Indicadores de Gestión

PAF has been found to activate numerous inflammatory cells, including neutrophils, eosinophils, platelets, monocytes, T-lymphooytes, basophils and macrc^jhages as well as vascular endothelial cells and smooth muscle cells. In addition, PAF stimulates cells to release mediators that can activate other cells to release further mediators vîhich in turn activate more inflammatory cells.

IN VITRO

a) Neutrc^iiils

PAF has been shewn to induce adhesion of neutrophils to endothelial cells in culture (Ingraham et al, 1982; Valone & Goetzl,

1983). PAF induces human neutrc^Ail chemotaxis and chemokinesis (Czametzki & Benveniste, 1981) and stimulates neutrophil aggregation (Goetzl et al, 1980). Furthermore, PAF induces degranulation of both azurophil granules (Jouvin Marche et al, 1982) and specific granules (Dewald & Baggiolini, 1986) and stimulates the release of oxygen free radicals (Jouvin Marche et al, 1982).

Interaction with other neutrc^ahil stimulants has been r^»rted. Thus, PAF increases the su^)eroxide generation induced by IMLP

(Ingraham et al, 1982) as well as enhancing the chemotaxis produced by ECF-A (Czametzki, 1982). The latter effect is thou^t to be via the stimulation of KTB^ synthesis in the neutrophil by PAF (Lin et al, 1982) since the enhancement is abolished by a dual cyclooxygenase

and lipoxygenase inhibitor (BW755C) (Czametzki, 1982).

PAF also stimulates the synthesis of other products of arachidonic acid metabolism, such as 5-HETE, hydroxy and dihydroxy eicosatetraenoic acids (Ingraham et al, 1982).

b) Eosinc^iiils

PAF stimulates the adherence of human eosinc^diils to human vascular endothelial cells in concentrations as lew as

(Kimani et al, 1988; Lamas et al, 1988). Pretreatment with a monoclonal antibody directed against the Mac-1 complex reduced the PAF induced adherence suggesting that PAF acts at least in part by increasing the e)q)ression of these membrane surface receptors (Kimani et al, 1988).

PAF is the most potent chemotactic and chemokinetic factor for eosinophils r^xDrted to date inducing eosinophil movement at concentrations as lew as 10”% (Wardlaw et al, 1986; Czametzki & Posenbaoh, 1986; Taraura et al, 1987).

PAF will also induce degranulation of eosinqgjiils with the release of proteins such as eosinophil peroxidase (Kroegel et clL, 1988), glucuronidase and alkaline phosgjiatase from the specific granules and aryl sulphatase and acid phosphatase from the small granules (Kroegel et eil, 1989) as well as stimulating the release of oxygen free radicals (Bruijnzeel et al, 1986).

Finally, PAF has been shewn to induce the release of IITC^ from human eosinc^hils (Verhagen et al, 1984), vhilst other eosinophil chemotactic factors are ineffective in this regard (Bruijnzeel et al, 1987; Taraura et al, 1988).

PAF causes a greater activation of eosinophils from asthmatic patients than other atcpic patients (Chanez et al, 1990). PAF has been reported to activate eosinophils in vitro by inducing a change in the density of eosinophils from normodense to hypodense (KLoprogge et al, 1989) and preliminary investigations in man indicate that the inhalation of PAF induces a change in the profile of the eosinophil of normal volunteers from normodense to hypodense in vivo (B O'Connor, personal communication).

c) Platelets.

The ability of PAF to aggregate human platelets was first described by Benveniste et al (1975). It is the most potent platelet aggregating agent identified to date, with a threshold of 0.8 hM in washed human platelets (Valone et al, 1982) and 2 hM in human platelet rich plamsa (McManus et al, 1981a). PAF also induces the release reaction in platelets, the release of 5-HT (Henson & Cades, 1976), ATP (Vargaftig et al, 1980), PF^ (McManus et al, 1979) and thromboxane (Cazenave et al, 1979) have been described. Cydo-oxygenase products are released if platelets are stimulated with doses of PAF that are sufficient to cause irreversible aggregation (MacIntyre et al, 1982).

However, the release reaction is not a prerequisite for aggregation in response to PAF (Cazenave et al, 1979) and throamboxane release does not accompany the aggregation response (Vargaftig et al, 1980). The aggregatory and release reactions to PAF esdiibit selective desensitisation following exposure to PAF (Lalau-Keraly & Benveniste, 1982). Experiments using tritiated PAF have shewn that exposure of

platelets results in the internalisation of PAF and its prc^xDsed receptor thus reducing the effective number of sites available for binding further PAF (Valone et al, 1982).

d) Macrophages

PAF stimulates stç)eroxide generation in macrophages and has been shewn to enhance prostanoid release (Hartung et al, 1983). Furthermore, PAF has delayed effects on macrophages. Several grotçxs have demonstrated modulation of monocyte cytokine production (IL-1 and INF algha) by PAF (Barrett et al, 1987; Valone et al, 1988;Salem et al, 1990). A positive feedback locp is thus set up, since macrophages, endothelial cells and neutrcphils will synthesise PAF v^en stimulated with these cytokines in vitro (Caraussi et al, 1987;

Bussolino et al, 1986; Valone et al, 1988).

e) lymphocytes

Unlike other leucocytes, lyitçAiocytes are unable to generate PAF and the literature on the effect of PAF on lymphocytes is conflicting, althou^ this may be because mixed cell populations have been studied since Rola-Pleszczynski et al (1988) have recently demonstrated that PAF induces a decrease in CD 4+ T-cells accompanied by a marked increase in CD 8+ T cells.

IN VIVO

Ihe administration of PAF by i.v. injection results in peripheral blood neutropenia and thrombocytopenia in a variety of experimental animals including the guinea pig (Vargaftig et al, 1980; Demopoulos

et al, 1981), rakibit (McManus et al, 1980) and baboon (McManus et cil, 1981b). Similar changes in peripheral blood cells have been r^xDrted after i.p. administration of PAF in the guinea pig (Morley et al, 1988).

Ihe fall in peripheral blood leucocytes and platelets is accompanied ty the accumulation of neutrc^hils, eosinophils and platelets in the lungs. By using a technique for continuously monitoring radio-labelled platelets and neutrophils in vivo, the transient nature of the accumulation of both platelets and neutrophils in the thoracic region of ejç)erimental animals following the i.v. adminsitration of 10-100 ng PAF could be examined. The response was dose related, and the kinetics varied with dose, with maximal accumulation occurring between 18 and 48 seconds and returning to baseline within 1-2 to > 10 minutes with increasing doses of PAF (Page et al, 1984). Furthermore, both the platelet and neutrc^hil accumulation in response to i.v. PAF can be potentiated by pretreatment with the NO synthesis inhibitor, L-NAME, suggesting that NO derived from the pulmonary circulation physiologically regulates the extent of cell trapping within the lung vasculature (May et cil,

1991).

The kinetics of cell accumulation are in agreement with the time course of the thrombocytopenia observed following intravenous PAF and with the histological studies which have confirmed platelet accumulation in the pxilmonary vasculature following PAF treatment. Aggregates of platelets and neutrqpAiils were noted within one minute of PAF injection (Dewar et al, 1984) and recently evidence of

platelet diapedesis and extravasation has been observed histologically following systemic administration of PAF (Lellouch Tubiana et al, 1985, 1988).

Both histological studies and the analysis of cells collected in BAL fluid after PAF challenge have confirmed the emigration of eosinophils into the lungs of e5^)erimental animals after i.v.

(Lellouch-Tubiana et al, 1985), i.t. (Amoux et al, 1988; Denjean et al, 1988) and inhalation challenge (Coyle et al, 1988a, 1990b; Takizawa et al, 1988; Sanjar et al, 1990a; Nieminen et al, 1991b; Seeds et al, 1991; Spina et al, 1991).

Injection of PAF into the skin of experimental animals (i.d.) results in intravascular neutrx^jhil accumulation within 15 minutes and these cells emigrate to the extravascular space over a period of hours. Aggregates of platelets have also been noted (Dewar et al, 1984; Huitçtoey et al, 1984).

In man, peripheral blood thrombocytopenia was noted in a preliminary study in subjects presenting with cerebral death follcwing i.t. PAF administration (Gateau et al, 1984). Hcwever, thrombocytopenia has not been noted in studies in normal volunteers follcwing the inhalation of PAF, althou^ platelet activation may accompany PAF inhalation, since von Willebrand Factor (vWF) ej^ression by platelets is induced (Wilson et al, 1990).

Transient but profound neutropenia has been reported follcwing the inhalation of PAF (Kioumis et al, 1988; Chung et al, 1988, 1989 a & b; Chung & Bames, 1989; Spencer et al, 1990) and this is acccmpanied

by the emigration of neutrophils into the lungs, as evidenced hy increased cell numbers in BAL fluid (Wardlaw et al, 1990).

I.d. injection of PAF in human skin results in the infiltration