Background/Purpose:

TAM receptors are a small family of receptor tyrosine kinases comprising Tyro-3, Axl and Mer, which are activated by GAS6 and Protein-S. TAM signaling is involved in phagocytosis, apoptosis, efferocytosis, and leukocyte stabilization and adhesion, and thus has been implicated in inflammatory disease. In fact, our findings from a previous study show apoptosis of neutrophils in the murine joint during inflammatory arthritis induces polarization of synovial macrophages, a key effector cell in arthritis. These data strongly suggest TAM signaling is involved in inflammatory arthritis in mice. However, TAM signaling can also induce immunoregulatory effects via inhibition of TLR signaling and blockade of the cytokine cascade. To that end, we used a number of genetic mouse models lacking TAM receptors Tyro3, Axl and Mer, or GAS6 to investigate the disparate functionalities of TAM signaling in inflammation using the KBxN model of inflammatory arthritis.

Methods:

Female GAS6^-/-, Tyro-3^-/-, Mer^-/-Axl^-/- and C57Bl/6 mice were bred in house until 8-10 weeks of age. Serum transfer induced arthritis was induced in mice using intravenous administration of KBxN sera at a dose of 85µl/20g. Inflammatory arthritis was monitored and scored for severity from day 0-21 or until all mice had 0 clinical score of disease. Clinical score was measured by scoring each distal joint from 0-3 based on inflammation, and summed to give a maximum clinical score of 12 per mouse. Mean clinical scores of N=4 mice in each treatment group were compared using GraphPad Prism. Differences were considered significant if P ≤ 0.05.

Results:

Tyro-3^-/- mice developed less severe inflammatory arthritis than C57Bl/6 controls. Mean clinical score in Tyro-3^-/- was less than that of C57Bl/6 controls from day 7 until day 14 although this did not reach statistical significance. Both groups had resolved all inflammation by day 21. Mer^-/-Axl^-/- mice also had reduced mean clinical score compared to C57Bl/6 controls which was statistically significant from day 11-23 (P < 0.05). Inflammation in the Mer^-/-Axl^-/- group resolved more quickly at day 21 vs day 25. Conversely, the Gas6^-/- group had increased disease compared to control mice from day 2 until day 11, which became statistically significant at peak disease on day 7 (P < 0.05).

Conclusion:

These findings demonstrate the pro-inflammatory effects of Tyro-3, Mer and Axl. In these models Tyro3 deletion inhibited early stage disease, whereas Mer Axl deletion resulted in faster disease resolution, thus indicating these receptors induce different downstream effects. Meanwhile, loss of GAS6 in these studies resulted in significantly increased inflammatory arthritis, suggesting GAS6 induces an anti-inflammatory or modulatory effect. Previous studies have shown TAM signaling causes both pro-inflammatory effects and induces a negative feedback loop to reduce inflammation via blockade of TLR signaling. The findings presented here indicate that TAM receptors Tyro-3, Mer and Axl have direct pro-inflammatory functions, whilst specific ligation of GAS6 is required to activate the anti-inflammatory negative feedback process.

« Back to 2018 ACR/ARHP Annual Meeting

ACR Meeting Abstracts - https://acrabstracts.org/abstract/the-pleiotropic-effects-of-tam-signaling-on-inflammatory-arthritis-in-mice-using-kbxn-serum-transfer-induced-arthritis/