Autophagy in thymic epithelium and T cell tolerance
Final Report Abstract
During development in the thymus, each T lymphocyte is equipped with one, essentially unique, T cell receptor (TCR)-specificity. This random process also leads to the emergence of potentially dangerous T cells that recognize ‘self’. Nevertheless, autoimmune tissuedestruction, the cause of diseases such as multiple sclerosis, is the exception rather than the rule. This is so because immature T cells test their receptor on self-antigens within the thymic microenvironment, and TCR engagement at this immature stage elicits an apoptotic suicide program, a process called ‘negative selection’. Intuitively, one may assume that negative selection is unlikely to cover T cells specific for self-antigens whose expression is confined to peripheral tissues. However, the range of selfantigens that are actually expressed in the thymus covers an unexpectedly large fraction of the genome. This ‘promiscuous gene expression’, which is confined to medullary thymic epithelial cells (mTECs), substantially broadens the scope of central tolerance. In the present project, we asked how T cells ‘see’ self-antigens that are expressed by mTECs. Generally, T cells recognize proteolytically processed peptides embedded in one of two types of major histocompatibility complex (MHC) molecules. Generally, MHC I molecules present peptides generated through proteasomal degradation of cytoplasmic proteins, whereas MHC II molecules carry peptides derived from lysosomal processing of endocytosed proteins. By inference, ‘promiscuously expressed’ cytoplasmic gene products in mTECs would thus be unlikely to be loaded onto MHC II for CD4 T cell tolerance. However, in previous experiments, we had obtained evidence that mTECs may have evolved means to violate these classical rules of MHC loading. This led us to hypothesize that, instead of shuttling exogenous material into the MHC II loading machinery, TECs may preferentially use an unconventional pathway of ‘endogenous’ MHC II presentation. Macroautophagy turned out to be an attractive candidate mechanism. Autophagosomes deliver their content to endosomal/lysosomal compartments for proteolytic degradation. The same applies to endocytic compartments derived from ingestion of extracellular material ultimately destined for loading onto MHC II. Consequently, an intersection of autophagy with the MHC II loading pathway is conceivable. Intriguingly, autophagy displays a curiously high constitutive activity in TECs, whereas in essentially all other tissues it reaches comparable levels only upon nutrient deprivation. In the present project, we set out to visualize whether T cell specificities that are otherwise negatively selected through recognition of self-antigens expressed and presented by mTECs in fact escape from central tolerance when autophagy is abolished. In a first series of experiments, we followed the fate of T cells that recognize the model antigen pigeon cytochrome c (PCC). It turned out that when PCC was expressed in its native form as a mitochondrial self-antigen in TECs, direct presentation on MHC II for negative selection of specific CD4 T cells indeed required an intact macroautophagy pathway. In a second set of experiments, we used a novel model system in which ‘the same’ mTEC-specific antigen was deliberately targeted for autophagosomal degradation in one setting but not in a second setting. Indeed, when expressed at identical levels specifically in mTECs, direct presentation of the autophagosomally targeted model antigen and negative selection of specific CD4 T cells turned out to be substantially more efficient as compared to the mutant form. Taken together, our observations indicate that the immune system co-opts an evolutionarily ancient cellular housekeeping and catabolic process in order to ‘boost’ the tolerogenic presentation of self-antigens within the thymic microenvironment.
Publications
- (2009) Antigen presentation in the thymus for positive selection and central tolerance induction. Nature Reviews Immunology, 9 (12): 833-844
Klein L., Hinterberger M., Wirnsberger G. and Kyewski B.
- (2010) Autophagy-mediated antigen processing in CD4+ T cell tolerance and immunity. FEBS Letters, 584 (7): 1405-1410
Klein L., Münz C. and Lünemann J.D.
- (2013) Macroautophagy substrates are loaded onto MHC Class II of medullary thymic epithelial cells for central tolerance. Journal of Experimental Medicine, 210(2): 287-300
Aichinger M., Wu C., Nedjic J. and Klein L.
(See online at https://doi.org/10.1084/jem.20122149)
