To the Editor: We have read with interest the article by Cantisán and co-workers 1 reporting an association between certain human leukocyte antigen class I (HLA-I) alleles (HLA-A1 and HLA-A2) and the presence of pretransplant reactivity in the QuantiFERON-CMV (QF-CMV) assay in cytomegalovirus (CMV)-seropositive solid organ transplant (SOT) candidates. As they acknowledged, this finding might be confounded by the fact that the QF-CMV assay measures CMV-specific interferon-γ (IFN-γ)–producing CD8+ T cells mainly targeting viral epitopes on pp65 and immediate-early 1 (IE-1) proteins, whose presentation is restricted through the above-referred alleles 1, 2. We sought to determine whether this association remains when CMV-specific IFN-γ–producing CD8+ T cell response is assessed by flow cytometry for intracellular staining (ICS) using 15-mer overlapping peptide libraries spanning the entire sequence of pp65 and IE-1 viral proteins as the stimulating antigen. The ICS method is considered the reference procedure for measuring CMV-specific T cell responses, and 15-mer overlapping peptides are potentially presented in a broader HLA-I allele background 2, 3. CMV-specific IFN-γ–producing CD4+ and CD8+ T cells were enumerated by ICS in 43 CMV-seropositive patients listed for kidney transplantation (details available as Supporting Information). The HLA-A1 and HLA-A2 alleles were present in 10 (24.4%) and 17 (39.5%) patients. The magnitude of CMV-specific CD8+ T cell responses in patients harboring HLA-A1 and/or HLA-A2 alleles was comparable to that observed in patients with HLA-I alleles of other specificities (median 6.59 vs. 4.20 cells/µL; p-value = 0.309). Interestingly, harboring the HLA-B44 allele (present in 11 [25.6%] patients) was associated with robust CMV-specific CD8+ T cell responses (median 9.88 vs. 4.06 cells/µL; p-value = 0.016) (Figure 1A). In fact, it was the only factor found to be associated with the magnitude of such response in a linear regression model (β = 0.366; p-value = 0.020). In turn, older age was not correlated with CMV-specific CD8+ T cell counts (Spearman's rho = −0.223; p-value = 0.155). As opposed to our study, the cohort analyzed by Cantisan and co-workers also included liver and lung transplant candidates, although kidney candidates had a higher probability of being QF-CMV reactive than the remaining patients 1. The skewed association between CMV-specific response and HLA-A1 and A2 genotypes reported by these authors 1, which has not been confirmed in our study, might have been related to the fact that the QF-CMV assay primes the detection of CD8+ T cell responses against the epitopes NLVPMVATV on pp65 (HLA-A2) and VLEETSVML on IE-1 (HLA-A2), which are considered immunodominant in Caucasians. Further studies assessing CMV-specific IFN-γ–producing CD8+ T cell responses measured in parallel by means of QF-CMV and ICS in homogeneous SOT cohorts are needed to resolve this discrepancy. Unexpectedly, CMV-specific IFN-γ–producing CD4+ T cell counts in HLA-A1 and/or HLA-A2 patients were significantly higher that that in patients displaying other HLA-class I allele specificities (median 5.35 vs. 1.59 cells/µL; p-value = 0.010) (Figure 1B), whereas age was inversely correlated with such a subset (Spearman's rho = −0.517; p-value < 0.0001). Linear regression confirmed that age (β = −0.331; p-value = 0.034) and harboring HLA-A1 and/or HLA-A2 alleles (β = 0.275; p-value = 0.068) were independently associated with the CMV-specific CD4+ T cell count. The finding that increasing age is linked to a poorer CMV-specific CD4+ T cell response is congruent with experimental studies 4 and with the clinical experience showing that the CMV disease is more often diagnosed in older kidney transplant recipients 5, and further supports the instrumental role of CD4+ T cells in the long-term maintenance of protective T cell immunity 2. The interpretation of the association found between HLA-I alleles and the CMV-specific CD4+ T cell response is not straightforward. It could be hypothesized that HLA-A1 and A2 alleles might be in linkage disequilibrium with single nucleotide polymorphisms in HLA class II genes promoting a more efficient antigen processing and presentation. In view of the increasingly recognized role for assessing the pretransplant CMV-specific immunity among CMV-seropositive SOT candidates, further studies are warranted to precisely characterize the effect of HLA class I and II alleles on the magnitude of CMV-specific T cell responses. This study was supported by the Spanish Ministry of Economy and Competitiveness, Instituto de Salud Carlos III (Fondo de Investigaciones Sanitarias [FIS] 12/02269 and Proyecto Integrado de Excelencia [PIE] 13/00045). M.F.R. holds a clinical research contract “Juan Rodés” (JR14/00036) from the Spanish Ministry of Economy and Competitiveness, Instituto de Salud Carlos III. M. Fernández-Ruiz1,*, I. Corrales2, P. Amat3, E. González4, A. Andrés4, D. Navarro2 and J. M. Aguado1 1Unit of Infectious Diseases, Hospital Universitario “12 de Octubre“, Instituto de Investigación Hospital “12 de Octubre“ (i+12), School of Medicine, Universidad Complutense, Madrid, Spain 2Department of Microbiology, Hospital Clínico Universitario, Instituto de Investigación Sanitaria INCLIVA, School of Medicine, Universidad de Valencia, Valencia, Spain 3Department of Hematology and Medical Oncology, Hospital Clínico Universitario, Instituto de Investigación Sanitaria INCLIVA, School of Medicine, Universidad de Valencia, Valencia, Spain 4Department of Nephrology, Hospital Universitario “12 de Octubre“, Instituto de Investigación Hospital “12 de Octubre“ (i+12), School of Medicine, Universidad Complutense, Madrid, Spain *Corresponding author: Mario Fernández-Ruiz, mario_fdezruiz@yahoo.es The authors of this manuscript have no conflicts of interest to disclose as described by the American Journal of Transplantation. 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