C5A destabilizes HIV particles. (a) NL4.3 virus (20 ng of p24) was incubated with or without C5A for 30 min at 37°C and loaded over a sucrose density gradient (see Materials and Methods). Each gradient fraction was analyzed for capsid by p24 ELISA (Top) and immunoblot, RT by exoRT assay (Middle), and gp41 content by immunoblot. (b) Same as a except that the virus was treated with decreasing concentrations of C5A for 30 min at 37°C (Upper Left), with C5A for 15, 30, and 60 min at 37°C (Lower Left), for 30 min at 4, 25, or 37°C (Upper Right), and for 30 min at pH 8, 7, 6, and 5 (Lower Right). Gradient fractions were analyzed for HIV capsid content by p24 ELISA. (c) Same as a except that the virus was treated either with C5A or control peptide. (d) Same as a except that the virus was first trypsinized for 15 min at 37°C, incubated in 10% FCS to neutralize trypsin, microcentrifuged for 90 min at 4°C, resuspended, and loaded over a sucrose gradient for virus integrity evaluation by p24 ELISA.