Impact of asymmetric cell division on T cell differentiation

Successful immune responses rely on diversity. Being equipped with highly variable T cell receptors (TCRs), which convey antigen specificity, CD8+ T cells exhibit an immense repertoire of different naïve cells even before antigen encounter, but for an immune response to be effective, both clonal expansion, and differentiation must take place. Upon cognate antigen activation, one single naïve CD8+ T cell can give rise to different progenies, including effector and memory cells. How this diversity is generated, is still a phenomenon incompletely elucidated, but some mechanisms were reported to be involved in fate determination, such as differential strength of TCR activation, and differential exposure to inflammatory stimuli, leading to differential transcriptional and metabolic profiles, epigenetic control of gene expression, and asymmetric cell division (ACD). Concerning ACD as a means to foster diversification, most studies were limited to describe the mitotic polarization of a variety of components in activated naïve T lymphocytes, leading to the rise of two daughter cells inheriting distinct potential fates. We set out to study whether the ability to undergo ACD is limited to certain CD8+ T cell subsets and developed a strategy to modulate ACD rates. Using the murine Lymphocytic Choriomeningitis virus (LCMV) infection model, we established a correlation between ACD and cellular stemness, as naïve and memory CD8+ T cells (which exhibit stemness in terms of self-maintenance and being able to generate progenies with different fates) were found to divide asymmetrically, while terminally differentiated cells, as short-lived effector and exhausted cells, lacked this ability. ACD modulation was achieved by transient mTOR inhibition, leading to higher ACD rates in naïve and memory cells, and reestablishment of ACD in pre-terminally exhausted PD-1int CD8+ T cells. The ability to undergo ACD correlated with memory potential. Upon adoptive transfer, progenies of mTOR-inhibited LCMV-specific TCR transgenic P14 cells, exhibiting increased ACD rates, showed improved expansion upon acute or chronic LCMV infection, resulting in more efficient viral control. mTOR inhibition also led to higher ACD rates in stimulated naïve and memory human CD8+ T cells. Thus, ACD modulation might be a potential useful tool to enforce memory differentiation in T cells. Memory formation is known to be impaired upon aging as a consequence of profound remodelling of immune responses, in particular CD8+ T cells. This led us to investigate how aging affects the ability of CD8+ T cells from aged individuals to undergo ACD and whether transient mTOR inhibition could reinvigorate memory features that are impaired during immunosenescence. Analysis of CD8+ T cells isolated from young and old naïve P14 mice showed that aging led to overall decreased ACD rates, which could be increased upon transient mTOR inhibition and also led to better memory potential. Being aware of the heterogeneity within CD8+ T cell populations, and that aging leads to an increased CD44hi/CD44lo T cell ratio, we further dissected the ability of CD8+ T cells with high or low CD44 expression levels to undergo ACD. We found that CD44hi cells showed intrinsic higher ACD rates compared to their CD44lo counterparts. Moreover, ACD modulation by transient mTOR inhibition was only effective in CD44lo cells. Phenotypical analysis of CD44hi cells revealed them as virtual memory T cells (TVM). Functionally, TVM showed better expansion potential in adoptive transfer experiments, which is likely explained by their ability to survive better upon limiting availability of homeostatic cytokines when compared to “truly naïve” CD44lo CD8+ T cells. In this perspective, TVM emerge as a possible solution on how the adaptive immune system counteracts immunosenescence by endowing this T cell subset with long-term survival and function during advanced aging. As enhanced ACD proved to correlate with superior long-term survival and function, our results open new perspectives on vaccination and adoptive transfer therapies, of particular relevance in the context of tumours, chronic infections and improvement of immune responses in the elderly.