Inhibition of NL4-3 and gp120-induced syncytium formation. (a-c) Inhibition of virus induced syncytium formation. GHOST-CXCR4 cells were infected with 1000 TCID of the HIV laboratory isolates NL4-3. Infection was inhibited by adding mHSA (final concentration 50 μg/ml). Cell cultures were monitored 5 days after infection by visualization of induced syncytia and cell layer destruction after formalin fixation followed by a standard staining procedure (Hemacolor®, Merck). (d-f) To study inhibition of syncytia formation following transfection with pSVATGrev, cells were transfected with the expression plasmid and mHSA was added 30 min later. Syncytia were monitored after 28 h by standard phase contrast microscopy.