Plasmalogen-mediated activation of GPCR21 regulates cytolytic activity of NK cells against the target cells.

Hossain MS, Mawatari S, and Fujino T. (2022) Plasmalogen-mediated activation of GPCR21 regulates cytolytic activity of NK cells against the target cells. The Journal of Immunology

Natural killer (NK) cells are a type of cytotoxic lymphocyte within the immune system and make up 5-20% of total circulating lymphocytes in humans. The role of NK cells is to provide rapid responses to virus-infected cells, to other intracellular pathogens that act 3 days after infection, and to tumor formation. Their response is considered rapid because NK cells are able to kill most abnormal cells within 24 hours of entry. The activity of these cells has been found to reduce with age and is associated with a decline in immune response. The complete understanding of age-related factors that regulate NK cell activity is unknown, however reduced lipid levels in blood have been found in older subjects. Specifically, a class of lipids called plasmalogens, phospholipids with a vinyl-ether bond at the sn-1 position, are known to decrease with age. Plasmalogens are involved in membrane structure and function, vesicular fusion, and have anti-oxidative properties. They are also neuroprotective and have been shown to induce cellular signaling such as ERK and Akt via G protein-coupled receptors (GPCRs), specifically GPCR21. Because of this association, Hossain et al were interested in the relationship between plasmalogens, GPCR21, and NK cell function.

To demonstrate the role of GPCR21 in NK cells, endogenous GPCR21 was knocked down in the KHYG-1 cell line to 60% of what is normally in these cells. This knockdown also reduced interleukin-2 (IL-2) -mediated cytolytic activity of the KHYG-1 cells and inhibited proliferation of the cells. However, ectopic expression of GPCR21 enhanced cytolytic activity of KHYG-1 cells against cancer cells, K562 cells, and increased proliferation of the KHYG-1 cells. These findings indicate that GPCR21 can regulate NK cell cytolytic activity against target cancer cells.

To investigate the role of plasmalogens in NK cell activity, plasmalogens derived from scallops was administered to the cells. When the KHYG-1 cells were treated with plasmalogens their proliferation increased in the presence of IL-2. Also, the treatment was able to stimulate cytolytic activity against the target cancer cells. In GPCR21-overexpressing KHYG-1 cells, the plasmalogen treatment increased cytolytic activity. This work suggests that plasmalogens enhance GPCR21 regulated cytolytic activity in NK cells against target cells. Plasmalogen treatment was also able to increase perforin-1 (PRF-1) expression in the KHYG-1 cells. PRF-1 encodes a pore-forming cytolytic protein in NK cells and once they have translocated to the target cell and bound to its membrane, PRF-1 forms pores along the cell allowing pro-apoptotic proteases into the target cell causing its degradation. PRF-1 expression is known to increase in NK cells when they recognize cancer cells or virus-infected cells. PRF-1 expression was highest in cells where GPCR21 was over expressed and in cells that were treated with plasmalogens in the presence of IL-2. This would suggest that plasmalogens and GPCR21 can assist cytolytic activity of NK cells during the recognition of target cells.

Although the above in vitro work was compelling, the authors also wanted to determine if plasmalogens regulate IL-2-mediated signaling in immune cells. To do this, they tested a plasmalogen treatment on peripheral blood mononuclear cells (PBMCs), such as lymphocytes, B cells, and NK cells, collected from human adult volunteers. The plasmalogen and IL-2 treatment was able to induce PRF-1 expression and increase GPCR21 expression. This indicates that plasmalogens can regulate endogenous expression of GPCR21, but it was not clear which cell type this was occurring in.

Hossain et al were interested in the relationship between plasmalogens, GCPR21, and NK cell activity. Plasmalogens have many important roles in cells especially with regards to membrane structure and function, but they also have many roles in the expression and activity of proteins that is not well known. In this work it was shown that a plasmalogen treatment was able to increase production of many important proteins in immune cells such as NK cells including GPCR21 and PRF-1. With increasing age it is known that immune cell function is reduced and plasmalogen levels decrease, which may be related activities. Future work looking at the ability of plasmalogens to enhance an immune response may present a new treatment route against viruses and cancer cell proliferation.