Histamine is a mediator of allergic inflammation released mainly from mast cells. Although histamine strongly increases vascular permeability, its precise mechanism under in vivo situation remains unknown. We here attempted to reveal how histamine induces vascular hyperpermeability focusing on the key regulators of vascular permeability, blood flow and endothelial barrier. Degranulation of mast cells by antigen-stimulation or histamine treatment induced vascular hyperpermeability and tissue swelling in mouse ears. These were abolished by histamine H1 receptor antagonism. Intravital imaging showed that histamine dilated vasculature, increased blood flow, while it induced hyperpermeability in venula. Whole-mount staining showed that histamine disrupted endothelial barrier formation of venula indicated by changes in vascular endothelial cadherin (VE-cadherin) localization at endothelial cell junction. Inhibition of nitric oxide synthesis (NOS) by L-NAME or vasoconstriction by phenylephrine strongly inhibited the histamine-induced blood flow increase and hyperpermeability without changing the VE-cadherin localization. In vitro, measurements of trans-endothelial electrical resistance of human dermal microvascular endothelial cells (HDMECs) showed that histamine disrupted endothelial barrier. Inhibition of protein kinase C (PKC) or Rho-associated protein kinase (ROCK), NOS attenuated the histamine-induced barrier disruption. These observations suggested that histamine increases vascular permeability mainly by nitric oxide (NO)-dependent vascular dilation and subsequent blood flow increase and maybe partially by PKC/ROCK/NO-dependent endothelial barrier disruption.

We investigated the role of prostaglandin D 2 (PGD 2 ) signaling in acute lung injury (ALI), focusing on its producer-effector interaction in vivo. Administration of endotoxin increased edema and neutrophil infiltration in the WT mouse lung. Gene disruption of hematopoietic PGD synthase (H-PGDS) aggravated all of the symptoms. Experiments involving bone marrow transplantation between WT and H-PGDS-deficient mice showed that PGD 2 derived from alveolar nonhematopoietic lineage cells (i.e., endothelial cells and epithelial cells) promotes vascular barrier function during the early phase (day 1), whereas neutrophil-derived PGD 2 attenuates its own infiltration and cytokine expression during the later phase (day 3) of ALI. Treatment with either an agonist to the PGD 2 receptor, DP, or a degradation product of PGD 2 , 15-deoxy-Δ 12,14 -PGJ 2 , exerted a therapeutic action against ALI. Data obtained from bone marrow transplantation between WT and DP-deficient mice suggest that the DP signal in alveolar endothelial cells is crucial for the anti-inflammatory reactions of PGD 2 . In vitro, DP agonism directly enhanced endothelial barrier formation, and 15-deoxy-Δ 12,14 -PGJ 2 attenuated both neutrophil migration and cytokine expression. These observations indicate that the PGD 2 signaling between alveolar endothelial/ epithelial cells and infiltrating neutrophils provides anti-inflammatory effects in ALI, and suggest the therapeutic potential of these signaling enhancements. vascular permeability | pneumonia | lipid mediator | respiratory infection

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Graphical AbstractHighlights d Alanine is an essential amino acid for T cells to exit quiescence d Alanine deprivation during early activation leads to functional impairment d T cells do not catabolize alanine during early activation d Extracellular alanine supports protein synthesis