Asymmetric cell division shapes naive and virtual memory T-cell immunity during ageing.

Efficient immune responses rely on heterogeneity, which in CD8+ T cells, amongst other mechanisms, is achieved by asymmetric cell division (ACD). Here we find that ageing, known to negatively impact immune responses, impairs ACD in murine CD8+ T cells, and that this phenotype can be rescued by transient mTOR inhibition. Increased ACD rates in mitotic cells from aged mice restore the expansion and memory potential of their cellular progenies. Further characterization of the composition of CD8+ T cells reveals that virtual memory cells (TVM cells), which accumulate during ageing, have a unique proliferation and metabolic profile, and retain their ability to divide asymmetrically, which correlates with increased memory potential. The opposite is observed for naive CD8+ T cells from aged mice. Our data provide evidence on how ACD modulation contributes to long-term survival and function of T cells during ageing, offering new insights into how the immune system adapts to ageing.

Chronic viral infections impinge on naive bystander CD8 T cells.

INTRODUCTION: Epidemiological data suggest that persistent viral infections impair immune homeostasis and immune responsiveness. Previous studies showed that chronic virus infections negatively impact bystander T-cell differentiation and memory formation but there is limited knowledge of how chronic virus infections impinge on heterologous naive T-cell populations. METHODS: We used adoptive transfer of naive CD8 T cells with defined nonviral specificity into hosts, which were subsequently chronically infected with lymphocytic choriomeningitis virus, followed by analyses of numeric, phenotypic, and functional changes provoked in the chronically infected host. RESULTS: We demonstrate that chronic virus infections have a profound effect on the number and phenotype of naive bystander CD8 T cells. Moreover, primary expansion upon antigen encounter was severely compromised in chronically infected hosts. However, when naive bystander CD8 T cells were transferred from the chronically infected mice into naive hosts, they regained their expansion potential. Conversely, when chronically infected hosts were supplied with additional antigen-presenting cells (APCs), primary expansion of the naive CD8 T cells was restored to levels of the uninfected hosts. CONCLUSIONS: Our results document numeric, phenotypic, and functional adaptation of bystander naive CD8 T cells during nonrelated chronic viral infection. Their functional impairment was only evident in the chronically infected host, indicating that T-cell extrinsic factors, in particular the quality of priming APCs, are responsible for the impaired function of naive bystander T cells in the chronically infected hosts.

Early primed KLRG1- CMV-specific T cells determine the size of the inflationary T cell pool.

Memory T cell inflation is a process in which a subset of cytomegalovirus (CMV) specific CD8 T cells continuously expands mainly during latent infection and establishes a large and stable population of effector memory cells in peripheral tissues. Here we set out to identify in vivo parameters that promote and limit CD8 T cell inflation in the context of MCMV infection. We found that the inflationary T cell pool comprised mainly high avidity CD8 T cells, outcompeting lower avidity CD8 T cells. Furthermore, the size of the inflationary T cell pool was not restricted by the availability of specific tissue niches, but it was directly related to the number of virus-specific CD8 T cells that were activated during priming. In particular, the amount of early-primed KLRG1- cells and the number of inflationary cells with a central memory phenotype were a critical determinant for the overall magnitude of the inflationary T cell pool. Inflationary memory CD8 T cells provided protection from a Vaccinia virus challenge and this protection directly correlated with the size of the inflationary memory T cell pool in peripheral tissues. These results highlight the remarkable protective potential of inflationary CD8 T cells that can be harnessed for CMV-based T cell vaccine approaches.

Modulation of asymmetric cell division as a mechanism to boost CD8+ T cell memory.

Asymmetric partitioning of fate determinants is a mechanism that contributes to T cell differentiation. However, it remains unclear whether the ability of T cells to divide asymmetrically is influenced by their differentiation state, as well as whether enforcing asymmetric cell division (ACD) rates would have an impact on T cell differentiation and memory formation. Using the murine LCMV infection model, we established a correlation between cell stemness and the ability of CD8+ T cells to undergo ACD. Transient mTOR inhibition was proven to increase ACD rates in naïve and memory cells and to install this ability in exhausted CD8+ T cells. Functionally, enforced ACD correlated with increased memory potential, leading to more efficient recall response and viral control upon acute or chronic LCMV infection. Moreover, transient mTOR inhibition also increased ACD rates in human CD8+ T cells. Transcriptional profiling revealed that progenies emerging from enforced ACD exhibited more pronounced early memory signatures, which functionally endowed these cells with better survival in the absence of antigen exposure and more robust homing to secondary lymphoid organs, providing critical access to survival niches. Our data provide important insights into how ACD can improve long-term survival and function of T cells and open new perspectives for vaccination and adoptive T cell transfer therapies.

Oncolytic virotherapy enhances the efficacy of a cancer vaccine by modulating the tumor microenvironment.

The efficacy of cancer vaccines has been limited by the immunosuppressive tumor microenvironment, which can be alleviated by immune checkpoint inhibitor (ICI) therapy. Here, we tested if oncolytic viruses (OVs), similar to ICI, can also synergize with cancer vaccines by modulating the tumor microenvironment. VSV-GP, a chimeric vesicular stomatitis virus (VSV) pseudotyped with the glycoprotein (GP) of the lymphocytic choriomeningitis virus, is a promising new OV candidate. Here, we show that in mouse B16-OVA melanoma, combination treatment of VSV-GP with an ovalbumin (OVA) peptide-loaded dendritic cell (DC) vaccine (DCVacc) significantly enhanced survival over the single agent therapies, although both DCVacc and DCVacc/VSV-GP treatments induced comparable levels of OVA-specific CD8 T cell responses. Virus replication was minimal so that direct viral oncolysis in B16-OVA did not contribute to this synergism. The strong therapeutic effect of the DCVacc/VSV-GP combination treatment was associated with high numbers of tumor-infiltrating, highly activated T cells and the relative reduction of regulatory T cells in treated and contra-lateral nontreated tumors. Accordingly, depletion of CD8 T cells but not natural killer cells abrogated the therapeutic effect of DCVacc/VSV-GP supporting the crucial role of CD8 T cells. In addition, a drastic increase in several proinflammatory cytokines was observed in VSV-GP-treated tumors. Taken together, OVs, similar to ICI, have the potential to markedly increase the efficacy of cancer vaccines by alleviating local immune suppression in the tumor microenvironment.

NK cells negatively regulate CD8 T cells via natural cytotoxicity receptor (NCR) 1 during LCMV infection.

Besides their function in recognizing cancerous and virally infected cells, natural killer (NK) cells have the potential to shape adaptive immune responses. However, the mechanisms employed by NK cells to negatively regulate virus-specific CD8 T cell responses remain to be fully defined. Using activating receptor natural cytotoxicity receptor (NCR) 1 deficient (NCR1gfp/gfp) mice, we found increased numbers of virus-specific CD8 T cells, leading to enhanced virus control during acute LCMV infection. Furthermore, virus-specific CD8 T cells were more activated in the absence of NCR1, resulting in exacerbated immunopathology, documented by weight loss, and superior virus control early during chronic LCMV infection. Transfer experiments of virus-specific CD8 T cells into NCR1 deficient hosts revealed a direct cross talk between NK and CD8 T cells. Studies on the splenic microarchitecture revealed pronounced disorganization of T cells in infected NCR1gfp/gfp mice, resulting in enhanced immunopathology and disruption of the T cell niche upon chronic LCMV infection. Our data show a novel pathway employed by NK cells to regulate antiviral CD8 T cell responses, namely direct recognition and elimination of activated CD8 T cells via NCR1 early during infection to protect the host from an overshooting T cell response.

Chronic virus infection compromises memory bystander T cell function in an IL-6/STAT1-dependent manner.

Chronic viral infections are widespread among humans, with ∼8-12 chronic viral infections per individual, and there is epidemiological proof that these impair heterologous immunity. We studied the impact of chronic LCMV infection on the phenotype and function of memory bystander CD8+ T cells. Active chronic LCMV infection had a profound effect on total numbers, phenotype, and function of memory bystander T cells in mice. The phenotypic changes included up-regulation of markers commonly associated with effector and exhausted cells and were induced by IL-6 in a STAT1-dependent manner in the context of chronic virus infection. Furthermore, bystander CD8 T cell functions were reduced with respect to their ability to produce inflammatory cytokines and to undergo secondary expansion upon cognate antigen challenge with major cell-extrinsic contributions responsible for the diminished memory potential of bystander CD8+ T cells. These findings open new perspectives for immunity and vaccination during chronic viral infections.

Tissue maintenance of CMV-specific inflationary memory T cells by IL-15.

Cytomegalovirus (CMV) infection induces an atypical CD8 T cell response, termed inflationary, that is characterised by accumulation and maintenance of high numbers of effector memory like cells in circulation and peripheral tissues-a feature being successfully harnessed for vaccine purposes. Although stability of this population depends on recurrent antigen encounter, the requirements for prolonged survival in peripheral tissues remain unknown. Here, we reveal that murine CMV-specific inflationary CD8 T cells are maintained in an antigen-independent manner and have a half-life of 12 weeks in the lung tissue. This half-life is drastically longer than the one of phenotypically comparable inflationary effector cells. IL-15 alone, and none of other common γ-cytokines, was crucial for survival of inflationary cells in peripheral organs. IL-15, mainly produced by non-hematopoietic cells in lung tissue and being trans-presented, promoted inflationary T cell survival by increasing expression of Bcl-2. These results indicate that inflationary CD8 T cells are not just simply effector-like cells, rather they share properties of both effector and memory CD8 T cells and they appear to be long-lived cells compared to the effector cells from acute virus infections.