Physiologic Thymic Involution Underlies Age-Dependent Accumulation of Senescence-Associated CD4+ T Cells

Abstract

Immune aging may underlie various aging-related disorders, including diminished resistance to infection, chronic inflammatory disorders, and autoimmunity. PD-1+ and CD153+ CD44high CD4+ T cells with features of cellular senescence, termed senescence-associated T (SA-T) cells, increasingly accumulate with age and may play a role in the immune aging phenotype. In this article, we demonstrate that, compared with young mice, the aged mouse environment is highly permissive for spontaneous proliferation of transferred naive CD4+ T cells, and it drives their transition to PD-1+ and CD153+ CD44high CD4+ T cells after extensive cell divisions. CD4+ T cells with essentially the same features as SA-T cells in aged mice are also generated from naive CD4+ T cells after extensive cell divisions under severe T-lymphopenic conditions by gamma irradiation or in developmental T cell defect, often in association with spontaneous germinal centers, as seen in aged mice. The increase in SA-T cells is significantly enhanced after thymectomy at the young adult stage, along with accelerated T cell homeostatic proliferation, whereas embryonic thymus implantation in the late adult stage markedly restricts the homeostatic proliferation of naive CD4+ T cells in the host and delays the increase in SA-T cells. Our results suggest that reduced T cell output due to physiologic thymic involution underlies the age-dependent accumulation of SA-T cells as a result of increasing homeostatic proliferation of naive CD4+ T cells. SA-T cells may provide a suitable biomarker of immune aging, as well as a potential target for controlling aging-related disorders.

CXCR3high CD8+ T cells with naïve phenotype and high capacity for IFN‐γ production are generated during homeostatic T‐cell proliferation

“…We found that CXCR3 + NP CD8 + T cells were confined to a “slow” fraction that underwent fewer than 4 cell division cycles before high CD44 expression occurred. CXCR3 + NP CD8 + T cells were eventually converted to CXCR3 + MP (CD44 high ) CD8 + T cells as the cell cycles further progressed, as reported previously for CD8 + as well as CD4 + T cells , known as virtual memory T cells , suggesting that CXCR3 + NP CD8 + T cells represent a distinct transitional stage from NP to MP cells during multiple rounds of cell division. It remains unclear whether fast proliferation, mostly initiated by foreign antigen recognition and IL‐2, is too robust for CXCR3 + NP CD8 + T cells to be detected at high levels, or whether HP, induced by the tonic TCR signal and IL‐7, preferentially generated this specific NP CD8 T cell population.…”

“…Multicolor flow cytometric analysis was performed using the FACSCanto II (BD Biosciences, San Jose, CA, USA) as previously described . The following antibodies were used: fluorescein isothiocyanate (FITC)‐conjugated anti‐CD62L (MEL14), phycoerythrin (PE)‐conjugated anti‐CD127 (A7R34), PE‐conjugated anti‐CD45.1 (A20), PECy7‐conjugated anti‐CD62L (MEL14), PE‐conjugated anti‐CD4 (GK1.5), PE‐conjugated anti‐CD11b (M1/70), PE‐conjugated anti‐B220 (RA3‐6B2), PE‐conjugated anti‐Ter119 (TER‐119), PE‐conjugated anti‐γd TCR (eBioGL3), PECy7‐conjugated anti‐Eomes (Dan11mag), PE‐conjugated GATA3 (TWAJ), PE‐conjugated RORγt (AFKJS‐9), PE‐conjugated FOXP3 (FJK‐16s), PE‐conjugated Ki67 (SolA15), PE‐conjugated CD122 (5H4), allophycocyanin (APC)‐conjugated anti‐CD183 (CXCR3‐173), PE‐conjugated anti‐TNFα (MP6‐XT22), PECy7‐conjugated anti‐IFN‐γ (XMG1.2), PerCpCy5.5‐conjugated anti‐IFN‐γ (XMG1.2), BV421‐conjugated T‐bet (4B10), PE‐conjugated anti‐Ly‐6G (RB6‐8C5), FITC‐conjugated anti‐Vα2 TCR (B20.1) (eBioscience, San Diego, CA, USA), PE‐conjugated anti‐CD19 (6D5), PE‐conjugated anti‐CD5 (53‐7.3), PE‐conjugated anti‐NK1.1 (PK136), PE‐conjugated anti‐CD105 (MJ7/18), PE‐conjugated anti‐CD4 (RM4‐5), FITC‐conjugated anti‐IL‐17 (TC11‐18H10.1), FITC‐conjugated anti‐granzymeB (GB11) (BioLegend, San Diego, CA, USA), PECy7‐conjugated anti‐CD8a (53‐6.7), PE‐conjugated anti‐CD11c (N418) (Tonbo Biosciences, San Diego, CA, USA), FITC‐conjugated anti‐BrdU (3D4), PE‐conjugated anti‐Vβ5.1, and 5.2 TCR (MR‐9‐4) (BD Biosciences).…”

“…In this study, we investigated the phenotypic and functional changes of NP CD8 + cells during persistent HP using adult‐thymectomized (ATx) mice that undergo accelerated T‐cell HP . We found that NP CD8 + cells expressing CXCR3, which represent a very minor population in normal adult mice, were markedly increased in ATx mice before the phenotypic conversion to MP cells, indicating that the generation of CXCR3 + NP CD8 + T cells was preferentially associated with the HP of the slow cell division rate rather than antigen‐driven robust T cell proliferation.…”

“…In mice, the HP of NP T cells results in efficient phenotypic conversion to the memory phenotype (MP) , and most T cells with the central memory (CM) phenotype in aged mice are considered to represent those that have developed via HP rather than authentic antigen‐driven memory cells, and thus are referred to as virtual memory cells . We previously reported that unique MP CD4 + T cells with characteristic features of cellular senescence, named as senescence‐associated (SA)‐T cells, are developed and accumulated with age as a consequence of extensive cell divisions during the HP of CD4 + NP T cells . Although SA‐T cells are devoid of TCR‐mediated proliferation capacity, likely because of the high expression of senescence‐related genes such as Cdkn1a and 2b , these T cells can secrete abundant proinflammatory cytokines in response to self‐B cells, reminiscent of the senescence‐associated secretory phenotype, and promote autoantibody production and tissue inflammation under certain genetic backgrounds and tissue stresses, respectively .…”

Solid Tumor–Induced Immune Regulation Alters the GvHD/GvT Paradigm after Allogenic Bone Marrow Transplantation

“…Thymectomy of C57BL/6 mice (i.e., utilized mice that were thymectomized at 4 to 6 weeks of age, i.e., before they achieve adult body weight, also referred to as preadult thymectomy) was performed according to the suction techniques (22), which allows compete visualization of the entire thymus and its complete removal.…”