Impaired immune surveillance accelerates accumulation of senescent cells and aging.

Cellular senescence is a stress response that imposes stable cell-cycle arrest in damaged cells, preventing their propagation in tissues. However, senescent cells accumulate in tissues in advanced age, where they might promote tissue degeneration and malignant transformation. The extent of immune-system involvement in regulating age-related accumulation of senescent cells, and its consequences, are unknown. Here we show that Prf1-/- mice with impaired cell cytotoxicity exhibit both higher senescent-cell tissue burden and chronic inflammation. They suffer from multiple age-related disorders and lower survival. Strikingly, pharmacological elimination of senescent-cells by ABT-737 partially alleviates accelerated aging phenotype in these mice. In LMNA+/G609G progeroid mice, impaired cell cytotoxicity further promotes senescent-cell accumulation and shortens lifespan. ABT-737 administration during the second half of life of these progeroid mice abrogates senescence signature and increases median survival. Our findings shed new light on mechanisms governing senescent-cell presence in aging, and could motivate new strategies for regenerative medicine.

p53 in Bronchial Club Cells Facilitates Chronic Lung Inflammation by Promoting Senescence.

The tumor suppressor p53 limits tumorigenesis by inducing apoptosis, cell cycle arrest, and senescence. Although p53 is known to limit inflammation during tumor development, its role in regulating chronic lung inflammation is less well understood. To elucidate the function of airway epithelial p53 in such inflammation, we subjected genetically modified mice, whose bronchial epithelial club cells lack p53, to repetitive inhalations of lipopolysaccharide (LPS), an exposure that leads to severe chronic bronchitis and airway senescence in wild-type mice. Surprisingly, the club cell p53 knockout mice exhibited reduced airway senescence and bronchitis in response to chronic LPS exposure and were significantly protected from global lung destruction. Furthermore, pharmacological elimination of senescent cells also protected wild-type mice from chronic LPS-induced bronchitis. Our results implicate p53 in induction of club-cell senescence and correlate epithelial cell senescence of chronic airway inflammation and lung destruction.

NKG2D ligands mediate immunosurveillance of senescent cells.

Cellular senescence is a stress response mechanism that limits tumorigenesis and tissue damage. Induction of cellular senescence commonly coincides with an immunogenic phenotype that promotes self-elimination by components of the immune system, thereby facilitating tumor suppression and limiting excess fibrosis during wound repair. The mechanisms by which senescent cells regulate their immune surveillance are not completely understood. Here we show that ligands of an activating Natural Killer (NK) cell receptor (NKG2D), MICA and ULBP2 are consistently up-regulated following induction of replicative senescence, oncogene-induced senescence and DNA damage - induced senescence. MICA and ULBP2 proteins are necessary for efficient NK-mediated cytotoxicity towards senescent fibroblasts. The mechanisms regulating the initial expression of NKG2D ligands in senescent cells are dependent on a DNA damage response, whilst continuous expression of these ligands is regulated by the ERK signaling pathway. In liver fibrosis, the accumulation of senescent activated stellate cells is increased in mice lacking NKG2D receptor leading to increased fibrosis. Overall, our results provide new insights into the mechanisms regulating the expression of immune ligands in senescent cells and reveal the importance of NKG2D receptor-ligand interaction in protecting against liver fibrosis.

CD151 Regulates T-Cell Migration in Health and Inflammatory Bowel Disease.

The continuous recirculation of mature lymphocytes and their entry into the peripheral lymph nodes are crucial for the development of an immune response to foreign antigens. Occasionally, the entry and the subsequent response of T lymphocytes in these sites lead to severe inflammation and pathological conditions. Here, we characterized the tetraspanin molecule, CD151, as a regulator of T cell motility in health and in models of inflammatory bowel disease. CD151 formed a cell surface complex with VLA-4 and LFA-1 integrins, and its activation led to enhanced migration of T cells. Picomolar levels of CCL2 that were previously shown to inhibit T-cell migration to lymph nodes suppressed CD151 expression and dissociated CD151-integrin complexes in T lymphocytes, resulting in attenuated migration toward T-cell attractant chemokines. To directly inhibit CD151 function, a truncated CD151 peptide fragment mimicking of the CD151 extracellular loop was designed. CD151 extracellular loop inhibited T-cell migration in vitro and in vivo and attenuated the development of dextrane sulfate sodium-induced colitis. Thus, CD151 is a key orchestrator of T cell motility; interference with its proper function results in attenuated progression of inflammatory bowel disease.

Fingerprint of Lung Fluid Ultrafine Particles, a Novel Marker of Acute Lung Inflammation.

BACKGROUND: Acute lung inflammation can be monitored by various biochemical readouts of bronchoalveolar lavage fluid (BALF). OBJECTIVE: To analyze the BALF content of ultrafine particles (UFP; <100 nm) as an inflammatory biomarker in early diagnosis of acute and chronic lung diseases. METHODS: Mice were exposed to different stress conditions and inflammatory insults (acute lipopolysaccharide inhalation, tobacco smoke and lethal dose of total body irradiation, i.e. 950 rad). After centrifugation, the cellular pellet was assessed while cytokines and ultrafine particles were measured in the soluble fraction of the BALF. RESULTS: A characteristic UFP distribution with a D50 (i.e. the dimension of the 50th UFP percentile) was shared by all tested mouse strains in the BALF of resting lungs. All tested inflammatory insults similarly shifted this size distribution, resulting in a unique UFP fingerprint with an averaged D50 of 58.6 nm, compared with the mean UFP D50 of 23.7 nm for resting BALF (p < 0.0001). This UFP profile was highly reproducible and independent of the intensity or duration of the inflammatory trigger. It returned to baseline after resolution of the inflammation. Neither total body irradiation nor induction of acute cough induced this fingerprint. CONCLUSIONS: The UFP fingerprint in the BALF of resting and inflamed lungs can serve as a binary biomarker of healthy and acutely inflamed lungs. This marker can be used as a novel readout for the onset of inflammatory lung diseases and for complete lung recovery from different insults.

Immunosurveillance of senescent cells: the bright side of the senescence program.

Cellular senescence, a state of irreversible cell cycle arrest, is a robust mechanism used to mediate tumor suppression and control the tissue damage response following short-term insults. In addition, the senescence associated-secretory phenotype (SASP), one of the most profound characteristics of the senescence program, facilitates the immunosurveillance of senescent cells. The SASP includes many chemokines, cytokines and adhesion molecules that can recruit and activate distinct immune cells from both the innate and adaptive immune system such as NK cells, monocytes/macrophages and T cells. Furthermore, senescent cells can upregulate specific immune ligands on their cell surface that can mediate the recognition of these cells by specific immune cell subsets and lead to activation of the immune cells. Consequently, the activated immune cells engage explicit regulatory mechanisms to eliminate senescent cells. For example, recent work from our laboratory showed that perforin-granzyme exocytosis mediates NK-cell killing of senescent cells. Here, we summarize the current advances in our knowledge of the mechanisms underlying specific immune-mediated elimination of senescent cells.

Occupancy of lymphocyte LFA-1 by surface-immobilized ICAM-1 is critical for TCR- but not for chemokine-triggered LFA-1 conversion to an open headpiece high-affinity state.

Lymphocyte arrest and spreading on ICAM-1-expressing APCs require activation of lymphocyte LFA-1 by TCR signals, but the conformational switches of this integrin during these critical processes are still elusive. Using Ab probes that distinguish between different LFA-1 conformations, we found that, unlike strong chemokine signals, potent TCR stimuli were insufficient to trigger LFA-1 extension or headpiece opening in primary human lymphocytes. Nevertheless, LFA-1 in these TCR-stimulated T cells became highly adhesive to both anchored and mobile surface-bound ICAM-1, although it failed to bind soluble ICAM-1 with measurable affinity. Rapid rearrangement of LFA-1 by immobilized ICAM-1 switched the integrin to an open headpiece conformation within numerous scattered submicron focal dots that did not readily collapse into a peripheral LFA-1 ring. Headpiece-activated LFA-1 microclusters were enriched with talin but were devoid of TCR and CD45. Notably, LFA-1 activation by TCR signals as well as subsequent T cell spreading on ICAM-1 took place independently of cytosolic Ca(2+). In contrast to LFA-1-activating chemokine signals, TCR activation of LFA-1 readily took place in the absence of external shear forces. LFA-1 activation by TCR signals also did not require internal myosin II forces but depended on intact actin cytoskeleton. Our results suggest that potent TCR signals fail to trigger LFA-1 headpiece activation unless the integrin first gets stabilized by surface-bound ICAM-1 within evenly scattered actin-dependent LFA-1 focal dots, the quantal units of TCR-stimulated T cell arrest and spreading on ICAM-1.

Lymph node chemokines promote sustained T lymphocyte motility without triggering stable integrin adhesiveness in the absence of shear forces.

Lymphocyte motility in lymph nodes is regulated by chemokines, but the contribution of integrins to this motility remains obscure. Here we examined lymphocyte migration over CCR7-binding chemokines that 'decorate' lymph node stroma. In a shear-free environment, surface-bound lymph node chemokines but not their soluble counterparts promoted robust and sustained T lymphocyte motility. The chemokine CCL21 induced compartmentalized clustering of the integrins LFA-1 and VLA-4 in motile lymphocytes, but both integrins remained nonadhesive to ligands on lymphocytes, dendritic cells and stroma. The application of shear stress to lymphocytes interacting with CCL21 and integrin ligands promoted robust integrin-mediated adhesion. Thus, lymph node chemokines that promote motility and strongly activate lymphocyte integrins under shear forces fail to stimulate stable integrin adhesiveness in extravascular shear-free environments.