eThe presence of inherited chromosomally integrated human herpesvirus 6 (ciHHV-6) in hematopoietic cell transplant (HCT) donors or recipients confounds molecular testing for HHV-6 reactivation, which occurs in 30 to 50% of transplants. Here we describe a multiplex droplet digital PCR clinical diagnostic assay that concurrently distinguishes between HHV-6 species (A or B) and identifies inherited ciHHV-6. By applying this assay to recipient post-HCT plasma and serum samples, we demonstrated reactivation of HHV-6B in 25% (4/16 recipients) of HCT recipients with donor-or recipient-derived inherited ciHHV-6A, underscoring the need for diagnostic testing for HHV-6 infection even in the presence of ciHHV-6.
Human herpesvirus 6A and -B (HHV-6A and -B) are known to be able to integrate within human chromosomal subtelomeric regions through homologous recombination at HHV-6 direct repeat regions (1). If this integration occurs within germ line cells, the offspring produced from those cells carry a copy of HHV-6A or -B in every nucleated cell; this condition is called inherited chromosomally integrated HHV-6A or -B (ciHHV-6A or -B) (2). The presence of ciHHV-6A or -B in every cell of the body has unknown effects (3), but two recent studies suggested a link between inherited ciHHV-6A or -B and angina pectoris (4, 5). Also, several reports have demonstrated HHV-6 reactivation and associated disease from inherited ciHHV-6 (6, 7).HHV-6B reactivation (but not HHV-6A reactivation) occurs in 30 to 50% of hematopoietic cell transplant (HCT) recipients, typically within the first 2 to 6 weeks posttransplantation. This has been associated with complications, such as central nervous system dysfunction, fever and rash, myelosuppression, graft rejection, and acute graft-versus-host disease (GVHD), some of which have also been reported after solid organ transplantation (8). While further research is needed to determine the clinical significance of inherited ciHHV-6A or -B after transplantation, it is well recognized that inherited ciHHV-6A or -B complicates molecular diagnostic testing for HHV-6B reactivation (9, 10). Specifically, tissue and blood samples from patients with inherited ciHHV-6A or -B will have high levels of HHV-6A or -B detected by routine quantitative PCR (qPCR), obscuring detection of HHV-6B reactivation.To aid in the recognition of inherited ciHHV-6 in the transplant setting, we recently developed a clinical droplet digital PCR (ddPCR) assay to identify inherited ciHHV-6 in cellular patient specimens (11). A subsequent study demonstrated the utility of ddPCR in distinguishing HHV-6A and -B (12). Here we describe an improved assay that identifies inherited ciHHV-6 and determines which species, HHV-6A or HHV-6B, is responsible for the integration by using a single reaction mixture. We also show that this assay can aid in the diagnosis of reactivated HHV-6B in HCT recipients with inherited ciHHV-6A, which further highlights the need for improved diagnostics for HHV-6 reactivation, particularly in immunosuppressed patien...