Page 6 - Read Online

Page 2 Bozzetti et al. Neuroimmunol Neuroinflammation 2021;8:1-13 I http://dx.doi.org/10.20517/2347-8659.2020.26

INTRODUCTION
Inflammatory demyelinating diseases (IDD) represent a spectrum of heterogeneous disorders affecting
the central nervous system (CNS). Multiple sclerosis (MS) is classified as a chronic, immune-mediated,
[1]
demyelinating disorder, and it is the most well-known disease of this group . Neuromyelitis optica
[2]
spectrum disorders (NMOSD), which preferentially involve the spinal cord and optic nerve , and acute
[3]
disseminated encephalomyelitis (ADEM), a typically monophasic disease of children , are also part of
[4]
CNS IDD. Other acute inflammatory conditions including idiopathic optic neuritis and acute transverse
[5]
myelitis also enter in the differential diagnosis. A major discovery in this field was the association between
NMOSD and serum aquaporin 4 IgG (AQP4-IgG), confirming that it is a different disease from MS and needs
[6]
different treatment . On the other hand, the association of serum anti-myelin oligodendrocyte glycoprotein
(MOG)-Abs with ADEM, NMOSD and other demyelinating events also clarify the final diagnosis in
[7]
many conditions previously classified as “idiopathic” . In addition to the difficulty in the diagnostic
process, one of the main issues of IDD is the correct assessment of disease activity and the prediction of
long-term prognosis. Different clinical scales, radiological parameters, and biological markers have been
studied, with the aim of identifying reliable and easily accessible measures of disease activity, treatment
response, and prognosis in these conditions. Neurofilament proteins recently emerged as a promising
biomarker in this context. Neurofilaments are cylindrical proteins located in dendrites, soma and, in
particular, axons of neurons, with their specific role in conferring structural stability and promoting axonal
growth and intracellular transport. They are classified as intermediate filaments (i.e., 10 nm in diameter,
intermediate between actin and myosin) and include neurofilament light chain (NfL), neurofilament
middle chain, neurofilament heavy chain (NfH), and α-internexin, depending on the length of the carboxy-
terminal region. Since NfLs are the most abundant and soluble subunit among intermediate filaments,
[8]
research has mainly focused on them . Low levels of NfL are constantly released from axons under
normal conditions, in an age-dependent manner. However, as a consequence of axonal damage due to
inflammatory, degenerative, vascular, or traumatic injury, NfL release significantly increases. After reaching
[9]
the interstitial fluid, NfL are detectable in CSF and in serum at lower but comparable levels . Enzyme-
linked immunosorbent assay (ELISA) technology allows the measurement of the higher NfL values present
in CSF; however, it is not sensitive enough to measure the significantly lower serum/plasma levels. The
recent development of ultrasensitive electrochemiluminescence-based immunoassay [in particular, single-
molecule immunoassay, single-molecule array (SiMoA) technology] enables reliable measurement of the
low NfL concentrations in serum and the monitoring of minor changes over time . This single-molecule
[10]
immunoassay is based on antibody capture agents bound to the surface of paramagnetic microbeads
containing approximately 250,000 attachment sites. The beads are added to the sample solution and then
incubated with a second biotinylated detection antibody and beta-galactosidase-labeled streptavidin. In
this manner, each bead that has captured a single protein molecule is labeled with an immunocomplex.
During the detection process, a fluorescent signal is generated in sealed wells that contain beads combined
with immuno-captured and enzyme-labeled protein molecules. Concentrations are determined digitally to
further increase the sensitivity of the assay. With this recently developed technique, a significant increase
in serum NfL levels has been demonstrated in different conditions, including Alzheimer’s disease ,
[11]
[12]
[14]
[13]
Creutzfeldt-Jakob disease , frontotemporal dementia , amyotrophic lateral sclerosis , parkinsonian
disorders , traumatic brain injury , stroke , peripheral neuropathies , autoimmune encephalitis ,
[19]
[18]
[16]
[15]
[17]
[21]
[20]
and in particular MS , in correlation with disease activity and post-mortem neurodegeneration . The
lower invasiveness of serum NfL measurement allows repeatable analyses over time and expands the
potential utility of NfL as a biomarker of disease activity and treatment response in a wide spectrum
of neurological disorders. Although the actual applicability of this assay in daily clinical practice is still
limited and influenced by critical aspects that need to be considered for the correct interpretation of this
[22]
measurement , the development of novel ultrasensitive assays and the extensive applicability of serum
NfL measurement have provided major advantages in this field.