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 gain-of-function mutation in MYD88 (S222R) in a patient with a progressively deforming arthritis characterized by marked bony overgrowth in arthritic lesions. To further investigate the contribution of MyD88 to arthritis pathogenesis, we created a novel Myd88 gene-edited (S209R) mouse using CRISPR/Cas.

Methods: We induced arthritis with collagen (CIA) in wild-type (WT), Myd88^S209R/WT, and Myd88^S209R/S209R C57BL/6 mice to assess differences in incidence, duration, and severity of arthritic phenotype. Arthritis was assessed via clinical scoring, caliper measurements of limb swelling, and H&E and safranin-O staining of limb sections. Since CIA elicits a directly-pathogenic adaptive immune response, anti-collagen antibodies were quantified from serum samples. To bypass the role of MyD88 in collagen-antibody production, we induced arthritis with anti-collagen antibodies (CAIA) in WT, Myd88^S209R/WT, and Myd88^S209R/S209R mice and assessed arthritis severity as above. In vitro assays were conducted to determine the effect of the mutation on T cells, including supernatant cytokine quantification, and measurement of proliferation rates by dye dilution.

Results: Mice harboring the Myd88-S209R mutation did not develop a spontaneous arthritic phenotype. CIA was achievable in 75% of Myd88^S209R/S209R mice, 56% of Myd88^S209R/WT, and 38% of WT mice. Myd88^S209R/S209R mice produced significantly higher amounts of anti-collagen antibodies than WT mice. Histological scoring revealed significantly more bone overgrowth in Myd88^S209R/S209R and Myd88^S209R/WT mice compared to WT. All mice that received exogenous anti-collagen antibodies developed arthritis, however arthritis in Myd88^S209R/S209R and Myd88^S209R/WT persisted longer than WT mice. Data from these experiments indicate enhancement of both the lymphocyte-dependent production of antibodies and their downstream pathogenic effects. In vitro analysis showed that Myd88^S209R/WT T cells secreted more GM-CSF, IL-1b and CCL4 in response to stimulation. Additionally, naive T cells from Myd88^S209R/S209R and Myd88^S209R/WT mice showed increased rates of proliferation.

Conclusion: The Myd88-S209R mutation impacts CIA and CAIA models by increasing the incidence and duration of the arthritic phenotype, as well as changes suggestive of new bone formation. 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-enhances-inflammation-in-mouse-models-of-arthritis/