Epithelial permeability induced by neutrophil transmigration is potentiated by hypoxia: role of intracellular cAMP

Mucosal tissues, such as the lung and intestine, are primary targets for ischemic damage. Under these conditions, neutrophil (polymorphonuclear leukocyte; PMN) infiltration into the protective epithelium has been implicated as a pathophysiologic mediator. Because PMN transepithelial migration results in increased paracellular permeability, and because our previous data revealed that epithelial hypoxia enhances PMN transmigration, we hypothesized that macromolecular permeability may be altered in epithelium exposed to hypoxia and reoxygenation (H/R) in the presence of PMNs. Human intestinal epithelia (T84) were grown on permeable supports, exposed to cellular hypoxia (pO2 20 torr) for 0-72 hr, and examined for increases in PMN-evoked permeability by using standard flux assays. Increasing epithelial hypoxia potentiated PMN-induced permeability of labeled paracellular tracers (size range 3-500 kD). Such increases were blocked by monoclonal antibody (mAb) to the PMN integrin CD11b (82 +/- 1% decreased compared with control mAb) and were partially blocked by anti-CD47 mAb (51 +/- 1%). Assessment of barrier recovery revealed that monolayers exposed to H/R were significantly diminished in their ability to reseal following PMN transmigration (recovery of 36 +/- 6% in H/R vs. 94 +/- 2% in normoxic controls). Because intracellular cyclic AMP (cAMP) has been demonstrated to regulate epithelial permeability, and because PMN-derived compound(s), (i.e., 5'-adenosine monophosphate; AMP) elevate epithelial cAMP, we examined the impact of hypoxia on epithelial cAMP responses. These experiments revealed that hypoxic epithelia were diminished in their ability to generate cAMP, and pharmacologic elevation (8-bromo-cAMP) of intracellular cAMP in hypoxic cells normalized both PMN-induced permeability changes and restoration of barrier function. These results support a role for PMN in increased intestinal permeability associated with reperfusion injury and imply a substantial role for cAMP signaling in maintenance of permeability during PMN transmigration.

Cryopreserved aortic homografts contain viable smooth muscle cells capable of expressing transplantation antigens

Frozen aortic tissue is increasingly used as homografts in reconstructive cardiovascular surgical procedures. The viability of cells within these cryopreserved tissues, their identity, and their potential immunogenicity have been the subject of controversy. We cultured cells from cryopreserved human aortic homografts that reacted with a monoclonal antibody that recognizes muscle actin isoforms, identifying them as smooth muscle cells. Under basal conditions, these smooth muscle cells contained messenger ribonucleic acid for class I human leukocyte antigens detected by northern blotting and expressed class I human leukocyte antigen on their surfaces as measured by enzyme-linked immunoassay and immunohistochemistry. Unstimulated smooth muscle cells contained no class II human leukocyte antigen messenger ribonucleic acid as determined by northern blotting and displayed almost no class II surface antigen as determined by enzyme-linked immunoassay. Interferon gamma (1000 U/ml, 72 hours), a product of activated T lymphocytes, not only increased the expression of class I human leukocyte antigens by smooth muscle cells, but induced class II human leukocyte antigen messenger ribonucleic acid and elevated surface expression from 22 +/- 7 to 819 +/- 35 enzyme-linked immunoassay units (n = 4). Immunohistochemistry revealed few class II-positive smooth muscle cells under basal culture conditions, but all cells showed high levels of DR antigen after exposure to interferon gamma for 3 days. Similar results were obtained in two independent isolates. We conclude that cryopreserved aortic homografts can contain viable smooth muscle cells capable of expressing major histocompatibility antigens that might render them immunogenic and susceptible to rejection by the recipient's immune system.

Regulation of major histocompatibility gene expression in human vascular smooth muscle cells

Human atheromata, but not normal blood vessels, contain numerous smooth muscle cells (SMC) that bear class II major histocompatibility (MHC) antigens. These lesions also contain leukocytes that can secrete cytokines, which may modulate SMC functions. Because of morphologic evidence for immune-activated (class II+) SMC in vascular lesions, we studied the regulation by cytokines of MHC gene expression in SMC cultured from human vessels. Under basal conditions, these SMC contained mRNA for class I MHC (detected by Northern blotting with a cDNA probe for HLA-B7) and expressed surface class I MHC product determined by enzyme-linked immunoassay with monoclonal antibody (MAb) W6/32. Unstimulated SMC contained little or no class II MHC mRNA (probed with HLA-DR alpha cDNA) or surface antigen (examined using MAb I2). Secretory products of activated human leukocytes (the cell-free supernatant of a mixed leukocyte reaction) induced class II MHC antigen expression by SMC after 3 days. Treatment of SMC with interferon (IFN)-alpha or -beta (1000 U/ml for 72 hours) increased class I MHC mRNA content and surface antigen but did not alter class II expression. Immune IFN (IFN-gamma), a leukocyte product known to induce class II MHC expression in classical antigen presenting cells as well as epithelial and endothelial cells, not only increased class I MHC expression by SMC but also induced substantial levels of class II MHC mRNA and surface antigen. IFN-gamma (ED50 approximately 10 U/ml) increased class II MHC mRNA maximally after 2 to 3 days and surface expression linearly from 1 to 4 days. Immunohistochemical study demonstrated few class II+ SMC in cultured human SMC under basal conditions but homogeneous expression of high levels of DR antigen after exposure to IFN-gamma for 3 days. Neither interleukin-1 (IL-1 alpha or beta), tumor necrosis factor alpha (TNF), nor endotoxin altered class II expression by SMC. Local secretion of IFN-gamma by activated leukocytes may account for the presence of HLA-DR+ SMC in the human atheroma. Immune activation of SMC might participate in the pathogenesis of vasculitis and arteriosclerosis, particularly in the form found in the coronary arteries of transplanted hearts.

Immune interferon inhibits proliferation and induces 2'-5'-oligoadenylate synthetase gene expression in human vascular smooth muscle cells

Proliferation of vascular smooth muscle cells (SMC) contributes to formation of the complicated human atherosclerotic plaque. These lesions also contain macrophages, known to secrete SMC mitogens, and T lymphocytes. Many of the SMC in the lesions express class II major histocompatibility antigens, an indication that activated T cells secrete immune IFN-gamma locally in the plaque. We therefore studied the effect of IFN-gamma on the proliferation of cultured SMC derived from adult human blood vessels. IFN-gamma (1,000 U/ml) reduced [3H]thymidine (TdR) incorporation into DNA by SMC stimulated with the well-defined mitogens IL 1 (from 15.3 +/- 0.7 to 6.2 +/- 0.7 dpm X 10(-3)/24 h) or platelet-derived growth factor (PDGF) (from 18.5 +/- 1.0 to 7.3 +/- 0.7 dpm X 10(-3)/24 h). Kinetic and nuclear labeling studies indicated that this effect of IFN-gamma was not due to altered thymidine transport or specific radioactivity of TdR in the cell. In longer term experiments (4-16 d) IFN-gamma prevented net DNA accumulation by SMC cultures stimulated by PDGF. IFN-gamma also delayed (from 30 to 60 min) the time to peak level of c-fos RNA in IL 1-treated SMC. It is unlikely that cytotoxicity caused these effects of IFN-gamma, as the inhibition of growth was reversible and we detected no cell death in SMC cultures exposed to this cytokine. Activation of 2'-5' oligoadenylate synthetase gene expression may mediate certain antiproliferative and antiviral effects of interferons. Both IFN-gamma and type I IFNs (IFN-alpha or IFN-beta) induced 2'-5' oligoadenylate synthetase mRNA and enzyme activity in SMC cultures, but with concentration dependence and time course that may not account for all of IFN-gamma's cytostatic effect on SMC. The accumulation of SMC in human atherosclerotic lesions is a long-term process that must involve altered balance between growth stimulatory and inhibitory factors. The cytostatic effect of IFN-gamma on human SMC demonstrated here may influence this balance during human atherogenesis, because T cells present in the complicated atherosclerotic plaque likely produce this cytokine.

Interleukin 1: a mitogen for human vascular smooth muscle cells that induces the release of growth-inhibitory prostanoids

There is much interest in defining the signals that initiate abnormal proliferation of cells in a variety of states characterized by the presence of mononuclear phagocytes. Since IL-1 is a major secretory product of activated human monocytes we examined whether this cytokine can stimulate the growth of human vascular smooth muscle cells (SMC). Neither recombinant IL-1 (rIL-1) alpha (less than or equal to 5.0 ng/ml) nor beta (less than or equal to 100 ng/ml) stimulated SMC growth during 2-d incubations under usual conditions. IL-1 did stimulate SMC to produce prostanoids such as PGE1 or PGE2 that can inhibit SMC proliferation. When prostaglandin synthesis was inhibited by indomethacin or aspirin both rIL-1 alpha and beta (greater than or equal to 1 ng/ml) markedly increased SMC growth. In longer-term experiments (7-28 d) rIL-1 stimulated the growth of SMC even in the absence of cyclooxygenase inhibitors. The addition of exogenous PGE1 or PGE2 (but not PGF1 alpha, PGF2 alpha, PGI2) to indomethacin-treated SMC blocked their mitogenic response to rIL-1. Antibody to IL-1 (but not to platelet-derived growth factor [PDGF]) abolished the mitogenic response of SMC to rIL-1. Exposure of SMC to rIL-1 or PDGF caused rapid (maximal at 1 h) and transient (baseline by 3 h) expression of the c-fos proto-oncogene, determined by Northern analysis. We conclude that IL-1 is a potent mitogen for human SMC. Endogenous prostanoid production simultaneously induced by IL-1 appears to antagonize this growth-promoting effect in the short term (2 d) but not during more prolonged exposures. IL-1 produced by activated monocytes at sites of tissue inflammation or injury may thus mediate both positive and negative effects on SMC proliferation that are temporally distinct.