Macroautophagy regulates CD4+ T cell function

Abstract

Protein turnover is imperative for the diminution and removal of accumulated damaged proteins in the cell. Turnover is also essential to facilitate recycling of amino acids for new protein synthesis and to allow for the modification of protein levels in response to extracellular signals. Autophagy has been shown to be critical for cellular homeostasis and for survival during stress conditions. Macroautophagy is a form of autophagy accountable for the degradation of cytosolic proteins and whole organelles. Recent data has shown that macroautophagy may be regulated during T cell activation. The overall goal of my thesis project is to elucidate the role(s) of macroautophagy during T cell activation.;We have found that macroautophagy is induced following effector T cell activation. Using three discrete methods, we have found that engagement of both the T cell receptor (TCR) and CD28 results in enhanced LC3 processing, increased numbers of LC3+-containing vesicles and increased LC3 flux, indicating active autophagosome formation and clearance. The autophagosomes formed in stimulated T cells actively fuse with lysosomes to degrade their cargo. Using a conditional knockout mouse model where Atg7, a critical gene for macroautophagy, is specifically deleted in T cells, we have found that macroautophagy-deficient effector T helper cells have defective IL-2 and IFNgamma production. Furthermore, these cells have reduced proliferation following stimulation, with no significant increase in apoptosis. Interestingly, we have found that ATP generation is decreased when autophagy is blocked, and that defects in activation- induced cytokine production are restored when an exogenous energy source is added to macroautophagy-deficient T cells. Furthermore, we present evidence demonstrating that the nature of the cargo inside the double-membrane autophagic vesicles found in resting T cells differs from the cargo of autophagosomes in activated T cells, where mitochondria and other organelles are selectively excluded. These results indicate that macroautophagy is an actively regulated process in T cells that can be induced in response to TCR engagement to regulate the bioenergetic requirements of activated T cells.;Also, we have found that macroautophagy may regulate immunological synapse (IS) formation. LC3, which incorporates into the nascent double membrane structure that will form the autophagosome, is present at the central supermolecular activation center (cSMAC) and T cells from Atg7-deficient mice lack LC3 at the IS and have dysregulated synapse formation. Consequent to a decrease in synapse formation, macroautophagy-deficient CD4+ T cells have defects in several key signaling molecules including activation of ERK, degradation of IKBalpha as well as phosphorylation of AMPK. However, CD28-mediated signaling appears intact as phosphorylation of AKT is not perturbed when autophagy is blocked. Thus, macroautophagy may have a dual role in T cell function: providing energy to the cell as well as regulating the immunological synapse through several different mechanisms.;The immune system undergoes numerous age-associated changes. Immunosenescence is defined as the age-dependent deterioration of the immune system which results in a diminished ability to respond to infections. Evidence suggests that decreased macroautophagic activity in aged organisms may be responsible for the deterioration of cell function with age that results in alterations of the function of many organs and systems. We have demonstrated that aged mice have decreased IL-2 production and proliferative responses as well as a decrease in long-lived protein degradation and LC3-II formation. Understanding how and why macroautophagy is regulated during T cell activation has many implications for immunosenescence. Therefore, it may be critical in understanding how the age-dependent dysregulation of autophagy may account, at least in part, for the defective T cell function that defines immunosenescence.