Conclusive evidence that ciHHV-6 activates in vivo: pathogenic ciHHV-6A infection in an infant with X- SCID
Since scientists first observed the phenomenon of chromosomal integration of HHV-6 (ciHHV-6), it has been unclear whether individuals with this condition could experience reactivation of the virus with pathogenic effects on the human body. Now, for the first time, a group in Japan has presented convincing molecular and virological evidence that a patient’s integrated strain can activate to produce infectious virions. The activated integrated strain triggered hemophagocytic syndrome (HPS), which was treated successfully with antivirals.
Earlier reports on ciHHV-6 suggested that it was unlikely that the integrated virus could activate and that all patients determined to have ciHHV-6 should have their antiviral medications withdrawn. These assumptions have now been proven wrong on both counts, and the new findings have significant implications for the management of patients with ciHHV-6 moving forward. In an accompanying editorial, ciHHV-6 expert Louis Flamand has called for transplant centers to initiate ciHHV-6 screening programs (Flamand 2014).
In the case report, after an immunocompromised infant developed HPS and HHV-6A was identified in the plasma. The patient was put on antiviral therapy with some improvement, but antivirals were discontinued when ciHHV-6 status was determined. The patient then worsened, prompting the investigators to determine if the HHV-6A infection was indeed active or merely a reflection of the fact that the patient was born with ciHHV-6A genomes integrated in the chromosome of every nucleated cell. First, infectious HHV-6A was isolated from the patient’s PBMC and cultured with cord blood cells, and confirmed by IFA. Second, the group performed reverse transcription–PCR (RT–PCR) and identified viral RNA in whole blood samples. Finally, immunostaining was used to detect intermediate early (IE) viral antigens in a bone marrow sample taken at the time of HPS.
The group then undertook further investigation to determine whether the patient with ciHHV-6 was infected with a community-acquired strain of HHV-6, or whether HHV-6 was activated from the ciHHV-6 genome itself. To their surprise, the sequences and subsequent phylogenetic analysis revealed that the isolated virus was identical to the original integrated HHV-6A strain present in both the patient and his father. Furthermore, this HHV-6A strain demonstrated polymorphisms that could distinguish it from all other HHV-6 strains analyzed when key viral genes (gB, U94, and DR) were analyzed. This thorough analysis leaves little doubt that the isolated HHV-6A virus originated from the patient’s own integrated strain of ciHHV-6A.
History and Course
Their report describes a 2-month-old boy hospitalized for recurrent fever, cough, diarrhea and failure to thrive, which did not resolve with supportive care. After 20 days of admission, pancytopenia was observed concurrently with elevated AST/ALT and ferritin. A subsequent bone marrow biopsy indicated hemophagocytic syndrome (HPS), while immunological evaluation revealed an absence of T cells and decreased levels of immunoglobulin. Genetic analysis revealed a mutation in the common gamma chain of IL2R, consistent with a diagnosis of X-linked SCID. Severe combined immunodeficiency (SCID) is a group of genetic disorders that result in a combined absence of T- and B-cell immunity, and is characterized by life-threatening infections during the first year of life unless treated, usually with hematopoietic stem-cell transplantation (HSCT).
High levels of HHV-6 DNA were simultaneously detected in the peripheral blood of the patient, and when antiviral treatment (ganciclovir and foscarnet) did not reduce the viral load, ciHHV-6 was suspected. To confirm this diagnosis, high levels of HHV-6 DNA was detected in the patient’s fingernails, and also in the hair follicles of his father. Additional analysis via FISH indicated vertical transmission of HHV-6 integration, which was found at chromosome 22 in both the patient and his father.
The discontinuation of antiviral treatment led to the recurrence of the patient’s HPS. Because no other pathogen was detected, activation of HHV-6 was suspected.
Antiviral treatment was resumed, along with prednisolone, and the patient’s HPS resolved. At 7 months, antiviral treatment was continued while the patient underwent HSCT. TMA and GI bleeding developed following engraftment, with subsequent elevation of HHV-6A DNA and RNA (HHV-6A was re-isolated at this time). However, the patient’s HHV-6A DNA and RNA titers were successfully reduced with immunological reconstitution, no HHV-6A was isolated from subsequent blood samples, and the patient was asymptomatic when discharged at 12 months.
Cause for concern in all ciHHV-6 individuals?
It is not known whether ciHHV-6 individuals with mild or selective immune deficiencies might fall victim to activation of their integrated viral genomes. Concern was raised in 2010 when a group from University of Rochester reported that pregnant women with ciHHV-6 may shed active virus from their integrated genomes which was then transmitted transplacentally to their infants. It was determined at that time that mothers with ciHHV-6 could transmit the virus to the fetus transplacentally, to infants that did not inherit the condition but were at risk for symptomatic congenital HHV-6 infection (Hall 2010). This was confirmed last year when it was determined that the sequences of the mothers’ integrated strains were identical to those of their infants who had acquired the infection from active virus shed from their mothers but different from other known HHV-6 isolates (Gravel 2013).
Note from the Foundation staff: We asked the authors of this important case report about the child’s current status. The patient continues to suffer from developmental delays. A follow-up examination of the patient’s CSF post-transplant has revealed no mRNA.
For more information, read the full paper here, and visit the HHV-6 Foundation’s webpage on ciHHV-6.