Background: Giant Cell Arteritis (GCA) is characterized by inflammation of large and medium arteries. Classic symptoms include headaches, malaise and, in severe cases, blindness and aortic aneurysms. Corticosteroids (CS) are the first line of treatment. Relapsing disease patients undergo multiple courses of CS therapy increasing their CS exposure and toxicity. A significant unmet need for disease-modifying CS-sparing therapy remains in GCA as the efficacy of current treatment options, including tocilizumab have limitations.

We have previously reported elevated expression of granulocyte-macrophage colony stimulating factor (GM-CSF) pathway transcriptomic signature in GCA vessels. GM-CSF may contribute to underlying disease mechanisms by regulating inflammatory macrophages, dendritic cells (DCs) and T helper (T _H 1/T _H 17) cells which are involved in GCA pathogenesis. GM-CSF produced by T cells ¹ can promote polarization of inflammatory macrophages ² and recruitment and differentiation of monocytes into inflammatory DCs ² that can in turn recruit T cells and stimulate T _H 1/T _H 17 differentiation creating a feedback loop. GM-CSF may also exert direct effects on angiogenesis ³ and vessel wall remodeling ⁴ .

Objectives: To demonstrate the contributing role of GM-CSF pathway to inflammation in GCA arteries.

Methods: Immunostaining was used to examine expression of GM-CSF and GM-CSF-Rα proteins in temporal artery biopsies (TABs) from GCA and controls (patients with suspected but not confirmed GCA and a negative TAB). Costaining with cell markers such as CD31, CD3, and CD68 allowed visualization of cells expressing GM-CSF and GM-CSF-Rα. Expression of GM-CSF pathway molecules such as phospho-JAK2 and PU.1 proteins was detected by immunohistochemical staining of GCA and control TABs. Ex vivo cultured GCA arteries treated (10 each) with mavrilimumab (anti-GM-CSF-Rα) or placebo for 5 days were assayed for gene expression by qPCR, and culture supernatants were analyzed by ELISA.

Results: Endothelial cells and macrophages were the main cell types expressing GM-CSF and GM-CSF-Rα. Increased expression of phospho-JAK2 (activated signaling molecule) and nuclear-localized PU.1 (transcription factor) in GCA TABs compared to controls indicated the presence of active GM-CSF signaling pathway in GCA.

Inhibition of PU.1 mRNA expression in ex vivo cultures of GCA arteries treated with mavrilimumab indicated blockade of GM-CSFR signaling pathway. Mavrilimumab induced decrease in mRNA expression of key cell type markers including DC and macrophage activation markers CD83 and HLA-DRA, monocyte markers CD14 and CD16, T cell marker CD3ε, and B cell marker CD20 in these GCA artery cultures. Expression of inflammatory T _H 1/T _H 17 factors IFNγ (mRNA), TNFα, CXCL10 (IFNγ-stimulated chemokine) and IL-6 (mRNA and protein) was also inhibited by mavrilimumab in GCA artery cultures.

Conclusion: Increased GM-CSF, GM-CSF-Rα, and downstream pathway-associated protein levels in GCA biopsies were consistent with previously-observed increased transcriptome signature. Expression of genes associated with inflammatory cells was suppressed by mavrilimumab in cultured GCA arteries. These data implicate the GM-CSF pathway in GCA pathophysiology and increase confidence in rationale for targeting the GM-CSF pathway in GCA.

REFERENCES:

[1]GM-CSF and T-cell responses: what we do and don’t know. Shi et al. , Cell Res 2006

[2]GM-CSF-Dependent Inflammatory Pathways. Hamilton, Front Immunol 2019

[3]GM-CSF increases tumor growth and angiogenesis. Zheng et al. , Tumour Biol 2017

[4]GM-CSF deficiency affects vascular elastin production and integrity of elastic lamellae. Weissen-Plenz et al ., J Vasc Res 2008

Disclosure of Interests: Maria C. Cid Grant/research support from: Kiniksa Pharmaceuticals, Consultant of: Janssen, Abbvie, Roche, GSK, Speakers bureau: Vifor, Sujatha Muralidharan Shareholder of: Kiniksa, Employee of: Kiniksa, Marc Corbera-Bellalta: None declared, Georgina Espigol-Frigole Consultant of: Roche and Janssen, Javier Marco Hernandez: None declared, Amanda Denuc: None declared, Roberto Rios-Garces: None declared, Nekane Terrades-Garcia: None declared, John F. Paolini Shareholder of: Kiniksa, Employee of: Kiniksa, Annalisa D’Andrea Shareholder of: Kiniksa, Employee of: Kiniksa