Natural killer T cells: Atherogenic Mechanisms and Therapeutic Targeting

Atherosclerosis is a chronic inflammatory disease of medium and large-sized arteries. The immune system, including innate and adaptive components participates in development and progression of atherosclerosis. Immune cells, including dendritic cells, monocytes/macrophages, CD4/CD8⁺ T cells, B cells, natural killer cells and natural killer T cells present in both human and mouse atherosclerotic lesion. NKT cells have been shown to be pro-atherogenic in many studies and our group have identified that CD4⁺ iNKT cells are the NKT cell subset responsible for pro-atherogenic activity. However it is not clear how exactly these NKT cells exert their pro-atherogenic effect and whether NKT cells are therapeutically targeted to improve atherosclerosis.
       Adoptive transfer of CD4⁺ iNKT cells isolated from wild type mice into Rag2^-/-Apoe^-/- and Rag2^-/-γc^-/-Apoe^-/- mice developed bigger lesions compared to vehicle control mice. It indicated that CD4⁺ iNKT cells can promote atherosclerosis independently of T, B and NK cells. Ja18^-/-Apoe^-/- mice, which are deficient only in iNKT cells, were adoptively transferred with CD4⁺ iNKT cells isolated from wild type or mice selectively deficient in interferon-γ, interleukin-4, interleukin-21, perforin or granzyme B. Compared to wild type and cytokine-deficient NKT cells, cytotoxin-deficient NKT cells failed to promote atherosclerosis in Ja18^-/-Apoe^-/- mice, suggesting that NKT cells utilise perforin and granzyme B in their atherogenic function. Moreover, smaller necrotic core area and less apoptotic cells were observed in mice that received cytotoxin-deficient NKT cells suggesting NKT cell-induced lesion cell death via cytotoxins.
       NKT cells are activated via CD1d-assisted lipid antigen presentation by antigen presenting cells. DPPE-PEG₃₅₀ (NKT cell antagonist) is a chemical lipid antagonist which blocks NKT cell activation via competeing with lipid antigens in CD1d binding. Thus I investigated whether DPPE-PEG₃₅₀ can therapeutically prevent NKT cell activation in atherosclerosis. DPPE-PEG₃₅₀ was administrated into Apoe^-/- mice at the beginning of 8-week high fat diet. The lesion size decreased significantly in DPPE-PEG₃₅₀-treated mice. Also, DPPE-PEG₃₅₀ treatment reduced necrotic core without affecting the content of smooth muscle cells and collagen. To investigate a potential of DPPE-PEG₃₅₀ in clinical translation, I designed an experiment where mice with established atherosclerosis were treated with DPPE-PEG₃₅₀. Apoe^-/- mice were fed a high fat diet for 6 weeks to establsih atherosclerosis and then treated with DPPE-PEG₃₅₀ for another 6 weeks while fed a high fat diet. DPPE-PEG₃₅₀ treatment attenuated atherosclerosis without affecting smooth muscle cells and collagen and also reduced necrotic core and apoptotic cells. To test whether atherosclerosis reduced by DPPE-PEG₃₅₀ treatment is via blocking NKT cell activation, DPPE-PEG₃₅₀-treated mice were challenged with α-GalCer (NKT cell agonist). Strikingly, α-GalCer failed to increase atherosclerosis in mice treated with DPPE-PEG₃₅₀ compared to mice that received α-GalCer alone, confirming that DPPE-PEG₃₅₀ treatment ameliorates atherosclerosis via NKT cell-dependent manner.
       In conclusion, this study defines the mechanism by which CD4⁺ iNKT cells promote atherosclerosis. It also shed light on therapeutic potential of DPPE-PEG₃₅₀ in targeting NKT cells in atherodclerosis and may lead to the finding of novel clinical treatment for atherosclerosis.