MECHANISMS OF PMN ACTIVATION MEDIATED BY GMP-140 AND PAF

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The first step in migration of neutrophils (PMNs) from the blood stream to sites of inflammation is adhesion to vascular endothelial cells (ECs). An impairment in PMN-EC adhesion at sites of inflammation results in recurrent and severe infection such as occurs in individuals with genetic defects in PMN adhesion proteins, and in neonates, where the defect is poorly characterized. Alternatively, PMN adhesion to ECs may lead to vascular injury as in regional or global ischemia-reperfusion injury, necrotizing enterocolitis, or hyaline membrane disease with subsequent bronchopulmonary dysplasia. There are two mechanisms for this adhesive interaction: 1) Dependent on PMN activation by fluid-phase chemotactic factors; 2) Dependent on proadhesive molecules expressed by activated ECs. The more physiologically-relevant mechanism may be that dependent on EC activation because it targets PMNs in a spatially-specific way. ECs stimulated by rapidly-acting agonists express two proadhesive molecules on their surface, granule membrane protein-140 (GMP-140) and platelet activating factor (PAF). PAF expressed on the EC surface binds to its PMN receptor, activating the PMN and upregulating the CD11/CD18 integrins that mediate adhesion. The adhesive mechanism involves a novel example of juxtacrine cell-cell interaction in which cell-associated PAF acts as a signal that activates PMNs. GMP-140 has only recently been identified as a proadhesive molecules expressed on rapidly-activated ECs. Preliminary data indicates that its expression is essential for the PAF-mediated binding to occur and that GMP-140 enhances PAF-stimulated CD11/CD18-dependent adhesion. However, the effect of GMP-140 on PMNs appears to be complex and it may inhibit PMN-CD11/CD18 upregulation and other activation responses under some conditions. The functional consequences of binding of GMP-140 to its receptor on the PMN have not been defined. These alterations are important given the central role GMP-140 may play in the initial adhesion of PMNs to ECs at the site of inflammation. The current project has 4 specific objectives: 1) Characterize the alteration in PMN CD11/CD18 adhesive responses resulting from ligation of the GMP-140 receptor; 2) Determine the molecular mechanism(s) of GMP-140 potentiation of PAF-stimulated adhesion; 3) Characterize the alterations in PMN functional responses other than adhesion and aggregation that occur as a result of ligating the GMP-140 receptor; 4) Characterize the ability of neonatal PMNs to respond to mediators of adhesion expressed on activated ECs. The information obtained in the first three specific objectives will be applied to the last specific objective. By elucidating the normal EC-PMN adhesive mechanisms, new therapeutic strategies for treating diseases involving defective or unregulated PMN adhesion to EC may be fashioned. In addition, the work proposed will form the basis for a program that will lead to my development as a biomedical investigator.