Selective effects of mTOR inhibitor sirolimus on naïve and memory T cells extending its applicable range beyond immunosuppression

Cytomegalovirus (CMV) infection and reactivation remain the most important complications of transplantation, particularly in patients undergoing intense immunosuppression. Recent observations indicate that mammalian target of rapamycin (mTOR) inhibition with sirolimus may improve the outcome of CMV infection following transplantation. This study was designed to elucidate this research question by investigating the role of mTOR complex 1 (mTORC1) signaling in CMV-specific cytotoxic CD8+ T cells (CTLs).The influence of sirolimus on naïve and memory T cells was determined by CD3/CD28 crosslinking and alloreactivity assays. After stimulating CD8+ T cells with artificial antigen-presenting cells (aAPCs) loaded with CMVpp65 peptide, we measured the effect of sirolimus on the proliferative capacity, phenotype and functionality of CTLs. Furthermore, next-generation sequencing (NGS) was used to monitor T-cell receptor (TCR) repertoire dynamics, as well as detection of signaling pathways and expression of target and effector molecules were assessed in response to mTOR inhibition.Sirolimus showed a selective mechanism of action: it inhibited naïve T cells while enhancing the functionality of memory T cells. Despite inhibiting virus-specific CD8+ CTL expansion, the drug induced strong T-cell activation without affecting effector memory phenotype, and significantly increased effector memory T-cell responses. Sirolimus treatment had differential effects on key elements of T-cell biology such as (1) the dynamics of the TCR repertoire, (2) the phosphorylation of kinases and proteins, and (3) the expression of micro-RNAs (miRNAs) and genes.Modulation of environmental cues during CTL development via interleukin-2 receptor (IL-2R)-driven signal transducer and activator of transcription 5 (STAT-5) signaling under the cover of mTORC1 inhibition allowed to fine-tune CTL programming for enhanced antiviral T-cell response with stable dynamics of the TCR repertoire. These study findings suggest that patients with an increased risk of pathogenic infection may benefit from treatment with sirolimus, providing a starting point for further individualization of immunosuppressive therapy.