Human Herpes Virus-6 and Clinical Manifestations After Allogeneic Hematopoietic Stem Cell Transplantation

Human herpesvirus 6 (HHV-6) is increasingly recognized as an opportunistic and potentially life-threatening pathogen in recipients of allogeneic Hematopoietic Stem Cell Transplantation (HSCT). Reported clinical manifestations of HHV-6 infection in transplanted patients are skin rash, interstitial pneumonia, bone marrow suppression and encephalitis. Moreover, an increasing number of clinical reports suggest that HHV-6 can facilitate the occurrence of other severe clinical complications of allogeneic HSCT, including Graft-versus-Host Disease (GvHD), ultimately increasining transplant-related mortality. Still, the actual incidence of HHV-6 infection in recipients of HSCT and the causative link between infection and clinical complications remain elusive, mostly due to the small and heterogeneous patient cohorts analyzed to date.

From January 2009 to July 2011, we retrospectively evaluated 43 consecutive adult patients (median age 51 years) who developed positivity to HHV-6 after allogeneic HSCT for high-risk hematological malignancies. Stem cell donor was for 30 patients family haploidentical, for 5 an HLA identical sibling, and for 8 an unrelated volunteer (1 of which cord blood). The viral load was determined by quantitative PCR in cell-free body fluids such as plasma, bronchoalveolar lavage, cerebrospinal fluid, bone marrow aspirates or in gastrointestinal biopsies. At the time of positivity all patients were receiving acyclovir as viral prophylaxis except 5. Sixteen patients had clinical acute GvHD at time of HHV-6 positivity (grade III-IV in 14), and 33 were profoundly immunosuppressed with variable association of 2–4 immunosuppressive drugs (steroids included). Moreover concomitant CMV positivity was detected in 11 patients, while a severe neutropenia in 12.

Median time from allogeneic HSCT to HHV-6 reactivation was 36 days (range: 7–625). In 19 patients HHV-6 was detected in plasma, with a median viral load of 19,454 cp/mL (34-4,524,600); 15 had concomitant fever, 5 skin rash of new onset, 4 impaired liver function, and 5 developed cytopenia subsequently to the infection. In 7 patients HHV-6 was detected in the bone marrow: the median viral load was 163'800 cp/mL (568-1'552'982). In 8 patients, all febrile, HHV-6 was observed in bronchoalveolar lavage samples with a median of 4'149 cp/mL (85–39250). In 16 patients, 10 with documented gut aGvHD, 11 with diarrhoea, HHV-6 was detected in gastrointestinal biopsies with a median of 7'510 cp/mL (120-4'524'600). HHV-6 was found in cerebrospinal fluid in 4 cases (all within 30 days after HSCT); the median viral load was 29'352 cp/mL (4'508-1'552'982); all these patients experienced encephalitis with confusion and anxiety, 2 suffered seizures and 3 showed abnormal findings on brain MRI. Amongst patients with organ localizations of HHV-6 only 28% had concomitant plasma positivity. HHV-6 positivity led to antiviral pharmacological treatment only when associated with clinical manifestations (n=21), using as first choice therapy foscarnet. Amongst the total 43 patients with documented HHV-6 positivity 11 completely solved the clinical event, whereas 19 (44%) died.

HHV-6 infection/reactivation is associated with high morbidity and mortality in patients who undergo allogeneic HSCT. HHV-6 infection typically occurred close to the time of neutrophil engraftment. HSCT from an HLA-mismatched donor and steroid administration were associated with increased risk of active HHV-6 infection. Development of encephalitis was associated with high HHV-6 viral load. The regular monitoring of HHV-6 DNA in allogeneic HSCT recipients, using a real-time PCR assay, may be useful for identifying active HHV-6 infection and for the introduction of a pre-emptive treatment, possibly reducing the incidence of the most severe clinical complications.

No relevant conflicts of interest to declare.