Soluble Factors Released From Activated T Lymphocytes Regulate C2C12 Myoblast Proliferation and Cellular Signaling, but Effects Are Blunted in the Elderly.

The key objective of this work was to investigate the impact of young and old human lymphocyte secretomes on C2C12 myoblasts regeneration. Conditioned media were harvested from isolated young and older lymphocytes treated with (activated [AC]) or without (nonactivated [NA]), anti-CD3/CD28 activators for 4 days. AC conditioned media from older lymphocytes had decreased levels of amphiregulin (367 ± 208 pg/mL vs 904 ± 323 pg/mL; p = .018) and IGF-I (845 ± 88 ng/mL vs 1100 ± 48 ng/mL; p = .032) compared with younger AC lymphocytes. AC older versus younger lymphocytes had reduced expression of CD25 (24.6 ± 5.5%; p = .0003) and increased expression of FoxP3 (35 ± 15.7%; p = .032). Treatment of C2C12 myoblasts with young AC lymphocytes resulted in decreased expression of MyoD (0.46 ± 0.12; p =.004) and Myogenin (0.34 ± 0.05; p = .010) mRNA, increased activation of MEk1 (724 ± 140 mean fluorescent intensity [MFI]; p =.001) and ERK1/2 (3768 ± 314 MFI; p =.001), and a decreased activation of Akt (74.5 ± 4 MFI; p = .009) and mTOR (61.8 ± 7 MFI; p = .001) compared with old AC lymphocytes. By contrast, C2C12 myoblasts treated with older AC lymphocytes displayed increased expression of MyoD (0.7 ± 0.08; p =.004) and Myogenin (0.68 ± 0.05; p =.010) mRNA, decreased phosphorylation of MEk1 and ERK1/2 (528 ± 80 MFI; p = .008, and 1141 ± 668 MFI; p = .001, respectively), and increased Akt/mTOR activation (171 ± 35 MFI; p = .009, and 184 ± 33 MFI; p = .001, respectively). These data provide new evidence that differences between older and younger lymphocyte secretomes contribute to differential responses of C2C12 myoblasts in culture.

Regenerative function of immune system: Modulation of muscle stem cells.

Ageing is characterised by progressive deterioration of physiological systems and the loss of skeletal muscle mass is one of the most recognisable, leading to muscle weakness and mobility impairments. This review highlights interactions between the immune system and skeletal muscle stem cells (widely termed satellite cells or myoblasts) to influence satellite cell behaviour during muscle regeneration after injury, and outlines deficits associated with ageing. Resident neutrophils and macrophages in skeletal muscle become activated when muscle fibres are damaged via stimuli (e.g. contusions, strains, avulsions, hyperextensions, ruptures) and release high concentrations of cytokines, chemokines and growth factors into the microenvironment. These localised responses serve to attract additional immune cells which can reach in excess of 1×10(5) immune cell/mm(3) of skeletal muscle in order to orchestrate the repair process. T-cells have a delayed response, reaching peak activation roughly 4 days after the initial damage. The cytokines and growth factors released by activated T-cells play a key role in muscle satellite cell proliferation and migration, although the precise mechanisms of these interactions remain unclear. T-cells in older people display limited ability to activate satellite cell proliferation and migration which is likely to contribute to insufficient muscle repair and, consequently, muscle wasting and weakness. If the factors released by T-cells to activate satellite cells can be identified, it may be possible to develop therapeutic agents to enhance muscle regeneration and reduce the impact of muscle wasting during ageing and disease.

The lymphocyte secretome from young adults enhances skeletal muscle proliferation and migration, but effects are attenuated in the secretome of older adults.

Older people experience skeletal muscle wasting, in part due to impaired proliferative capacity of quiescent skeletal muscle satellite cells which can be reversed by exposure to young blood. To investigate the role of immune cells in muscle regeneration, we isolated lymphocytes from whole blood of young and older healthy volunteers and cultured them with, or without, anti-CD3/CD28 activators to induce release of cytokines, interleukins, and growth factors into the media. The secreted proteins were collected to prepare a conditioned media, which was subsequently used to culture C2C12 myoblasts. The conditioned media from the activated young lymphocytes increased the rate of proliferation of myoblasts by around threefold (P < 0.005) and caused an approximate fourfold (P < 0.005) increase in migration compared with nonactivated lymphocyte control media. These responses were characterized by minimal myotube formation (2%), low fusion index (5%), low myosin heavy chain content, and substantial migration. In contrast, myoblasts treated with conditioned media from activated old lymphocytes exhibited a high degree of differentiation, and multi-nucleated myotube formation that was comparable to control conditions, thus showing no effect on proliferation or migration of myoblasts. These results indicate that secreted proteins from lymphocytes of young people enhance the muscle cell proliferation and migration, whereas secreted proteins from lymphocytes of older people may contribute to the attenuated skeletal muscle satellite cell proliferation and migration.

Activated lymphocytes secretome inhibits differentiation and induces proliferation of C2C12 myoblasts.

BACKGROUND/AIMS: ageing is associated with a marked decline in immune function which may contribute to the local environment that can influence the regenerative process of skeletal muscle cells. METHODS: Herein, we focused on determining the effect of an activated immune system secretome on myoblast differentiation and proliferation as possible means to attenuate adverse effects of muscle aging. C2C12 myoblasts were used as model to assess the impact of lymphocyte conditioned media (CM) following anti-CD3/IL-2 activation. RESULTS: Myoblasts cultured with activated lymphocytes CM exhibited reduced morphological and biochemical differentiation (98±20, p<0.005) and increased entry to the S Phase of the cell cycle (61%±7, p<0.001), when compared with myoblasts cultured with non-activated lymphocytes CM. Associated with increased proliferation and reduced differentiation, muscle specific transcription factors MyoD and myogenin were significantly reduced in C2C12 treated with activated lymphocytes CM vs control CM, respectively (myoD: 0.5±0.12 fold reduction P<0.005); myogenin: 0.38±0.08 fold reduction; p<0.005). Moreover, key protein of proliferation pERK1/2 increased (46±11U/ml, p<0.05) whereas mediator of differentiation pAkt decreased (21±12U/ml, p<0.05) in C2C12 treated with activated vs. non-activated CM. CONCLUSION: our data demonstrate that, following activation, secretome of the immune system cells elicit marked regulatory effects on skeletal muscle growth and differentiation; enhancing the former with the loss of the latter.