CD44 as a Conserved Biomarker and Functional Modulator of Neuroinflammation in Multiple Sclerosis and Beyond

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS) characterized by demyelination, blood-brain barrier disruption, and leukocyte infiltration. The transmembrane glycoprotein cluster of differentiation 44 (CD44) has been implicated in neuroinflammation, but its functional role remains unclear. Here, we examined CD44 expression and function across both non-immune-mediated and immune-mediated MS animal models, including cuprizone-induced demyelination, experimental autoimmune encephalomyelitis (EAE), and combined cuprizone/EAE (Cup/EAE), as well as in human MS tissue and cerebrospinal fluid (CSF), using immunohistochemistry, flow cytometry and enzyme-linked immunosorbent assay. CD44 expression increased in parallel with demyelination and was most pronounced in the forebrain of Cup/EAE mice, where it localized to perivascular cuffs and lesion-associated parenchyma. CD44 expression co-localized to IBA1⁺ microglia/monocytes and CD3⁺ T cells. Flow cytometry analyses revealed high CD44 expression on myeloid-derived suppressor cells and regulatory T cells, which further increased upon immune activation. On the functional level, Cd44-deficient mice exhibited exacerbated disease severity in EAE, accompanied by increased immune cell infiltration and tissue damage. In human MS samples, CD44 expression and soluble CD44 levels in CSF were significantly elevated. Notably, CD44 upregulation was also observed in other neurological disease models, including ischemic stroke and APPswe/PS1dE9 mice, a model of Alzheimer's disease. Together, these findings identify CD44 as a conserved marker of neuroinflammatory activity and a context-dependent regulator of neuroinflammation with partially protective functions, rather than an exclusively pro-inflammatory molecule, highlighting its potential relevance in MS and related neurological disorders.