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Guidelines for HHV-6 infections
tion. Normally this would be accompanied by HHV-6 seroconversion, but severely immunocompromised HSCT recipients may not develop antibodies. Donor-derived CIHHV-6 must be excluded.
Human herpesvirus 6 reactivation
Given the difficulty distinguishing between reactivation of latent virus (endogenous) and reinfection (exogenous), in clinical practice the term HHV-6 reactivation is applied to both scenarios and is defined as new detection of HHV- 6 in individuals with evidence of previous infection; this latter can be assumed in individuals older than two years. The diagnosis usually relies on the presence of HHV-6 DNA in peripheral blood but other methods and samples are sometimes used. Reactivation is not proven if newly detected HHV-6 DNA is due to donor- or recipient- derived CIHHV-6 since latently-integrated viral DNA can- not be distinguished from replicating virus DNA. See below for tests for CIHHV-6 and its reactivation.
Human herpesvirus 6 diagnostic testing
Antibody tests cannot distinguish between HHV-6A and HHV-6B and are not indicated in HSCT patients. Table 2 gives an overview of possible diagnostic tests.
DNA tests
Polymerase chain reaction (PCR) is the mainstay of HHV-6 diagnosis and a variety of real-time PCR assays for HHV-6 DNA load are available.22,23 Not all differentiate
Table 2. Human herpesvirus 6 (HHV-6) diagnostic tests. Method
Virus culture*
Viral antigen test (immunohistochemical staining)* Detection of viral mRNA by reverse transcription PCR*
Quantitative viral DNA PCR
Droplet digital PCR*
Fluorescence in situ hybridization*
between HHV-6A and HHV-6B, and agreement between laboratories for HHV-6 DNA levels is poor.22,24 However, a World Health Organization standard for HHV-6B DNA is now available (http://www.nibsc.org/documents/ ifu/15- 266.pdf).
• Quantitative PCR that distinguishes between HHV-6A and HHV-6B DNA is recommended for diagnosis of infec- tion.
• For a given patient, repeat HHV-6 DNA testing should be performed using the same DNA extraction method, quantitative PCR and type of specimen.
Interpretation of DNA testing post-hematopoietic stem cell transplantation in the presence of chromosomally integrated human herpesvirus 6
If a HSCT donor has CIHHV-6, HHV-6 DNA load in blood will increase post-HSCT in parallel with leukocyte engraftment,13,16,25 and antivirals will have no effect on the quantity of the latently integrated viral DNA.26 Alternatively, if the recipient has CIHHV-6, high levels of HHV-6 DNA will be detected pre-HSCT in blood and will decrease alongside recipient leukocytes post-transplant.14,27 Importantly, in this latter situation, HHV-6 DNA will con- tinue to be detected at high levels in non-hematopoietic tissue throughout the body28 (Table 3).
Tests for chromosomally integrated human herpesvirus 6
Currently there is no indication for routine testing of HSCT donors or recipients for CIHHV-6. However, in clinically ambiguous cases, such testing can be important
Use and limitations
Diagnosis of infection: gold standard, specialized, labor-intensive
Diagnosis of infection: limited sensitivity, slow turn-around time
Late gene transcripts to confirm virus replication. No international standardization or specific thresholds for virus replication, especially for CIHHV-6
Longitudinal studies, comparison of HHV-6 DNA levels in blood vs. organs. Can discriminate between HHV-6A and HHV-6B*
Precise method for DNA levels, identification of CIHHV-6 Confirmation of CIHHV-6
*Not available to most diagnostic laboratories. PCR: polymerase chain reaction; CIHHV-6: chromosomally integrated HHV-6.
Table 3. Human herpesvirus 6 (HHV-6) test results after allogeneic hematopoietic stem cell transplantation that indicate naturally acquired HHV- 6 infection versus chromosomally integrated HHV-6 (CIHHV-6).
Laboratory observations
One HHV-6 copy/leukocyte
HHV-6 status
Prior childhood infection*
No
Donor CIHHV-6 positive
Yes**
Recipient CIHHV-6 positive
No
Yes§
*Human herpesvirus 6B (HHV-6B) primary infection usually occurs in childhood. **HHV-6 found persistently in hematopoietic tissue, e.g. blood, bone marrow, spleen. §HHV-6 found persistently at extremely high levels in all nucleated non-hematopoietic cells. ***A low level in peripheral blood in cases of organ damage and cell death or hematologic malignancy relapse.
Donor and recipient CIHHV-6 positive
Yes **
Yes§
One HHV-6 copy/non-hematopoietic cell
HHV-6species B AorB AorB AorB Persistent HHV-6 DNA in blood No Yes +/-*** Yes Response of HHV-6 DNA level to antiviral drugs Yes No No No
No
No
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