An immunohistochemical analysis of folate receptor beta expression and distribution in giant cell arteritis – a pilot study

Abstract
Background
Giant cell arteritis (GCA) is a chronic vasculitis affecting large and medium-sized blood vessels, but currently, there are no identifiable biomarkers for targeted treatment, disease activity prediction, or therapeutic monitoring. The temporal artery (TA) can be biopsied, enabling morphological studies to analyze macrophages and T cells within the arterial wall microenvironment. In this study, we investigated the expression of folate receptor beta (FRB), a potential diagnostic and therapeutic biomarker, and compared it to key macrophage markers while correlating it with the severity of GCA.

Methods
We examined formalin-fixed, paraffin-embedded tissue sections from six GCA patients and two controls. Immunohistochemistry was conducted using FRB, ETB, CD68, and CD3 antibodies to assess activated macrophages and T cells, analyze FRB distribution across the intima, media, and adventitial layers, and characterize the inflammatory infiltrates. We compared the expressions of FRB, ETB, and CD68 in GCA patients to negative controls, as well as between severe cases (with visual loss) and mild cases (without visual loss).

Results
In GCA, moderate to severe inflammation led to over 90% destruction of the internal elastic lamina. Macrophages made up 36.3 ± 4.1% and CD3+ lymphocytes constituted 61.7 ± 4.1% of total leukocytes. FRB was selectively expressed in macrophages and localized to the adventitia. Compared to controls, GCA patients showed a trend toward increased median expression of FRB (9.8 cells/hpf vs. 0; p=0.095), ETB (20.5 vs. 0; p=0.095), and CD68 (38.8 vs. 5; p=0.071). ETB was observed in endothelial cells, smooth muscle cells, and macrophages in the intima/media. Notably, FRB expression positively correlated with both ETB (r=0.90; p=0.037) and CD68 levels (r=0.90; p=0.037). Additionally, ETB expression showed a perfect correlation with CD68 (r=1.0; p<0.0001). There were no significant differences in FRB levels between severe and mild GCA cases.

Conclusion
FRB appears to be a promising diagnostic and therapeutic biomarker with specific expression in macrophages associated with GCA. The localization of FRB+ macrophages in the adventitia, along with their correlation AM 095 with ETB and CD68, suggests their involvement in the pathogenesis of GCA.