Background/Purpose: MYD88 is a critical adaptor protein that connects Toll-like and IL-1 receptor signaling to activation of NF-kB. We previously reported a heterozygous de novo mutation in MYD88 (S222R) in a female pediatric patient with progressively deforming arthritis. To further investigate the contribution of MYD88 to arthritis pathogenesis, we created a novel Myd88 gene-edited (S209R ortholog) mouse using CRISPR/Cas.

Methods: In an attempt to elicit an arthritic phenotype, wild-type (WT), Myd88^S209R/WT (S209R+/-), and Myd88^S209R/S209R (S209R+/+) C57BL/6 mice were acutely stimulated with LPS or repeated CpG injections. We induced arthritis with collagen (CIA) in the mice to assess whether Myd88-S209R could drive differences in incidence, duration, or severity of arthritic phenotype. Arthritis was assessed via clinical scoring, caliper measurements of limb swelling, H&E staining of limb sections, and measurement of anti-collagen antibody production. Since CIA is a Th17 dependent model, draining lymph node IL-17+ and IFN-g+ T cells were measured during peak arthritis using flow cytometry. Serum IFN-g was measured by ELISA. To test the effect of Myd88-S209R on IFN-g production, which is protective in the CIA model, non-immunized splenic NK cells were stimulated ex vivo with IL-12, IL-18, or IL-12+IL-18.  To bypass the role of MYD88 in collagen-antibody production, we induced arthritis with anti-collagen antibodies (CAIA) in WT, S209R+/-, and S209R+/+ mice and assessed arthritis severity as above.

Results: Myd88-S209R+ mice did not develop a spontaneous arthritic phenotype, nor with acute TLR stimulation. CIA was achievable in 83% of S209R+/+ mice, 59% of S209R+/-, and 54% of WT mice. Interestingly, this difference in incidence is sex dependent; 100% of female S209R+/+ mice developed arthritis, whereas male S209R+/+ incidence did not differ from WT. S209R+/+ mice experienced significantly worsened clinical scores, histological scores, and increased number of affected limbs relative to WT mice, and these differences were most pronounced in female mice. Serum IFN-g was nearly undetectable in all S209R+/+ mice at peak arthritis.  Furthermore, S209R+/+ mice showed a deficit in IFN-g production in draining lymph node CD8+ T cells. This impaired IFN-g production was also observed in stimulated ex vivo splenic NK cells of non-immunized S209R+/+ mice. While all mice in the CAIA model developed arthritis, it persisted longer in mutant mice than in WT. Data from these experiments indicate enhancement of both the lymphocyte-dependent production of antibodies, likely influenced by lack of IFN-g, and their downstream pathogenic effects.

Conclusion: The Myd88-S209R mutation impacts the CAIA and CIA models by increasing the incidence, severity, and duration of the arthritic phenotype, likely due to a defect in IFN-g production.  Recapitulating the sex bias of juvenile idiopathic arthritis, female Myd88-S209R develop a higher incidence and severity compared to males. This is the first description of a mouse model harboring the Myd88-S209R mutation, which will allow for further investigation of the mechanism(s) by which innate and adaptive immune activation through MYD88 contributes to the complex phenotype of arthritis.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/myd88-s209r-mediated-immune-dysregulation-in-mouse-models-of-arthritis/