Key words
neuropathy - node of Ranvier - autoantibody - CIDP
Inflammatory polyneuropathies are present in clinical practice as a heterogeneous
group of diseases, not only in terms of clinical symptoms, but also in terms of their
therapeutic response. Reliable biomarkers are lacking both for diagnostics and as
markers of response to specific therapies. This is primarily due to the fact that
the pathogenesis of the immunneuropathies is still largely unexplained. T cell-mediated,
auto-antibody-induced, complement-mediated damage mechanisms as well as a damaged
blood-nerve barrier are discussed as possible causes of autoimmune-inflammatory neuropathies
[1]. Therefore, immuno-neuropathies, in particular the chronic inflammatory demyelinating
polyradiculoneuropathy (CIDP), are not only clinically but also pathophysiologically
a heterogeneous disease group. A subform of the CIDP are e. g., ganglioside autoantibody
neuropathies, which account for only a few percent of the CIDP cases but, depending
on the antigen, exhibit characteristic disease patterns [2]. In recent years, a further subform of autoantibody-associated neuropathies has
been identified: immuno-neuropathies with autoantibodies against paranodal proteins
[3].
The node of Ranvier – more than just a gap in the Myelin Sheath
The node of Ranvier – more than just a gap in the Myelin Sheath
Paranodium is the portion of the nerve fiber directly adjoining the sides of the node
of Ranvier. It demarcates the node, on which the sodium channels important for the
saltatorial excitation are localized, from the juxtaparanode, where potassium channels
are located [4]. The myelin sheath is connected to the axon via a complex of the paranodal proteins
neurofascin-155, contactin-1 and caspr-1 ([Fig. 1]). This complex serves, for example, as diffusion barrier for ion channels [5]. It has been described in various studies [6]
[7]
[8]
[9]
[10] that especially in the case of demyelinating neuropathies, changes in the node of
Ranvier architecture can occur. It has been known for a long time that, in the case
of primarily demyelinating polyneuropathies, segmental demyelination begins with an
elongation of the node and extends in the direction of internodes, and a decrease
in node length occurs again during remyelination [11]
[12]
[13]. Recently, however, proteins of the node of Ranvier have been identified as antigens
for inflammatory neuropathies [14]
[15]
[16]
[17]
[18].
Fig. 1 Molecular structure of the node of Ranvier. The node of Ranvier is divided into the
section juxtaparanode (potassium channels), paranode (paranodal protein complex) and
node (sodium channels). The paranodal protein complex consists of the proteins Caspr,
neurofascin-155 (NF-155) and contactin and connects the myelin sheath to the axon
[40].
Neurofascin-155 Autoantibodies
Neurofascin-155 Autoantibodies
Autoantibodies to neurofascin-155 were first described in multiple sclerosis, since
neurofascin-155 is also present in paranodes of the CNS [19]. The extent to which these contribute to the pathogenesis of multiple sclerosis
as part of the humoral immune response has not yet been finally clarified. In 2012,
Ng et al. showed the presence of autoantibodies against neurofascin-155 (the paranodal
isoform) and −186 (the nodally localized isoform) in 4% of patients with Guillain-Barré
syndrome and CIDP using ELISA and cell-based assay [17]. Although the 2 isoforms differ only slightly, isotype-specific autoantibodies are
present in almost all patients [17]. A uniform clinical phenotype, consisting of severe motor symptoms, acute onset,
poor response to intravenous immunoglobulins (IVIG) and cerebellar tremor, was reported
by Querol et al. [20]. This clinical phenotype was confirmed in follow-up studies [21]
[22]. Neurofascin-155 autoantibodies have also been described in patients with CIDP in
combination with a central involvement, but there are contradictory findings with
regard to a preferential occurrence of neurofascin-155 autoantibodies in this subgroup
[23]
[24]. The prevalence of neurofascin-155 autoantibodies in patients with CIDP was approximately
4% [20] in a European cohort, 3% in a cohort with Japanese and European patients, and 7%
in Japanese cohorts [21] and 18% [22]. The extent to which ethnic differences and a different sensitivity and specificity
of the different assays play a role currently cannot be determined with certainty.
However, it can be assumed that neurofascin-155 autoantibodies occur in a subgroup
of patients diagnosed with a CIDP and are associated with a characteristic phenotype.
Contactin-1 Autoantibody
In 2013, Querol et al. detected autoantibodies to contactin-1 in 2 patients with CIDP,
and autoantibodies to the protein complex of contactin-1 and Caspr in a further patient
[16]. Clinically, the patients were characterized by acute onset severe polyneuropathy,
predominantly motor in character. A study carried out in our clinic found that in
3 out of 4 patients identified with contactin-1 autoantibodies also exhibited intention
tremor [25] whereas in a Japanese cohort, sensory ataxia was clinically important [26]. The prevalence in the cohorts studied so far is 2.4–7.5% [16]
[25]
[26].
Caspr-1 Autoantibody
We recently identified in our cohort of patients with inflammatory neuropathies autoantibodies
to the third paranodal protein Caspr-1 [27]. While neurofascin-155 and contactin-1 autoantibodies were almost exclusively detectable
in patients with chronic inflammatory polyneuropathy, Caspr-1 autoantibodies were
detected in a patient with Guillain-Barré syndrome and a patient with CIDP. Both exhibited
severe, acutely onset polyneuropathy, predominantly motor, and with pronounced neuropathic
pain. The prevalence in our cohort was 3.5%, but both prevalence and clinical phenotype
must be confirmed in further studies ([Table 1]).
Table 1 Overview of the characteristics of each autoantibody against paranodal proteins.
|
Neurofascin-155
|
Contactin-1
|
Caspr-1
|
|
Prevalence in CIDP
|
3–18%
|
2.4–7.5%
|
ca. 3%
|
|
Literature
|
Ng et al., 2012, Kawamura et al., 2013, Querol et al., 2014, Ogata et al., 2015, Devaux
et al., 2016
|
Querol et al., 2013, Doppler et al., 2015, Miura et al., 2015
|
Doppler et al., 2016
|
|
Clinical phenotype
|
Acute onset
|
Acute onset
|
Acute onset
|
|
Motor>sensory
|
Motor>sensory
|
Motor>sensory
|
|
Cerebellar tremor
|
(Intention tremor)
|
Neuropathic pain
|
|
IgG-Subclasses
|
IgG4>IgG3>IgG1
|
IgG4>IGg3
|
IgG4
|
Assays for the detection of autoantibodies to paranodal proteins
So far, various assays have been used for the detection of autoantibodies to paranodal
proteins. The ELISA was predominantly used as a screening instrument [17]
[25]
[26]. However, the selection of the protein has to be taken into account: For neurofascin-155
it has been shown in several studies that the use of neurofascin-155-NS0 of the rat
can result in a nonspecific binding [17]
[28]
[29]. Therefore, coating the ELISA plates with human neurofascin-155 is recommended.
Another frequently used test is the binding experiment with HEK293 cells transfected
with the DNA of the respective antigen, also in combination with flow cytometry for
quantification [16]
[17]
[27]. For detection of the paranodal binding, binding assays were carried out with murine
teased fibers in almost all studies [16]
[20]
[25]
[27]. Thus, although with these experiments the specific antigen cannot be determined,
a paranodal binding can be demonstrated. Western blot was only used in a few studies
and showed false-negative results in individual patients, possibly due to the denaturation
of the protein [25]
[29]. In summary, the ELISA appears to be a reliable screening tool, and binding experiments
with transfected HEK293 cells, or directly with the nerve tissue, are suitable as
confirmatory tests.
Pathophysiology: Concept of Paranodopathy
Pathophysiology: Concept of Paranodopathy
Although neuropathies with autoantibodies to paranodal proteins are classified as
subtypes of CIDP, it is not a classical demyelinating polyneuropathy. In contrast
to the CIDP, in nerve biopsies, there are no signs of demyelination/remyelination,
such as onion bulb formations or thinly myelinated fibers (22,25,27). This is because
the myelin sheath is not the point of attack of the autoantibodies. Electrophysiologically,
there are conduction blocks, prolonged F-latencies and distal motor latencies, and
reduced nerve conduction velocity, so that the electrophysiological criteria of a
CIDP are usually met [16]
[20]
[25]
[27]. This apparent discrepancy between histology and electrophysiology is explained
by the concept of paranodopathy/nodopathy, a disease targeting the complex nodal region
[30]
[31]. The hypothesis is that the autoantibodies interfere with the adhesion of the paranodal
proteins and thus interfere with the connection between axon and myelin sheath, which
could lead to elongation of the nerve and dispersion of ion channels at the node of
Ranvier of the elongated nerve. Thus, the presence of conduction blocks and the extended
distal motor latency can be explained by the lengthening of the myelin sheath gaps
as well as the reduced nerve conduction velocity [31]. In fact, it was shown in vitro that autoantibodies to contactin-1 inhibit the adhesion
of the paranodal proteins and lead to the destruction of their architecture [32]. The latter was confirmed in skin nerves and nerve teased nerve fibers of patients
with contactin-1, Caspr-1, and neurofascin-155-associated neuropathy [22]
[25]
[27] ([Fig. 2]).
Fig. 2 Destruction of the node of Ranvier architecture in the teased fiber of sural nerve
biopsy of a patient with autoantibodies to the paranodal protein Caspr, fluorescence
double staining with myelin basic protein (MBP, myelin marker, red) and anti-Caspr
c, d, anti-neurofascin a and Anti-sodium channel b (in each case green), in e in comparison the staining of a control nerve. There is a misdistribution of neurofascin
a and the sodium channels b. The double arrows in b, c mark elongated nodes. Some myelin sheath gaps show an almost complete loss of Caspr
immunoreactivity (c, arrows), partly also on one side (d, asterisks). Other fibers show a distribution of Caspr into the juxtaparanode and
internode c [27]. Bars 10 μm.
Role of IgG subclasses
In the majority of cases, autoantibodies to paranodal proteins are immunoglobulins
of subclass IgG4 [17]
[20]
[25]
[26]
[27]. This has a special role among the immunoglobulins in that it does not activate
complement and is predominantly functionally monovalent and thus does not lead to
cross-linking of antigens [33]
[34]. Therefore, in the majority of patients, one cannot assume the presence of a complement-mediated
inflammatory reaction, which is pathogenically relevant in neuropathies with ganglioside
autoantibodies. A possible pathophysiological mechanism, however, is antibody binding-induced
transformational processes in the node of Ranvier, as described above. It is assumed
that the lack of efficacy for IVIG in these patients is associated with the lack of
complement activation [20]. It is known that IVIG inhibits complement activation even if the exact mode of
IVIG action is still not elucidated in detail [35]
[36]. However, not all autoantibodies to paranodal proteins belong to the IgG4 subclass.
Patients with contactin-1 and Caspr autoantibodies with IgG3 predominance have also
been described. Also, most patients with predominant IgG4 autoantibodies are by no
means exclusively IgG4. Thus, it was shown that sera from patients with predominant
IgG4 lead to a complement binding, presumably by IgG3, though not abundantly present
[37]. The extent to which IgG3 autoantibodies are pathogenic remains to be clarified
in vivo. For IgG4 autoantibodies to contactin-1, a potential pathogenicity has already
been demonstrated in the passive transfer model, but not yet for IgG3 [38]. The cause for the presence of different IgG subclasses with the same antigen is
not yet known.
Therapy of neuropathy with antibodies to paranodal antibodies
Therapy of neuropathy with antibodies to paranodal antibodies
The number of patients known to have autoantibodies to paranodal proteins is just
as small as the number of studies. It is not surprising that there are no studies
as yet on therapeutic strategies. However, in the available case reports, impressive
and unifrom response to rituximab of patients with IgG4 autoantibodies against neurofascin-155,
contactin-1 and Caspr is described [25]
[27]
[39]. A temporary response to corticosteroids and plasmapheresis has also also been reported,
and occasionally in patients with IgG3 autoantibodies also response to IVIG [25]
[27]
[37]. However, the clearest and most sustained improvement is seen after administration
of rituximab, so that in patients with IgG4 autoantibodies to paranodal antigens,
contrary to the guidelines for CIDP therapy, this can well be the first choice drug.
The extent to which this is also true of the rare occurrence of predominantly IgG3
autoantibodies to paranodal proteins cannot as yet be estimated. In patients with
relatively acute onset of predominantly motor inflammatory neuropathy, the presence
of autoantibodies to paranodal proteins should be suspected, especially in the case
of poor response to IVIG. If there is evidence for the presence of autoantibody, given
available evidence, we consider initiation of therapy with rituximab as reasonable.
Commercial tests for the detection of autoantibodies to paranodal proteins are not
currently available, but research labs (for example, Würzburg University Hospital,
Neurology) can help demonstrate the presence of these antibodies.
Conclusion
Neuropathies with autoantibodies to paranodal proteins are a new subgroup of immune
neuropathies. The characteristic clinical picture is a relatively acute onset of severe
motor neuropathy, partly together with cerebellar tremor. The autoantibodies predominantly
belong to the subclass IgG4 and patients typically respond well to therapy with rituximab
but most often not to IVIG.
