Background/Purpose: In Giant Cell Arteritis (GCA), granulomatous infiltrates occupy the vessel wall and elicit maladaptive vascular remodeling with intimal hyperplasia. The major cell types of the granulomatous lesions are CD4⁺ T cells and macrophages, some of which differentiate into multinucleated giant cells. Lesional CD4⁺ T cells undergo clonal expansion and vasculitic arteries contain mRNA and protein of multiple T cell effector cytokines, but it is unclear whether GCA patients possess specialized T cell subsets that promote macrophage multinucleation and granuloma formation in the vessel wall.

Methods: Patients with a positive temporal artery biopsy or unequivocal evidence for GCA aortitis were enrolled into the study. Patients with granulomatosis with polyangiitis served as disease controls and age-matched healthy controls were recruited through the Biobank. Tissue lesions were analyzed by immunofluorescence staining of temporal artery sections. T cell phenotyping relied on multiparametric flow cytometry and T cell reactivity was tested against anti-CD3-loaded antigen-presenting cells. The functional relevance of CD4⁺ T cell subsets was examined in immunodeficient mice engrafted with human arteries and immuno-reconstituted with immune cell populations from GCA patients (human artery-SCID chimeric mice).

Results: Memory CD4⁺ T cells isolated from GCA patients and age-matched controls fell into 10 clusters based on the combinatorial expression of 8 immunoreceptors (CD45RA, CCR7, PD1, LAG3, CD226, CD96, TIGIT, TIM3). GCA patients selectively expanded CD4⁺CD96⁺ memory T cells (10.9% control, 16.9% GCA), while CD4⁺TIGIT^high populations were reduced (24.5% control, 17.4% GCA). CD4⁺CD96^low T cells, generated by siRNA transfection, induced vascular inflammation in artery-SCID chimeric mice (p=0.0044), indicating that CD96 delivers a negative signal and opposes T cell activation. The expansion of CD4⁺CD96⁺ T cells was dependent on interaction of CD96 with its ligand CD155 on the surface of antigen-presenting cells. Maldifferentiation of CD4⁺ T cells in GCA patients was associated with the excessive production of three effector cytokines: IL-9 (p=0.02), IL-21 (p=0.028), and IFN-γ (p=0.03). In vivo testing identified IL-9 as a strong driver of vascular inflammation, associated with marked damage of the vessel wall smooth muscle cell layer. Anti-IL-9 treatment efficiently suppressed vascular inflammation (p=0.0025). In single cell RNA sequencing from tissue derived T cells, CD96 expression mapped to the T follicular helper cell population.

Conclusion: In GCA patients, the differentiation of CD4⁺ memory T cells is abnormal, leading to the selective expansion of immature and multifunctional T cells, while the transition into effector T cells is decelerated. The underlying defect lies in antigen-presenting cells that withdraw opposing signals as T cells progress through their differentiation cycle. Resulting CD4⁺ memory T cells hyperproduce IL-9, IL-21, and IFN-γ. Blocking T cell effector functions in GCA will therefore require targeting an array of cytokines.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/cd4-cd96-t-cells-are-pathogenic-effector-cells-in-giant-cell-arteritis/