HHV-6 and Multiple Sclerosis

Wisconsin Viral Research Group, Ltd.

Physician's Corner: MS

HHV-6 and Multiple Sclerosis

Background

Multiple sclerosis (MS) is a chronic disease of the central nervous system, i.e. brain and spinal cord, that is caused by progressive loss of myelin, the main insulating material for the nerves that are responsible for the functioning of the CNS. The reason for the destruction of the myelin in the CNS tissues of patients with MS is unknown. However, risk for developing MS is related to the major histocompatibility complex, and a virus infection of the CNS has long been suspected to be involved as a trigger of the disease process. Over the past few years evidence has been obtained by our laboratory and other laboratories in the United States and Europe that suggests involvement of a specific member of the herpesvirus family, human herpesvirus six or HHV-6, in MS. HHV-6 is a frequent cause of severe and often fatal CNS infections in immunologically normal people as well as in patients with various forms of immunodeficiency. In those cases in which it has been examined, the damage to the CNS tissues associated with active HHV-6 infections has been diffuse or focal demyelination, similar to that seen in MS.

Studies by Wisconsin Viral Research Group

In studies performed by our laboratory (Abstract Number 1960; 39th Interscience Conference on Antimicrobial Agents and Chemotherapy and manuscript in press), we have documented that at least 59% (36/61) of patients with MS have active HHV-6 infections compared to 2% (2/89) of healthy control subjects and patients with other forms of neurologic disease. This difference is highly statistically significant (p < 0.0001 by two-sided Fisher’s Exact Test).

HHV-6 in MS CNS Tissues

More specifically, using an immunohistochemical staining procedure with brain tissues from patients with MS, we documented that active HHV-6 infections are not only present in the brains of most such patients, but that they are very closely involved in the actual damage being done to the brain tissue. Only two other neurologic disease controls had cells actively infected with HHV-6 in their CNS tissues, and both of these were diagnosed as having HHV-6 encephalitis. The vast majority of the MS CNS tissues containing active demyelinative disease contained cells actively infected with HHV-6 compared to only a few of the tissues free of active demyelination (p < 0.0001 by two-sided Fisher’s Exact Test).

HHV-6 in MS Lymphoid Tissues and Blood

Somewhat surprisingly, we also found that the active HHV-6 infections were present in the lymphoid tissues such as spleen and lymph nodes in most patients with MS but not in the lymphoid tissues of normal controls (p< 0.015). Consistent with this observation, we have also documented by means of a rapid HHV-6 culture assay that active HHV-6 infections are present in the blood of most patients with MS, but not in the blood of normal controls (p < 0.0001). There was no significant difference between the incidence of active HHV-6 infection in the bloods of MS patient with chronic progressive and relapsing/remitting MS.

Critical Review of HHV-6/MS Scientific Literature

In order to evaluate the results of these studies and place them in the context of other published reports on the role of HHV-6 in the pathogenesis of MS, a MEDLINE search of the National Library of Medicine for two MESH headings [(1) Multiple Sclerosis and (2) Herpesvirus Six, Human] was performed. A total of 39 articles were identified by this search, and these were classified as follows: (1) 15 review articles/commentaries, (2) 2 case reports, (3) 5 basic science reports and (4) 17 diagnostic technique applications. The articles describing diagnostic technique applications were retrieved and analyzed with respect to the methodologies used and the conclusions drawn. A manuscript describing these findings is currently in press.

Polymerase Chain Reaction (PCR) of Cellular Samples

In studies using diagnostic technologies that could not distinguish between active and latent HHV-6 infections, i.e. PCR analysis of blood leukocytes, CSF containing cells or CNS tissue, essentially no differences were found between samples from patients with MS and control individuals. Interpretation of these negative data with respect to the pathogenesis of MS is unclear and is made even more uncertain by the wide range of positive results seen with normal cells and tissues, e.g. normal leukocyte positivity rates of 5% to 95% and normal CNS tissue positivity rates of 15% to 85%.

Classical Serology

Studies in which classical serological methods were used, i.e. viral specific serum IgG titers and viral specific serum IgM detection, suggest a special role for HHV-6 in MS. Six of eight studies showed either increased HHV-6 IgG titers or a higher rate of positive HHV-6 IgM in MS patients compared to controls. An additional study detected HHV-6 IgG in samples of CSF from patients with MS but not in controls, supporting the idea of a special role for the virus. It should be noted that, while the serological methods used in these studies suggest a special role for HHV-6 in MS, results obtained for any one individual must be interpreted with caution since healthy individuals can have high titers of HHV-6 IgG and can occasionally be positive for HHV-6 IgM. Conversely, patients with MS who have an active HHV-6 viremia may have low titers of HHV-6 IgG and may be negative for HHV-6 IgM.

Polymerase Chain Reaction (PCR) of Acellular Samples (Serum and CSF)

PCR analysis of serum samples yielded mixed results with some studies suggesting a special role for HHV-6 in MS and others failing to show such an association. Importantly, the two negative serum PCR studies failed to include positive serum controls, raising the question of whether their procedures could detect HHV-6 DNA in serum. Also, neither of these two studies controlled for inhibition of the PCR reaction by substances in the patient specimens.

Three of four studies using PCR analysis of acellular CSF samples demonstrated a clearly increased positivity of the MS patients compared to other neurologic disease (OND) controls. While the negative acellular CSF study used a control for PCR inhibition, the control DNA used was 500 times higher than the lower limit of sensitivity of the PCR technique. Significant inhibition of the PCR reaction could have occurred that would not have been detected.

The three positive investigations using PCR analysis of acellular CSF samples deserve special comment. In the three studies, 14% (3/21), 11% (4/36) and 17% (2/12) of CSF samples from patients with MS were positive for HHV-6 DNA demonstrating active infection within their CNS tissues. This reduced rate of positivity compared to that observed with immunohistochemical staining of CNS tissues probably reflects the relatively low level of active infection present in the CNS tissues of MS patients compared to that seen in the brains of immunocompromised patients with HHV-6 encephalitis. It is well known that analysis of CSF samples by PCR in cases of herpes simplex encephalitis can give false negative results if the sample is obtained too early in the disease course when the infection is focal and limited in size.

Immunohistochemical Staining of MS CNS Tissues

All three independent studies using immunohistochemical staining of CNS tissues from patients with MS and controls, demonstrated active HHV-6 infections only in the MS patients. Also striking is the fact that the percentage of MS patients who were positive in the three studies were quite similar, i.e. 50% (16/32), 73% (8/11) and 47% (7/15).

Conclusion

Thus, when appropriate diagnostic technologies are used, i.e. those that detect only active HHV-6 infections, a strong relationship between HHV-6 and the pathogenesis of MS is reproducibly observed.

In summary, this work from our laboratory, in combination with previous work by other investigators, demonstrates that a sizable proportion of patients with MS suffer from active, disseminated infections of HHV-6, including infection of their CNS. We believe that this infection is the fundamental cause of the disease which raises the possibility that MS may be amenable to effective treatment with pharmacologic agents capable of suppressing the replication of HHV-6.