Competition but not cross-competition controls T cell proliferation in transplant patients following cytomegalovirus reactivation

Competition between antigen-specific T cells for peptide:MHC (p:MHC) complexes shapes the ensuing T cell response. T cells also compete for access to antigen presenting cells (APCs), which results in competition between T cells of different antigen specificities and is referred to as cross-competition. When both mechanisms are in place, a memory T cell population specific for just a single epitope can effectively limit de novo antigen-specific T cell responses. Whether human cytomegalovirus (CMV)-specific T cell responses prevent other de novo T cell responses is unknown, but of interest in a transplant context as well as the development of CMV-based vaccines. We assessed the extent of T cell competition and cross-competition in a cohort of non-myeloablative hematopoietic stem cell transplant (nmHSCT) patients. In our cohort, HSCT donors and recipients were CMV-seronegative and -positive, respectively, thus providing genetically distinct memory and naïve T cell populations. We used single-cell transcriptomics to track donor versus recipient-derived T cell clones. We found that recipient-derived CMV-specific memory T cells outcompeted donor-derived responses of identical specificity; however, expansion of other donor-derived T cell clones still occurred averaging 24 rounds of cell division. The transcriptional programs between expanding donor and recipient-derived cells were largely congruent and consistent with an effector T cell response. Our data further indicated that the composition of the T cell compartment changed due to expanding and contracting clones in the first 90 days post-nmHSCT. Overall, our data suggest that CMV-specific memory T cells compete for p:MHC but not APC access during CMV reactivation. CD8+ T cells were isolated from cryopreserved PBMCs collected longitudinally post non-myeloablative hematopeotic stem cell transplant. These cells were then process for transcriptional profiling and/or V(D)J sequencing using either 3' or 5' whole transcriptomic approaches (10x)