Introduction:
Recognizing foreign antigens as peptides in a complex with MHC class I is indispensable
for CD8+ T-lymphocytes to eliminate infected cells. Tight cellular co-regulation of
metabolism and immunity is required to control effector function, but the mechanisms
regulating organ-specific immunity in tissues rich in nutrients such as liver remained
unclear. CD8+ T cells expressing the chemokine receptor CXCR6 are important for staying
as tissue-resident memory T cells (Trm) in the liver and providing front-line defense
for infections. Trm express high levels of effector molecules like GzmB or IFN-γ but
how Trm regulates their effector function in order to avoid immune pathology is largely
unknown. Here we identify Foxo1-activity in CXCR6+CD8+ T cells as critical regulator
of CXCR6 expression, metabolism and effector function during liver disease states.
Material and Methods:
Extracellular flux analysis, cytokine expression, cytotoxicity assays were performed
to study co-regulation of metabolism and immunity of CXCR6+CD8+ T cells and its dependence
on Foxo1 in vitro and ex vivo studies. Murine models of viral hepatitis and non-alcoholic
steatohepatitis (NASH) were used to explore Foxo1-dependent T cell immunopathology.
Results:
RNA-seq and KEGG pathway analysis of CXCR6+ memory CD8+ T cells of the liver compared
to memory CD8+ T cells from other tissues revealed high GzmB level accompanied with
downregulation of Foxo1-dependent pathways. Flow cytometric analysis of liver-specific
lymphocytes revealed a Foxo1lowCXCR6+GzmBhighCD69+CD8+ T cell population. Using germ-free
mice and in vitro studies we found that high GzmB level in Foxo1lowCXCR6+ CD8+ T cells
were dependent on the microbiota. We further identified that TGFβ and IL15 are critical
cytokines to downregulate Foxo1 via PI3K/pAkt pathway and inducing CXCR6+CD8+ T cells
derived from CXCR6-CD122+ CD8+ T cells. To address the functional consequence of CXCR6+Foxo1lowGzmbhigh
CD8 T cells we performed in vitro killing assays of infected and non-infected hepatocytes
demonstrating antigen-independent immune pathology against hepatocytes in the presence
of elevated levels IL15 and acetate. This development of autoimmune CD8 T cell effector
function could was also be observed in NASH where we detected high numbers of Foxo1lowCXCR6+GzmBhighCD69+CD8+
T cells that caused liver damage. Since TCR sequencing of hepatic T cells did not
reveal presence of particular T cell-clones in NASH, we assume that increased effector
function in CXCR6+ CD8+ T cells in the absence of Foxo1-control caused antigen-independent
hepatic immunopathology.
Conclusion:
Our results provide evidence for a critical role of Foxo1 in controlling metabolism
in CD8 T cells that is required to prevent liver immunopathology and may explain metabolic
CD8 T cell activation in NASH causing sterile inflammation.
