Tactics 2: War Activation Bypassl
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The side effects inherent to CPB originate mostly from exposure of blood to a large foreign surface, the non-physiological blood-air interphase, mandatory anti-coagulation, physical trauma to blood components, micro-embolization and hypothermia . These primary processes induce activation of multiple cascades such as the intrinsic and extrinsic coagulation system, the complement and kalikrein systems as well as cellular components, leading to intense systemic inflammatory response. Altogether, these pathophysiological processes result in coagulopathy and end-organ dysfunction .
Inflatable Decoy: Proximity activated decoy mine. Upon activation, the decoy is rapidly deployed to confuse and distract enemy Operators. It can also be manually activated to fool enemies into attacking it on sight.
UAV: Upon activation, the UAV flies high above the playable space, revealing enemy locations on the minimap every few seconds for all squad members until it needs to refuel. It then stops sweeping for locations and quickly flies away.
Cruise Missile: Upon activation, your Operator will launch a Cruise Missile via tablet and release it from its air carrier. While airborne, the missile can be steered and accelerated via its built-in boost function. Enemy Operators without the Cold-Blooded Perk will appear with a red icon for assisted targeting.
Following activation, circulating naive T cells have three major fates in the periphery (Fig. 1). First, the effector T cell population can contract through apoptosis as the immune response resolves (cytokine withdrawal) or following repeated high-dose stimulation (restimulation-induced cell death)287,288,289. T cells can also exhibit an exhausted phenotype induced by repeated low-dose and low-affinity stimulation, as seen in chronic infections and neoplastic processes88. Lastly, a subset of these effector cells are involved in long-term immunological memory. Memory T cells are primed to react more vigorously to the same antigen during a subsequent encounter, making them critical mediators of immune recall responses to pathogens and tumours290. Leveraging the power of technological advances in molecular biology, recent single-cell RNA sequencing and epigenomic studies have provided additional molecular insight into T cell fates and the corresponding features of immunotherapy-responsive T cells. These studies collectively implicate that complex transcriptomic, epigenomic and clonotypic changes of tumour-infiltrating T cells determine the success of immunotherapy291,292,293,294.
After the discovery of T cell co-stimulation mediated by the surface protein CD28 (Box 1), the search for additional immune regulators led to the identification of CTLA4, a receptor with structural and biochemical similarities to CD28, as a new immunoglobulin superfamily member9,10. The CTLA4 and CD28 genes are found in the same region of chromosome 2 (2q33.2) and are selectively expressed in the haematopoietic compartment11. However, in contrast to the high levels of basal CD28 expression on conventional T cells, CTLA4 is expressed at a low basal level and is strongly induced following antigen activation. Interestingly, CD4+CD25+ regulatory T (Treg) cells, which have an immunosuppressive function, express CTLA4 constitutively. Structurally, both CTLA4 and CD28 form membrane-bound homodimers comprising an extracellular immunoglobulin-like domain, a transmembrane region and a cytoplasmic tail capable of recruiting signalling proteins and controlling surface expression10,12,13. The trafficking of CTLA4-containing vesicles to the cell surface after activation is controlled by a physical interaction with the lipopolysaccharide-responsive and beige-like anchor protein (LRBA)13. The sequence similarity between CTLA4 and CD28 is highest within their extracellular binding domain and they therefore bind to the same ligands, called B7-1 (also known as CD80) and B7-2 (also known as CD86), which are expressed by antigen-presenting cells (APCs; Box 1). However, CTLA4 has greater affinity and avidity than CD28 for B7 ligands, representing a key difference in their biology14,15,16.
Before activation, antigen-presenting cells (APCs) load antigen onto MHC molecules to prepare for contact with a T cell that displays a cognate T cell receptor (TCR) while also providing necessary co-stimulatory ligands B7-1 and B7-2. The inhibitory molecule cytotoxic T lymphocyte antigen 4 (CTLA4) is contained within intracellular vesicles in naive T cells, whereas it is constitutively expressed on the cell surface of CD4+CD25+ regulatory T (Treg) cells. Both classes of T cells express the co-stimulatory receptor CD28. Early after activation, generally in the lymphoid tissue, T cells are activated when their TCRs bind to their cognate antigen presented by APCs in conjunction with CD28 binding to B7-1/B7-2. Also, the activated T cells begin the process of displaying CTLA4 on the cell surface. T cells within peripheral tissues upregulate PD1 at the mRNA level early after activation. Late after activation, in lymphoid tissue, CTLA4 expressed by activated T cells binds to the B7-1 and B7-2 molecules on APCs, thereby preventing their binding to CD28 and promoting anergy by decreasing the T cell activation state. At the same time, constitutive expression of CTLA4 on Treg cells leads to trans-endocytosis of B7 ligands and interferes with the CD28 co-stimulatory ability of APCs. Late after activation in peripheral tissues, PD1 is further upregulated transcriptionally, leading to greater surface expression of programmed cell death 1 (PD1), which binds to its ligands PDL1 and PDL2, thereby promoting T cell exhaustion at sites of infection or when confronted with neoplasms. Image courtesy of the National Institute of Allergy and Infectious Diseases.
The recognition of CTLA4 as a negative regulator of T cell activation gave rise to the idea that blocking its actions could unleash a therapeutic response of T cells against cancer45 (Fig. 3). James Allison and colleagues first tested this idea and demonstrated that neutralizing anti-CTLA4 antibodies enhanced antitumoural immunity in mice against transplanted and established colon carcinoma and fibrosarcoma46. In addition, during rechallenge, animals treated with anti-CTLA4 were able to rapidly eliminate tumour cells through immune mechanisms, providing evidence that blocking of CTLA4 induces long-lasting immunological memory46,47. Although CTLA4-targeted monotherapy was shown to confer benefit in animal models of brain48, ovarian49, bladder50, colon46, prostate47 and soft tissue46 cancers, less immunogenic cancers, including SM1 mammary carcinoma51 and B16 melanoma52, did not respond as favourably. Furthermore, heterogeneity between cancer models yielded discordant tissue-specific results45,53. In addition, a greater tumour burden correlated with reduced tumour responses to anti-CTLA4 treatment because larger tumours foster a more robust anti-inflammatory tumour microenvironment45,49.
Cytotoxic T lymphocyte antigen 4 (CTLA4)-blocking antibodies (α-CTLA4), especially when bound to an Fc receptor (FcR) on an antigen-presenting cell (APC), can promote antibody-dependent cellular cytotoxicity (ADCC). CD4+CD25+ regulatory T (Treg) cells express higher amounts of CTLA4 than conventional T cells and are therefore more prone to α-CTLA4-induced ADCC than conventional T cells. In addition, α-CTLA4 can bind to CTLA4 on the surface of the Treg cell and prevent it from counter-regulating the CD28-mediated co-stimulatory pathways that are playing a role in T cell activation. At the same time, α-CTLA4 can also promote T cell responses by blocking CTLA4 on the surface of conventional T cells as they undergo activation. TCR, T cell receptor. Adapted from ©2019 Fritz, J. M. & Lenardo, M. J. Originally published in J. Exp. Med. (ref.135).
PD1 restrains immune responses primarily through inhibitory intracellular signalling in effector T cells and Treg cells81. The immunoreceptor tyrosine-based switch motif and the immunoreceptor tyrosine-based inhibitory motif of PD1 are phosphorylated and recruit the phosphatases SHP1 and SHP2, which dephosphorylate, and thereby inactivate, downstream effectors (that is, the CD3 ζ-subunit and ZAP70) that are important for early T cell activation76 and CD28 signalling82. Both CTLA4 and PD1 inhibit protein kinase B (PKB; also known as AKT) signalling to reduce glucose uptake and utilization, the former through PP2A and the latter by reducing phosphoinositide 3-kinase (PI3K) activity83. In contrast to CTLA4, the PD1 axis is essential for controlling the continued activation and proliferation of differentiated effectors; when PD1 engages its ligands, it can induce a state of T cell dysfunction called T cell exhaustion84,85,86. However, what determines whether PD1 mediates exhaustion or apoptosis in certain contexts is still an active area of research. One model suggests that the interaction between PI3K signalling and the mitochondrial B cell lymphoma-extra large (BCL-XL) protein is a critical control point at which PD1-mediated P13K inhibition reduces BCL-XL and promotes apoptosis25,83. Beyond regulating conventional T cells, PDL1 on APCs can control Treg cell differentiation and suppressive activity87. Unfortunately, tumour cells can exploit this mechanism by upregulating PD1 ligands to induce T cell exhaustion and generate a tumour microenvironment that facilitates tumour growth and invasion88.
Activated T cells express programmed cell death 1 (PD1), which engages with its specific ligand (PDL1 or PDL2) to dampen activation. Blocking of the PD1 axis through the administration of an anti-PD1 (or anti-PDL1 or anti-PDL2) antibody prevents this inhibitory interaction and unleashes antitumoural T lymphocyte activity by promoting increased T cell activation and proliferation, by enhancing their effector functions and by supporting the formation of memory cells. Consequently, more T cells bind to tumour antigens presented on tumour cells by MHC molecules via their T cell receptors (TCRs). This ultimately leads to the release of cytolytic mediators, such as perforin and granzyme, causing enhanced tumour killing. APC, antigen-presenting cell. Adapted from ©2019 Fritz, J. M. & Lenardo, M. J. Originally published in J. Exp. Med. (ref.135). 2b1af7f3a8