*Purpose: Cytomegalovirus (CMV) is a major complication for organ transplant recipients. T cells reacting to the CMV antigen immediate-early (IE-1) are key in controlling reactivation of the virus after the transplant. However, our understanding of the CMV-specific T cell response is incomplete and the characteristics of the TCR repertoire conferring protection to CMV are not known. The aim of this study is to identify protective specificity groups in the IE-1-specific TCR repertoire pretransplantation in A02:01 kidney transplant patients.

*Methods: PBMC from three healthy donors and six kidney transplant recipients that are CMV seropositive, IE-1-ELISPOT+, and HLA A02:01 were stimulated in vitro with IE-1 peptides. After 24h the IE1-specific CD8+ T cells were single cell FACS-sorted based on the expression of the CD137 activation marker. We sequenced the IE-1-specific TCRαβ chains, defined the IE-1-TCR repertoire clonality for each patient and analysed the IE-1 TCRβ sequences using the GLIPH biocomputational clustering algorithm.

*Results: The IE-1-TCR repertoire analysis revealed clonally expanded T cell populations in both the healthy seropositive donors and kidney transplant patients. Interestingly, the IE1-TCR repertoire of patients who did not have CMV reactivation was more diverse prior to transplant than that of the patients who did have CMV-reactivation post-transplant (Gini index: 0.2 vs 0.7, respectively). Analysis of post-transplant CMV-specific T cells revealed a decrease of IE-1-TCR diversity in the group of ‘non- reactivating’ patients to similar levels than that of reactivating patients. To note, most immunodominant T cell clones detected prior to transplantation were also detected 6 months post-transplant in both CMV reactivating and CMV non-reactivating patients.GLIPH identified multiple clusters of different TCRβ sequences putatively binding the same IE-1 epitope as a “public” IE-1 TCR sequence reported in the literature as highly conserved among the HLA A2:01 CMV-seropositive population.

*Conclusions: In summary, a single cell TCR sequencing and computational approach has the potential to identify and cluster TCR sequences in transplant patients that control CMV replication and facilitate prediction of CMV risk in the transplant setting.

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