To study inhibition, virus was given to the cells after preincubation of cells with modified serum albumin for 30 min. usage. Viral infection was monitored by a standard p24 or X-gal staining assay. Our data demonstrate that HOCl-modified mouse-, bovine- and human serum albumins all bind to the HIV-1 NL4-3 gp120 (LAV) glycoprotein in contrast to non-modified albumin. Binding of HOCl-modified albumin to gp120 correlated to the blockade of CD4 as well as that of V3 loop specific monoclonal antibody binding. In neutralization experiments, HOCl-modified serum albumins inhibited replication and syncytium formation Nadolol of the X4- and R5-tropic NL4-3 isolates in a dose dependent manner. Conclusions Our data indicate that HOCl-modified serum albumin veils the binding site for CD4 and the V3 loop on gp120. Such masking of the viral gp120/gp41 envelope complex might be a simple but promising strategy to inactivate HIV-1 and therefore prevent infection when HOCl-modified serum albumin is applied, for example, as a topical microbicide. Background An important event in HIV-1 infection is the step-by-step binding of the external envelope complex, the gp120/gp41 trimer, to (i) CD4 [1] and to (ii) a family of seven-transmembrane chemokine receptors including CXCR4 and CCR5 which are the two major coreceptors [2] on the cellular membrane as Rabbit Polyclonal to APC1 well as to (iii) heparan sulfate structures [3]. HIV-1 entry can therefore be inhibited by heparan sulfate [4], its analogs [5] or other synthetic polyanions [6], ligands for CD4 Nadolol [7], CXCR4 [8] or CCR5 [9], and by factors that bind to gp120 like neutralizing antibodies [10]. Since HIV-1 variability is prodigious, viral escape to all these antiviral substances has been documented and therefore there is a pressing need to find new strategies to efficiently block HIV-1 infection. In 1992, Klebanoff and coworkers [11] showed that stimulated polymorphonuclear (PMN) cells released an unknown factor which neutralized HIV-1. PMN from patients with hereditary myeloperoxidase (MPO) deficiency indicated that the antiviral activity was correlated with the presence and the release of the enzyme into cell culture medium. Adding MPO to PMN-(MPO-deficient) cell cultures restored the ability to block HIV-1 infection [11,12]. In addition to MPO, the presence of two other factors, H2O2 and chloride, was absolutely necessary to observe the antiviral activity in cell culture supernatants of stimulated PMN cells. Since the enzyme MPO catalyzes the reaction between H2O2 and chloride to generate HOCl (bleach), Klebanoff and coworkers [12] suggested that this product of the MPO/H2O2/halide system was directly responsible for HIV inactivation. A clue as to the substrate of the MPO/H2O2/halide system was provided by the Nadolol detection of HOCl-modified proteins in human tissue by a specific monoclonal antibody (clone 2D10G9) [13]. This antibody recognized an HOCl-modified protein in glomerular and tubulointerstitial inflammatory and fibrotic lesions and its binding was inhibited by HOCl-modified human serum albumin (mHSA) [14,15]. Recently we demonstrated that mHSA was active against West Nile virus (WNV) [16]. Low doses of HOCl-modified human serum albumin (mHSA) inactivated WNV entry into VeroB4 cells em in vitro /em and Nadolol we observed an interaction between mHSA and the domain III of the WNV external envelope. Similar to WNV, HIV-1 also enters its target cell following membrane fusion of the viral envelope and the plasma membrane and it is conceivable that enveloped viruses share common functions to promote membrane-membrane contact to allow binding of cell membrane-anchored receptors. Here we report, that mHSA, mBSA (bovine) and mMSA (mouse) bind to recombinant HIV-1 gp120 of the laboratory strain NL4-3. Binding of HOCl-modified serum albumins to gp120 also inhibited binding of recombinant CD4 (rCD4) and V3 loop-specific antibodies. All three HOCl-modified serum albumins inhibited viral infection and syncytium formation in a dose dependent manner. Method Viruses, Cell Lines, Env expression plasmids, and HIV Reagents HIV-1 strain NL4-3, an X4-monotropic laboratory isolate, was used for virus inhibition tests. HeLa-P4 cells expressing human CD4, CXCR4 were kindly provided by Matthias T..