, such as the transfer of a novel resistance locus, de-repression of expression of a resistance allele from one strain when it is introduced onto the reciprocal genetic background, or the suppression of A/J- or B6-derived susceptibility alleles upon transfer to a different genetic background. This strain also displays a gender-specific effect, with females tending to be more resistant than their male counterparts. The effect of gender on the ability of a host to resist infectious challenge with C. albicans has been noted previously, with male mice of different inbred strains tending to be more susceptible to infection, even among C5-sufficient strains, where serum C5 concentrations are higher in males than in females.42, 43 This gender specificity appears to be much stronger in BcA72 than in the A/J and B6 controls. Among the C5-sufficient strains, BcA67 displayed a marginally enhanced susceptibility, compared to the B6 controls. The pursuit of further gene mapping in BcA67 would necessitate the generation of crosses with a highly resistant inbred strain, such as BALB/c. Based on the failure to identify any highly susceptible C5-sufficient mice, it appears that wild-type C5 alone is sufficient to confer resistance to infection with C. albicans in the experimental model used herein, and that the presence of any additional putative susceptibility or resistance alleles may only become relevant in the absence of functional C5.

The results of the whole-genome scan and the survey of the set of AcB/BcA RCS clearly illustrate the importance of C5 as the major gene controlling the differential susceptibility to systemic C. albicans infections seen in A/J and B6 mice. The physiological relevance of a C5 deficiency in humans with systemic candidiasis is unclear, since although this deficiency renders patients increasingly susceptible to a range of microbial infections, an increased incidence of candidiasis has not been reported.44 As resistance to antifungal agents becomes a growing concern, a better understanding of the host–pathogen interaction is invaluable for the development of any new therapies. Animal models provide a useful tool to study the response to systemic candidiasis and studies in mice have indicated the importance of the innate immune system, particularly the complement pathway, in controlling infection. The involvement of neutrophils in the initial stages of infection parallels the importance of neutrophils in human infections and validates the mouse as a physiologically relevant model of systemic candidiasis.

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Materials and methods

Mice

Inbred, pathogen-free 8- to 12-week-old A/J and C57BL/6J (B6) mice were purchased from The Jackson Laboratory (Bar Harbor, ME, USA). [A/J times B6] F2 progeny were bred by systematic brother–sister mating of [A/J times B6] F1 mice. The AcB/BcA RCS were purchased from Emerillon Therapeutics (Montreal, Quebec, Canada). All housing and experimental procedures were approved by the Biotechnology Research Institute (BRI) Animal Care Committee, operating under the guidelines of the Canadian Council of Animal Care. Mice were age and sex matched for all experiments.

C. albicans infections

We performed infections with C. albicans as described previously.8 C. albicans strain SC5314 was grown overnight in YPD medium (1% yeast extract, 2% Bacto Peptone and 2% dextrose) at 30°C and harvested by centrifugation. Blastospores were washed twice and resuspended in phosphate-buffered saline (PBS) at the required density. For experimental infections, mice were injected via the tail vein with 200 mul of either 5 times 104 (low dose) or 3 times 105 (high dose) of C. albicans blastospores in PBS. For survival experiments, we monitored infected animals daily for up to 28 days after injection of a high dose of C. albicans. Moribund animals were killed and survival times were recorded for use in statistical analysis. For the determination of organ fungal load, we killed mice 48 h after injection with a low dose of C. albicans. At the time of killing, mice were anesthetized and exsanguinated by cardiac puncture. Serum was isolated by collection of blood in serum-gel microtubes (Sarstedt, Montreal, Quebec, Canada), followed by centrifugation and storage at -20°C until used to measure cytokine levels. To measure TNF-alpha levels during infection, mice were injected with a low dose of C. albicans and killed at 48 h, with serum collected as described above. Tail biopsies were taken from all mice at the time of killing for all experiments and stored at -80°C.

Determination of organ fungal load

Target organs were removed aseptically and homogenized in 4 ml of PBS. We determined fungal load in the resulting suspensions by making serial 10-fold dilutions in PBS and plating 40 mul of each dilution on YPD-agar plates containing 34 mg/l chloramphenicol. Plates were incubated at 30°C for 24–48 h. Total colony-forming units (CFU) were determined and counts were expressed as the log10 CFU per organ. For the initial determination of differences in fungal burden between the organs of A/J and B6, where we tested brain, heart, kidney, lung, liver, and spleen, the minimum detectable number of CFU was 17 per organ. For all other experiments the maximum sensitivity of this assay was 100 CFU per organ. For subsequent analyses, mice below the detection limit of the assay were assigned an arbitrary value of 100 CFU, the minimum number of detectable colonies. This assumption overestimates the mean for groups having titres below detectable levels.

Cytokine detection

We determined levels of IFN-gamma, IL-6, IL-12, and TNF-alpha in the serum of healthy and infected mice using a sandwich enzyme-linked immunosorbant assay (ELISA) (BD Biosciences, Mississauga, Ontario, Canada). Measurements were performed according to the manufacturer's instructions.

Genotyping

Genomic DNA was isolated from the tail tips of individual F2 mice, which we obtained at the time of killing, as described previously.45 A total of 138 polymorphic microsatellite markers informative for A/J and B6, and distributed over all chromosomes except chromosome Y, were selected to give an approximately 10 cM coverage of the genome (Invitrogen, Burlington, Ontario, Canada) (Table 1). The genetic locations of these microsatellite markers were obtained from the Mouse Genome Database Project (http://www.informatics.jax.
org
).27 We used the nonpolymorphic Y chromosome marker, SRY, to verify the gender of all the F2 mice. Genotyping was performed using a standard PCR-based method with trace amounts of [32P]alpha-dATP, followed by separation on denaturing polyacrylamide gels.45

The A/J strain has a 2-bp deletion in exon 6 of the Hc gene31 that introduces a BsgI restriction site (5'-GTGCAG(N)16-3'). To determine the C5 status of the F2 mice, this polymorphism was analyzed by PCR amplification of genomic DNA using the forward primer: 5'-CCGAAGTTATCATTGGTCCTTT-3' and the reverse primer: 5'-CCCCACCCTCTTCTGGTACT-3'. The PCR product was digested with BsgI and visualized by electrophoresis on a 2% agarose gel followed by staining with ethidium bromide. The expected fragment size for wild-type samples was 446 bp, while the sizes for samples containing the deletion were 318 and 126 bp.

Statistical analysis

An unpaired, two-tailed t-test was used to establish the significance of differences in mean CFU per organ between A/J and B6. One-way ANOVA and Dunnett's multiple comparison test were used to determine the effect of genotype at C5 on the various phenotypes assessed in the F2 and RCS mice. A P-value of 0.05 or less was considered significant. All statistical analyses were performed using GraphPad Prism version 4 (San Diego, CA, USA).

Genetic markers were assigned to and mapped within the chromosomes using multipoint linkage analysis in Mapmaker/EXP version 3.0.46 A log10 transformation of the data for brain and kidney fungal load was performed in order to approximate a normal distribution. Genome-wide simple interval mapping between the transformed kidney and brain fungal load phenotypes and genetic markers was performed using Mapmaker/EXP and Mapmaker/QTL 1.1,46 to identify quantitative trait loci (QTLs). Logarithm of odds (LOD) scores were calculated using the expectation/maximization algorithm in Mapmaker/QTL. Initial linkage analyses were conducted using a free genetic model. Map Manager QT47 was used to test for single-locus association between C5 genotype and the heart fungal load, serum TNF-alpha, and survival phenotypes assessed in additional F2 mice. To reduce skewness and more closely approximate a normal distribution, we performed a log10 transformation on the phenotypic data for each of these traits. Single-locus associations were tested by simple regression analysis between trait values and genotypes at the C5 locus and the significance of each potential association was measured using the likelihood ratio statistic (LRS, chi2). LOD scores were calculated as chi2/2ln(10).

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Acknowledgements

This work was supported by research grants from the Canadian Genetic Diseases Network to PG. PG is a James McGill Professor of Biochemistry and is a distinguished Investigator of the Canadian Institutes of Health Research. AT is supported by a studentship from the Natural Sciences and Engineering Research Council of Canada. Work in AM's lab was supported by the Genomics and Health Initiative of the National research Council of Canada.

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