Neurofilament-light chain (NfL) and glial fibrillary acidic protein (GFAP) are two of the main fluid biomarkers currently being examined for their clinical utility in evaluating disease activity, progression and treatment response. However, there are a number of other biomarkers under investigation. The following is a summary of some of the more promising candidates (reviewed in Maroto-Garcia et al. Clin Chim Acta 2023;548:117471).

Chitinase-3-like 1 (CHI3L1) is expressed in astrocytes, macrophages and neutrophils and is involved in inflammation and tissue remodelling. CSF levels of CHI3L1 are elevated in MS (Burman et al. J Neuroimmunol 2016;292:52-57). CHI3L1 has been shown to be predictive of physical and cognitive disability in patients with optic neuritis and clinically isolated syndrome (Modvig et al. Mult Scler 2015;21:1761-1770). [See also Emerging role of CHI3L1 as MS biomarker, NeuroSens, May 2, 2023.]

S100β is a binding protein produced by astrocytes and oligodendrocytes that promotes neuronal proliferation and oligodendrocyte differentiation. S100β levels are elevated in blood and CSF at MS diagnosis and during periods of disease activity (Barateiro et al. Mol Neurobiol 2016;53:3976-3991). It may be useful to distinguish disease phases (levels RRMS > SPMS > PPMS) and treatment response (O’Connell et al. Autoimmunity 2014;47:505-511). This marker is not specific to MS: S100β levels are elevated post-stroke and following traumatic brain injury. Recent studies have suggested that S100β is a Damage-Associated Molecular Pattern (DAMP) molecule that contributes to tissue damage in numerous neurological diseases, including AD, PD, ALS, epilepsy and MS (Michetti et al. Int J Mol Sci 2023;24:9605).

Osteopontin is a protein secreted by activated macrophages, leukocytes and activated T cells. Blood and CSF levels are reportedly higher in MS patients (Sinclair et al. Neuropathol Appl Neurobiol 2005;31:292-303). Elevated osteopontin levels are associated with larger lesions at 10 years (Orsi et al. Mult Scler Relat Disord 2021;51: 102923); elevated levels in CSF are associated with lower regional brain volumes (Orsi et al. Sci Rep 2021;11:23604). Preclinical data have recently reported that there are at least two isoforms that separately affect myelination and maintenance of axonal integrity (Nilsson et al. FASEB Bioadv 2023;5:336-353).

CD163 is a membrane component on monocytes and macrophages that is cleaved from the surface of activated macrophages during periods of CNS inflammation. Plasma levels of the soluble form (sCD1639) are elevated in MS patients (Fabriek et al. J Neuroimmunol 2007;187:179-186). CD163 is highly expressed in white-matter lesions (Elkjaer et al. Front Immunol 2022;13:761225). The sCD163 CSF/serum ratio may be useful as a marker of macrophage activation (Stilund et al. PloS One 2014;9:e98588).

Heat shock proteins (HSP) are chaperone molecules that have a cytoprotective role by refolding proteins damaged by various stressors. HSP70 has been shown to prevent the aggregation of amyloid, tau proteins, huntingtin and alpha-synuclein fibres (reviewed in Mansilla et al. Mol Med 2012;18:1018-1028). However, in MS, extracellular HSP70 triggers an innate immune response by interacting with toll-like receptors and activating the NF-kappaB pathway (Basu et al. Int Immunol 2000;12:1539-1546). HSP70 also primes the adaptive immune response by activating dendritic cells and promoting a cytotoxic CD8+ T cell response (Moroi et al. Proc Natl Acad Sci U S A 2000;97:3485-3490). Serum levels of HSP70 are significantly higher in CIS/RRMS vs. PPMS, suggesting that it may be a useful biomarker for monitoring CNS inflammation earlier in the disease course (Lechner et al. Mult Scler J Exp Transl Clin 2018;4:2055217318767192).