Selective CD4+ T Cell Loss Promotes Liver Cancer Development
CD4 T cells die in nonalcoholic fatty liver disease through ROS overproduction in the mitochondria leading to enhanced tumor growth.
Hepatocellular carcinoma (HCC), the second leading cause of cancer deaths worldwide, commonly develops in patients with underlying chronic liver disease, such as hepatitis B or C virus infection or non-alcoholic fatty liver disease (NAFLD). With rising obesity rates, NAFLD is becoming more common, but how the metabolic changes in the disease promote HCC is unclear. To begin addressing this question, Tim Greten, M. D., of CCR’s Thoracic and Gastrointestinal Oncology Branch and his colleagues examined a series of mouse NAFLD models along with human HCC samples.
Similar to humans, the researchers found that diet-induced NAFLD in mice led to HCC development. They next examined the immune cell changes in NAFLD livers because, although HCC is not generally considered an immunogenic tumor, certain immune cell populations are important for NAFLD progression. Surprisingly, the investigators found a significant decrease in CD4+, but not CD8+, T cells in the livers of mice with NAFLD, and further characterization of the CD4+ T cells revealed that NAFLD enhanced their activation status. The scientists then used antibodies to deplete CD4+ T cells and found more liver tumors in mice lacking CD4+ T cells, suggesting that their loss contributes to HCC development.
To understand why CD4+ T cells are lost in NAFLD, the researchers studied the cells’ survival and observed increased cell death with NAFLD. They hypothesized that excess lipid accumulation in liver cells in NAFLD might cause CD4+ T cell loss, and studies with isolated cells showed that CD4+ T cell survival was affected by soluble factors from NAFLD liver cells. Since free fatty acids are known to alter immune cell survival, the investigators incubated CD4+ T cells in the fatty acids predominantly secreted by NAFLD liver cells. They found that only linoleic acid increased CD4+ T cell death, and mice fed a high linoleic acid diet had reduced numbers of CD4+, but not CD8+, T cells.
The scientists then assessed the mechanism of linoleic acid-induced CD4+ T cell death. When incubated with linoleic acid, mouse CD4+ T cells and a human CD4+ T cell line showed changes in gene expression that implicated mitochondria. While mitochondrial functioning was similar in CD4+ and CD8+ T cells, the increased number of mitochondria in CD4+ T cells and a shift to ATP synthase independent oxygen consumption with linoleic acid increased total reactive oxygen species (ROS) production in CD4+ T cells. Knocking down CPT1a, the rate-limiting enzyme for importing fatty acids into mitochondria, blocked linoleic acid-induced ROS production, suggesting that increased ROS leads to CD4+ T cell death in NAFLD. Encouragingly, blocking ROS with N-acetylcysteine (NAC) or mitoTEMPO prevented the loss of CD4+ T cells and delayed tumor development in an NAFLD mouse model.
Finally, the researchers asked whether linoleic acid has the same effect in human disease. They found that, as in mice, linoleic acid selectively killed CD4+, but not CD8+, T cells. Likewise, linoleic acid treatment increased ROS production in CD4+ T cells. The investigators also observed fewer CD4+ T cells in liver biopsies from patients with NAFLD but not viral hepatitis. Importantly, the CD4+-to-CD8+ T cell ratio was lower in NAFLD biopsies.
These studies identified a novel link between obesity-induced lipid accumulation and selective CD4+ T cell loss, which impairs tumor immunity and enhances HCC development. In addition, they provide new information on how polyunsaturated fatty acids, like linoleic acid, affect T cell function.Summary Posted: 03/2016
Ma C, Kesarwala AH, Eggert T, Medina-Echeverz J, Kleiner DE, Jin P, Stroncek DF, Terabe M, Kapoor V, ElGindi M, Han M, Thornton AM, Zhang H, Egger M, Luo J, Felsher DW, McVicar DW, Weber A, Heikenwalder M, and Greten TF. NAFLD causes selective CD4+ T lymphocyte loss and promotes hepatocarcinogenesis. Nature. March 2, 2016. PubMed Link