CD69 and regulation of the immune function.

CD69, also known as activation inducer molecule, very early activation antigen, MLR-3 and Leu-23, is a member of the natural killer (NK) cell gene complex family of signal transducing receptors. CD69 is as a type II transmembrane glycoprotein with a C-type lectin binding domain in the extracellular portion of the molecule. CD69 expression is induced in vitro on cells of most hematopoietic lineages, including T and B lymphocytes, NK cells, murine macrophages, neutrophils and eosinophils, while it is constitutively expressed on human monocytes, platelets and epidermal Langerhans cells. Although a specific ligand for CD69 has not been identified, its wide cellular distribution and the induction of intracellular signals upon CD69 crosslinking suggest a role for the receptor in the biology of hematopoietic cells. Moreover, certain results indicate that CD69 may be involved in the pathogenesis of such diseases as rheumatoid arthritis, chronic inflammatory liver diseases, mild asthma, and acquired immunodeficiency syndrome.

Nitric oxide production in experimental gram-negative infection: studies with cytokine receptor-deficient mice.

Overproduction of nitric oxide (NO) upon expression of inducible NO synthase (iNOS) may be responsible for refractory hypotension in septic shock. Whereas high levels of NOS activity have been documented in experimental models of endotoxemia or intravenous challenge with Escherichia coil, much less is known concerning tissue models of Gram-negative infection. We examined NO production (measured as the accumulation of plasma NO3- + NO2-) in a murine model of Gram-negative peritonitis. Plasma NO3- + NO2- increased progressively from 25 microM to peak levels of 50-150 microM 24 h after intraperitoneal challenge with E. coli 0111:B4, similar to values reported for septic shock patients. Treatment of infected mice with NG-monomethyl-L-arginine, an inhibitor of NOS activity, resulted in the efficient inhibition of NO3- + NO2- production. In order to evaluate the roles of interferon-gamma (IFN-gamma) and tumor necrosis factor (TNF-alpha) in the induction of NO synthesis in murine peritonitis, mice deficient in the respective cytokine receptors were studied. In control in vitro experiments, macrophages from IFN-gammaR- or TNFR55-deficient mice, while failing to respond to IFN-gamma or TNF-alpha, respectively, produced high levels of NO under appropriate stimulation. When challenged intraperitoneally with E. coli, IFN-gammaR- or TNFR55-deficient mice exhibited similar levels of bacteremia and NO production as their wild-type controls. These data thus suggest that enhanced NO production during focal Gram-negative infection may occur in the absence of signaling through either IFN-gammaR or TNFR55.

Competition between bactericidal/permeability-increasing protein and lipopolysaccharide-binding protein for lipopolysaccharide binding to monocytes.

The bactericidal/permeability-increasing protein (BPI) inhibits the lipopolysaccharide (LPS)-mediated activation of monocytes. Due to its inhibitory activity for various LPS, BPI has therapeutic potential in endotoxic shock. To be efficient in vivo, BPI should overcome the action of LPS-binding protein (LBP), a serum molecule that increases the expression of LPS-inducible genes via CD14 of monocytes, rBPI23, a recombinant fragment of BPI, prevented in a dose-dependent manner the binding and the internalization of LPS mediated by LBP. Consequently, rBPI23 also inhibited LPS-induced tumor necrosis factor (TNF alpha) synthesis from monocytes. LPS- and LBP-mediated activation of monocytes was totally inhibited when LPS was preincubated with rBPI23. Adding rBPI23 at the same time as LBP resulted in an important but partial inhibition of TNF alpha release, but this inhibition vanished with delaying the time of addition of rBPI23. These studies suggest that the inhibitory activity of BPI is related to its ability to compete with LBP for LPS.

Differential effects of prostaglandins on macrophage activation induced by calcium ionophore A23187 or IFN-gamma.

Calcium ionophore A23187 can mimic IFN-gamma-induced macrophage activation for intracellular Leishmania killing and secretion of L-arginine-derived nitrite. Because the effects of ionophore are not restricted to calcium mobilization but also involve alterations of phospholipid metabolism, we have examined the role of PGE2 in the activation process. Macrophages exposed to A23187 or IFN-gamma in the presence of LPS and FCS secreted significant amounts of PGE2 independently of the presence of L-arginine in the incubation medium. The addition of the cyclooxygenase inhibitor indomethacin or omission of FCS abrogated PGE2 secretion but had little effect on nitrite production or intracellular killing. The addition of exogenous PGE2, of agents increasing PGE2 production such as arachidonic acid and colchicine, or of an analogue of cAMP, dibutyryl cAMP inhibited A23187 + LPS-induced activation whereas that mediated by IFN-gamma + LPS remained unimpaired. Our results indicate that PGE2 can modulate activation induced by A23187 but not by IFN-gamma, probably by a process involving cAMP. Conceivably, ionophore can mimic IFN-gamma for the induction of activation but lacks the capacity to help maintain the activated state because of its inability to desensitize macrophages to negative regulation by PGE2, as suggested previously for IFN-gamma-dependent activation.

3-amino-1,2,4-triazole inhibits macrophage NO synthase.

Murine macrophages activated by interferon-gamma and lipopolysaccharide become leishmanicidal through a process involving L-arginine-derived nitrogen oxidation products. Both nitrite secretion and parasite killing by activated macrophages were inhibited by 3-amino-1,2,4-triazole as well as the related compound, 3-amino-1,2,4-triazine. Moreover, NO synthase activity in cytosolic extracts of activated cells was inhibited by both compounds. 4-amino-1,2,4-triazole, an isomer of 3-amino-1,2,4-triazole, was without effect. Our results suggest that besides its known inhibitory effect on catalases and peroxidases, 3-amino-1,2,4-triazole is an inhibitor of NO synthase. The resemblance between the tautomeric form of 3-amino-1,2,4-triazole and the guanidino group of L-arginine, the natural substrate for NO synthase, might be responsible for the observed inhibition.