Functional requirements for CD1 in development and function of invariant Natural Killer T (iNKT) cells

Abstract

Activation of the adaptive immune response primarily depends on the presentation of peptide antigens via Class I and II major histocompatibility (MHC) complexes. However, some pathogens and tumor cells have evolved evasive mechanisms to escape these ‘conventional routes’ of antigen presentation. The research described in this thesis focuses on the ‘alternative route’ of lipid antigen presentation on CD1d molecules for the development of invariant Natural Killer T (iNKT) cells and their activation during pathogenic and autoimmune diseases. Activated iNKT cells represent highly potent immunoregulatory cells, stimulating both innate and adaptive immune responses, which makes them attractive targets for immunotherapy. It is therefore, important to understand the mechanisms behind the development and activation of iNKT cells. Thymic selection and peripheral maturation of iNKT cells in mice requires CD1d presentation of antigen acquired in endosomal compartments. CD1d molecules reach these compartments through their intrinsic tyrosine motif and by association with the Class II MHC chaperone, invariant chain (Ii). Using a new mouse model in which all CD1d is replaced by CD1d-enhanced yellow fluorescent fusion protein (EYFP), we clarified that the CD1d-encoded tyrosine motif influences the thymic selection of iNKT cells, but is dispensable for their peripheral maturation. In combination with mouse models deficient for Ii or the Ii-processing enzyme cathepsin S (CatS), we showed that both the CD1d-encoded and Ii-encoded endosomal sorting motifs are necessary for peripheral activation of iNKT cells and that thymic selection of iNKT cells not only occurs on CD4+CD8+ thymocytes but also on Ii and CatS-expressing cells. Patients with deletions or mutations in the CD1d tyrosine motif, invariant chain, or accessory molecules that interact with the cytosolic tail of CD1d, could therefore display severe iNKT cell deficiencies and susceptibility to certain pathogen infections. Invariant NKT cells play a role in the early immune response against Mycobacterium tuberculosis (Mtb) by suppressing intracellular bacterial growth when activated by antigenic CD1d/lipid complexes on Mtb-infected cells. Mtb, which causes one of the most prevalent deadly diseases, tuberculosis, avoids degradation by preventing fusion of phagosomes with lysosomal compartments. Using Ii-deficient mice, we demonstrated that Mtb replication occurs in lysosomes and requires Ii-mediated phagolysosomal fusion. However, we also showed that lysosomal presence of Mtb is equally pivotal for induction of CD1d-dependent iNKT cell responses in the host. Lysosomes could therefore be a potential target in the development of therapeutic strategies against Mtb infections. Invariant NKT cells also regulate immune tolerance as during autoimmunity and inflammation. Using transferrin-ovalbumin (Tf-mOVA) mice as an autoimmune hepatitis model, we demonstrate how rapid cytokine production by activated CD1d-restricted iNKT cells stimulates intrahepatic CD8+ T cell effector responses to self-antigen expressed in liver. We thus highlight the rapid cytokine production by iNKT cells as a potential intervention point for targeted immunotherapy to autoimmune and possibly (viral) infectious liver diseases. The studies in this thesis together highlight the importance and mechanisms behind CD1d-restricted iNKT cell development, and their function during infectious disease and autoimmunity. Our findings also elucidate previously unknown targets for therapeutic strategies against autoimmune hepatitis and mycobacteria infections.