"New kid on the block": UL36-specific T cells contribute significantly to the HCMV-specific T-cell repertoire in healthy individuals

Complications due to infection or reactivation with human cytomegalovirus (HCMV) remain a challenging problem in immunocompromised patients, mainly due to insufficient or absent T-cell immunity. Knowledge of immunodominant viral targets is crucial to classify T-cell responses to improve monitoring of high risk patients and optimize antiviral T-cell therapy. This study aims to evaluate whether the anti-apoptotic HCMV protein UL36 is suitable to expand the spectrum of immunogenic targets.

Overlapping 15-mer peptides covering the whole UL36 protein were synthesized and assembled into one overlapping peptide pool (UL36_pp) and 31 sub-pools containing 7─8 peptides (UL36_sp1-31). UL36-specific T-cell responses were analyzed in comparison to the immunodominant HMCV peptide pools pp65_pp and IE1_pp in ≥40 healthy donors by IFN-γ-EliSpot assay. Peptides derived from UL36 sub-pools that elicited high T-cell responses were further evaluated for their binding strength to common HLA I and II alleles using established databases. Phenotypic and functional characteristics of UL36-specific T cells were assessed after antigen exposure by multicolor flow cytometry and multiplex assays to determine their immunogenicity and cytotoxic potential.

Overall, high UL36-specific T-cell responses were detected in HCMV-seropositive donors. Surprisingly, more than 50% of all donors showed a stronger T-cell response to UL36_pp (316.4 spw) than to pp65_pp (244 spw). Compared to IE1_pp (86.6 spw), the T-cell response was even stronger in 80% of donors. For strong responding UL36 sub-pools, we identified peptides with high binding scores covering a broad range of HLA alleles. In-depth profiling of phenotype and functionality of UL-36-specific memory T cells revealed an increased expression of activation markers (e.g. CD25, CD69) and a high proliferative and cytotoxic capacity after antigen contact. This was indicated by increased secretion of cytotoxic effector molecules (e.g. IFN-γ, granzyme B).

Our results show that UL36 is a novel immunodominant target that exhibits an even stronger anti-HCMV T-cell immune response compared to the known responses against pp65 and IE1. Our data underline the impressive potential of UL36 in expanding the repertoire of immunogenic target antigens for improved monitoring of high-risk patients and optimization of adoptive antiviral T-cell therapy.

The authors declare no conflict of interest.