| dc.description.abstract | Natural Killer (NK) cells are cytotoxic, innate lymphocytes that play a crucial role in
mediating viral and tumour responses as well as supporting tissue homeostasis. Due to
their fast acting immune response independent of antigen recognition against tumour
cells, NK cells hold great potential for developing new treatments and cancer
immunotherapies. Our knowledge of NK cell metabolism has evolved over the recent
years and has highlighted the significant connection between metabolism and NK cell
effector function. For instance, Sterol Regulatory Element Binding Proteins (SREBPs)
were found to be crucial regulators of NK cell metabolic and effector function. SREBP is
required for cytokine-mediated induction of NK cell metabolism supporting elevated
rates of glycolysis and OxPhos. In NK cells, SREBP support elevated rates of glycolysis
and OXPHOS by supporting the citrate malate shuttle, a non-canonical TCA cycle,
through inducing the expression of Slc25a1 and ATP citrate lyase. Inhibition of SREBP or
the citrate malate shuttle prevents elevated rates of NK cell metabolism and impairs the
production of IFNγ and NK cell cytotoxicity.
Oxysterols are oxygenated derivatives of cholesterol known for their conventional role
in mediating cholesterol homeostasis. However, there is emerging evidence and
ongoing studies revealing diverse immunoregulatory roles of oxysterols and the host
immune response in multiple pathological settings. Indeed, in multiple diseases, but not
limited to infection, cancer, autoimmunity and atherosclerosis, there are aberrant levels
of oxysterols which contribute to immune dysregulation and dysfunction. This study
aims to understand the immunoregulatory role of oxysterols for NK cell biology.
Two oxysterols were focused upon in this study, 25-Hydroxycholesterol (25HC) and 27-
Hydroxycholesterol (27HC). Both oxysterols can become elevated in certain
immunological situations such as viral infection and cancer and have the potential to
regulate NK cell responses. 25HC is generated by the interferon inducible gene, Ch25h
and is expressed by macrophages during viral infection. 27HC is generated by the
cytochrome p450 enzyme Cyp27a1 and is the most abundant oxysterol in the human
serum. 27HC was identified as the first endogenous Selective Estrogen Receptor
Modulator (SERM), known for its potency in ER+ breast cancer progression. Previously,
v
27HC had only been shown to inhibit SREBP activity in hepatocytes. This study identified
27HC as a potent inhibitor of SREBP activation and as a result SREBP-controlled
metabolism, glycolysis and OxPhos in NK cells. 27HC or 25HC exposure at any stage of
NK cell activation, downregulate SREBP activity, as determined by reduced SREBP target
genes expression. 27HC and 25HC exposure results in reduced rates of NK cell
metabolism and loss of NK cell effector function, inhibition of IFNγ production and a
near complete loss of NK cytotoxicity. The data show that irrespective of whether
oxysterols were added during NK cell activation or after previous activation, the capacity
of NK cell to kill tumour target tumour cells was significantly decreased. Overall, this
study reveal that NK cells exposed to immunosuppressive oxysterols have severe
metabolic defects resulting in significant loss in IFNγ production and cytotoxicity.
This study also revealed that oxysterol exposure led to a significant decrease in amino
acid uptake through Slc7a5. Previous studies argue that amino acid uptake into NK cells
is important for NK cells as it supports cellular signalling transduction important for
supporting metabolism and function. To examine the impact of impaired amino acid
uptake into NK cells for antitumour functions a transgenic mouse was generated in
which NK cells selectively lack the expression of Slc7a5. Loss of Slc7a5 in NK cells did not
affect NK cell development and maturation nor initial activation. However, NK cells
deficient of Slc7a5 activated in vivo fail to complete blastogenesis and fail to normally
upregulate metabolic pathways. Consequently, these NK cells have impaired
antitumour effector functions including the production of IFNγ and cytotoxicity against
tumour cells. Indeed, mice containing Slc7a5-null NK cells show significantly increased
tumour burden when challenged with B16 melanoma tumour cells.
Taken together, my research has made important discoveries in (1) its study of
oxysterols and their influence on NK cell metabolism and cytotoxicity and (2) the
importance of Slc7a5-mediated amino acid uptake for antitumour NK cell responses.
The increased understanding of cellular mechanisms of NK cell regulation as a direct
result of this research will have important implications in developing new and improved
NK cell based immunotherapies. | en |
