Immune cell subsets that predict cytomegalovirus reactivation magnitude after haematopoietic stem cell transplantation.

Haematopoietic stem cell transplantation (HSCT) is a curative-intent treatment for otherwise lethal haematological malignancies. The first 100 days post-transplant is a period of high risk for opportunistic infections, including reactivation of latent viruses, as the recipient's immune system reconstitutes. Human cytomegalovirus (HCMV) is the most frequent virus to reactivate post-HSCT, however, there are no simple and well validated biomarkers that predict HCMV reactivation and its subsequent course. In this study, we identified potential predictive immune signatures for HCMV reactivation occurrence and viral load magnitude. We developed a 21-parameter spectral cytometry antibody panel to characterise peripheral blood mononuclear cells (PBMCs) from adult allogeneic HSCT-recipients (n=25) at four timepoints over the course of HCMV reactivation, including before and at initial detection of HCMV DNAemia. Prior to HCMV reactivation, CD4+ T, Vδ2pos γδT and mucosal-associated invariant T (MAIT) cells from patients who would later reactivate HCMV exhibited CD69+ phenotypes. At the initial detection of HCMV reactivation, patients who went on to experience high-level HCMV DNAemia had lower CD45RO+CD27- CD8+ T-cell, and greater CD57+ Vδ2pos γδT and TIM-3+ CD4-CD8+ MAIT cell compartments than those with controlled infection. At later timepoints, MAIT cells acquired a chronically activated (CD38, granzyme B) and terminally exhausted (CD57, TIM-3) phenotype during high-level HCMV DNAemia. In summary, we present phenotypic distinctions in MAIT, γδT, conventional T and NK cells dependent on the subsequent trajectory of HCMV reactivation. We propose these cell subsets as predictive biomarkers for HCMV reactivation post-HSCT to inform clinical decision-making.