The role of natural killer cells in atherosclerosis.

Atherosclerosis is a progressive multifactorial disease of large elastic and muscular arteries in which lesions are characterized by the deposition of cholesterol, leukocyte influx, smooth muscle cell proliferation, cell death and collagen accumulation. Inflammatory cells of the innate and adaptive immune systems contribute to lesion development and progression. NK cells have been detected in human atherosclerotic lesions in low numbers and also detected in atherosclerotic lesions of LDLR-deficient mice fed a high fat diet. However their significance for atherosclerotic lesion development is still unclear. The studies presented in this thesis unequivocally demonstrate an important role for NK cells in the development of atherosclerosis and provide novel insights as to how these cells contribute to atherosclerosis. Chapter 3 examines the role of NK cell in the development of atherosclerosis in ApoE-deficient mice fed a high fat diet. Despite being very minor lymphocyte population in developing atherosclerotic lesions, the study has definitely demonstrated that NK cells are proatherogenic in mice. Specific depletion of NK cells by more than 90% attenuated lesion size and macrophage accumulation. Whilst not specifically examined the reduction in macrophage accumulation could be due to lack of production of NK cell derived cytokines and chemokines in the lesions. Chapter 4 examines whether the activation of NK cells further augments development of atherosclerosis. Activation of NK cells with Poly IC exacerbates the development of atherosclerosis. Since Poly IC can also activate T cells, the effects of Poly IC were determined in T and B cells deficient ApoE-/- Rag2-/- mice. Poly IC augmented atherosclerosis in these mice which suggest a strong dependency on NK cells. To confirm the dependency of Poly IC effects on atherosclerosis on NK cells, the effects of Poly IC in NK cell depleted mice were also determined. The depletion of NK cells reduced atherosclerotic lesion development. Therefore the study indicates a dependency of Poly IC’s proatherogenic effects on NK cells. Chapter 5 examines the mechanisms by which NK cells are activated during development of atherosclerosis. NK cells can be activated by via NKT cells and/or via their activating receptor NKG2D. Activation of NKT cells has been shown to stimulate NK cell proliferation and cytotoxicity. This study demonstrates that NKT cells contribute to NK cell activation during development of atherosclerosis since the atherosclerotic lesions were similar in mice deficient in invariant NKT cells in ApoE-/- Jalpha18-/- mice and in NK cell depleted ApoE-/- Jalpha18-/- mice. As to the role of NKG2D receptors in activating NK cells during the development of atherosclerosis, the study indicate that these receptors are not involved despite the high expression of its activating ligands Rae-1δ, Rae-1ε and MULT-1. Chapter 6 examines the mechanisms by which NK cells promote the development of atherosclerosis. Activated NK cells produce cytokines and cytotoxicity. This study examined whether NK cells participate in atherosclerosis via production of cytokines implicated in atherosclerosis, or through secretion of cytotoxic molecules. IL-2 activated NK cells from mice deficient in cytokines interferon-gamma (IFN-γ) were adoptively transferred to NK-, NKT-, T- and B- cell-deficient ApoE-/- Rag2-/- γ-/- mice. Interferon-gamma was found to be unnecessary for the proatherogenic actions of NK cells, as NK cells deficient in IFN-γ were capable of exacerbating atherosclerosis to the same extent as wild-type IL-2 activated NK cells. Conversely, adoptive transfer of IL-2 activated NK cells deficient in cytotoxic molecules perforin or granzyme B did not increase atherosclerosis in ApoE-/- Rag2-/- γ-/- mice. Therefore the atherosclerosis-promoting effects of NK cells appear to be dependent on their cytotoxic properties. Increased cytotoxicity in atherosclerotic lesions can lead to increased necrosis and inflammation via activation of inflammasomes. Lesions from mice that received perforin- or granzyme B-deficient NKT cells had smaller necrotic areas and lower expression of pro-IL-1β and caspase-1 mRNA. In summary, the studies indicate that NK cell within the developing lesions augment atherosclerosis by secreting cytotoxic molecules perforin and granzyme B. The magnitude of effects of NK cells on lesion development is dependent not only on NK cell numbers but also on their activation status. Increased understanding of the mechanisms of NK cell actions during atherogenesis may lead to new strategies to develop immunotherapeutic treatments to attenuate atherosclerosis.