Complex genetic control in a rat model for rheumatoid arthritis

Rheumatoid arthritis: identifying and characterising polymorphisms using rat models

Rheumatoid arthritis is a chronic inflammatory joint disorder characterised by erosive inflammation of the articular cartilage and by destruction of the synovial joints. It is regulated by both genetic and environmental factors, and, currently, there is no preventative treatment or cure for this disease. Genome-wide association studies have identified ∼100 new loci associated with rheumatoid arthritis, in addition to the already known locus within the major histocompatibility complex II region. However, together, these loci account for only a modest fraction of the genetic variance associated with this disease and very little is known about the pathogenic roles of most of the risk loci identified. Here, we discuss how rat models of rheumatoid arthritis are being used to detect quantitative trait loci that regulate different arthritic traits by genetic linkage analysis and to positionally clone the underlying causative genes using congenic strains. By isolating specific loci on a fixed genetic background, congenic strains overcome the challenges of genetic heterogeneity and environmental interactions associated with human studies. Most importantly, congenic strains allow functional experimental studies be performed to investigate the pathological consequences of natural genetic polymorphisms, as illustrated by the discovery of several major disease genes that contribute to arthritis in rats. We discuss how these advances have provided new biological insights into arthritis in humans.

The early days of NK cells: an example of how a phenomenon led to detection of a novel immune receptor system - lessons from a rat model

In this review, I summarize some of the early research on NK cell biology and function that led to the discovery of a totally new receptor system for polymorphic MHC class I molecules. That NK cells both could recognize and kill tumor cells but also normal hematopoietic cells through expression of MHC class I molecules found a unifying explanation in the “missing self” hypothesis. This initiated a whole new area of leukocyte receptor research. The common underlying mechanism was that NK cells expressed receptors that were inhibited by recognition of unmodified “self” MHC-I molecules. This could explain both the killing of tumor cells with poor expression of MHC-I molecules and hybrid resistance, i.e., that F1 hybrid mice sometimes could reject parental bone marrow cells. However, a contrasting phenomenon termed allogeneic lymphocyte cytotoxicity in rats gave strong evidence that some of these receptors were activated rather than inhibited by recognition of polymorphic MHC-I. This was soon followed by molecular identification of both inhibitory and stimulatory Ly49 receptors in mice and rats and killer cell immunoglobulin-like receptors in humans that could be either inhibited or activated when recognizing their cognate MHC-I ligand. Since most of these receptors now have been molecularly characterized, their ligands and the intracellular pathways leading to activation or inhibition identified, we still lack a more complete understanding of how the repertoire of activating and inhibitory receptors is formed and how interactions between these receptors for MHC-I molecules on a single NK cell are integrated to generate a productive immune response. Although several NK receptor systems have been characterized that recognize MHC-I or MHC-like molecules, I here concentrate on the repertoires of NK receptors encoded by the natural killer cell gene complex and designed to recognize polymorphic MHC-I molecules in rodents, i.e., Ly49 (KLRA) receptors.

Identification of candidate risk gene variations by whole-genome sequence analysis of four rat strains commonly used in inflammation research

Background

The DA rat strain is particularly susceptible to the induction of a number of chronic inflammatory diseases, such as models for rheumatoid arthritis and multiple sclerosis. Here we sequenced the genomes of two DA sub-strains and two disease resistant strains, E3 and PVG, previously used together with DA strains in genetically segregating crosses.

Results

The data uncovers genomic variations, such as single nucleotide variations (SNVs) and copy number variations that underlie phenotypic differences between the strains. Comparisons of regional differences between the two DA sub-strains identified 8 genomic regions that discriminate between the strains that together cover 38 Mbp and harbor 302 genes. We analyzed 10 fine-mapped quantitative trait loci and our data implicate strong candidates for genetic variations that mediate their effects. For example we could identify a single SNV candidate in a regulatory region of the gene Il21r, which has been associated to differential expression in both rats and human MS patients. In the APLEC complex we identified two SNVs in a highly conserved region, which could affect the regulation of all APLEC encoded genes and explain the polygenic differential expression seen in the complex. Furthermore, the non-synonymous SNV modifying aa153 of the Ncf1 protein was confirmed as the sole causative factor.

Conclusion

This complete map of genetic differences between the most commonly used rat strains in inflammation research constitutes an important reference in understanding how genetic variations contribute to the traits of importance for inflammatory diseases.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-391) contains supplementary material, which is available to authorized users.

What Have We Learned about the Pathogenesis of Rheumatoid Arthritis from TNF-Targeted Therapy?

Studies of cytokine regulation in rheumatoid arthritis led to the development of TNFα inhibitors which are now used for a number of indications, including rheumatoid arthritis, inflammatory bowel disease, psoriasis, psoriatic arthritis, and ankylosing spondylitis. The widespread use of biologics in the clinic offers unique opportunities for probing disease pathogenesis and this paper provides an overview of rheumatoid arthritis, with a particular emphasis on the impact of anti-TNFα therapy on pathogenetic mechanisms. An overview is also provided on the most commonly used animal models that mimic RA, including adjuvant-induced arthritis, collagen-induced arthritis, TNFα-transgenic mice, and the K/BxN and SKG models. These models have led to significant discoveries relating to the importance of pro-inflammatory cytokines in the pathogenesis of rheumatoid arthritis, resulting from disregulation of the normally finely tuned balance of pro- and anti-inflammatory cytokine signalling. In addition, experimental evidence is discussed suggesting how genetic and environmental factors can contribute to disease susceptibility. The role of effector and regulatory T cells is discussed in the light of the relatively disappointing therapeutic effects of T cell modifying agents such as anti-CD4 antibody and cyclosporin. It is concluded that comprehensive analyses of mechanisms of action of biologics and other drugs entering the clinic will be essential to optimise therapy, with the ultimate aim of providing a cure.

Finemapping of the arthritis QTL Pia7 reveals co-localization with Oia2 and the APLEC locus

In this study, we sought to determine the effect of the quantitative trait locus Pia7 on arthritis severity. The regulatory locus derived from the arthritis-resistant E3 rat strain was introgressed into the arthritis-susceptibility DA strain through continuous backcrossing. Congenic rats were studied for their susceptibility to experimental arthritis using pristane and adjuvant oil. In addition, cell number and function of various leukocyte populations were analyzed either under naive or stimulated conditions. We found that the minimal congenic fragment of DA.E3-Pia7 rats overlapped with the minimal fragment in DA.PVG-Oia2 congenic rats, which has been positionally cloned to the antigen-presenting lectin-like receptor complex (APLEC) genes. DA.E3-Pia7 congenic rats were protected from both PIA and OIA, but the protection was more pronounced in OIA. In adoptive transfer experiments we observed that the Pia7 locus controlled the priming of arthritogenic T cells and not the effector phase. In addition, Pia7 congenic rats had a significant higher frequency of B cells and granulocytes as well as TNFalpha production after stimulation, indicating a higher activation state of cells of the innate immune system. In conclusion, this study shows that the APLEC locus is a major locus regulating the severity of experimentally induced arthritis in rats.

Identification of gene regions regulating inflammatory microglial response in the rat CNS after nerve injury

Local CNS inflammation takes place in many neurological disorders and is important for autoimmune neuroinflammation. Microglial activation is strain-dependent in rats and differential MHC class II expression is influenced by variations in the Mhc2ta gene. Despite sharing Mhc2ta and MHC class II alleles, BN and LEW.1N rats differ in MHC class II expression after ventral root avulsion (VRA). We studied MHC class II expression and glial activation markers in BN rats after VRA. Our results demonstrate that MHC class II expression originates from a subpopulation of IBA1(+), ED1(-), and ED2(-) microglia. We subsequently performed a genome-wide linkage scan in an F2(BNxLEW.1N) population, to investigate gene regions regulating this inflammatory response. Alongside MHC class II, we studied the expression of MHC class I, co-stimulatory molecules, complement components, microglial markers and Il1b. MHC class II and other transcripts were commonly regulated by gene regions on chromosomes 1 and 7. Furthermore, a common region on chromosome 10 regulated expression of complement and co-stimulatory molecules, while a region on chromosome 11 regulated MHC class I. We also detected epistatic interactions in the regulation of the inflammatory process. These results reveal the complex regulation of CNS inflammation by several gene regions, which may have relevance for disease.

Detection of arthritis-susceptibility loci, including Ncf1, and variable effects of the major histocompatibility complex region depending on genetic background in rats

OBJECTIVE

To characterize the arthritis-modulating effects of 3 non-major histocompatibility complex (MHC) quantitative trait loci (QTLs) in rat experimental arthritis in the disease-resistant E3 strain, and to investigate the disease-modulating effects of the MHC region (RT1) in various genetic backgrounds.

METHODS

A congenic fragment containing Ncf1 along with congenic fragments containing the strongest remaining loci, Pia5/Cia3 and Pia7/Cia13 on chromosome 4, were transferred from the arthritis-susceptible DA strain into the background of the completely resistant E3 strain. The arthritis-regulatory potential of the transferred alleles was evaluated by comparing the susceptibility to experimental arthritis in congenic rats with that in E3 rats. The RT1(u) haplotype from the E3 strain was transferred into the susceptible DA strain (RT1(av1)), and various F(1) and F(2) hybrids were generated to assess the effects of RT1 on arthritis susceptibility.

RESULTS

The DA allele of Ncf1 did not break the arthritis resistance of the E3 rats, although it led to enhanced autoimmune B cell responses, as indicated by significantly elevated levels of anticollagen antibodies in congenic rats. Introgressing Pia5 and Pia7 loci on chromosome 4 broke the resistance to arthritis, and the MHC locus on chromosome 20 in DA rats enhanced arthritis when RT1 interacted with E3 genes.

CONCLUSION

The findings in these congenic lines confirm the existence of 3 major QTLs that regulate the severity of arthritis and are sufficient to induce the transformation of a completely arthritis-resistant rat strain into an arthritis-susceptible strain. This study also reveals a dramatic difference in the arthritis-regulatory potential of the rat MHC depending on genetic background, suggesting that strong epistatic interactions occur between MHC and non-MHC genes.

Genetic and molecular basis of quantitative trait loci of arthritis in rat: genes and polymorphisms

Rheumatoid arthritis (RA) is an autoimmune disease, the pathogenesis of which is affected by multiple genetic and environmental factors. To understand the genetic and molecular basis of RA, a large number of quantitative trait loci (QTL) that regulate experimental autoimmune arthritis have been identified using various rat models for RA. However, identifying the particular responsible genes within these QTL remains a major challenge. Using currently available genome data and gene annotation information, we systematically examined RA-associated genes and polymorphisms within and outside QTL over the whole rat genome. By the whole genome analysis of genes and polymorphisms, we found that there are significantly more RA-associated genes in QTL regions as contrasted with non-QTL regions. Further experimental studies are necessary to determine whether these known RA-associated genes or polymorphisms are genetic components causing the QTL effect.

Genetic influences on joint contractures secondary to immobilization

The primary research question of this study queries whether, beyond environmental conditions, genetic factors affect the development of joint contractures. We hypothesized that intrinsic genetic factors influence the severity of joint contractures developing secondary to joint immobilization. Forty rats from four inbred rat strains had one leg immobilized in knee flexion for 4 weeks. The contracture was measured mechanically as the lack of range of motion to a standardized torque. Using the contralateral leg as a control, the average severity of the contracture could be calculated and compared between strains. All immobilized legs presented knee contractures after 4 weeks of immobilization. Two strains (Dark Agouti and Fisher 344) showed a larger mean knee contracture than those of the two other rat strains (Augustus Copenhagen Irish and Brown Norway). Environmental factors, such as immobility, are usually identified as a cause of a joint contracture. These results demonstrate that, in addition to mechanical factors in the environment of a joint, intrinsic genetic factors participate in the process leading to joint contracture. This demonstration has important consequences for directing future research and may lead to interventions to help patients at risk of developing joint contractures.

Quest for arthritis-causative genetic factors in the rat

Experimental rat models of arthritis are extensively studied with a view to understand the genetic underpinnings of rheumatoid arthritis (RA). Genome scans using these models have led to the detection of arthritis regulatory quantitative trait loci (QTLs) on all but three chromosomes of the rat. Whereas some of the QTLs are model specific, others overlap between models. Some arthritis susceptibility and/or severity QTLs identified by genetic linkage analyses are corroborated by substitution mapping using congenic strains, whereas others are not. In these cases, testing alternate arthritis models proved to be useful to identify QTL effects. Nevertheless, development and testing of congenic substrains containing progressively shorter introgressed regions have not only fine mapped the location of the arthritis QTLs but also resulted in the identification of multiple QTLs within several originally identified individual QTL. Most of these studies progressed rapidly since 2001, when the rat genome sequence was published. Proof of principle for substitution mapping as a successful method for QTL gene discovery is provided by the positional cloning of Ncf1 as one of the arthritis QTLs in rats. This finding is encouraging for similar sustained dissection of all the other arthritis QTLs mapped in the rat. Identification of rat arthritis QTLs is expected to pave the way for discovery of yet-unidentified arthritis-causative genetic elements and/or pathways for RA in humans and potential development of targeted therapeutics. This review catalogs some of the recent advances made in QTL discovery projects of experimentally induced rat models of arthritis.

Involvement of antibody production quantitative trait loci in the susceptibility to pristane-induced arthritis in the mouse

Mice obtained by bidirectional selective breeding for high (HIII) or low (LIII) antibody (Ab) production are resistant or extremely susceptible to pristane-induced arthritis (PIA), respectively. Several quantitative trait loci regulating Ab production (Ab QTL) have been mapped in these lines, which were used to investigate the influence of these Ab QTL in PIA. Parental HIII and LIII mice and their F1 and F2 intercrosses were injected twice with pristane, and arthritis was observed for 200 days. In LIII mice PIA was more severe and incidence was 100% at day 105, while F1 and F2 mice showed intermediate values. HIII mice were totally resistant. Microsatellite polymorphisms of Ab QTL were analysed and D3Mit100 alleles cosegregated significantly with PIA incidence, severity and onset in F2 intercross mice, while the other four markers showed suggestive values. Results indicate colocalization of QTL for Ab production and PIA susceptibility. Moreover, the different cytokine and IgG isotype profiles observed in HIII and LIII lines after PIA induction are useful to candidate genes endowed with the regulation of the Ab production and arthritis phenotypes.

Accelerating effect of an MRL gene locus on the severity and onset of arthropathy in DBA/1 mice

OBJECTIVE

To analyze the influence of the genetic background of an arthritis-prone strain of mice, MRL, on the spontaneous development of arthropathy in DBA/1 mice, which histopathologically resembles enthesopathy in humans, and to clarify the strain-specific gene loci and their interactions that confer susceptibility to arthropathy.

METHODS

MRL, DBA/1, (MRL x DBA/1)F(1), and (MRL x DBA/1)F(2) intercross mice were prepared, and the severity and onset of arthropathy of the ankle joints in individual mice were quantified (0-3 and 0-5 scale, respectively). A genome-wide scan of 271 male F(2) intercross mice with polymorphic microsatellite markers was performed.

RESULTS

Only male DBA/1, (MRL x DBA/1)F(1), and (MRL x DBA/1)F(2) mice developed arthropathy. The macroscopic and histopathologic findings of arthropathy in the F(2) mice were similar to those in the parental DBA/1 mice, but the onset was significantly earlier. In the quantitative trait locus analysis of male F(2) mice, 1 susceptibility locus for both the severity and early onset of the disease in the region of an MRL allele, Amd1, was located at marker D10Mit259 (map position 40.0 cM), which was common to 1 of the sialadenitis susceptibility loci in MRL mice, Asm1. Another susceptibility locus for the severity and early onset of arthropathy in the region of a DBA allele, Amd2, was located at D3Mit46 (29.5 cM). These loci manifested an additive effect on the development of arthropathy.

CONCLUSION

Arthropathy in DBA/1 mice is under the control of an allelic combination of gene loci, one of which is common to the locus for sialadenitis in MRL/MpJ-lpr/lpr mice.

Resolution of a 16.8-Mb Autoimmunity-Regulating Rat Chromosome 4 Region into Multiple Encephalomyelitis Quantitative Trait Loci and Evidence for Epistasis

To investigate effects of a 16.8-Mb region on rat chromosome 4q42-43 on encephalomyelitis, we performed a high-resolution mapping using a 10th generation advanced intercross line between the susceptible DA strain and the MHC identical but resistant PVG.1AV1 strain. Clinical signs of myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis (EAE) developed in 29% of 772 F(10) rats. Three regions controlling disease, Eae20, Eae21, and Eae22, were mapped using 15 microsatellite markers spanning 16.8 Mb. Eae20 was a major genetic determinant within the region whereas Eae21 modified disease severity. Eae22 was identified as an epistatic region because it only displayed an effect together with Piebald Virol Glaxo (PVG) alleles on Eae20. Disease down-regulation by PVG alleles in the telomeric part of Eae20 was also demonstrated in DA rats made congenic for a approximately 1.44-Mb chromosomal region from PVG. As the region containing Eae20-Eae22 also regulates arthritis, together with the fact that the syntenic mouse 6F(2)-F(3) region regulates experimental lupus and diabetes, and the syntenic human 12p13.31-13.2 region regulates multiple sclerosis and rheumatoid arthritis, the present data point to genes that control several inflammatory diseases. The pairscan analyses of interaction, which here identified Eae22, are novel in the encephalomyelitis field and of importance in the design of further studies of this region in other diseases and species. The limited number of genes identified in Eae20, Eae21, and Eae22 enables focused examination of their relevance in mechanistic animal studies and screening of their association to human diseases.

Two-loci interaction confirms arthritis-regulating quantitative trait locus on rat chromosome 6

A form of genetic interaction, or epistasis, occurs when one gene interferes with the phenotypic effect of another nonallelic gene. In pristane-induced arthritis (PIA) in rats we have previously identified Pia3, on chromosome 6, to be a locus that regulates onset of disease. In a single congenic strain containing Pia3 on the arthritis-susceptible DA background, DA.Pia3, no difference in onset of disease or early disease severity could be detected. After a two-loci interaction analysis of (E3 x DA)F2 intercross data, Pia3 was found to interact with Pia4 (chromosome 12). Subsequently, the DA.Pia3 congenic strain was combined with the DA.Pia4 congenic strain so that an effect of Pia3 could be observed. The effect of heterozygosity in Pia4 results in lower severity and thus in combination with Pia3 made it possible to observe that Pia3 alleles from the arthritis-resistant E3 strain rendered more severe arthritis into the otherwise 100% susceptible DA strain. As the introduction of Pia4 heterozygosity results in a lower level of arthritis severity we regard this as an additive interaction with a severity threshold-lowering effect.

The genetic control of rheumatoid factor production in a rat model of rheumatoid arthritis

OBJECTIVE

To investigate the genetic regulation of rheumatoid factor (RF) in a rat model of rheumatoid arthritis, in order to gain understanding of the enigmatic role of RF in the disease.

METHODS

IgM-RF and IgG-RF, as well as total levels of immunoglobulins of different subclasses, were measured in sera from rats with pristane-induced arthritis (PIA). The major gene regions were identified by linkage analysis of genetically segregating crosses.

RESULTS

The production of RF was found to correlate with development of arthritis and to be higher in females than in males. Surprisingly, the relatively arthritis-resistant E3 strain had higher levels of RF than the arthritis-susceptible DA strain. In an (E3 x DA)F(2) cohort a major locus controlling the levels of IgM-RF in serum was identified on chromosome 11 (Rf1) and another on chromosome 16 (Rf3), and these were not related to arthritis susceptibility. However, the Rf2 locus on chromosome 4 controlled IgG-RF levels, IgG2a levels, and chronic arthritis in males (Pia5). Some previously defined arthritis loci (Pia4, Pia6, Pia7, and Pia8) were found to also control immunoglobulin levels in serum.

CONCLUSION

RFs are produced in the rat PIA model and correlate with development of arthritis. Gene regions controlling RF and serum immunoglobulin levels were identified, of which some cosegregated with arthritis. This suggests a new focus of study to elucidate the role of RF in the pathogenesis of arthritis.

Differential gene expression in pristane-induced arthritis susceptible DA versus resistant E3 rats

Arthritis susceptibility genes were sought by analysis of differential gene expression between pristane-induced arthritis (PIA)-susceptible DA rats and PIA-resistant E3 rats. Inguinal lymph nodes of naïve animals and animals 8 days after pristane injection were analyzed for differential gene expression. mRNA expression was investigated by microarray and real-time PCR, and protein expression was analyzed by flow cytometry or ELISA. Twelve genes were significantly differentially expressed when analyzed by at least two independent methods, and an additional five genes showed a strong a tendency toward differential expression. In naïve DA rats IgE, the bone marrow stromal cell antigen 1 (Bst1) and the MHC class II beta-chain (MhcII) were expressed at a higher level, and the immunoglobulin kappa chain (Igkappa) was expressed at a lower level. In pristane-treated DA rats the MHC class II beta-chain, gelatinase B (Mmp9) and the protein tyrosine phosphatase CL100 (Ptpn16) were expressed at a higher level, whereas immunoglobulins, the CD28 molecule (Cd28), the mast cell specific protease 1 (Mcpt1), the carboxylesterase precursor (Ces2), K-cadherin (Cdh6), cyclin G1 (Ccng1), DNA polymerase IV (Primase) and the tumour associated glycoprotein E4 (Tage) were expressed at a lower level. Finally, the differentially expressed mRNA was confirmed with protein expression for some of the genes. In conclusion, the results show that animal models are well suited for reproducible microarray analysis of candidate genes for arthritis. All genes have functions that are potentially important for arthritis, and nine of the genes are located within genomic regions previously associated with autoimmune disease.

Dissection of the genetic complexity of arthritis using animal models

The exploding progress in genomic technology and knowledge now opens the possibility to actually identify the molecular mechanisms in disease. However, inflammatory diseases such as rheumatoid arthritis (RA) and multiple sclerosis (MS), are complex and polygenic and remain a challenge. One possible shortcut could be the use of inbred animals as models for RA and MS for the genetic analysis. These models have been extensively characterized and show a similar degree of complexity as the corresponding human diseases. Using these models linkage analysis followed by isolation of the loci in congenic strains have been shown to be highly efficient and have provided fundamental new knowledge on the genetic control of these diseases. The genetically controlled congenic strains are also useful as scientific tools. They can be used for the identification of the disease-associated genes and, thereby, the essential disease pathways that have been selected by nature. We know that this is possible since we have succeeded in identifying the genes within two of the congenic regions; the MHC class II gene Aq controlling immune response and the Ncf1 gene controlling oxidative burst. Both of these genes are associated with T cell activation and arthritis severity.

A comparative genetic analysis between collagen-induced arthritis and pristane-induced arthritis

OBJECTIVE

To compare the genetic regulation of collagen-induced arthritis (CIA) with that of pristane-induced arthritis (PIA) in rats.

METHODS

A genome-wide linkage analysis of an (E3 x DA)DA backcross of rats with CIA (n = 364 male rats; the same strain combinations as previously used to determine the genetic control of PIA) was performed. The strongest loci in both CIA and PIA (i.e., Cia12/Pia4 and Cia13/Pia7) were isolated in congenic strains. Susceptibility in both congenic strains was tested in rats with CIA and in rats with PIA.

RESULTS

We found a striking, although not complete, similarity of the arthritis-controlling loci in CIA and in PIA, as well as the previously defined loci associated with cartilage destruction, antibody production, and the acute-phase response. All major PIA quantitative trait loci (QTLs) identified in early severe arthritis were also strong regulators of CIA. The 2 strongest QTLs, Cia12/Pia4 on chromosome 12 and Cia13/Pia7 on chromosome 4, were also analyzed in congenic strains with DA or E3 as the background genome. Consistent with the results of linkage analysis, the congenic strain experiments showed that the chromosome 4 locus was more penetrant in CIA than in PIA, while the chromosome 12 locus almost completely dominated the control of PIA severity.

CONCLUSION

The underlying genetic control of CIA was found to have many, but not all, pathogenic mechanisms in common with PIA, despite the use of a cartilage-specific antigen (type II collagen) to induce CIA but not PIA.

Identification and isolation of dominant susceptibility loci for pristane-induced arthritis

Rheumatoid arthritis is a chronic inflammatory autoimmune disorder, controlled by multiple genes as well as environmental factors. With animal models, like the pristane-induced arthritis (PIA) in rats, it is possible to reduce the environmental effects and the genetic heterogeneity to identify chromosomal regions harboring genes responsible for the arthritis development. The PIA model has proved to be useful for identifying gene regions controlling different phases of the disease based on intercrosses between the resistant E3 and the susceptible DA rat. We have now performed a high-powered backcross analysis that confirms previous intercross-based data but also identifies additional loci. Earlier identified PIA loci were reproduced with high significance; Pia1 (MHC region on chromosome 20), Pia4 (chromosome 12), and Pia7 (chromosome 4) are all major regulators of PIA severity and were also found to operate in concert. These three loci were verified in congenic strains using both disease- and arthritis-inflammatory-related subphenotypes as traits. We were also able to detect five new quantitative trait loci with dominant effects on PIA: Pia10, Pia12, Pia13, Pia14, and Pia15 on chromosomes 10, 6, 7, 8, and 18, respectively. These data highlight the usefulness of the statistical power obtained in a backcross of a complex disease like arthritis.

Mapping and functional characterization of rat chromosome 4 regions that regulate arthritis models and phenotypes in congenic strains

OBJECTIVE

DA rats are highly susceptible to experimental models of rheumatoid arthritis (RA). Linkage analyses in different models have identified several quantitative trait loci (QTLs) within a 70-cM region of DA rat chromosome 4 (C4). We produced congenic strains for these QTLs in order to map and characterize their impact on arthritis development.

METHODS

Selective breeding was used to transfer C4 intervals from arthritis-resistant PVG.1AV1 rats onto DA rats. These congenic strains were evaluated for susceptibility to arthritis induced by intradermal injection of rat type II collagen, pristane (a well-defined synthetic adjuvant oil), mycobacteria, or squalene (an endogenous adjuvant oil used in human vaccine).

RESULTS

Rats congenic for PVG.1AV1 genes in the 70-cM region were less susceptible than DA rats to collagen-induced arthritis (CIA), pristane-induced arthritis, adjuvant-induced arthritis, and squalene-induced arthritis (SIA). Experiments in subcongenic strains indicated a gene regulating arthritis in males located in a 20-cM interval overlapping the QTL Pia5. A second gene, located in a 10-cM interval harboring the QTL Oia2, attenuated SIA and CIA. The latter caused a change in anticollagen antibody isotype levels toward a pattern similar to that seen in PVG.1AV1 rats.

CONCLUSION

The QTL Oia2 regulates arthritis induced both by the nonimmunogenic immunostimulant squalene and by cartilage collagen. In CIA, it also skews anticollagen isotype profiles, suggesting qualitative regulation of autoimmunity. Interestingly, the homologous human chromosome region 12p12-p13 has also been linked to RA, suggesting that genetic and functional dissection of this locus will provide clues to disease pathways that lead to joint inflammation.

New loci regulating rat myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis

Myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis (EAE) is an inflammatory disease in rats that closely mimics many clinical and histopathological aspects of multiple sclerosis. Non-MHC quantitative trait loci regulating myelin oligodendrocyte glycoprotein-induced EAE have previously been identified in the EAE-permissive strain, DA, on rat chromosomes 4, 10, 15, and 18. To find any additional gene loci in another well-known EAE-permissive strain and thereby to assess any genetic heterogeneity in the regulation of the disease, we have performed a genome-wide linkage analysis in a reciprocal (LEW.1AV1 x PVG.1AV1) male/female F(2) population (n = 185). We examined reciprocal crosses, but no parent-of-origin effect was detected. The parental rat strains share the RT1(av1) MHC haplotype; thus, non-MHC genes control differences in EAE susceptibility. We identified Eae16 on chromosome 8 and Eae17 on chromosome 13, significantly linked to EAE phenotypes. Two loci, on chromosomes 1 and 17, respectively showed suggestive linkage to clinical and histopathological EAE phenotypes. Eae16 and Eae17 differ from those found in previously studied strain combinations, thus demonstrating genetic heterogeneity of EAE. Furthermore, we detected a locus-specific parent-of-origin effect with suggestive linkage in Eae17. Further genetic and functional dissection of these loci may disclose critical disease-regulating molecular mechanisms.

Positional identification of Ncf1 as a gene that regulates arthritis severity in rats

The identification of genes underlying quantitative-trait loci (QTL) for complex diseases, such as rheumatoid arthritis, is a challenging and difficult task for the human genome project. Through positional cloning of the Pia4 QTL in rats, we found that a naturally occurring polymorphism of Ncf1 (encoding neutrophil cytosolic factor 1, a component of the NADPH oxidase complex) regulates arthritis severity. The disease-related allele of Ncf1 has reduced oxidative burst response and promotes activation of arthritogenic T cells. Pharmacological treatment with substances that activate the NADPH oxidase complex is shown to ameliorate arthritis. Hence, Ncf1 is associated with a new autoimmune mechanism leading to severe destructive arthritis, notably similar to rheumatoid arthritis in humans.

Both common and unique susceptibility genes in different rat strains with pristane-induced arthritis

Pristane-induced arthritis (PIA) in rats is an animal model for rheumatoid arthritis (RA). We have previously identified seven quantitative trait loci (QTLs), which regulate arthritis development using a cross between the susceptible DA strain and the resistant E3 strain of rats (Pia2-8). In the present study the inbred rat strain LEW.1F was used as the susceptible strain in a cross with the E3 strain. The results confirmed the locus Pia4 on chromosome 12, which previously was shown to be associated with PIA, and also with experimental allergic encephalomyelitis, in crosses between the rat strains E3 and DA. On chromosome 1, linked to the albino locus, we identified a novel QTL, Pia9 in the LEW.F1 cross. This locus was associated with arthritis severity in the early phase of disease. A locus on chromosome 16, denoted Pia11, was also associated with arthritis severity in the early phase of the disease. A suggestive locus was detected on chromosome 14, which was associated with arthritis severity at the time when PIA progresses into a chronic phase. Using a congenic LEW.1F strain, which carries E3 alleles at the Pia9 locus, we confirmed that the E3 allele significantly suppresses arthritis severity during the early phase of the disease. The results revealed synergistic effects between different susceptibility loci using ANOVA analysis. These interactions were influenced by gender. Rats with Pia9 alleles from LEW.1F and Pia11 alleles from E3, were shown to suffer from much more severe arthritis in the early stage of the disease. On the other hand, the Pia9 and the suggestive locus on chromosome 14 affected only males during the chronic phase of the disease. These findings provide clues to how genetic factors by themselves, and in interaction with each other, regulate the development of a disease, which displays many similarities to RA.

The arthritogenic adjuvant squalene does not accumulate in joints, but gives rise to pathogenic cells in both draining and non-draining lymph nodes

A single intradermal injection of the adjuvant-oil squalene induces T cell-mediated arthritis in DA rats. The chain of events leading from non-specific provocation of the immune system to arthritis, with clinical similarities to rheumatoid arthritis, is largely undetermined. Here, we combined in vivo tracking of tritium-labelled squalene with lymph node (LN) cell transfer experiments to determine where critical activation events may take place. The majority of squalene remained at the injection site (79%). The amounts recovered in peripheral joints (<1%) were equal to that recovered in other organs that can be targets in autoimmune diseases. This argues that arthritis does not develop as a consequence of adjuvant accumulation in joints. In contrast, substantial amounts of squalene were recovered in hyperplastic LN draining the injection site (1-13%). The adjuvant was deposited to a larger extent in cells than in extracellular matrix. The draining LN cells could transfer arthritis to naïve irradiated DA rats following in vitro stimulation with conA. Interestingly, non-draining LN were also hyperplastic and harboured arthritogenic cells, although they contained low amounts of squalene (<1%). Consequently, the amount of arthritogenic adjuvant in a particular LN is not closely linked to the development of pathogenic cells. The distribution pattern of squalene was similar in MHC-identical but arthritis-resistant PVG.1AV1 and LEW.1AV1 rats, and it was unaffected by T cell depletion with a monoclonal antibody (R73). Thus, T cells and non-MHC genes do not regulate dissemination of squalene, but rather determine arthritis development at the level of adjuvant response.

Polymorphisms of the tumor necrosis factor alpha locus among autoimmune disease susceptible and resistant inbred rat strains

Inbred rat strains manifest remarkable differences in susceptibility/severity to autoimmune disease. MHC alleles strongly influence the pathogenesis of autoimmune disease in rats, but the precise mechanism(s) remain inadequately defined. The TNFalpha gene is located in the class III region of the MHC. Polymorphisms, influencing either the structure or expression of the TNF protein, might contribute to differences in autoimmune disease susceptibility/severity. We therefore sequenced the Tnf locus using genomic DNA from ACI, BB(DR), BN, DA, F344, and LEW rats that vary in susceptibility/severity to autoimmune diseases. We found 42 polymorphisms among these six strains. Although none of these polymorphisms are predicted to change the amino acid sequence of the TNF protein, several reside in potential non-coding regulatory regions and may influence expression levels. These polymorphisms may serve as good candidates for analysis of TNF expression to elucidate the mechanism(s) by which the MHC regulates susceptibility and/or severity of autoimmune diseases.