Tissue-type plasminogen activator regulates macrophage activation and innate immunity

Abstract

Tissue-type plasminogen activator (tPA) is the major intravascular activator of fibrinolysis and a ligand for receptors involved in cell signaling. In cultured macrophages, tPA inhibits the response to lipopolysaccharide (LPS) by a pathway that apparently requires low-density lipoprotein receptor-related protein-1 (LRP1). Herein, we show that the mechanism by which tPA neutralizes LPS involves rapid reversal of IκBα phosphorylation. tPA independently induced transient IκBα phosphorylation and extracellular signal-regulated kinase 1/2 (ERK1/2) activation in macrophages; however, these events did not trigger inflammatory mediator expression. The tPA signaling response was distinguished from the signature of signaling events elicited by proinflammatory LRP1 ligands, such as receptor-associated protein (RAP), which included sustained IκBα phosphorylation and activation of all 3 MAP kinases (ERK1/2, c-Jun kinase, and p38 MAP kinase). Enzymatically active and inactive tPA demonstrated similar immune modulatory activity. Intravascular administration of enzymatically inactive tPA in mice blocked the toxicity of LPS. In mice not treated with exogenous tPA, the plasma concentration of endogenous tPA increased 3-fold in response to LPS, to 116 ± 15 pM, but remained below the approximate threshold for eliciting anti-inflammatory cell signaling in macrophages (∼2.0 nM). This threshold is readily achieved in patients when tPA is administered therapeutically for stroke. In addition to LRP1, we demonstrate that the -methyl-D-aspartic acid receptor (NMDA-R) is expressed by macrophages and essential for anti-inflammatory cell signaling and regulation of cytokine expression by tPA. The NMDA-R and Toll-like receptor-4 were not required for proinflammatory RAP signaling. By mediating the tPA response in macrophages, the NMDA-R provides a pathway by which the fibrinolysis system may regulate innate immunity.

Tissue-type plasminogen activator neutralizes LPS but not protease-activated receptor-mediated inflammatory responses to plasmin

“…In the absence of plasminogen, EI‐tPA (12 nM) reversed the effects of LPS (0.1 μg/mL) on expression of TNF‐α, IL‐1β, IL‐6, and CCL2 (Fig. B), confirming our earlier results . EI‐tPA also reversed the effects of LPS on expression of IL‐10 and IL‐1ra (Fig.…”

“…We previously demonstrated that BMDMs express the NMDA‐R, which serves as an essential receptor in the pathway by which tPA suppresses cytokine expression by BMDMs in response to LPS . We therefore tested whether the NMDA‐R is involved in the proinflammatory response to plasmin by treating BMDMs with the noncompetitive NMDA‐R antagonist, Dizocilpine/MK‐801 (1.0 μM).…”

“…Tissue‐type plasminogen activator (tPA) is one of the two major activators of fibrinolysis in humans and rodents . We previously demonstrated that in addition to its role in fibrinolysis, tPA suppresses innate immune system responses to LPS in bone marrow‐derived macrophages (BMDMs) and in vivo in mice . The identified pathway did not require tPA proteinase activity but instead, interaction of tPA with the N ‐methyl‐ d ‐aspartate receptor (NMDA‐R) in macrophages.…”

“…In order to examine the effects of tPA on the response to LPS independently of plasmin, in our previous study, we performed most of our experiments using enzymatically inactive (EI) tPA (EI‐tPA). Our results were replicated with enzymatically active, nonmutated tPA; however, we were careful to exclude sources of plasminogen.…”

Brentuximab: is it time for a new “B” in ABVD?

“…No unexpected toxicities were detected in the brentuximab vedotin plus AVD arm; only 2 of the patients relapsed, and both have since achieved second remission. 1 Although these phase 1/2 data are limited by the small number of patients (n 5 51), they have spurred significant interest in brentuximab vedotin plus AVD as front-line therapy for advanced Hodgkin lymphoma. Because of the potential of enhanced efficacy and the elimination of bleomycin-induced pulmonary toxicity, could brentuximab vedotin replace bleomycin in ABVD as the standard of care for these patients?…”