Impaired natural killer cell subset phenotypes in human obesity

Obesity is associated with alterations in functionality of immune cells, like macrophages and natural killer (NK) cells, leading to an increased risk for severe infections and several cancer types. This study aimed to examine immune cell populations and functional NK cell parameters focusing on NK cell subset phenotypes in normal-weight and obese humans. Therefore, peripheral blood mononuclear cells (PBMCs) were isolated from normal-weight and obese individuals and analyzed by flow cytometry. Results show no significant changes in the frequency of monocytes, B lymphocytes, or NKT cells but a significantly increased frequency of T lymphocytes in obesity. The frequency of total NK cells was unaltered, whereas the number of low cytotoxic CD56bright NK cell subset was increased, and the number of high cytotoxic CD56dim NK cell subset was decreased in obese subjects. In addition, the frequency of CD56bright NK cells expressing the activating NK cell receptor NKG2D as well as intracellular interferon (IFN)-γ was elevated in the obese study group. In contrast, the frequency of NKG2D- and IFN-γ-positive CD56dim NK cells was lower in obesity compared to normal-weight individuals. Moreover, the expression of the activation marker CD69 was decreased in NK cells, which can be attributed to a reduction of CD69-positive CD56dim NK cells in obese subjects. In conclusion, data reveal an impaired NK cell phenotype and NK cell subset alterations in obese individuals. This NK cell dysfunction might be one link to the higher cancer risk and the elevated susceptibility for viral infections in obesity.

SIRT1 and HIF1α signaling in metabolism and immune responses

SIRT1 and HIF1α are regarded as two key metabolic sensors in cellular metabolism pathways and play vital roles in influencing immune responses. SIRT1 and HIF1α regulate immune responses in metabolism-dependent and -independent ways. Here, we summarized the recent knowledge of SIRT1 and HIF1α signaling in metabolism and immune responses. HIF1α is a direct target of SIRT1. Sometimes, SIRT1 and HIF1α cooperate or act separately to mediate immune responses. In innate immune responses, SIRT1 can regulate the glycolytic activity of myeloid-derived suppressor cells (MDSCs) and influence MDSC functional differentiation. SIRT1 can regulate monocyte function through NF-κB and PGC-1, accompanying an increased NAD⁺ level. The SIRT1-HIF1α axis bridges the innate immune signal to an adaptive immune response by directing cytokine production of dendritic cells in a metabolism-independent manner, promoting the differentiation of CD4⁺ T cells. For adaptive immune cells, SIRT1 can mediate the differentiation of inflammatory T cell subsets in a NAD⁺-dependent manner. HIF1α can stimulate some glycolysis-associated genes and regulate the ATP and ROS generations. In addition, SIRT1-and HIF1α-associated metabolism inhibits the activity of mTOR, thus negatively regulating the differentiation and function of Th9 cells. As immune cells are crucial in controlling immune-associated diseases, SIRT1-and HIF1α associated-metabolism is closely linked to immune-associated diseases, including infection, tumors, allergic airway inflammation, and autoimmune diseases.

The multifaceted role of PIP2 in leukocyte biology

Phosphatidylinositol 4,5-bisphosphate (PIP2) represents about 1 % of plasma membrane phospholipids and behaves as a pleiotropic regulator of a striking number of fundamental cellular processes. In recent years, an increasing body of literature has highlighted an essential role of PIP2 in multiple aspects of leukocyte biology. In this emerging picture, PIP2 is envisaged as a signalling intermediate itself and as a membrane-bound regulator and a scaffold of proteins with specific PIP2 binding domains. Indeed PIP2 plays a key role in several functions. These include directional migration in neutrophils, integrin-dependent adhesion in T lymphocytes, phagocytosis in macrophages, lysosomes secretion and trafficking at immune synapse in cytolytic effectors and secretory cells, calcium signals and gene transcription in B lymphocytes, natural killer cells and mast cells. The coordination of these different aspects relies on the spatio-temporal organisation of distinct PIP2 pools, generated by the main PIP2 generating enzyme, phosphatidylinositol 4-phosphate 5-kinase (PIP5K). Three different isoforms of PIP5K, named α, β and γ, and different splice variants have been described in leukocyte populations. The isoform-specific coupling of specific isoforms of PIP5K to different families of activating receptors, including integrins, Fc receptors, toll-like receptors and chemokine receptors, is starting to be reported. Furthermore, PIP2 is turned over by multiple metabolising enzymes including phospholipase C (PLC) γ and phosphatidylinositol 3-kinase (PI3K) which, along with Rho family small G proteins, is widely involved in strategic functions within the immune system. The interplay between PIP2, lipid-modifying enzymes and small G protein-regulated signals is also discussed.