TABLE 5.

Criteria for causality and strength of evidence linking HHV-6A/B to multiple sclerosis

Disease causation criterion	Evidencea
HHV-6A/B nucleic acid is present in diseased tissue, in most cases, in higher abundance than in nondiseased tissue (by qPCR or other means).	Viral DNA in plaques  Positive evidence: 69, 139–146  Negative evidence: 147, 148
	Viral DNA in CSF  Positive evidence: 151–154, 159, 162  Negative evidence: 148, 155–158
	Viral DNA in cell-free CSF  Positive evidence: 159  Negative evidence: 160, 161
	Viral DNA in serum more often in MS than healthy controls  Positive evidence: 152, 163–169, 171  Negative evidence: 155–157, 170
	Viral DNA in circulating WBCs  Positive evidence: 166, 172  Negative evidence: 142
	Viral mRNA in circulating WBCs (178)
	Viral proteins indicating active infection in circulating WBCs (149)
The amount of HHV-6A/B nucleic acid in diseased tissue or blood, and/or antibody levels, correlates with the severity of the disease.	Viral load in blood correlates with disease activity (164, 167, 174, 175, 177).
	IgM and IgG antibodies to HHV-6A/B are more often and at higher levels in MS patients than in healthy controls and correlate with disease activity.  Positive evidence: 154, 168, 169, 180–186, 189–193, 195  Negative evidence: 187, 188, 194
HHV-6A/B nucleic acid is demonstrated in cells relevant to disease pathology.	HHV-6A/B can infect adult neurons (230, 232), adult astrocytes (109, 230, 231), microglial cells (30, 229, 230), primary adult oligodendrocytes (229, 230, 232), primary oligodendrocyte precursor cells (190, 230, 307), oligodendrocyte cell lines (30, 212, 230, 308), CD4+ T cells (18, 20), monocytes/macrophages (18, 20), endothelial cells (18, 20, 33) …  … and is found in MS lesions in neurons (139), oligodendrocytes (139, 145), microglia (145), and lymphocytes (145).
HHV-6A/B mRNA (by RT-PCR or other means) and antigens by immunohistochemistry are present in diseased tissue.	Viral mRNA and/or antigens/proteins in plaques (69, 143, 144, 149, 150)
Exposure to and then presence of the viruses and their gene products in affected tissue precede the development of the disease (temporal relationship).	95% of humans are infected with HHV-6B in very early childhood. The number of humans infected with HHV-6A, and the typical age of primary infection, is less clear. Thus, people with MS have almost surely been infected with HHV-6A/B before the development of the disease.
Infectious agents other than HHV-6A/B are not detected in diseased tissue in a substantial number of cases.	EBV and endogenous retroviruses, as discussed separately
There are cellular and/or humoral immune responses to HHV-6A/B in diseased tissue and/or in blood, and these responses correlate with the severity of the disease.	Lymphocytes in CSF proliferate in response to HHV-6A/B antigens (196).
	Intrathecal antibodies/oligoclonal bands against HHV-6A/B (162, 200–202).
	HHV-6A/B proteins share potential epitopes with host tissue (molecular mimicry) (206, 207).
	No studies correlate these immune responses to the severity of disease.
HHV-6A/B affect cellular function in diseased tissue in a manner known to cause or augment the disease pathology (in vitro or in vivo studies).	HHV-6A, by engaging the complement receptor, may explain the increased activation of the complement system seen in MS (27, 166, 208, 209).
	HHV-6A infects oligodendrocytes (and precursor cells), affecting their ability to remyelinate (190, 210).
	HHV-6A induces apoptosis in neuronal, astrocytic, and oligodendrocyte cell lines (212).
Specific antiviral therapy both reduces viral load in diseased tissue or blood and is followed by clinical improvement.	No evidence yet.

^aAll evidence cited is positive evidence in support of the assertion, unless specifically identified as negative evidence.