MS is a neuroinflammatory disorder affecting the central nervous system (CNS), demolition of the myelin sheath, and axonal injury accompanying with diverging degrees of neurological impairment.1 Viral infection has long been a dilemma in the etiology of multiple sclerosis (MS) and related neuroinflammatory disorders. Epidemiological, pathological, and molecular investigations in the last decades have profoundly associated the human herpesvirus 6 (HHV-6) and human herpesviruses Epstein–Barr virus (EBV) with MS.2 HHV-6 is a neurotropic virus that frequently reactivates in immunocompromised patients.3 Two variants of HHV-6 have been recognized: HHV-6A, with a more prominent preference for infecting neural cells, and HHV-6B. However, it is less often associated with disease than HHV-6B.4 HHV-6A has been found predominantly in MS lesions. Primary evidence backing a convincing pathogenic role for HHV-6 in MS was based on cerebrospinal fluid (CSF) detection of viral DNA by polymerase chain reaction (PCR).4 Observation of viral messenger RNA and protein expression in oligodendrocytes were additional contribution to the hypothesis of HHV-6 as an operator of MS.5 Subsequent studies have revealed drastic variability in IgG- and IgM-specific responses to viruses in CSF, to viral antigens, as well as DNA detection from MS lesions.6–9

Furthermore, activation of HHV-6 and other herpes viruses (e.g., varicella-zoster, herpes simplex viruses, and herpesvirus-7 and 8) can trigger MS progression. Interestingly, marmosets (a small nonhuman primate) inoculated with HHV-6 developed an MS-like experimental neuroinflammatory condition when compared to controls.9,10 The role of vitamin D in MS etiology draws growing attention. The geographical distribution of the disease suggested the involvement of vitamin D in disease pathogenesis.11 Several studies have shown that vitamin D levels are lower in MS patients than in controls.12–16 Moreover, several studies of 25-hydroxyvitamin D levels after MS onset have noted correlations with MS risk, progression, disability, and severity.11 MMPs are a big family of zinc-containing endopeptidases that degenerate extracellular matrix and basement membrane composites (collagens, gelatin, laminin, fibronectin). Expression and activity of MMPs are tightly controlled.17 Among of all MMPs, MMP-9 and MMP-2 have been extensively investigated in MS given their capacity to degenerate components of the basal lamina and their involvement in blood-brain barrier (BBB) damage.18,19 Remarkably, increasing experimental evidence proposes the engagement of MMP-9 in the pathogenesis of MS, since the circulating levels of MMP-9 have been shown to be upregulated in MS patients, in contrast with other noninflammatory neurological disorders (NIND) and healthy subjects.20–23 To investigate the roles of MMP-9, MMP-2, and vitamin D in the pathogenesis of MS associated with HHV-6 infection, we determined serum MMP-9, MMP-2, vitamin D levels, and compared them among MS patients with or without HHV-6 infection. In addition, we assessed HHV-6 IgG titers in MS patients with HHV-6 infection.

Environmental factors, such as viral and bacterial infections can accelerate, aggravate, or protect patients from MS.25 The vast majority of studies have suggested that HHV-6 is involved in the etiology of MS. Nevertheless, this association is controversial and, as is the case, experimental evidence linking this virus to MS is opaque.4,26 Pioneering studies describing HHV-6 viral DNA in the brains27,28 and CSF29 of MS patients and controls indicated that HHV-6 has strong neurotropism that was correlated with a CNS reservoir.27 Several clinical studies have proposed that MS and episodes of disease exacerbation are associated with concurrent viral or bacterial infections.30–32 However, there is still debate regarding the role of HHV-6 in MS pathogenesis. In this study, we demonstrated that the seropositivity of HHV-6 in MS patients was significantly higher than healthy subjects, as was observed in other studies.29,33–36 On the other hand, some studies have not shown significantly higher seropositivity rate of HHV-6 in MS patients.36–39 Besides, a number of studies found no correlation between MS and HHV-6.40–42 The hallmarks of MS are BBB disruption, migration of inflammatory cells to the CNS accompanying neuroinflammation, and neurodegeneration. Substantial experimental evidence suggests that MMPs play an active role in MS Experimental autoimmune encephalomyelitis pathogenesis.43 In vivo studies have revealed that MMP-2 null mice are far more susceptible to experimental autoimmune encephalomyelitis (EAE) because of the compensatory rise in MMP-9 in these animals.44 In EAE mice, increased levels of MMP-9 favors development and progression of the disease, while shifts in the levels of MMP-2 have a role in its resolution.45 These data proposed that the distinct MMPs enzymes might have distinguished roles in EAE development since the experiments have confirmed that the dual MMP-2 and MMP-9 knockout mice are resistant to EAE progression.46 Furthermore, we have demonstrated that serum levels of MMP-9 and MMP-2 in MS patients are significantly higher than in healthy controls. Furthermore, seropositive MS patients have significantly higher levels of MMP-9, but not MMP-2, than seronegative MS patients. Kittaka et al. reported a positive association between HHV-6 DNA and MMP-9/TIMP-1 levels in infants infected with HHV-6. These researchers stated that high serum levels of MMP-9 and TIMP-1 in infants infected with HHV-6 might influence the dysfunction of the BBB, ultimately inducing febrile seizures.47 Sanchooli et al. showed that the expression levels of CD154 isoforms, MMP-9, and MMP-2 in MS patients are significantly higher than in healthy subjects. Their results demonstrated that following an exacerbation period, sCD154 concentration increases, which is mutually related to the MMPs/TIMP-1 ratio.48 Moreover, our results demonstrated that there was no correlation between serum levels of MMP-2 and MMP-9 with EDSS in both seropositive and seronegative MS patients. The vast majority of observational research has proposed a relationship between vitamin D level in serum and MS risk, as well as disease activity.49 We have indicated that serum levels of vitamin D in MS patients are significantly lower than healthy subjects. Also, there were no significant differences in vitamin D levels among seropositive and seronegative MS patients. Besides, we observed no correlation between vitamin D levels and EDSS. Our results are in line with previous studies.16,50,51

All in all, our results illustrate that the HHV-6 seropositivity in MS patients was significantly higher than in healthy subjects. In addition, MMP-9 levels and EDSS in seropositive MS patients were markedly increased in relation to seronegative MS patients. However, the mechanism as to how HHV-6 could affect MS risk is uncertain, but several hypotheses have been suggested to explain how HHV-6 may play a causative role in MS development, including (1) direct cytopathic effect; (2) molecular mimicry; and (3) modulation of cytokine production during acute infection or virus reactivation, and an increase in an already active immune response during virus reactivation, a phenomenon also known as bystander effect. Finally, we suggest that HHV-6 infection may play a role in MS pathogenesis. Further work should be undertaken to further test the possible role of HHV-6 in MS pathogenesis.

None.

This study was approved by the ethical committee of the Hamadan University of Medical Science approved (IR.UMSHA.REC.1398.409).

The authors declare no conflicts of interest.