Experiment
A previously identified experimental allergic encephalomyelitis QTL on mouse Chromosome 15, Eae2, was demonstrated to have an affect on collagen-induced arthritis (CIA). A congenic strain carrying RIIIS/J-derived DNA at the Eae2 locus on the genetic background of B10.RIII was constructed. Background strain B10.RIII is susceptible to CIA whereas donor strain RIIIS/J is resistant. The Eae2 congenic was immunized with bovine CII antigen and found to be resistant to collagen-induced arthritis. CIA resistancewas conferred in both male and female homozygous congenic animals, but heterozygous congenic female animals were susceptible to CIA. This suggests that the B10.RIII-derived allele at Eae2 confers CIA susceptibility in female animals.
The Eae2 region wasdissected using partial advanced intercross (PAI) lines. Eae2 was previously shown to interact with the Eae3/Cia5 locus on mouse Chromosome 3, so Eae2 congenic animals were crossed to animals congenic for the Eae3 locus (derived from the same donor and background strains as the Eae2 congenic). Bicongenic F1 animals were intercrossed for 8 generations and examined for CIA susceptibility. Four distinct loci within the Eae2 locus show linkage to collagen-induced arthritis. These loci are named Cia26, Cia30,Cia31, and Cia32.
Cia26 mapped to the distal portion of the Eae2 congenic region near D15Mit182 (19.8 cM). Homozygous RIIIS/J-derived alleles at Cia26 interact with B10.RIII-derived alleles at Cia30 to promote severe CIA and increase disease incidence in female animals. Cia26 also interacts with Cia31.
Cia30 mapped to the proximal portion of the Eae2 congenic region near D15Mit10 (9.9 cM). Homozygosity for RIIIS/J-derived alleles at Cia30 confers CIA resistance when animals are also homozygous for B10.RIII-derived alleles at Eae3/Cia5.
Cia31 mapped to the central portion of the Eae2 locus near D15Mit21 (9.6 cM). (According to this mapping experiment D15Mit21 is proximal to D15Mit10.) Animals heterozygous at Cia31 and homozygous for B10.RIII-derived alleles at Eae3/Cia5 exhibit earlier CIA onset and increased disease severity before boost. A potential candidate gene for Cia31 is Ank at 14.4 cM. Mutations in Ank are known to result in inflammation and arthritis.
Cia32 mapped to the distal portion ofthe Eae2 locus near D15Mit111 (17.8 cM). Female animals heterozygous at Cia32 and homozygous for B10.RIII-derived alleles at Eae3/Cia5 exhibit aggressive development of CIA.
Interacting T cell ratio modifier QTLs Trmq4 and Trmq5 mapped to mouse Chromosome15 near D15Mit80 (9.4 cM) and mouse Chromosome 3 near D3Mit187 (42.4 cM), respectively. Homozygosity for RIIIS/J-derived alleles at both Trmq4 and Trmq5 confer increased CD4+/CD8+ T cell ratios. Trmq4 colocalizes with Cia30 and Trmq5 colocalizes with Eae3/Cia5. Trmq4 also shows linkage to percentage of CD8+ T cell numbers in peripheral blood leukocytes.
Trmq6 mapped proximal to Trmq5 on mouse Chromosome 3 near D3Mit103 (51.1 cM). This locus affects relative numbers of CD4+ and CD8+ T cells in peripheralblood leukocytes and also shows interaction with Trmq4 on chromosome 15.
In addition, animals from a (B10.RIII x RIIIS/J)F1 x B10.RIII backcross were analyzed for CD86 expression on peritoneal macrophages. Parental strain B10.RIII exhibits increased CD86 expression in activated macrophages 48 hours after stimulation whereas CD86 expression in parental strain RIIIS/J is near the level of unstimulated macrophages. Genome scan revealed significant linkage to mouse Chromosome 17 in male animals near D17Mit20 (34.3 cM) and suggestive linkage to Cia31 on mouse Chromosome 15 near D15Mit21. The chromosome 17 locus is named Cxam (CD86 expression in activated macrophages.) Animals heterozygous at Cia31 exhibit decreased CD86 expression in 48-hour activated macrophages. Eae2 congenic animals heterozygous or homozygous for RIIIS/J-derived alleles exhibit increased CD86 expression in activated macrophages. This observation may confirm the existence of the D15Mit21 locus in the backcross study, however the CD86 increasing effect of the RIIIS/J-derived allele in Eae2 congenic mice is opposite of what was observed in parental and backcross mice.
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