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Background:  The relative importance of cell-associated and cell-free virus in sexual transmission of HIV-1 is unknown. Our lab has previously demonstrated that cell-associated transmission of HIV-1 by monocytes is the most efficient route of transmission across a model cervical epithelial monolayer and in a hu-PBL-SCID model of vaginal HIV-1 transmission. Antibody to intercellular adhesion molecule-1 (ICAM-1) has been shown to block transmission of cell-associated HIV-1 in both of these systems; in addition, it has been shown that the presence of ICAM-1 on the virion surface increases infectivity, and that antibody to ICAM-1 can reduce the efficiency of virus entry 100-fold.  Because of antibody to ICAM-1’s potential use as a microbicide, we have been developing a lactobacillus-based delivery system for in situ secretion of this antibody in the female genitourinary tract.  Bacteria can only express antibodies as single-chain variable regions (scFv), which are analogous in function to Fab molecules.  In order to determine the likely effectiveness of scFv, we compared the transmission- and cell-migration-blocking capabilities of monoclonal anti-ICAM-1 to the corresponding Fab’s and examined the ability of recently obtained scFv from lactobacilli to efficiently bind ICAM-1 on cell surfaces.

Methods:  Peripheral blood mononuclear cells (PBMC) infected with HIV-1 BaL were added to the apical side of confluent monolayers of HT-3 cervical epithelial cells grown on permeable transwell supports. After 24 hours, PBMC in the basal compartment were counted, and HIV transmission was measured by p24 ELISA on the basal supernatant (sup). scFv secreted into culture medium by growing transformed lactobacilli were evaluated for their ability to bind ICAM-1 expressing cells, as determined by flow cytometry. Data were analyzed using one-way analysis of variance with Bonferroni correction by the STATA statistical package.

Results:  Anti-ICAM Fab were capable of blocking HIV-1 transmission and migration of PBMC from infected cultures (p < 0.05 compared to untreated and isotype-control treated cells). scFv, both in purified form and in bacterial culture sup, efficiently bound cell-associated ICAM-1 on expressing cells even at a 10-fold dilution of the crude sup.

Conclusions:  These findings indicate that lactobacilli-expressing scFv specific for ICAM-1 have potential application as a microbicide to block cell-mediated HIV-1 sexual transmission.

Keywords: Microbicide; ICAM-1; Transmission