Natural Killer Cells PDF

Natural killer cells Dr Felix N. Toka Professor, Veterinary Immunology & Virology Department of Biomedical Sciences Objectives for this topic Define the origin of Natural Killer (NK) cells and their role innate immunity Understand the role of NK cell receptors in target cell recognition Understand t...

Natural killer cells Dr Felix N. Toka Professor, Veterinary Immunology & Virology Department of Biomedical Sciences Objectives for this topic Define the origin of Natural Killer (NK) cells and their role innate immunity Understand the role of NK cell receptors in target cell recognition Understand the killing mechanisms engaged by NK cells Understand the regulatory role of NK cells in innate immune response Cell therapy: administration of living cells in humans with therapeutic purpose Worldwide, a total of 372 clinical trials were undertaken in 2015/16 of which 10 were NK cell therapy trials Natural cells (NK) Originate from bone marrow Live for about a week Are found in the blood, spleen and liver Migrate to tissues in large numbers only when an inflammatory reaction is underway Do not posses receptors generated through gene segment rearrangement (i.e., no receptors such as TCR or BCR) Cell trafficking receptors on NK cells Cell trafficking receptors on NK cells Chemokine receptors NK cell morphology Also referred to as large granular lymphocytes (LGL), because morphologically, they resemble lymphocytes They contain granules NK cell identification in vitro They are identified by expression of CD56, CD16 and lack of CD3 mouse and human NK cells In pigs there is no exclusive marker but CD2+/CD8+/CD3- are used (NKp46 also) In cattle, CD335 (NKp46) has recently been reported to be expressed on bovine NK cells (Horses too) Role of NK cells in innate immunity 1. They kill virus-infected cells, tumor cells and stressed cells – cells that have been deemed “abnormal” Following activation in vitro with cytokines such as IL-2 and IFNγ, they become lymphokine activated killer cells (LAK) 2. Cytokine and chemokine production - IFNγ, TNFα, IL-17 and IL-22, MIP-1α, MIP-1β - regulatory role ME How do NK cells recognize abnormal cells? An “abnormal cell” should be understood as one which has been stressed by any means, a cell that is infected or a transformed cell (tumor) Cells that are stressed, infected or transformed alter expression of certain cell surface markers All cells, i.e., normal cells, express the Major Histocompatibility Complex class I (MHC I) molecule on their surface How do NK cells recognize abnormal cells? Cell surface marker alteration on abnormal cells In cells that are infected, particularly with a virus, expression of MHC I is suppressed Tumor cells often fail to express MHC I Proteins such as MHC I chain-related A (MICA) and MICB, Rae-1, H60 are highly expressed on the surface of stressed cells How do NK cells recognize abnormal cells? NK cell receptors NK cells express two types of recognition receptors Activating receptors Inhibitory receptors Activation of NK cells is regulated by a balance between signals from activating receptors and inhibitory receptors How do NK cells recognize abnormal cells? – cont’d A high level of activating signals will lead to killing of the target cells – high expression of stress molecules A strong inhibitory signal preserves the target cell – normal MHC I expression and absence of stress markers Generally, activating signals are blocked by inhibitory signals so as to prevent killing of normal cells „To kill or not to kill?” NK cell Decisions, Decisions! Normal cell NK cell „do not kill” !! MHC I Inhibitory receptor The „do not kill” signal is transmitted by the inhibitory receptors when normal expression of the MHC I molecule is detected „To kill or not to kill?” NK cell Decisions, Decisions! Activating receptor Stress protein Abnormal cell „kill” NK cell „do not kill” !! MHC I Inhibitory receptor The “kill” signal is transmitted by the activating receptors when they sense change on the target cell surface e.g., expression of stress molecules or lack of MHC I expression Dominant inhibition to prevent killing of normal cells What is the outcome of the encounter below? ? NO KILLING!! What is the outcome of the encounter below? ? NK cell KILL! What is the outcome of the encounter below? ? NK cell KILL! Activating receptors on NK cells bind to ligands expressed on stressed, infected or transformed cells Inhibitory receptors on NK cells recognize MHC-I molecules expressed on normal cells Reduction of or lack of MHC I expression renders the cell vulnerable to destruction by NK cells Effector functions of NK cells Lysis of infected, stressed or transformed cells = abnormal cells Activation of macrophages through secretion of cytokines Functions of NK cells NK cell cytotoxicity mechanisms 1. Perforin-dependent mechanism Once triggered the NK cell releases perforin from its granules Perforin then creates a lesion in the target cell membrane – perforin channels NK cell cytotoxicity mechanisms - cont’d Later granulysin, NK-lysin (granzymes) and fragmentin (protease) are released from cytotoxic granules and passed through perforin channels Granzymes and the protease induce apoptosis of the target cell 2. CD95/CD95L (Fas/FasL) mechanism NK cells normally express CD95L (FasL) on the cell surface Target cells (abnormal cells) may have increased expression of CD95 (Fas) Binding of CD95L on the NK cell to CD95 on the target cell induces apoptosis of the target cell 3. CD16 killing pathway (ADCC) NK cells also recognize target cells through an antibody dependent pathway using CD16 CD16 is an Fc receptor (FcγIII) expressed on NK cells Antibodies bind to an antigen on an infected cell, NK cells bind to antibodies through CD16 leading to NK cell cytotoxicity Antibody-dependent cellular cytotoxicity ADCC Target recognition through CD16 will occur only when antibodies are present Regulation of NK cell activity NK cells are activated by cytokines such as IL-1, IL-2, IL-12, IL-15, IL-18, IL-21 and type I and II IFNs in vitro Treatment of isolated NK cells with these cytokines turns them into lymphokine activated killer cells (LAK) which have increased cytotoxic capacity LAK are highly cytotoxic cells and are a potential tool in immunotherapy of tumors In certain virus infections activity of NK cells may be highly enhanced. However, some viruses inhibit NK cell cytotoxicity e.g., Foot-andmouth disease virus in swine Natural killer T cells (NKT) NKT cells have properties that are similar to NK cells and T lymphocytes They are of the same lineage as the T cells (thymic origin) Their specificity is directed against only a few pathogens They make up at least 0.5% to 1% of the peripheral blood mononuclear cells NKT cells They express an invariant α/β TCR They express NK1.1 and other KLR family receptors Mostly, they are CD4+ cells or may be double negative (CD4-/CD8-) NKT cells mostly recognize glycolipid antigens on bacteria NKT cells are activated by IL-15 They do not develop into memory cells (???) They serve to link the T cell system and NK cells They play a role in allergies, antitumor immunity, autoimmunity and antimicrobial immunity Natural killer dendritic cells (NK DCs) Dendritic cells that share some properties with NK cells They express NK1.1 – an NK cell marker They express CD11c – a DC marker Are present in spleen, liver, lymph nodes, and thymus They spontaneously lyse tumor cells They can present antigen to naïve T cells They produce large amounts of IFNγ upon stimulation through TLR9 (CpG) They link the innate and adaptive immunity