Blockade of PD‐1 or p38 MAP kinase signaling enhances senescent human CD8⁺ T‐cell proliferation by distinct pathways

Abstract

Immune enhancement is desirable in situations where decreased immunity results in increased morbidity. We investigated whether blocking the surface inhibitory receptor PD-1 and/or p38 MAP kinase could enhance the proliferation of the effector memory CD8 + T-cell subset that re-expresses CD45RA (EMRA) and exhibits characteristics of senescence, which include decreased proliferation and telomerase activity but increased expression of the DNA damage response related protein γH2AX. Blocking of both PD-1 and p38 MAPK signaling in these cells enhanced proliferation and the increase was additive when both pathways were inhibited simultaneously in both young and old human subjects. In contrast, telomerase activity in EMRA CD8 + T cells was only enhanced by blocking the p38 but not the PD-1 signaling pathway, further indicating that nonoverlapping signaling pathways were involved. Although blocking p38 MAPK inhibits TNF-α secretion in the EMRA population, this decrease was counteracted by the simultaneous inhibition of PD-1 signaling in these cells. Therefore, end-stage characteristics of EMRA CD8 + T cells are stringently controlled by distinct and reversible cell signaling events. In addition, the inhibition of PD-1 and p38 signaling pathways together may enable the enhancement of proliferation of EMRA CD8 + T cells without compromising their capacity for cytokine secretion. Keywords: Aging r Inhibitory receptors r T cells IntroductionHuman T-cell memory is maintained by episodes of repeated antigenic challenge throughout life [1,2]. These antigenic encounters drive proliferation and differentiation of memory T cells, leading to the accumulation of highly differentiated T cells in older subjects [1,3] responsive pools of specific T cells due to their dysfunctional ability to divide [10,11]. This raises the question of whether it is possible to enhance T-cell proliferation in older subjects. Human T cells at late stages of differentiation can be identified by the loss of surface markers CD27, CD28, and CCR7 and the reexpression of CD45RA [7,9, 12]. Furthermore, highly differentiated memory T cells have relatively short telomeres [13] and have changes in cell signaling pathways including defective Akt/PKB [7, 12] and increased p38 MAP kinase (MAPK) phosphorylation that are associated with cellular senescence [7,9, 12,14]. These changes are particularly evident in highly differentiated effector memory T cells that re-express CD45RA (EMRA T cells) [7, 12], which are considered to be an end-stage or senescent population [15,16] increased susceptibility to apoptosis [7,9, 12,17] and these cells increase significantly during ageing.Two processes have been shown to induce a loss of proliferative ability in T cells. First, repeated antigenic stimulation of T cells may induce a state of functional exhaustion where proliferative activity and cytokine production are lost [18]. This immune exhaustion is initiated by external cell surface inhibitory receptors such as PD-1 and Tim3 [19,20]. Second, repeated T-cell stimulation or gen...

Human CD8+EMRA T cells display a senescence‐associated secretory phenotype regulated by p38 MAPK

“…Of the inflammatory and immune‐modulatory cytokines and chemokines, TNF‐α, IL‐18, IL‐29, CCL5, CCL16 and CCL23 were all found to be significantly upregulated in the EMRA subset. We and others have previously published that EMRA T cells secrete high levels of TNF‐α (Hamann et al ., 1997; Henson et al ., 2015). However, we validate here the relative increase in gene expression of IL‐18 and CCL16 by flow cytometry, showing them both to be significantly increased in the CD8 + EMRA subset (Fig.…”

“…Heterogeneity within memory T cells has been documented to occur during aging (Henson et al ., 2015; Pulko et al ., 2016). A human memory population with a naïve‐like phenotype (CCR7 + CD45RA + CD28 int CD95 lo ) was found to increase during aging and exhibited a transcriptome distinct from other T‐cell subsets (Pulko et al ., 2016).…”

“…T cells that re‐express CD45RA within this subset have multiple characteristics of senescence, including a low proliferative activity, high levels of DNA damage and the loss of telomerase activity (Henson et al ., 2014, 2015). We have also shown that p38 MAPK signalling, which is increased in highly differentiated CD8 + T cells (Henson et al ., 2014), is involved in the loss of telomerase activity and proliferative capacity and that blockade of p38 MAPK activity with a specific small‐molecule inhibitor can restore both proliferation and telomerase activity (Lanna et al ., 2013; Henson et al ., 2014, 2015) in these cells. However, surprisingly the CD45RA‐re‐expressing senescent T cells do not have critically short telomeres (Di Mitri et al ., 2011; Riddell et al ., 2014), suggesting that senescence in these cells may be induced by other mechanisms including DNA damage by increased ROS production (Henson et al ., 2014).…”

Adaptive Memory of Human NK-like CD8+ T-Cells to Aging, and Viral and Tumor Antigens

“…An increased expression of NK cell markers on T-cells has been reported to be associated with aging and chronic activation of the immune system, as reflected in the accumulation of effector/senescent T-cells (30). This memory subpopulation is interesting, because the senescence of human T TE CD8 + T-cells is stringently controlled by distinct and reversible cell signaling events (31). Also, there is evidence of a differential regulation of NKR expression between T-cells and NK cells suggesting that NKR expression on T-cells is physiologically programed rather than a random event of the aging process (12).…”

CD8+ T-cell senescence: no role for mTOR

“…However, HIF1-null CD8 + Tcells were shown to have many characteristics of effector CD8 + T-cells, such as high levels of interferon (IFN)γ production; however, they lacked perforin and granzyme expression. A situation which is mirrored in senescent CD8 + EMRA T-cells isolated from old individuals, where EMRA T-cells display high levels tumour necrosis factor (TNF)α and IFNγ but lower levels of perforin and granzyme [34]. Furthermore, when the mTORC1 inhibitor rapamycin was incubated with senescent CD8 + EMRA T-cells, it had no effect on the production of IFNγ by these cells (Figures 2C and 2D) despite reducing its production in the other less differentiated CD8 + T-cell subsets.…”