DISCUSSION
In this study, we identified the elevations of serum PRX5 and PRX6 levels in patients with MS and NMOSD and their associations with blood–brain barrier dysfunction and disease duration in NMOSD patients. Pathological analyses of the spinal cords from EAE mice showed remarkable PRX5 staining in CD45+ infiltrating cells. These results indicate that PRXs may play some role in CNS inflammation during MS and NMOSD.
PRXs have recently received attention as novel DAMPs. Among the PRX subtypes (PRX1-6), PRX1, PRX2, PRX5, and PRX6 are expressed in the brain and can trigger the release of several cytokines. Furthermore, PRX5 has the strongest effect on the activation of Th17 activation via the secretion of IL-23 [12]. Th17 cells play a dominant role in the development of EAE [17] and are also thought to be involved in the pathogenesis of MS and NMOSD [5]. Therefore, we must consider that PRXs may play a role in triggering autoimmunity during MS and NMOSD.
Thus far, some papers about the role of PRXs in patients with MS have been published but not in patients with NMOSD. Holley et al. reported that PRX5+ hypertrophic reactive astrocytes were observed in the acute and chronic brain lesions of MS patients. They also speculated that ongoing oxidative stress occurred during the acute and chronic phases of MS, and PRX5 was upregulated in astrocytes to neutralize oxidative stress [18]. Voigt et al. reported that PRX2 was upregulated mainly in astrocytes of white matter lesions. Furthermore, its expression level was positively correlated with the degree of inflammation and oxidative stress in patients with MS, which suggests that PRX2 contributes to the resistance of astrocytes against oxidative damage [19]. Yun et al. reported that PRX6 was strongly expressed by cells with astrocyte-like morphology in the MS lesions of human patients [13]. The increased PRX6 expression in astrocytes of MS patients reduced MMP9 expression, fibrinogen leakage, chemokines, and free radical stress, leading to decreased blood–brain barrier disruption [13]. These findings suggest that PRX6 expression may represent a therapeutic way to restrict inflammation in the CNS and potentiate oligodendrocyte survival. Therefore, PRX6 may have potential as a new neuroprotective therapy for MS [13]. Taken together, upregulated PRXs may help protect astrocytes and maintain blood–brain barrier function. However, no paper has described the protein levels of PRXs in patients with MS and NMOSD thus far. In our study, serum PRX5 and PRX6 levels were significantly elevated in MS and NMOSD patients and partly associated with Qalb (as a marker of blood–brain barrier function), CSF protein, and disease duration in NMOSD patients. In general, longer disease duration correlates with more severe blood–brain barrier dysfunction in MS and NMOSD patients. Additionally, serum PRX6 levels were significantly associated with PRX5 levels in NMOSD patients. Our findings indicate that serum PRX6 and PRX5 may be associated with blood–brain barrier dysfunction in NMOSD, like pathological analyses in patients with MS [13]. However, there was no significant elevation in PRX levels within the CSF of MS and NMOSD patients. Further studies are needed to confirm the definite mechanism of PRXs in patients with MS and NMOSD.
Conversely, there is a limited number of papers regarding the role of PRXs in EAE. It has been reported that mRNA levels of PRX1, PRX3, and PRX6 were increased in the spinal cords of EAE mice compared with that of control mice. Also, PRX6 was strongly expressed on cells with astrocyte-like morphology in EAE lesions [13]. PRX6-transgenic EAE mice exhibited less severe pathology, which indicates that the upregulation of astrocytic PRX6 has an important role in inhibiting the destruction of myelin via microglial activation, blood–brain barrier breakdown, and immune cell infiltration [13]. Similar to pathological analyses in patients with MS [13], upregulated PRXs inside astrocytes may protect blood–brain barrier function and inhibit CNS inflammation. Contrariwise, PRXs have an opposite face: having the ability to induce inflammatory cytokines production as inflammatory mediators [12]. In our study, only PRX5 was upregulated in CD45+ cells (likely monocytes), which had infiltrated into the spinal cord lesions of EAE mice. We speculate that CD45+ cells may accumulate in the lesion and amplify the inflammatory response via PRX5. Another possible explanation is that CD45+ cells were dealing with ongoing oxidative stress. Although previous papers described the upregulation of astrocytic PRX6 in EAE [13], we did not observe the expression of PRXs in astrocytes.
Some limitations of our study need to be addressed. First, increased PRXs levels were not confirmed in the CSF but only the serum from patients with MS and NMOSD. The small sample size in our study may explain this discrepancy between serum and CSF levels of PRXs. Also, it is still unclear as to which cells secreted PRXs into the serum. Additional basic research studies are required to determine the source of these PRXs. Finally, our pathological findings of EAE were different from previous reports; EAE spinal cords showed PRX5 expression in CD45+ infiltrating cells but no PRX expression in astrocytes. One possible explanation for this discrepancy is species-specific differences between humans and mice. It is also possible that pathological characteristics may differ according to disease status. Further studies are needed to confirm the definite mechanism between PRXs and EAE pathogenesis.
From our results, we suggest that the elevations of serum PRX5 and PRX6 levels were associated with the pathogenesis of MS and NMOSD and partially responsible for blood–brain barrier dysfunction in patients with NMOSD. We also showed that upregulated PRX5 in CD45+ infiltrating cells amplify inflammation in EAE. In summary, the expression of PRXs was significantly altered in CNS inflammatory demyelinating diseases, which suggests that they may trigger of blood–brain barrier disruption or CNS inflammation. Consequently, the development of new treatment targeting PRXs may reduce symptoms of MS and NMOSD.