Human Herpesviruses 6A and 6B in Reproductive Diseases.

Human herpesvirus-6A (HHV-6A) and human herpesvirus-6B (HHV-6B)—collectively, HHV-6A/B—are ancient human viruses that were discovered only about 30 years ago ( 1 , 2 ). An expanding group of human illnesses have been definitively or provisionally linked to the viruses ( 3 , 4 ). Recently, studies have indicated that HHV-6A may be one cause of unexplained primary infertility and that both HHV-6A and HHV-6B may, in some cases, contribute to the pathogenesis of preeclampsia (PE). If the viruses play a role in these conditions, it may be due to their ability to infect endometrial epithelial cells, placental cells, natural killer (NK) cells and the endothelial cells of myometrial spiral arteries—and due to the immune response to infection. Both viruses also can cause transplacental infection of the newborn. We will briefly describe the biology of these viruses. Then, we will describe what is known about their relationship to primary infertility and to PE, as well as what is known about transplacental congenital infection of newborns and its possible health consequences.

RR reviewed all of the information (text and references) about the virology of HHV-6A/B and the information about endometrial infection and primary unexplained infertility. JE reviewed all the information about preeclampsia (text and references). AK drafted the manuscript, reviewed all the references, and integrated the editorial contributions of RR and JE. All authors contributed to the article and approved the submitted version.

HHV-6A and HHV-6B are betaherpesviruses, members of the Roseolovirus genus, as described in more detail elsewhere ( 3 , 5 – 7 ). Initial infection with HHV-6B occurs in early childhood, and somewhat later for HHV-6A. Infection of the respiratory tract, including the tonsils and olfactory-ensheathing cells of the nasal cavity, is the primary route of infection ( 5 , 8 ). Horizontal transmission, particularly from adult to child, is likely ( 7 ). Primary infection from breast feeding or blood transfusions has not been reported ( 3 ). HHV-6A/B infects a wide variety of cells and tissues including: 1) multiple immune system cells—including CD4+ T cells, CD8+ T cells and NK cells; 2) multiple cells of the nervous system—astrocytes, microglial cells, oligodendrocytes and neuronal cells; 3) and cells of other tissues—liver cells, human fibroblasts, epithelial cells and endothelial cells ( 4 , 6 ). The viruses also can infect multiple cells of the reproductive tract ( 9 ), as will be discussed shortly. Over 95% of adults are infected with HHV-6B. A smaller but substantial fraction are infected with HHV-6A. As with all herpesviruses, the infections are permanent: the viruses establish latency—a state in which they cannot be eradicated and from which they can periodically reactivate ( 7 ). Infection with HHV-6B in early childhood can cause roseola infantum (exanthem subitem) ( 10 ), cause minimal symptoms or be asymptomatic. HHV-6B causes encephalitis/encephalopathy and delirium in people undergoing hematopoietic stem cell transplantation ( 11 – 14 ). The viruses are a common trigger for febrile seizures in young children, including febrile status epilepticus ( 15 – 21 ). As summarized in detail elsewhere, they may be one trigger of mesial temporal lobe epilepsy; multiple sclerosis; drug rash with eosinophilia and systemic symptoms (DRESS); and Hashimoto’s thyroiditis ( 4 ). When the viruses are acquired by a person early in life, they permanently infect a small number of somatic cells, inserting their full genomes into the telomeric region of a host cell’s chromosomes, through molecular mechanisms recently identified ( 22 – 24 ). Remarkably, and of potential importance in reproductive disease, on multiple occasions in human history, both viruses inserted their genomes into a human germ cell. The earliest known occurrence of this appears to have been between 85,000-342,000 years ago, in Africa ( 25 ). Consequently, about 1% of the human race is born with the entire viral genome inside every cell—a condition called inherited chromosomally-integrated HHV-6A/B (iciHHV-6A/B) ( 22 , 26 , 27 ). This inherited viral genome can be transcriptionally active, producing viral proteins and even full virions. Diagnosis of iciHHV-6A/B can be made by viral load studies in whole blood that exceed 5.5 log10 copies/ml ( 26 ). Investigators have begun to explore the health consequences of iciHHV-6A/B ( 28 ). The infection of somatic cells by acquired virus, and the ancient infection of a germ cell leading to an inherited viral genome, are summarized in Figure 1 . Consequences of HHV-6A/B DNA integration into a chromosome of a somatic cell, and into the host chromosome of a germ cell. (A) Shows acquired virus integrating its DNA (in red) into the telomere of a chromosome of a somatic cell. It has been hypothesized that this is a mechanism by which HHV-6A/B achieves latency. Since the viral genome is not integrated into the DNA of sperm or ova, no vertical transmission of the viral genome occurs. (Note: This figure shows only one of the 23 chromosomes, in early metaphase, and assumes viral infection and integration occurs in the G1 phase, then is replicated in the S phase and passed to both of the daughter cells during mitosis. When the integration event occurs during S or G2 phases—not shown in the figure—the daughter cells create a mosaic, since one contains the viral genome and the other does not.) (B) Describes an ancient event: on several occasions in human history, the viral genome integrated into the DNA of a haploid germ cell chromosome. This led to a fertilized ovum containing the viral genome and, hence, to a human with the viral genome integrated into a chromosome in every cell: inherited chromosomally-integrated HHV-6 (iciHHV-6). In Mendelian fashion, the integrated viral DNA is present in 50% of gametes (whether sperm or ova). About 1% of humans are born with iciHHV-6. The inherited presence of the viral genome in every cell, including germ cells, contrasts with the integration of acquired virus into only a small fraction of target somatic cells. Adapted from, and reprinted with permission of the publisher, from: Komaroff AL, Pellett PE, Jacobson S. Human herpesvirus 6A and 6B in brain diseases: Association vs. causation. Clinical Microbiology Reviews. 2021; 34:e00143-20. https://doi.org/10.1128/CMR.00143-20 .

Investigators examined endometrial biopsy tissues from 30 women with unexplained primary infertility (no evidence of endometriosis, endometritis, recurrent miscarriage, ovulatory dysfunction or anatomical uterine pathologies) and 36 fertile women with at least one previous successful pregnancy. The two groups were similar with regard to age, length of menstrual cycle, smoking habits, and levels of FSH, LH, TSH, FT4 and progesterone ( 9 ). HHV-6A (but not HHV-6B) DNA was found in the endometrial epithelial cells of 43% of women with primary unexplained infertility vs. none of the fertile controls, a highly significant difference (P<0.00001). In vitro studies demonstrated that endometrial NK cells in the infertile, infected women attacked endometrial epithelial cells that were infected with the virus ( 9 ). One likely cause of primary unexplained infertility is defective endometrial receptivity ( 52 , 53 ). Inflammatory changes in the endometrium and/or in the placenta, triggered by infection, theoretically could inhibit implantation. Indeed, viral agents long have been postulated as possible environmental factors in infertility ( 54 , 55 ). HHV-6A infection of endometrial epithelial cells, endometrial NK cells ( 46 , 47 ) and trophoblast cells ( 35 , 36 ) promotes changes that can impair implantation. Indeed, women with primary unexplained infertility and endometrial HHV-6A infection have higher levels of pro-inflammatory cytokines in uterine washings ( 56 ). HHV-6A infection of the endometrium reduces levels of two decidualization markers, soluble human leucocyte antigen-G (sHLA-G) and mucin1 ( 48 , 57 ), possibly augmenting the maternal immune response against paternally-derived fetal antigens ( 56 ). Infection of endometrial epithelial cells also generates a pattern of microRNA expression that has been linked to implantation failure ( 35 ). HHV-6A infection of the endometrium in women with primary unexplained infertility is seen more often in women with a particular polymorphism for an ATP-gated ion channel, P2X7R. An antagonist of P2X7R has been shown to reduce the infectability of target cells by HHV-6A, and to reduce its replication, in vitro ( 56 ). HHV-6A infection of endometrial epithelial cells inhibited the ability of a human choriocarcinoma trophoblast cell line to attach to endometrial cells ( 35 ). Although HHV-6A also can infect syncytiotrophoblast cells in vitro ( 35 , 36 ), it has not been demonstrated that such infection occurs in vivo , and influences implantation. HHV-6A infection of both endometrial epithelial cells, endometrial NK cells and, possibly, trophoblasts may be one trigger of primary unexplained infertility, through multiple different mechanisms that may impair implantation. Further research is required to determine if therapeutic interventions directed at blocking the pathobiology produced by endometrial HHV-6A infection—for example, antiviral therapy—improve prognosis in women with primary unexplained infertility, particularly in those with documented endometrial infection. If antiviral therapy improved prognosis, it would strongly suggest an etiologic role for HHV-6A.

HHV-6A/B can cause “congenital HHV-6A/B infection”, as defined by the presence of HHV-6A/B DNA in cord blood or placental tissues ( 71 ). About 1-2% of normal neonates have congenital HHV-6A/B infections; in 10% of congenital HHV-6A/B infections there are elevated viral loads in the infant’s blood; one third are due to HHV-6A ( 30 , 72 – 74 ). Congenital HHV-6A/B infection can occur in three ways: 1) iciHHV-6A/B in a parent is passed to the infant, with the infants inheriting the viral genome in every cell—which is the cause of 86% of cases of congenital infection; 2) in a mother with iciHHV-6A/B, the inherited viral genome produces viruses that are passed transplacentally to the infant—even though the infant does not have iciHHV-6A/B (i.e., the haploid oocyte that produced the infant did not contain the integrated viral genome)—which is the cause of about 10% of cases of congenital infection; 3) the mother has acquired (not inherited) the virus and then passed it transplacentally to the infant—which is the cause of about 4% of cases of congenital infection, as occurs with cytomegalovirus ( 74 , 75 ). While it now is well established that congenital infection with HHV-6A/B occurs, it is uncommon. More important, it is unclear whether congenital HHV-6A/B infection affects the health of the baby (or mother) ( 74 ). One study did find that children with transplacental infection subsequently have significantly lower scores on the Bayley Scale of Infant Development MDI instrument ( 76 ), but additional studies of developmental and other health outcomes are needed.

Human herpesviruses-6A and -6B (HHV-6A/B) are capable of infecting a remarkably wide range of cells and tissues, and are being linked to an increasing number of diseases ( 4 ). We summarize evidence that they may be linked to reproductive diseases, as are multiple other viruses (e.g., rubella virus, cytomegalovirus ( 54 , 62 ). One study has found endometrial infection by HHV-6A in 43% of cases of primary unexplained infertility vs. 0% of fertile, well-matched control subjects, a highly significant difference ( 9 ). The effects of the virus on the endometrial epithelium, on endometrial NK cells and possibly trophoblasts make the observed association plausible, and suggest the viruses could plausibly play a role in spontaneous abortion and intrauterine growth retardation. The inherited form of infection (iciHHV-6A/B), and possibly the acquired form, are associated with PE, although only in a small fraction of cases. Effects of the viruses on placentation and endothelial function that could contribute to PE require further study, as well as whether the inherited form of the virus explains some cases of “dangerous fathers” more likely to be associated with cases of PE. Finally, we have summarized how congenital infection can occur in 1-2% of neonates: the possible consequences for the health of the baby or mother warrant further study. In short, we seek to alert readers to the possibility that these viruses can sometimes contribute to the pathogenesis of several reproductive diseases. Our emerging knowledge of the viruses does not justify any changes in current diagnostic testing and treatment practices for primary unexplained infertility, preeclampsia or congenital infection. However, the available evidence is provocative enough to justify further research.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The HHV-6 Foundation supported the costs of publication.