Human herpesvirus 6-specific T-cell immunity in allogeneic hematopoietic stem cell transplant recipients

Abstract

Human herpesvirus 6 (HHV-6) can reactivate after allogeneic hematopoietic stem cell transplant (allo-HSCT) and may lead to severe symptoms. HHV-6-specific immune responses after HSCT are largely unexplored. We conducted a prospective observational study on 208 consecutive adult patients who received allo-HSCT to investigate HHV-6 reactivations and specific immune responses. Interferon gamma-producing HHV-6-specific T cells were quantified using enzyme-linked immunospot assay (ELISpot). HHV-6 reactivation occurred in 63% of patients, at a median of 25 days from allo-HSCT. Only 40% of these presented a clinically relevant infection, defined by the presence of classical HHV-6 end-organ diseases (EODs), based on European Conference on Infections in Leukaemia (ECIL) guidelines, and other possible HHV6-related EODs. Using multivariate analysis, we identified risk factors for HHV-6 reactivation: previous allo-HSCT, posttransplant cyclophosphamide (PT-Cy), and time-dependent steroids introduction. The use of PT-Cy and steroids were associated with clinically relevant infections, whereas higher CD3+ cell counts seemed to be protective. Interestingly, circulating HHV-6-specific T cells were significantly higher in patients with reactivated virus. Moreover, HHV-6-specific T-cell responses, quantified at >4 days after the first viremia detection, predicted clinically relevant infections (P < .0001), with higher specificity (93%) and sensitivity (79%) than polyclonal CD3+ cells per μL. Overall survival and transplant-related mortality were not affected by time-dependent HHV-6 reactivation, whereas a significant association was observed between clinically relevant infections and acute graft-versus-host disease. These results shed light on the role of HHV-6 in allo-HSCT and may affect HHV-6 monitoring and treatment.

Graphical abstract

Figure 1.

HHV-6 reactivation and clinically relevant infection. (A) CI of total HHV-6 reactivation; (B) CI of HHV-6 clinically relevant infection.

Figure 2.

IFN-γ ELISpot for HHV-6–specific T cells. (A) Ex vivo T-cell responses in HDs upon stimulation with the phytohemagglutinin control (PHA), CMV antigens, and HHV-6. Results are expressed as SFC/10⁵ PBMC. Lines indicate medians with interquartile range. (B) HHV-6–specific T-cell responses in HD at steady state (day 0) and 14 days after in vitro stimulation. (C) Frequencies of HHV-6–specific T cells at the indicated time points from the first day of viremia in HSCT recipients. (D) HHV-6–specific SFC/10⁵ PBMC measured at ≥3 or at ≥7 days from the first day of viremia in samples collected from the same patient. (E) Frequencies of HHV-6–specific T cells in different groups: reactivating patients (HHV6+, red), controls (nonreactivating patients; HHV6-, green) and HD (violet). (F) Frequencies of HHV-6–specific T cells in patients developing HHV-6 clinically relevant infection (red) and in patients experiencing HHV-6 subclinical reactivation (blue). (G) Absolute counts of polyclonal CD3+ cells in patients developing HHV-6 clinically relevant infection (red) and in patients experiencing HHV-6 subclinical reactivation (blue). Statistical analysis in panels B and D were performed by Wilcoxon t test. Statistical analyses in panel E were performed using Kruskal-Wallis test, and in panels A, C, F, and G using Mann-Whitney test. ^∗P < .05; ^∗∗P < .01; ^∗∗∗∗P < .0001. ns, not significant.

Figure 3.

Correlation of HHV-6 reactivation and specific immune reconstitution. (A) Receiver operating characteristic (ROC) analysis for HHV-6–specific immune reconstitution by ELISpot (threshold of 18 SFCs per 10⁵ PBMCs; specificity 0.933; sensitivity 0.789); (B) CI of clinically relevant HHV-6 reactivations by the identified threshold (in blue, patients below threshold and in red, patients above the threshold of 18 HHV-6–specific SFC/10⁵ PBMC).