Impaired vaccine responses in older individuals are associated with alterations in both the quantity and quality of the T-cell compartment with age. As reviewed herein, the T-cell response to vaccination requires a fine balance between the generation of inflammatory effector T cells versus follicular helper T (T FH) cells that mediate high-affinity antibody production in tandem with the induction of longlived memory cells for effective recall immunity. During aging, we find that this balance is tipped where T cells favor short-lived effector but not memory or T FH responses. Consistently, vaccine-induced antibodies commonly display a lower protective capacity. Mechanistically, multiple, potentially targetable, changes in T cells have been identified that contribute to these age-related defects, including posttranscription regulation, T-cell receptor signaling, and metabolic function. Although research into the induction of tissue-specific immunity by vaccines and with age is still limited, current mechanistic insights provide a framework for improved design of age-specific vaccination strategies that require further evaluation in a clinical setting. (

CD4 T cell responses are composed of heterogeneous T cell receptor (TCR) signals that influence the acquisition of effector and memory characteristics. We sought to define early TCR-dependent activation events that control T cell differentiation. A polyclonal panel of TCRs specific for the same viral antigen demonstrated substantial variability in TCR signal strength, expression of CD25, and activation of nuclear factor of activated T cells and nuclear factor κB. After viral infection, strong TCR signals corresponded to T helper cell (T1) differentiation, whereas T follicular helper cell and memory T cell differentiation were most efficient when TCR signals were comparatively lower. We observed substantial heterogeneity in TCR-dependent CD25 expression in vivo, and the vast majority of CD4 memory T cells were derived from CD25 effector cells that displayed decreased TCR signaling in vivo. Nevertheless, memory T cells derived from either CD25 or CD25 effector cells responded vigorously to rechallenge, indicating that, although early clonal differences in CD25 expression predicted memory T cell numbers, they did not predict memory T cell function on a per cell basis. Gene transcription analysis demonstrated expression clustering based on CD25 expression and enrichment of transcripts associated with enhanced T follicular helper cell and memory development within CD25 effector cells. Direct enhancement of TCR signaling via knockdown of Src homology region 2 domain-containing phosphatase 1, a tyrosine phosphatase that suppresses early TCR signaling events, favored the differentiation of T1 effector and memory cells. We conclude that strong TCR signals during early T cell activation favor terminal T1 differentiation over long-term T1 and T follicular helper cell memory responses.

SUMMARY Induction of protective vaccine responses, governed by the successful generation of antigen-specific anti-bodies and long-lived memory T cells, is increasingly impaired with age. Regulation of the T cell proteome by a dynamic network of microRNAs is crucial to T cell responses. Here, we show that activation-induced upregulation of miR-21 biases the transcrip-tome of differentiating T cells away from memory T cells and toward inflammatory effector T cells. Such a transcriptome bias is also characteristic of T cell responses in older individuals who have increased miR-21 expression and is reversed by antagonizing miR-21. miR-21 targets negative feedback circuits in several signaling pathways. The concerted, sustained activity of these signaling path-ways in miR-21high T cells disfavors the induction of transcription factor networks involved in memory cell differentiation. Our data suggest that curbing miR-21 upregulation or activity in older individuals may improve their ability to mount effective vaccine responses.

Diversity and size of the antigen-specific T cell receptor (TCR) repertoire are two critical determinants for successful control of chronic infection. Varicella zoster virus (VZV) that establishes latency during childhood is able to escape control mechanisms, in particular with increasing age. We examined the TCR diversity of VZV-reactive CD4 T cells in individuals older than 50 years by studying three identical twin pairs and three unrelated individuals before and after vaccination with live attenuated VZV. While all individuals had a small number of dominant T cell clones, the breadth of the VZV-specific repertoire differed markedly. A genetic influence was seen for the sharing of individual TCR sequences from antigen-reactive cells, but not for repertoire richness or the selection of dominant clones. VZV vaccination favored the expansion of infrequent VZV antigen-reactive TCRs including those from naïve T cells with lesser boosting of dominant T cell clones. Thus, vaccination does not reinforce the in vivo selection occurred during chronic infection but leads to a diversification of the VZV-reactive T cell repertoire. However, a single booster immunization seems insufficient to establish new clonal dominance. Our results suggest that repertoire analysis of antigen-specific TCRs can be an important read-out to assess whether a vaccination was able to generate memory cells in clonal sizes that are necessary for immune protection.

Summary During CD4+ T cell activation, T cell receptor (TCR) signals impact T cell fate, including recruitment, expansion, differentiation, trafficking and survival. To determine the impact of TCR signals on the fate decision of activated CD4+ T cells to become end-stage effector or long-lived memory T-helper 1 (Th1) cells, we devised a deep sequencing-based approach that allowed us to track the evolution of TCR repertoires following acute infection. The transition of effector Th1 cells into the memory pool was associated with a significant decrease in repertoire diversity, and major histocompatibility complex (MHC) Class II tetramer off-rate, but not tetramer avidity, was a key predictive factor in the representation of individual clonal T cell populations at the memory stage. We conclude that stable and sustained interactions with antigen during the development of T-helper 1 (Th1) responses to acute infection are a determinative factor in promoting the differentiation of Th1 memory cells.

MicroRNAs have emerged as key regulators in T cell development, activation, and differentiation, with miR-181a having a prominent function. By targeting several signaling pathways, miR-181a is an important rheostat controlling T cell receptor (TCR) activation thresholds in thymic selection as well as peripheral T cell responses. A decline in miR-181a expression, due to reduced transcription of pri-miR-181a, accounts for T cell activation defects that occur with older age. Here we examine the transcriptional regulation of miR-181a expression and find a putative pri-miR-181a enhancer around position 198,904,300 on chromosome 1, which is regulated by a transcription factor complex including YY1. The decline in miR-181a expression correlates with reduced transcription of YY1 in older individuals. Partial silencing of YY1 in T cells from young individuals reproduces the signaling defects seen in older T cells. In conclusion, YY1 controls TCR signaling by upregulating miR-181a and dampening negative feedback loops mediated by miR-181a targets.

T cell differentiation involves the dynamic regulation of FOXO1 expression, which rapidly declines after activation and is subsequently restored. Reexpression is impaired in naïve CD4+ T cell responses from older individuals. Here, we show that FOXO1 promotes lysosome function through the induction of the key transcription factor for lysosomal proteins, TFEB. Subdued FOXO1 reexpression in activated CD4+ T cells impairs lysosomal activity, causing an expansion of multivesicular bodies (MVBs). Expansion of the MVB compartment induces the sequestration of glycogen synthase kinase 3β (GSK3β), thereby suppressing protein turnover and enhancing glycolytic activity. As a consequence, older activated CD4+ T cells develop features reminiscent of senescent cells. They acquire an increased cell mass, preferentially differentiate into short-lived effector T cells, and secrete exosomes that harm cells in the local environment through the release of granzyme B.

Highlights d miR-181a deficiency in naive T cells is a hallmark of human and murine T cell aging d miR-181a deficiency in T cells from young mice resembles aged T cell responses d miR-181a deficiency in T cells impairs expansion, viral clearance, and recall response d miR-181a deficiency impairs generation of liver-residing memory T cells