Lipid antigen presentation and thymic selection of iNKT cells

Abstract

The previous introductory part has reviewed the current knowledge on CD1 and lipid immunology. Although the last 20 years witnessed a dramatic expansion of the field, many aspects remain elusive and require additional studies. Little is known about the mechanisms by which stimulatory CD1:lipid antigen complexes are generated and how is the influence of CD1d lipid antigen presentation on thymic selection and peripheral activation of iNKT cells. The aim of this study is to investigate the mechanisms contributing to generation of lipid-specific complexes. In particular, I aim to investigate the impact of LTP residing in different cellular compartments on lipid antigen generation, loading on CD1 molecules, presentation to T cells, positive selection in the thymus as well as stimulation of peripheral CD1-restricted T cells. These studies are divided in three parts: 1. Impact of the absence of the lysosomal LTP NPC2 or of the enzyme β- Galactosidase on lipid antigen presentation and on thymic selection of iNKT cells. Recent studies demonstrate that different lysosomal lipid transfer proteins, like saposins and GM2-A, are implicated in loading of endogenous and exogenous lipid antigens onto CD1d and thus in CD1d-restricted presentation of lipid antigens important for iNKT cells thymic selection [84, 87]. Development of iNKT cells is also completely abolished in mice deficient in saposins [84]. Deficiencies in the lysosomal enzyme Hexb, which is involved in the lipid degradation pathway, or in saposins cause severe imbalances in lipid metabolism. It has never been addressed whether lipid metabolism alteration, and thus accumulation of storage lipids, could contribute to the impairment of lipid antigen presentation and iNKT cell thymic selection. This study has directly assessed the contribution of imbalance in lipid metabolism, consequence of ablation of the LTP NPC2 and of the enzyme β-Galactosidase, on generation of stimulatory CD1d:lipid antigen complexes, on their presentation to iNKT cells, and thus on thymic selection of iNKT cells. 2. Investigation of the functional interaction of CD1e with the other CD1 molecules CD1e is the fifth member of the CD1 family. It is the only member of the CD1 family that does not reach the plasma membrane and is soluble in lysosomes, therefore it might have functions going beyond presentation of lipid antigen to T cells. CD1e binds lipid molecules in the lysosomal compartment and facilitates processing of complex glycolipid antigens, thus participating in lipid editing of CD1b-presented antigens [16]. Nothing is known about the role of CD1e in editing CD1 restricted lipids, either of self or of microbial origin. This study has demonstrated the capacity of CD1e to interact with all lysosomal CD1 molecules and to influence the response of lipid specific-T cells, including that of iNKT cells. Generation of transgenic mice expressing CD1e in professional APC allowed us to study the contribution of CD1e in the formation of antigenic CD1d:lipid complexes and their capacity to activate iNKT cells. 3. Impact of the absence of the peroxisomal and cytoplasmic LTP SCP-2 on lipid antigen presentation and on thymic selection of iNKT cells. All the LTP influencing lipid antigenicity and thus iNKT cell thymic selection that have been investigated so far are localised within the lysosome. The role of LTP residing in other subcellular compartments, like the cytoplasm and the peroxisomes, on presentation of lipid antigens has never been investigated. Furthermore, whether cytoplasmic LTP are involved in generation and intracellular trafficking of endogenous-lipid antigens remains unknown. This study has directly investigated the role of a cytoplasmic/peroxisomal LTP in the generation of the repertoire of selecting lipids, and its impact on thymic selection of iNKT cells.