Roles of the tissue-type plasminogen activator in immune response

Alteplase (Actilyse or Activase) is the gold standard acute treatment of ischemic stroke1 and receives attention for hematoma resolution in hemorrhagic stroke. The active principle of Alteplase is the recombinant form of an endogenous protease, tissue-type plasminogen activator (tPA). Its intravascular thrombolytic activity is well known, but less are its multifaceted functions in the central nervous system (CNS). Endogenous tPA is not only released in the blood by endothelial cells but also expressed by many cells within the brain parenchyma (online-only Data Supplement), and it can act on all cell types of the brain virtually. Endogenous tPA has been involved in an ever-increasing number of brain functions, of which several are highly relevant during and after stroke. Importantly, knowledge on the mechanisms of action of endogenous tPA may hold true for recombinant tPA. In this review, we provide an up-to-date overview of the current knowledge on the enzymatic or cytokine-like effects of action of tPA in the CNS, its various molecular substrates or receptors, focusing on the processes occurring during and after ischemic or hemorrhagic stroke, including excitotoxicity, apoptosis, blood–brain barrier breakdown, inflammation, axonal damage, and demyelination. tPA is a mosaic protease of 527 amino acids consisting of 5 distinct modules: a Finger domain, an epidermal growth factor (EGF)–like domain, 2 kringle domains (K1 and K2), and a serine protease proteolytic domain. Through these domains, tPA can interact with a variety of binding proteins and receptors in the brain parenchyma, thus extending its functions above the conversion of plasminogen into plasmin (Figure 1). Figure 1. Main cellular and molecular targets of tissue-type plasminogen activator in the central nervous system. BBB indicates blood–brain barrier, EGFR, epidermal growth factor receptor; LRP, low-density lipoprotein receptor–related protein; NMDAR, N-methyl-d-aspartate receptors; and PDGFR, platelet-derived growth factor receptor. tPA is not fibrinolytic by itself. …

Aprominent pathology textbook used in the United States includes an image illustrating the renal histopathology caused by malignant hypertension. The legend describes striking “onion skin” changes of a renal arteriole. Curiously, a sea of mononuclear inflammatory cells surrounding this arteriole is overlooked both in the legend and in the related text. Moreover, nothing regarding inflammation or immune reactions is discussed. This lack of attention to inflammatory cells is, however, not surprising. Although many experimental studies have implicated inflammation in hypertension, these have largely been performed in experimental animals; there is no proof that inflammation contributes to human hypertension. In fact, some anti-inflammatory or immune-suppressing drugs (eg, nonsteroidal anti-inflammatory drugs and cyclosporine) paradoxically cause hypertension in humans, likely via off-target effects. Often the term “inflammation” is used in the context of cardiovascular disease as a catchall referring to nonspecific phenomena, such as elevation of C-reactive protein or the presence of macrophages in a tissue. Most clinicians and investigators find this vague and difficult to understand. Even more puzzling is that many studies now implicate the adaptive immune response, and in particular, lymphocytes, in hypertension and vascular disease. Traditionally, bacterial, viral, or tumor antigens activate this arm of immune defense. As such, it has been hard to imagine how adaptive immunity could be involved in a disease such as hypertension. In this article, we will attempt to address some of these puzzling questions. We will briefly review components of the innate and adaptive immune response, discuss data from many groups, including our own, that suggest that common forms of hypertension are immune mediated, and provide a working hypothesis of how signals from the central nervous system trigger an immune response that causes hypertension. ### General Concepts Regarding Inflammation and Immunity #### Innate Immunity The first line of defense against pathogens is the innate immune response. Important components of this system include epithelial …

Strategies to Modulate Immune Responses: A New Frontier for Gene Therapy

This paper presents a study of the flow of ice in wedge-shaped converging channels. Such flows are encountered in the relatively constricted waters of the Canadian Arctic Archipelago. Ridging, lead opening patterns, development of a highpressure area, and arch formation are some of the processes which take place during ice flow through converging channels. An idealized geometry and steady wind forcing were used in the testing. The results give ice cover velocity, distribution of stresses, ice thickness, area coverage and ridging. Some of the conditions leading to arch formation at the constricted exit of the channel are explored.

Genetically Programmed Biases in Th1 and Th2 Immune Responses Modulate Atherogenesis

IFN-α Enhances Poly-IC Responses in Human Keratinocytes by Inducing Expression of Cytosolic Innate RNA Receptors: Relevance for Psoriasis

Mucosal sites represent the primary routes of HIV transmission, yet the oral mucosa appears uniquely resistant to HIV-1 infection. Since the oral cavity is not conducive to either infection or transmission of HIV, characterizing the responsible resistance factors, particularly components of innate immunity, and their mechanisms of action may identify new opportunities for interference with viral acquisition at other mucosal sites. Although many of the innate immune factors described to date exhibit both antibacterial and antiviral activities, molecules that inhibit HIV-1 are largely HIV-1 specific. Nonetheless, if these oral innate factors can be identified, characterized, and harnessed, they may be useful in protection and/or intervention in acute HIV-1 infection. Endogenous molecules acting independently or synergistically, given their lack of substantive side effects, may represent an alternative novel approach for reducing HIV risk and suppressing infection. While HIV infection via breast milk does involve oral trafficking and is an important area of study, it will only be briefly discussed here. The major emphasis of this article is on the oral cavity and salivary substances that can influence HIV infection Epidemiology of oral transmission of HIV More than twenty-five years into the global HIV/AIDS epidemic, it is well established that in addition to blood, the reproductive and/or rectal mucosae represent the primary portals of HIV entry in adults. There is less definitive evidence regarding whether other mucosal sites serve as viral conduits and in particular, uncertainty remains as to the possibility of transmission of HIV-1 via the oral entrance to the gastrointestinal (GI) tract. This is not a trivial concern, since despite the successes of combination therapy for HIV/AIDS, infection rates remain high, compounded by persistence of latent viral reservoirs, antiretroviral drug resistance, and the lack of an effective vaccine or potential cure. Although it is often accepted that oral transmission is of little or no consequence, we have failed to capitalize on that information to identify the mechanism(s) of protection at the oral mucosal surface. These mechanisms could provide clues for novel strategies of prevention/intervention at other more vulnerable viral entry sites. Early in the epidemic, evidence of the presence of HIV-1 in the oral cavity suggested the possibility of transmissibility via this route [1, 2]. Whereas in some early studies [3–7], oral transmission could not be established, a few studies implicated this route as a potential, even likely mode of transmission (e.g., [8]). In larger cohorts of individuals and in well-designed studies to monitor oral transmission, the rarity of HIV-1 infection by this route was substantiated [9]. Nonetheless, potential shortcomings in these studies, including accurately documenting the definitive route of infection, underscore our lack of certainty regarding the relative risk of oral transmission. (Reviews: [10, 11]). The low frequency of documented oral transmission of HIV-1 could reflect low titers of infectious virus in saliva. While high levels of HIV-1 RNA and antigens can routinely be measured in oral fluids, infectious virus was not detected in early studies attempting to isolate infectious virus from saliva [12, 13] or more recently in large cohort studies, despite plasma viremia [14, 15].

Accessory cells unrelated to mononuclear phagocytes and not of bone marrow origin

The chemokine system: tuning and shaping by regulation of receptor expression and coupling in polarized responses.

Who am I? (re-)Defining fibroblast identity and immunological function in the age of bioinformatics.

4. Gastrointestinal mucosal immunity

Antigen processing and presentation in teleost immune responses

Plasmin Is Essential in Preventing Periodontitis in Mice

Neutrophil infiltration of the postischemic tissue considerably contributes to organ dysfunction on ischemia/reperfusion injury. Beyond its established role in fibrinolysis, tissue-type plasminogen activator (tPA) has recently been implicated in nonfibrinolytic processes. The role of this serine protease in the recruitment process of neutrophils remains largely obscure. Using in vivo microscopy on the postischemic cremaster muscle, neutrophil recruitment and microvascular leakage, but not fibrinogen deposition at the vessel wall, were significantly diminished in tPA(-/-) mice. Using cell transfer techniques, leukocyte and nonleukocyte tPA were found to mediate ischemia/reperfusion-elicited neutrophil responses. Intrascrotal but not intra-arterial application of recombinant tPA induced a dose-dependent increase in the recruitment of neutrophils, which was significantly higher compared with stimulation with a tPA mutant lacking catalytic activity. Whereas tPA-dependent transmigration of neutrophils was selectively reduced on the inhibition of plasmin or gelatinases, neutrophil intravascular adherence was significantly diminished on the blockade of mast cell activation or lipid mediator synthesis. Moreover, stimulation with tPA caused a significant elevation in the leakage of fluorescein isothiocyanate dextran to the perivascular tissue, which was completely abolished on neutrophil depletion. In vitro, tPA-elicited macromolecular leakage of endothelial cell layers was abrogated on the inhibition of its proteolytic activity. Endogenously released tPA promotes neutrophil transmigration to reperfused tissue via proteolytic activation of plasmin and gelatinases. As a consequence, tPA on transmigrating neutrophils disrupts endothelial junctions allowing circulating tPA to extravasate to the perivascular tissue, which, in turn, amplifies neutrophil recruitment through the activation of mast cells and release of lipid mediators.

In this study, we investigated the role of Ca2+ and G proteins in thrombin-induced acute release (regulated secretion) of tissue-type plasminogen activator (TPA) and von Willebrand factor (vWF), using a previously described system of primary human umbilical vein endothelial cells (HUVECs). The acute release of TPA and vWF, as induced by alpha-thrombin, was almost zero after chelation of Ca2+i, showing that an increase in [Ca2+]i was required. It did not matter whether the increase in [Ca2+]i came from an intracellular or extracellular Ca2+ source. Thrombin-induced release of TPA and vWF already started at low [Ca2+]i, around 100 nmol/L. Half-maximal release was found at a [Ca2+]i, of 261 nmol/L for TPA and at 222 nmol/L for vWF. The Ca2+ signal was transduced to calmodulin, as calmodulin inhibitors inhibited TPA and vWF release. The Ca2+ ionophore ionomycin dose dependently released vWF; half-maximal vWF release occurred at a [Ca2+]i of 311 nmol/L. In contrast, no TPA release was found at all below a [Ca2+]i of 500 nmol/L. Thus, below 500 nmol/L [Ca2+]i, an increase in [Ca2+]i alone was sufficient to induce vWF release but not sufficient to induce TPA release. Protein kinase C did not appear to be involved in TPA or vWF release, as neither an activator nor an inhibitor of protein kinase C significantly influenced release. Inhibition of phospholipase A2 also did not reduce thrombin-induced TPA and vWF release. The involvement of G proteins was studied by using both saponin-permeabilized and intact cells. GDP-beta-S, which inhibits heterotrimeric and small G proteins, significantly inhibited thrombin-induced vWF and TPA release from permeabilized cells. AlF-4, which activates heterotrimeric G proteins, induced TPA and vWF release in both intact and permeabilized HUVECs. Preincubation of HUVECs with pertussis toxin significantly inhibited thrombin-induced vWF release, due to inhibition of thrombin-induced Ca2+ influx. Pertussis toxin did not affect ionomycin-induced release. The inhibitory effect of pertussis toxin was less obvious in thrombin-induced TPA release, because it was counterbalanced by a positive effect of the toxin on TPA release. Thus, both inhibitory and stimulatory (pertussis toxin-sensitive) G proteins were involved in TPA release. Therefore, thrombin-induced acute release of TPA and vWF differed in two respects. First, below a [Ca2+]i of 500 nmol/L, an increase in Ca2+ was sufficient for vWF release but not for TPA release. Second, pertussis toxin-sensitive G proteins were differentially involved in acute TPA and vWF release.