Defects in invariant NKT cell function in lysosomal storage disorders

We are interested in a family of rare diseases called lysosmal storage disorders in which fatty molecules (lipids) build up over time, within cells in a specialised part of the cell called a lysosome. The lysosome is the waste disposal and recycling centre of the cell. These diseases tend to affect babies and young children and the main organ affected is the brain. In many ways these diseases can be considered as the childhod equivalent of Alzheimers disease combined with severe physical impairment.
We have found that in these storage diseases there is a gradual loss of a very specialised cell type of the immune system that plays an important role in regulating the bodies response to infection. These cells are called iNKT cells. This study aims to try and understand why these rare diseases cause the partial loss of this specific cell type and how this type of cell may be involved in these rare diseases. For example, we want to find out whether these cells enter the brain and contribute to the disease process leading to the severe neurological symptoms charactersitic of these diseases. We also aim to better understand how the fatty molceules recognised by iNKT cells move within the cell in healthy cells and cells storing lipids in the lysosomal diseases. These studies will give us unique insights into the normal biology of iNKT cells and the lipids they recognise so will be of fundamental importance as well as offering insights into storage disease pathology.

Glycosphingolipid (GSL) antigens presented via CD1d exert important regulatory immune functions in mouse and man, mediated by invariant Natural Killer T cells (iNKT). We currently have an incomplete knowledge of how lipids are presented by CD1d and recognised by the invariant TCR on iNKT cells. We also do not fully understand how lipid ligands traffic within cells, leading to efficient presentation at the cell surface. The lipid ligands responsible for thymic selection of iNKT cells and their peripheral activation remain controversial. The study of diseases states that interfere with the process of lipid presentation would greatly facilitate our understanding of this component of the immune system. One group of diseases in which we have identified alterations in lipid presentation are the GSL lysosomal storage diseases (LSDs). LSDs are inherited metabolic diseases that most typically result from deficiencies of lysosomal hydrolases responsible for the degradation of GSLs. We found that the frequency of iNKT cells is reduced in multiple murine models of lysosomal GSL storage diseases, irrespective of the specific lipid stored. Thymic selection of iNKT cells was found to be impaired, suggesting this to be the major mechanism through which GSL storage results in reduced iNKT cell frequencies in the periphery. We have proposed a model in which the storage of any GSL above a certain threshold in the late endosome/lysosome, irrespective of its structure, could potentially impair iNKT cell development. This could arise as a result of the entrapment of endogenous ligand(s) within storage bodies, or as a result of these ligand(s) being out-competed by the storage lipid for binding to CD1d.
In this proposal we aim to exploit these findings and aim to address have the following specific aims:

1. Do the residual iNKT cells in the Sandhoff disease mouse have a skewed TCR that recognises the storage lipids?

2. Does lipid storage alter lipid ligand trafficking in storage mice relative to control mice?

3. In Sandhoff disease do iNKT cells enter the brain and contribute to pathogenesis?

4. Do the changes in iNKT cell frequencies found in the mouse LSD models occur in patients?