Background/Purpose:

TLR2 signaling pathway has been suggested as a potential therapeutic target in RA.  However, studies with mice deficient in TLR2 (TLR2-/-) and IL-1Ra suggest that TLR2 may suppress arthritis mediated through increased interferon-γ, and reduced TGFβ and T regulatory cell function.  In order to determine the role of TLR2 deletion on the effector phase of arthritis, studies were performed with the K/BxN serum transfer model of RA, mediated by antibodies to glucose-6-phosphate isomerase (GPI) which results in an immune complex-mediated arthritis. 

Methods:

Wild type and homozygous Tlr2^tm1Kir mutation (Tlr2-/-) mice on the C57Bl/6 background were injected intraperitoneally with 100 ml anti-GPI serum and evaluated between days 0 to 9 post-induction by ankle swelling and clinical score.  Ankles were harvested and sections analyzed by hematoxylin and eosin, and TRAP activity staining for osteoclasts.  IL-1b, IL-10, RANKL and osteoprotegerin (OPG) in ankle homogenates were quantified by ELISA.  Bone marrow-derived macrophages (BMM) were generated from wild type and Tlr2-/- mice by in vitro differentiation in GM-CSF for 7 days.  BMM activation was induced by incubation with model immune complexes employing mouse IgG coated on plastic plates for 4 hours. Supernatants were assessed for TNFα and IL-10 expression by ELISA.  The macrophage Fc receptor mediated macrophage signaling was assessed by immunoblot analysis employing phospho-antibodies to Akt, p38, and ERK.  Fc receptor expression on cell membranes was determined employing antibodies to CD16/CD32, analyzed by flow cytometry.  Macrophage Fc receptor isoform expression at mRNA level was determined by quantitative real-time RT-PCR.

Results:

The transfer of anti-GPI serum resulted in significantly worse arthritis in TLR2-/- mice compared to wild type controls.  Histological exam demonstrated more inflammation and joint destruction.  Examination of the joints homogenates collected at the peak of inflammation (day7 post-induction) revealed increased IL-1b and decreased IL-10 in TLR2-/- mice.  TLR2 deficiency also resulted in increased osteoclasts, identified by TRAP staining.  There was a trend toward reduction of OPG (p = 0.056) in the ankles of TLR2-/- mice, and a strong negative correlation (p < 0.001) between OPG and joint swelling.  There was no difference in the expression of inhibitory or activating Fc receptors in TLR2-/- mice.  However, activation of TLR2-/- macrophages with immune complexes resulted in significantly reduced IL-10, while there was no difference in TNFa, compared to the controls.  Macrophages from the TLR2-/- demonstrated decreased activation of Akt, but not ERK or p38, following activation with immune complexes. 

Conclusion:

These observations demonstrate that deletion of TLR2 exacerbates serum transfer-induced arthritis.  In the absence of TLR2, there was a reduction of IL-10 in the joints, and this may be due to the reduced activation of Akt by immune complexes.  These observations demonstrate cross-talk between TLR2 and Fc receptor signaling modulates the effector phase of inflammatory arthritis.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/tlr2-deletion-promotes-arthritis-and-joint-destruction-through-reduction-of-il-10/