Background: Cluster of Differentiation 3 (CD3) is a multiunit protein complex expressed on the surface of T-cells that directly associates with the T-cell receptor (TCR). CD3 is composed of four polypeptides: ζ, γ, ε and δ. Engagement of TCR complex with antigens presented in Major Histocompatibility Complexes (MHC) induces tyrosine phosphorylation in the immunoreceptor tyrosine-based activation motif (ITAM) of CD3 proteins. CD3 phosphorylation is required for downstream signaling through ZAP-70 and p85 subunit of PI-3 kinase, leading to T cell activation, proliferation, and effector functions (1). Cluster of Differentiation 8 (CD8) is a transmembrane glycoprotein expressed primarily on cytotoxic T cells, but has also been described on a subset of dendritic cells in mice (2,3). On T cells, CD8 is a co-receptor for the TCR, and these two distinct structures are required to recognize antigen bound to MHC Class I (2). Cluster of Differentiation 4 (CD4) is expressed on the surface of T helper cells, regulatory T cells, monocytes, macrophages, and dendritic cells, and plays an important role in the development and activation of T cells. On T cells, CD4 is the co-receptor for the TCR, and these two distinct structures recognize antigen bound to MHC Class II. CD8 and CD4 co-receptors ensure specificity of the TCR–antigen interaction, prolong the contact between the T cell and the antigen presenting cell, and recruit the tyrosine kinase Lck, which is essential for T cell activation (2). Granzyme B is a serine protease expressed by CD8+ cytotoxic T lymphocytes and natural killer (NK) cells and is a key component of the immune response to pathogens and transformed cancer cells (4). Forkhead box P3 (FoxP3) is crucial for the development of T cells with immunosuppressive regulatory properties and is a well-established marker for T regulatory cells (Tregs) (5). CD19 is a co-receptor expressed on B cells that amplifies the signaling cascade initiated by the B cell receptor (BCR) to induce activation. It is a biomarker of B lymphocyte development, lymphoma diagnosis, and can be utilized as a target for leukemia immunotherapies (6,7). F4/80 (EMR1) is a heavily glycosylated G-protein-coupled receptor and is a well-established marker for mouse macrophages (8). CD11c (integrin αX, ITGAX) is a transmembrane glycoprotein highly expressed by dendritic cells, and has also been observed on activated NK cells, subsets of B and T cells, monocytes, granulocytes, and some B cell malignancies including hairy cell leukemia (9,10).
Background: Apoptosis is a regulated physiological process leading to cell death (1,2). Caspases, a family of cysteine acid proteases, are central regulators of apoptosis. Caspases are synthesized as inactive zymogens containing a pro-domain followed by large (p20) and small subunits (p10) that are proteolytically processed in a cascade of caspase activity. Initiator caspases (including 8, 9, 10, and 12) are closely coupled to proapoptotic signals. Once activated, these caspases cleave and activate downstream effector caspases (including 3, 6, and 7), which in turn cleave cytoskeletal and nuclear proteins like PARP, α-fodrin, DFF, and lamin A, and induce apoptosis. Cytochrome c released from mitochondria is coupled to the activation of caspase-9, a key initiator caspase. Apoptosis induced through the extrinsic mechanisms involving death receptors in the tumor necrosis factor receptor superfamily activates caspase-8. Activated caspase-8 cleaves and activates downstream effector caspases, such as caspase-1, -3, -6, and -7. Caspase-3 is a critical executioner of apoptosis, as it is either partially or totally responsible for the proteolytic cleavage of many key proteins, such as the nuclear enzyme poly (ADP-ribose) polymerase (PARP).Necroptosis, a regulated pathway for necrotic cell death, is triggered by a number of inflammatory signals, including cytokines in the tumor necrosis factor (TNF) family, pathogen sensors such as toll-like receptors (TLRs), and ischemic injury (3,4). Necroptosis is negatively regulated by caspase-8 mediated apoptosis in which the kinase RIP/RIPK1 is cleaved (5). Furthermore, necroptosis is inhibited by a small molecule inhibitor of RIP, necrostatin-1 (Nec-1) (6). Research studies show that necroptosis contributes to a number of pathological conditions, and Nec-1 has been shown to provide neuroprotection in models such as ischemic brain injury (7). RIP is phosphorylated at several sites within the kinase domain that are sensitive to Nec-1, including Ser14, Ser15, Ser161, and Ser166 (8). Phosphorylation drives association with RIP3, which is required for necroptosis (9-11). Mixed lineage kinase domain-like protein (MLKL) is a pseudokinase that was identified as downstream target of RIP3 in the necroptosis pathway (12). During necroptosis RIP3 is phosphorylated at Ser227, which recruits MLKL and leads to its phosphorylation at Thr357 and Ser358 (12). Knockdown of MLKL through multiple mechanisms results in inhibition of necroptosis (13). While the precise mechanism for MLKL-induced necroptosis is unclear, some studies have shown that necroptosis leads to oligomerization of MLKL and translocation to the plasma membrane, where it effects membrane integrity (14-17).