Autoimmunity to retinal, neuronal, and axonal antigens may play a role in inducing cellular degeneration. The specific AAbs against retinal antigens are most commonly associated with CAR and AR but their pathogenic effects were not fully established [14]. The current studies showed that serum anti-optic nerve AAbs are also present in more than 50% patients tested in patients with either CAR or AR. The major optic nerve antigenic targets for patients AAbs described in these studies can be divided into four categories as follows: (1) classical glycolytic enzymes involved in energy production, including α and γ enolases, glyceraldehyde 3-phosphate dehydrogenase, which also reacted with retinal antigens; (2) neuronal-specific myelin proteins; (3) CRMP5; (4) AQP4. Also other antibodies were present but their antigens have not yet been discovered and are under investigation. The newly identified AAbs labeled optic nerve components such as axons, astrocytes, and oligodendrocytes. Although their role in the pathogenicity of the optic neuropathy affecting these patients needs to be evaluated further, we believe that these unique anti-optic nerve AAbs can be valuable disease markers. We observed that the AAb presence correlated with various clinical (ophthalmoscopic changes at or around the disc), imaging (thinning or, in fact, swollen RNFL), and functional findings presented by affected patients, not usually seen in retinopathies. Moreover, distinct autoantigens may become novel therapeutic targets in disease, e.g., blocking GAPDH-mediated cell death in pathological conditions. Therefore, the identity of cellular targets is important in better understanding the etiology of autoimmune retinopathy associated with optic neuropathies, whether with or without retinopathy, and for developing better treatments. Our research was solely intended to identity of anti-optic nerve AAbs targets because this area was not explored before. The future studies will evaluate their responsiveness to immunosuppressive treatments for neuro-retinopathy targeting reducing the antibody titers on follow-up testing and clinical response to therapy.
The definition of the autoimmune neuro-retinopathy is not well defined. Autoimmune optic neuropathy (AON) was first described by Dutton et al. as recurrent episodes of optic neuropathy with AAbs but as a different entity distinct from demyelinating optic neuritis [15]. In general, AON has been characterized by chronically progressive or recurrent visual loss associated with serological evidence of autoimmunity and without a defined systemic autoimmune disorder [4]. Later, autoimmune-related retinopathy and optic neuropathy (ARRON) has been defined by the Keltner’s group as a syndrome that is characterized by visual loss and often the presence of antibodies against retinal or optic nerve antigens in the absence of cancer [16]. Our patients suffer from retinopathy and optic neuropathy, more than half had both anti-retinal and anti-optic nerve autoantibodies, and third of them has been diagnosed with cancer at the time of testing for antibodies. We recommend the following unique measurements that help improve the diagnosis for optic neuropathy in the context of retinopathy: delay VEPs, pattern of visual fields loss that is not clearly consistent with retinopathy, attenuated and/or thickened RNFL as measured by OCT imaging criteria, compromised photopic negative response (PhNR) of the photopic flash ERG, which is mediated by RGCs, and the presence of AAbs, some of which may label RGCs in the tissue. Our research identified new AAbs specific against optic nerve antigens not previously known in autoimmune neuropathy associated with retinopathy, suggesting the autoimmune nature of the syndrome also at the optic nerve level. If pathogenic, AAbs binding to proteins in optic nerve neurons may lead to disruption of protein function, apoptosis of cells, as well as inhibiting enzymes that preserve neuronal glycolytic activity and neuronal integrity.
A high prevalence of various AAbs in patients with neuro-retinopathy suggests a polyclonal activation of the humoral immune system. It is not clear what the role of each AAb is in autoimmune optic neuropathy and whether the AAbs contribute to pathogenicity at all. We hypothesize that autoimmune damage to the retina and optic nerve may be caused directly by the AAbs, or indirectly by a non-immune cause that contributes to tissues damage but may activate the secondary immune responses as a result of neuronal degeneration. During pathogenic processes, AAbs could bind to neuronal proteins in optic nerve and retina, inhibiting enzymes that preserve neuronal glycolytic activity and neuronal integrity, which may ultimately lead to apoptosis of those cells [17]. Thus, AAbs may be directly involved in the induction of pathogenic processes or could be related to the progression over time of ongoing pathological processes in the retina and optic nerve. In the latter, specific AAbs related to specific phenotype could be considered as biomarkers related to disease. We identified AAbs that belong to the group of neuronal glycolytic enzymes such as GAPDH (glyceraldehyde 3-phosphate dehydrogenase) [18] and neuronal-specific and non-neuronal enolases [19]. These are multifunctional proteins that are all expressed intracellularly but also on the neuronal cell surface [20, 21]. They are involved in energy metabolism, cell signaling, and synaptic neurotransmission [22]. Although these AAbs can be present in normal controls, they were present significantly more frequently in the sera of retinopathy patients [23]. The biological significance of anti-GADPH must be related to the roles of GADPH as a multifunctional enzyme that is involved in apoptosis, oxidative stress, activation of transcription, membrane fusion, and vesicle transport. GADPH is a cytosolic protein but can translocate to the nucleus in apoptotic processes. Interestingly, GADPH is present in large quantities in rod outer segments playing a role in the production of energy for these active cells [21]. Therefore, neuronal death induced during glycolysis inhibition involves calcium influx through NMDA receptors and calcium release from intracellular ER stores [17, 24]. Uncontrolled increases in intracellular calcium lets to apoptotic cell death. It has been suggested that GADPH plays a pathogenic role in human age-related neurodegenerative diseases, including Huntington's disease, Parkinson's disease, and Alzheimer's disease [25]. Neuronal damage related to aging and chronic neurodegenerative diseases has been suggested to be associated with decreased glucose metabolism due to AAb action [24, 26]. Such possibility exists because GADPH is a strong antigenic protein, especially during infections, which leads to elucidation of anti-GADPH AAbs [22] and similarity in the amino acid sequence of Streptococcal and human proteins explains a cross-reactive immune response against this protein [27]. Likewise, enolase and aldolase are also expressed on Streptococcal pyogenes and they too show significant identity between Streptococcal and human neuronal glycolytic enzymes (apart from aldolase C) [28].
Only a few of our patients have AAbs targeting the astrocytic water channel protein, AQP4. Anti-AQP4 AAbs has been found in a high percentage (~75%) of neuromyelitis optica patients (NMO-IgG), usually identified by immunofluorescence only [13], suggesting the importance of conformational epitopes, but also linear epitopes were later recognized in denatured protein [29]. The recognition of linear epitopes could explain the immunoreactivity discordances observed for some serum samples, which tested positive by immunocytochemistry but were negative by WB, and vice versa. NMO is a rare inflammatory demyelinating disease that selectively affects optic nerves and spinal cord, usually not seen in multiple sclerosis (MS) and other demyelinating syndromes of the CNS [30]. AAbs against AQP4, which is expressed in astrocytic endfeet at the blood brain barrier, are believed to induce damage to astrocytes [31]. The close contact of AAbs/AQP4 positive processes with oligodendrocytes and myelin tracts suggest that a bystander effect of AAbs-damaged astrocytes on oligodendrocytes might occur in the nervous tissues affected by autoimmune optic neuropathy.
Optic neuritis associated with anti-AQP4-Ab affected only females who had bilateral eye involvement, and all had severe visual impairment in the acute phase and delayed partial visual recovery [32]. Our patients with serum anti-AQP4 were male. Similar to our patients, NMO patients showed a predominant positive anti-MOG response and anti-MBP AAbs. There are some clinical features that overlap between autoimmune optic neuropathy and NMO [6]. Despite the limited number of samples, presence of such AAbs suggests a predominantly widespread acute immune activation, including a strong B-cell response [30]. However, the role of anti-AQP4 AAbs in neuro-retinopathy is not clear.
AAbs against CRMP5 were reported in paraneoplastic syndrome patients with optic neuropathy and retinopathy associated with the presence of vitreous and intrathecal cells [12]. The neurological syndromes associated with CRMP5 antibodies are very diverse (much like those associated with anti-Hu antibodies) and include peripheral neuropathy, limbic encephalitis, ataxia, as well as paraneoplastic chorea or optic neuritis [33]. Because anti-CRMP5 AAbs were found in almost 80% seropositive patients with lung cancer, CRMP5 has become an established biomarker for lung cancer-related paraneoplastic syndromes [33–35]. However, our CRMP5-seropositive patients did not have cancer at the time of testing.
In summary, our research identified novel AAbs specific against the optic nerve antigens unknown before in autoimmune neuro-retinopathy, suggesting the autoimmune nature of the syndrome. Indirect evidence, coming from clinical, functional, and imaging observations and immunology strongly suggest that these AAbs may play a direct role in the disease process. However, direct proof of the pathogenic potential of these anti-optic nerve antibodies is needed. If pathogenic, AAbs binding to proteins in optic nerve neurons may lead to disruption of protein function apoptosis of cells, as well as inhibiting enzymes that preserve neuronal glycolytic activity and neuronal integrity. Some of the neuronal antigens present in these patients coincide with molecular targets in other entities, such as NMO and other recognized paraneoplastic syndromes. These AAbs likely cause additional visual loss via an optic nerve-related component in autoimmune retinopathies that had been thus far underappreciated and underestimated in its frequency, and may offer additional treatment targets for patients with autoimmune neuro-retinopathies.
