Autophagy within the antigen donor cell facilitates efficient antigen cross-priming of virus-specific CD8+ T cells

Abstract

Cross-presentation of cell-associated antigen is important in the priming of CD8 þ T-cell responses to proteins that are not expressed by antigen-presenting cells (APCs). In vivo, dendritic cells are the main cross-presenting APC, and much is known regarding their ability to capture and process cell-associated antigen. In contrast, little is known about the way death effector pathways influence the efficiency of cross-priming. Here, we compared two important mechanisms of programmed cell death: classical apoptosis, as it occurs in wild-type (WT) fibroblasts, and caspase-independent cell death, which occurs with increased features of autophagy in Bax/Bak À/À fibroblasts. We assessed virally infected WT and Bax/Bak À/À fibroblasts as a source of cell-associated antigen. We found that immunization with cells undergoing autophagy before cell death was superior in facilitating the cross-priming of antigen-specific CD8 þ T cells. Strikingly, silencing of Atg5 expression inhibited priming. We interpret this to be a novel form of 'immunogenic death' with the enhanced priming efficiency being a result of persistent MHC I cross-presentation and the induction of type I interferons. These results offer the first molecular evidence that catabolic pathways, including autophagy, influence the efficiency of cross-priming. We predict that targeting the autophagy cascade may provide a therapeutic strategy for achieving robust cross-priming of viral and tumor-specific CD8 þ T cells. There is now evidence to suggest that antigen-presenting cells (APCs) have evolved mechanisms to capture immunologically relevant information from internalized apoptotic cells, thus offering a mechanism whereby cell death may influence subsequent immune response(s). 1-3 Specifically, it has been shown that antigen derived from phagocytosed dying cells may be processed by dendritic cells (DCs) to generate MHC I/peptide, resulting in the activation of CD8 þ T cells. This pathway is called crosspriming due to the 'crossing' of the classically defined restriction of MHC I for presentation of endogenously synthesized protein. 4 In addition, dying cells are also capable of skewing cytokine production in phagocytes. 5 For example, macrophages and DCs engulf dying cells through specific phagocytic receptors (e.g., CD36, a v b 5 , CR3 and others), which may result in the inhibition of proinflammatory cytokine secretion (e.g., IL-1b, IL-12 and TNF-a). [6][7][8][9] These studies have all addressed the downstream effects of dying cells on the immune system with little attention to the complexity of cell death pathways. We hypothesized that the molecular mechanism of cell death influences the immunologic instruction given to the phagocyte and we focused our attention on two of the major classes of PCD that have been described: type I PCD (or classical apoptosis) and type II PCD (referred to as autophagic cell death or caspase-independent cell death). 10 Distinct, yet intertwined pathways are involved in executing cell death programs. Briefly, apoptosis is mediated ...

HDLs inhibit endoplasmic reticulum stress and autophagic response induced by oxidized LDLs

“…39 Moreover, recent studies highlight the role of Ca 2 þ as a potent inducer of autophagy, which is triggered by Ca 2 þ mobilizing stimuli such as ATP, thapsigargin and ionomycin that activate the Ca 2 þ /calmodulin-dependent protein kinase-kinase and consequently inhibit mTOR. 14,31 The upstream mechanisms and potential targets of HDLs involved in Ca 2 þ deregulation are still unknown. However, the antioxidant properties of HDLs that inhibit intracellular oxidative stress mediated by oxLDLs, 20 could explain their protective effect, as oxidative stress is a well-known inducer of ER stress and calcium deregulation 9,39 In conclusion, our data show a potent implication of ER stress and CHOP in apoptosis mediated by oxLDLs in vascular cells, resulting from the deregulation of cytosolic Ca 2 þ .…”

Intersection of autophagy with pathways of antigen presentation

“…Dying cells from bax/bak-deficient mouse embryonic fibroblasts (MEFs) display enhanced autophagy. Infec-© Higher Education Press and Springer-Verlag Berlin Heidelberg 2012 tion of these cells with influenza A virus facilitates enhanced cross-presentation of influenza A virus-derived antigen to CD8 + T cells (Uhl et al, 2009). siRNA knockdown of Atg5 in bax/bak-deficient MEFs abrogates this response (Uhl et al, 2009).…”

“…Infec-© Higher Education Press and Springer-Verlag Berlin Heidelberg 2012 tion of these cells with influenza A virus facilitates enhanced cross-presentation of influenza A virus-derived antigen to CD8 + T cells (Uhl et al, 2009). siRNA knockdown of Atg5 in bax/bak-deficient MEFs abrogates this response (Uhl et al, 2009). In addition, rapamycin treatment of the human melanoma cell line FEMX enhances cross presentation of tumour antigen gp100, while this is abrogated by preventing autophagy with 3-MA or wortmannin, beclin 1 shRNA and Atg12 siRNA knockdown (Li et al, 2008).…”

Autophagy: for better or for worse

“…For example, during herpes simplex virus (HSV-1) infection, autophagy facilitates presentation of HSV-1 antigens on MHC class I molecules, and mice with Atg5-deficient dendritic cells show impaired CD4 + T-cell priming upon HSV-1 infection [115,116]. Also, immunization with cells undergoing autophagy enhances the efficiency of cross-priming of antigen-specific CD8 + T-cells [117]. Furthermore, autophagy functions in delivery of cytoplasmic viral nucleic acids to endosomal TLRs, which results in type I IFN production and IFN-dependent immune responses [118].…”