This project seeks to understand the molecular interactions of Haemophilus decreyi with human neutrophils (PMN) and host epithelial cells during the initial stages of infection. Attachment to, and internalization of, pathogens by host cells is often the first step in the infectious process. In vitro, H. ducreyi is bound and internalized by PMN in the absence of serum opsonins and by epithelial cells suggesting a ligand (adhesin)/receptor interaction(s). Immunological intervention early in the infectious process could modify or prevent cellular attachment and possibly disease. PMNs are the first cells recruited at the onset of chancroidal lesion and therefore might be an important defense against infection. These studies will enhance our overall understanding of the role of attachment to epithelial cells and PMN in the infectious process, the ability of PMN to kill H. ducreyi, and perhaps, lead to new vaccine strategies. There are five Specific Aims. In the first Specific Aim, an isogenic mutant of the 30 kDa outer membrane protein will be constructed. As described in detail in the following sections, the 30 kDa protein is related to cellular adhesins and proteins which mediate serum resistance in other bacteria. The second Specific aim is to determine the function of this protein. We will use mutant/parent comparisons to determine its possible role in adherence to PMN, epithelial cells, and cellular matrix proteins,. We will also perform 30 kDa mutant/parent comparisons for resistance to serum and phagocytic killing. In the third Specific aim, we will test an isogenic mutant of the H. ducreyi 30 kDa protein in the temperature dependent rabbit model of infection and in the human experimental model of H. ducreyi infection. The fourth Specific aim addresses certain vaccine related issues. We will determine if antibodies to the 30 kDa protein are elicited during chancroid infection to document in vivo expression. We will test the ability of purified recombinantly expressed 30 kDa to protect animals from a challenge infection. In Specific Aim 5 we will identify PMN and cellular adhesins other than the 30 kDa protein.