Abstract

The immune system is highly organized and multifunctional; it protects the organism against invading pathogens (immune response), while at the same time being tolerant to ‘self’ (immune tolerance). Central tolerance is achieved during T-cell development by negative selection via self-peptide-MHC complexes (pMHC) presented by thymic dendritic cells (tDCs) and medullary thymic epithelial cells (mTECs). However, how self-antigens are processed into peptides that further are shuttled onto major histocompatibility complex (MHC) class II in thymic epithelial cells (TECs) is not fully understood. One candidate pathway involved in this process is macroautophagy (referred to as autophagy hereafter), originally thought to solely be involved cellular housekeeping, has previously been implicated to contribute to MHC class II endogenous antigen presentation by TECs. This research project investigates the role of autophagy in negative selection by TECs. A model antigen was targeted specifically to the autophagosome in autoimmune regulator (Aire)+ mTECs. Transgenic mice expressing this antigen were generated. Experimental data indicated that the efficient direct presentation of this endogenous antigen by mTECs observed in vitro was sufficient to induce negative selection in vivo. By contrast, interference with autophagosomal processing of this antigen through exchange of one amino acid or disruption of an essential autophagy gene abolished endogenous antigen presentation by mTECs and impaired negative selection, which resulted in the escape of autoreactive CD4 T-cells. Moreover, further investigation showed that if the level of antigen expression was limited, autophagy-dependent direct presentation by mTECs was essential for T-cell deletion, while with sufficient amounts of antigen provided by mTECs, direct presentation by mTECs and indirect presentation by tDCs co-operated to induce central CD4 T-cell tolerance. These findings strongly suggest that the autophagy pathway contributes to CD4 T-cell tolerance induction by facilitating the loading of intracellular antigens onto MHC class II in mTECs.