CD44 is associated with muscle inflammation in polymyositis and skin damage in idiopathic inflammatory myopathy.

Objectives: Idiopathic inflammatory myopathy (IIM) is a group of systemic autoimmune diseases characterised by muscle involvement. This study aims to reveal the characteristics of IIM subtypes and explore the molecular mechanisms underlying IIM.

Methods: The STRING database was utilised to construct a protein-protein interaction network of differentially expressed genes obtained from the GSE128470, GSE3112, and GSE39454 datasets. The immune cell infiltration level was assessed by CIBERSORT in polymyositis (PM). Experimental autoimmune myositis (EAM) model mice were constructed for experimental verification. Serum levels of soluble CD44 (sCD44) were measured using enzyme-linked immunosorbent assay.

Results: The upregulated hub gene CD44 was highly expressed in inflammatory cells infiltrating the skeletal muscle of patients with PM and in EAM mice. CD44 was correlated with both M1 macrophages (r=0.57, p<0.0001) and M2 macrophages (r=0.57, p<0.0001) in PM. Additionally, CD44+F4/80+ macrophages in skeletal muscle were increased (p<0.0001) and CD44 showed a stronger association with markers of M1 macrophage in EAM mice. Moreover, serum sCD44 levels were elevated in patients with IIM (p=0.0024), PM (p=0.0332) and dermatomyositis (p=0.0001) notably in the anti-melanoma differentiation-associated gene 5 antibody positive subtype (p=0.0007). sCD44 levels also positively correlated with visual analogue score (r=0.4424, p=0.0013), myositis intention to treat activity index (r=0.3938, p=0.0047), skin damage score (r=0.3796, p=0.0101) and skin activity score (r=0.4625, p=0.0014) in patients with IIM.

Conclusions: This study suggests that macrophages expressing CD44 may be involved in the pathogenesis of PM, and sCD44 could serve as a potential marker for skin damage and activity in IIM.