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

Abstract

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 TNFα 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.

Introduction

Rheumatoid arthritis (RA) is an autoimmune disease characterized by chronic erosive inflammation of the peripheral joints.1 The causes of RA remain largely unknown, but considerable evidence suggests a multifactorial aetiology involving both environmental and genetic factors. Large efforts have been directed towards the understanding of the molecular mechanisms underlying RA and recent studies based on various gene-finding strategies have identified the first genes implicated in the susceptibility to RA.2, 3, 4 However, most susceptibility genes are yet to be discovered. Because of the complexity of the disease in humans, animal models for RA have become attractive tools for gene-identification. Use of such models not only overcomes genetic complications, but it also permits studies under stable environmental conditions and allow for experimental investigation. We have previously reported the location of 12 non-MHC quantitative trait loci (QTL) (Pia2–8, Pia10, Pia12–15) that regulate the day of onset, severity or chronicity of pristane-induced arthritis (PIA) in F2 offspring of the susceptible DA and the resistant E3 inbred rat strains.5, 6 Although some loci are specific for this particular arthritis model, others overlap with collagen-induced arthritis (CIA) or adjuvant oil-induced arthritis (OIA). One of the strongest QTLs of PIA is Pia7 on rat chromosome 4. The QTL was first identified in an F2 intercross population between the PIA-susceptible DA rat and the resistant DXEC rat, a recombinant inbred strain between DA and E3.7 The QTL Pia7 regulates the acute phase of PIA and overlaps with QTLs for CIA, Cia138 and OIA, Oia2.9 To positionally clone the underlying gene(s) and study the immunological effects of this arthritis-regulating QTL, we produced a congenic rat strain in which the regulatory locus was transferred from the arthritis-resistant E3 strain into the arthritis susceptible DA strain. Subsequently, we isolated a minimal congenic fragment that included the antigen-presenting lectin-like receptor complex (APLEC). APLEC genes, encoding C-type lectin like receptors preferentially expressed on professional antigen-presenting cells (dendritic cells, macrophages and B cells) and neutrophils,10 have been positionally identified as the arthritis-regulating genes of the Oia2 locus.11 Therefore, we investigated the susceptibility to arthritis using pristane and adjuvant oil. In addition, we studied cell numbers and function of various leukocyte populations to establish the mechanism conferring the arthritis protection in congenic rats.

Results

Construction of DA.E3-Pia7 congenic rats

To positionally clone the underlying gene(s), we generated DA.E3-Pia7 congenic rats harboring the arthritis-protecting E3 fragment at rat chromosome 4 on the DA background. After testing several congenic sublines, we generated a rat strain harboring a minimal congenic fragment with the microsatellite markers D4Mir55 and D4Arb23 representing the centromeric and telomeric outer borders, respectively (Figure 1). We found that the 2.3 Mb congenic fragment of DA.E3-Pia7 rats overlaps with the 0.6 Mb fragment in DA.PVG-Oia2 congenic rats. The Oia2 locus has been positionally cloned to APLEC, a cluster of genes encoding group II C-type lectin-like receptors. As genetic variation in APLEC might also underlie Pia7, we genotyped SNPs that were retrieved by comparison of DNA sequences from BN (Ensembl), DA and PVG. Several SNPs in Dcir3, Dcir2 and Mincle have been identified in the susceptible DA/Kiru and the resistant PVG/Kiru strains. When testing the parental rat strains of Pia7 congenic rats, we found identical SNP haplotypes in our DA/Rhd and E3/Rhd.

Figure 1
figure1

Physical map of the Pia7 locus in DA.E3-Pia7 congenic rats. It shows the size and location of the congenic fragment (left) in comparison with the location of the Oia2 locus. All genes inside the Oia2 locus are indicated (middle). All tested intragenic SNPs between BN/Kiru as reference strain and the parental strains for the DA.E3-Pia7 and DA.PVG-Oia2 congenic rats are shown at the right side, DA/Rhd and E3/Rhd, as well as DA/Kiru and PVG/Kiru, respectively.

DA.E3-Pia7 congenic rats are protected from arthritis

To test the arthritis-susceptibility in the DA.E3-Pia7 congenic strain, we immunized homozygous (E3/E3) and heterozygous (DA/E3) congenic rats as well as DA littermate controls (DA/DA) with a single dose of either 50 μl pristane or 150 μl incomplete Freund's adjuvant. Two experiments with identical set up were performed and thus, data were pooled. In PIA, we observed a mild, but reproducible reduction in arthritis severity in homozygous female congenic rats, which were significantly different from DA at scoring day 12–21 (P<0.05, Figure 2a). However, there was no significant difference between heterozygous animals and DA controls. In male rats, results were similar and we detected a reduced arthritis score in homozygous rats compared with DA rats (Figure 2b). However, due to the low number of rats (n=6) represented in this experiment, significant results were only observed for day 12 (P<0.05). In Supplementary Figure S1, we provide additional data for heterozygous DA.E3-Pia7 congenic male and female rats from backcross generation (N11+3), where we used DA littermates as controls. The data confirm that there is no difference in PIA susceptibility between heterozygous rats and DA controls.

Figure 2
figure2

DA.E3-Pia7 congenic rats are protected from severe arthritis. (a) Arthritis severity after PIA in female rats. Homozygous congenic rats (E3/E3, n=19) developed significantly less severe arthritis compared with DA littermate controls (DA/DA, n=23), whereas no difference between heterozygous DA.E3-Pia7 rats (DA/E3, n=11) and DA rats was observed. (b) Arthritis severity after PIA in male rats. Only homozygous congenic rats (n=6) were compared with DA rats (n=10). (c) Arthritis severity after OIA in female rats. A homozygous congenic rat (n=1) was completely protected from developing OIA. Heterozygous rats (n=15) had a significantly reduced arthritis severity compared with DA controls (n=22). (d) The same results were observed in male rats after OIA. Homozygous congenic rats (n=2) were completely protected from OIA. Heterozygous rats (n=16) had a significantly reduced arthritis severity compared with DA controls (n=25). Data from two independent experiments were pooled and are depicted as mean arthritis score±s.e.m. Mann–Whitney P values between E3/E3 and DA/DA for PIA and between DA/E3 and DA/DA for OIA are depicted. *P<0.05, **P<0.001, ***P<0.0001.

Because the congenic Pia7 fragment co-localized with Oia2 and both PVG and E3 rats show the same SNP haplotypes in the APLEC region, we sought to study OIA. As in PIA, the disease score was significantly reduced in homozygous animals (Figures 2c and d). Notably, all homozygous rats were completely protected from OIA. In addition, there was also a significant reduction in arthritis severity between heterozygous rats and DA controls (P<0.0001 in females and males). Taken together, Pia7 congenic rats were protected from both PIA and OIA, but the decreased arthritis severity was more pronounced in OIA.

The protective effect in DA.E3-Pia7 congenic rats is conferred in the priming phase of disease

To investigate, whether the Pia7 locus controls the immune priming of arthritis or is involved in regulating the effector phase of the disease locally in the joints, we carried out reciprocal adoptive transfers of primed splenocytes. In selective depletion experiments it has been shown previously, that it is the CD4+ αβT cell subset of activated splenocytes that transfer arthritis.12 Because the arthritis severity was more reduced in OIA compared with PIA, we used a single injection of IFA for priming of splenocytes. Two different experiments with similar results were performed and thus, data were pooled. Here, we show clearly that the Pia7 locus controlled the priming of arthritogenic T cells and not the effector phase, as spleen cells from Pia7 congenic donors did not transfer disease (P<0.0001), whereas naive Pia7 recipient rats receiving activated cells from DA rats developed severe arthritis (Figure 3). There was no significant difference between recipient groups that received cells from the same donor strain.

Figure 3
figure3

Mean arthritis severity after transfer of IFA-primed spleen cells. Female rats were primed with IFA and spleen cells were transferred into naive male donor rats. IFA-primed splenocytes from DA.E3-Pia7 donors were transferred into either DA (white square, n=16) or DA.E3-Pia7 (white circle, n=15) recipient rats. Simultaneously, IFA-primed splenocytes from DA donors were transferred into either DA (back square, n=14) or DA.E3-Pia7 (back circle, n=14) recipients. Two independent experiments were performed, data were pooled and are shown as mean arthritis score±s.e.m. All rats receiving splenocytes from DA.E3-Pia7 rats developed significantly less arthritis at all days between day 6 and day 32 (each day, P<0.0001).

DA.E3-Pia7 congenic rats have increased frequencies of granulocytes and B cells

As we observed such a dramatic reduction in the arthritis susceptibility in DA.E3-Pia7 congenic rats, we investigated the cell number and function of various cell populations of the immune system, which have been implicated in the pathogenesis of arthritis. We collected peripheral blood of 8 weeks old naive rats (at least 18 rats per group) and analyzed them by flow cytometry (Figure 4). First, we analyzed cells from the T and B cell compartment. We could not detect any obvious phenotypic difference in the αβT cells of the CD4 subset, which transfer the arthritis in adoptive transfer experiments. However, we found a significant difference in the CD8-positive subset, in which DA.E3-Pia7 congenic rats have a slightly higher percentage of CD8+ cells among αβT cells (P<0.05). When analyzing the frequency of αβT cells among all leukocytes, DA.E3-Pia7 congenic rats were found to have significantly less αβT cells compared with DA rats (P<0.05). Next, we analyzed the frequency of B cells in the total leukocyte population and detected a significant higher frequency of B cells in Pia7 congenic rats compared with DA controls (P<0.0001). In addition, Pia7 congenic rats were found to have significantly more granulocytes compared with DA controls (P<0.0001). No differences in the frequencies of dendritic cells/macrophages, CD25-expressing CD4 and CD8 cells, nor NK cells were observed (data not shown).

Figure 4
figure4

Frequencies of naive cell populations determined by flow cytometry. (a) Frequency of CD8α+ cells among αβT cells, (b) frequency of αβT cells among leukocytes, (c) frequency of B cells among leukocytes, and (d) frequency of granulocytes among leukocytes. Data represent results from two separate experiments with at least 18 rats per group. Each circle/square represents an individual rat. Horizontal lines display the mean. *P<0.05, ***P<0.0001. NS, not significant. No significant difference was detected in the absolute number of leukocytes.

Cells from DA.E3-Pia7 congenic rats show an enhanced proliferation and produced more cytokines upon stimulation

The pro-inflammatory cytokine TNFα has a pivotal role in the pathogenesis of PIA and OIA. Therefore, we examined the TNFα secretion using various stimulation protocols (Figures 5a–c). First, we stimulated whole blood with LPS and surprisingly, we found an increased secretion of TNFα in the protected DA.E3-Pia7 congenic rats compared with DA rats (P<0.05). We also measured the TNFα production in splenocytes after ConA stimulation or after cross-linking of CD3, which exclusively stimulates T cells. We found an increased TNFα secretion in congenic rats after ConA stimulation compared with DA rats (P<0.05), but we could not detect any difference after anti-CD3 treatment, indicating no difference in TNFα production of T cell from naive congenic and DA rats.

Figure 5
figure5

Cytokine production and proliferation after in vitro stimulation. (a) TNFα secretion after whole blood stimulation with LPS. (b) Splenocytes from naive DA and DA.E3-Pia7 congenic rats were cultured for 72 h and stimulated with ConA or (c) plate-bound anti-CD3. TNFα production was measured after 72 h. (d) IFNγ secretion after 72 h culture with medium alone (basal, left), with ConA (middle) and plate-bound anti-CD3 (right). (e) Cell proliferation in untreated cells (basal, left) and after stimulation with ConA (middle) or plate-bound anti-CD3 (right). Mean values±s.e.m are shown of triplicate cultures from at least 14 rats per group. Black bars, DA; white bars, DA.E3-Pia7. *P<0.05, **P<0.001, NS, not significant.

However, enhanced TNFα secretion is not the only known pathologic mechanism in RA and to investigate, whether the arthritis difference is in fact a result of an altered cell proliferation or shift in Th1/Th2 cytokine response, we performed in vitro proliferation and cytokine assays of naive splenocytes. First, we analyzed the cell proliferation and found a significant higher proliferation in non-stimulated (P<0.05) and ConA-stimulated (P<0.05) Pia7 congenic cells compared with DA controls. No difference was detected using anti-CD3 cross-linking (Figure 5d). In addition, we analyzed the proinflammatory cytokines IFNγ (Th1) and IL-4 (Th2), as well as the anti-inflammatory cytokine IL-10. Although IL-4 and IL-10 were under the detection limit of our assays and could not be detected in either of the strains, the Th1 cytokine IFNγ was secreted in large amounts. When measuring IFNγ production, we could only observe a significant difference in non-stimulated (P<0.001) cells, whereas IFNγ production in cells with either ConA or anti-CD3 stimulus was similar in both groups (Figure 5e). Taken together, Pia7 congenic rats showed an increased TNFα secretion after LPS and ConA stimulation, which was mainly due to cells of the innate immune system. In addition, when naive splenocytes were cultured for 72 h, congenic cells showed an increased cell proliferation and IFNγ production. However, when T cells were exclusively activated through CD3 cross-linking, they did not show any phenotypical differences.

Discussion

Two different arthritis models, PIA and OIA, were used to assess the arthritis-regulating effect of the DA.E3-Pia7 congenic strain. In PIA, the non-immunogenic mineral oil pristane is injected, causing arthritis which depends on polyclonal activation of T cells, as shown by depletion of αβT cells and disease transfer though CD4+ T cell.12, 13 In OIA, a poorly defined mixture of emulsifier and mineral oils (IFA) is injected, causing arthritis, which is also dependent on polyclonal activation of T cells.14 Although both models appear to involve same physiological pathways in the induction of arthritis, there is a profound difference in the mechanisms sustaining inflammation. In OIA, arthritis is transient and inflammation subsides 2–3 weeks after onset15 while PIA is a chronic disease with a relapsing and progressive disease course.13 Nevertheless, both models probably share a high similarity in their genetic control, as seen in linkage analyses with overlapping OIA and PIA QTLs.16 But one must also consider that some genetic loci will be different between the models, accounting for the development of chronic arthritis in PIA. In this study, we show that the Pia7 locus is co-localizing with the Oia2 QTL identified by Lorentzen et al.11 In fact, arthritis data obtained from our Pia7 congenic rats show that the protection is greater in OIA than in the original PIA model. This could be due to the higher arthritogenic potential from pristane compared with IFA. However, it could also be due to the loss of other arthritis-regulating genes in the original Pia7 locus. These genes would rather be controlling arthritis after pristane injection, and could have been lost in the process of generating congenic sublines. There is evidence to suggest that this was the case in our study. First, results from the linkage study as well as from previously published and unpublished experiments with larger congenic fragments showed a clear dominant inheritance, with one E3 allele protecting from pristane-induced arthritis.6, 7, 17 In this study however, PIA was only reduced in rats with two E3 alleles, thus showing a clear recessive mode of inheritance. Although linkage peaks most often do not very precisely predict the position of a QTN, it is still notable that both the LOD score interval as well as the maximum peak from Pia7 is centromeric from Oia2. This goes in hand with the second observation, where linkage analyses from advanced intercrossed lines between DA and PVG.DA-RT1av1 rats show two additional PIA QTL peaks centromeric to the APLEC region.18 Therefore, it is very likely that additional genes close to the APLEC locus, but excluded in the congenic fragment, are regulating pristane-induced arthritis as well.

We have shown previously that DA/OlaHsd and DA's derived from the Hanover rat colony (DA/ZtmRhd and DA/ZtmKiru) differ genetically and in their arthritis susceptibility.19 DA/OlaHsd, which have been used as a genetic background for the Pia7 congenic rats, are more susceptible to PIA. On the other hand, DA/Kiru and DA/Rhd, which have been used as background for Oia2 congenic rats and in earlier experiments with Pia7 congenic rats respectively, are less sensitive to PIA. Thus, the different backgrounds account for the discrepancies in disease severity of DA rats and consequently also the congenic rats, which have been observed when comparing the present results with our own previous results and with experiments from Guo et al.20

Congenic strains have a great potential in the identification of arthritis-regulating genes and with the rat genome sequenced, one can expect an acceleration in the number of identified genes within the next decade.21 So far, one arthritis-regulating gene in the rat has been positionally identified, that is, the Ncf1 gene,22 and another mapping study identified a gene complex on chromosome 4 including seven genes of the APLEC complex.11 With this study we can strengthen the finding of the APLEC complex. We observed a similar dramatic reduction in arthritis severity after OIA in congenic rats, with the similar susceptible background (DA strains) but with a different donor fragment (E3 strain). We detected a significantly higher frequency of granulocytes and B cells in Pia7 congenic rats compared with DA controls. Interestingly, it is these cell types in which APLEC-encoding group II C-type lectin-like receptors are predominantly expressed. Although we could not detect a difference in the cell number of dendritic cells and macrophages, we observed an increased TNFα secretion after LPS stimulation, accounting for a functional difference between the congenic and control rats. It may be surprising that both cell number as well as function indicates an enhanced activity of the immune system in congenic rats protected from arthritis. Especially the increased TNFα production stands in discrepancy to other studies, where TNFα appears to be a mediator of joint inflammation.23 However, our findings are in agreement with data from Guo et al.,24 where they found an enhanced IL-6 secretion of Oia2 congenic macrophages following LPS stimulation. It also confirms data from a mutant DA rat, which is protected from PIA, CIA and experimental allergic encephalomyelitis yet shows enhanced cell proliferation and Th1 cytokine production as well as an increased number of activated T cells (unpublished data). Further studies are required to elucidate the pathological mechanisms in which an apparently lower activation of the immune system leads to a greater arthritis severity. It is already apparent from studies of reactive oxygen species (ROS) that immune-modulating molecules, such as increased ROS production, can have both pathological effects in arthritis (joint destruction), but they can also be beneficial to the host (regulation of arthritogenic T cells).25 Thus, it is important to address not only the quantity of certain pro-inflammatory cytokines but also the quality, such as the source, the stimulus and the time point of the cytokine secretion. Taken together, Pia7 congenic rats showed an increased population size and activation state of cells of the innate immune system (granulocytes, LPS response). These cellular phenotypes were already present in naive animals, however, it is still unclear how these phenotypes would translate to a lower T-cell activation and reduced arthritogenic potential. In conclusion, this study confirms the association of the APLEC locus to experimentally induced arthritis in rats. Furthermore, it shows the great potential of congenic rats to identify genes of complex traits and provide models for studying their function.

Materials and methods

Animals

DA/ZtmRhd (DA) and E3/ZtmRhd (E3) rats originated from Zentralinstitut für Versuchstierkunde (Hanover, Germany) and were bred for more than 20 generations in the animal facility in Lund in a climate-controlled environment with 12 h light/dark cycles. In the same facility, breeding of DA/OlaHsd originating from Harlan Europe was performed. DA.E3-Pia7 (D4Wox49-D4Got136) congenic rats were obtained through conventional backcross breeding (N11 using DA/ZtmRhd plus additional N5 using DA/OlsHsd) with negative selection of all known PIA QTLs and positive selection using microsatellite makers. Heterozygous congenic rats were intercrossed and homozygous congenic rats and DA littermates were selected and bred one generation to obtain experimental rats. Rats were housed in polystyrene cages containing wood shavings and fed standard rodent chow and water ad libitum. The rats were free from common pathogens and experiments are approved by the local ethical committee (Malmö/Lund, Sweden).

Genotyping

DNA was prepared from toe biopsies by alkaline lysis,26 amplified with fluorescence-labeled microsatellite markers by Multiplex-PCR according to standard protocol and analyzed on MegaBACE 1000 (Amersham Bioscience, Buckinghamshire, UK). Sequences for microsatellite markers used for genotyping of congenic fragments were retrieved from http://www.ensembl.org/index.html. In addition, one marker was newly designed and designated D4Mir55. Sequences of the primers are as follows: forward primer 5′-IndexTermGCTGAGTTTTGGGGTTGATTT-3′, reverse primer 5′-IndexTermGGTACAGCCGCCTTCTT-3′. Single-nucleotide polymorphism (SNP) typing was performed using specific primers designed with PSQ Assay Design software (Biotage, Uppsala, Sweden). The reactions were amplified by PCR using biotinylated forward and reverse primers followed by separation with streptavidin. The SNPs were then analyzed in sequencing reactions using pyrosequencing equipment according to protocols supplied by the manufacturer (Biotage).

Induction and evaluation of arthritis

PIA was induced by a single intradermal injection of 50 μl pristane (2,6,10,14-tetramethylpentadecane; ACROS Organics, Geel, Belgium) at the base of the tail. OIA was induced by an intradermal injection of 150 μl incomplete Freund's adjuvant (IFA; Difco, Detroit, MI, USA). Arthritis was induced in rats at the age of 7–9 weeks and arthritis development was monitored in all four limbs, using a macroscopic scoring system. Briefly, one point was given for each swollen and red toe, one point for each affected midfoot, digit or knuckle and five points for a swollen ankle (maximum score per limb 15).27

Adoptive spleen cell transfer

Homogenized spleen cells from female donor rats 12 days after injection of 200 μl IFA were pooled, depleted of erythrocytes with ACK lysis buffer and cultured at a cell density of 4 × 106 per ml at 37 °C in D-MEM supplemented with streptomycin, D-penicillin, β-mercaptoethanol, 5% fetal calf serum and 3 μg ml–1 ConA. After 48 h, cells were collected, washed and resuspended in PBS and injected intraperitoneally into male recipient rats at a concentration of 40–47 × 106 per ml depending upon the performed experiment.

In vitro spleen cell stimulation and proliferation

Splenocytes were obtained from naive DA.E3-Pia7 congenic rats (eight females, eight males) and DA controls (eight females, six males). Tissues were homogenized, erythrocytes lysed with ACK lysis buffer. After washing with PBS, cells were plated in 96-well plates at 5 × 105 cells per well (200 μl per well) and cultured in X-VIVO 15 Medium (Bio-Whittaker, In Vitro AB, Stockholm, Sweden) supplemented with streptomycin, D-penicillin and β-mercaptoethanol. Splenocytes were left untreated or stimulated in triplicate with either ConA (3 μg ml–1) or plate-bound anti-CD3 (0.75 μg per well, BD Pharmingen, San Diego, CA, USA) for 72 h at 37 °C and 5% CO2. For the last 18 h of culture, cells were pulsed with 1 μCi [3H]thymidine (Amersham Bioscience). For LPS stimulation, blood was collected from naive DA.E3-Pia7 congenic rats (12 females, 6 males) and DA controls (12 females, 10 males). 50 μl heparinized whole blood in 150 μl D-MEM medium supplemented with streptomycin and D-penicillin was cultured in 96-well cell culture plates. After incubation for 18 h at 37 °C and 5% CO2, supernatant was collected and immediately used for cytokine detection.

Detection of cytokines

All cytokines in supernatant were analyzed by Europium3+-linked immunosorbent assays (ELISA). Commercially available anti-IFNγ, anti-TNFα, anti-IL4 and anti-IL-10 antibodies (R&D Systems, Minneapolis, MN, USA; BD Bioscience, San Jose, CA, USA) were used according to the manufacturer's protocols. Shortly, ELISA plates were coated overnight, blocked with 2% BSA and washed with PBS containing 0.1% BSA and 0.05% Tween 20. Undiluted supernatant (TNFα, IL4 and IL-10) or diluted supernatants (IFNγ) were added and incubated overnight. Biotin-labeled anti-rat antibodies were added and incubated. Eu3+-labeled streptavidin (Wallac, Turku, Finland) was added. For final detection enhancement solution was added and fluorescence emissions were read using Victor/Wallac (Wallac).

Antibodies for flow cytometry

Anti-rat mAbs were purchased from BD Bioscience as FITC, PE, PerCP or APC conjugates: anti-LCA (leukocyte common Ag) for staining of leukocytes (OX-1), anti-αβTCR (R73), anti-CD4 (OX-35), anti-CD8a (OX-8), anti-CD25 (OX-39), anti-CD11b/c (Ox-42), HIS48 for staining granulocytes and anti-CD161a (10/78) for staining NK cells. Anti-CD45RA for staining of B cells (OX-33) conjugated with Alexa Fluor 647 was purchased from Biolegend (San Diego, CA, USA).

Flow cytometry

Peripheral blood from DA.E3-Pia7 congenic rats (12 females, 6 males) and DA rats (12 females, 10 males) was obtained through tail bleeding. After depletion of erythrocytes with ACK lysis buffer, cells were washed with PBS containing 1% FCS and 0.2% NaN3. Cells were stained with conjugated mouse anti-rat mAbs for 20 min at 4 °C. After washing, cells were acquired on FACSort (BD Biosciences) using the BD Cell-Quest Pro, Version 4.0.1 software (BD Biosciences) and later analyzed by FlowJo (Tree Star Inc., Ashland, OR, USA) software.

Statistics

Statview software program was used for all statistical analyses. Incidence of arthritis was analyzed by Fisher's exact test and the non-parametrical Mann–Whitney U-test (comparison of two groups) was used in all other statistical analyzes. P<0.05 were considered significant.

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Acknowledgements

This work has been supported by grants from the Swedish Research Council, the Swedish Association against Rheumatism, the Swedish Foundation for Strategic Research and European Union Grants MUGEN (LSHG-CT-2005–005203), AUTOCURE (LSHM-CT-2005-018661) and EURATools (European Commission contract no. LSHG-CT-2005-019015).

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Correspondence to R Holmdahl.

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Rintisch, C., Kelkka, T., Norin, U. et al. Finemapping of the arthritis QTL Pia7 reveals co-localization with Oia2 and the APLEC locus . Genes Immun 11, 239–245 (2010). https://doi.org/10.1038/gene.2010.2

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Keywords

  • rheumatoid arthritis
  • experimental arthritis
  • rat
  • quantitative trait locus
  • antigen-presenting lectin-like receptor complex

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