A protein found in human placentas blocks HIV infection in laboratory studies and may be what naturally protects most fetuses from acquiring HIV from their infected mothers, Newsday reports. A study, presented at the 8th Annual Conference on Retroviruses and Opportunistic Infections in Chicago, found in an analysis of 22 placentas that leukemia inhibitory factor (LIF) may be the naturally occurring protective ingredient that keeps more than two thirds of newborns from acquiring HIV from HIV-positive pregnant women, even in the absence of antiretroviral drugs. Researchers analyzed 22 placentas, 14 from HIV-positive women who gave birth to HIV-negative infants, five from infected women whose infants tested positive for the virus, and three from uninfected mothers. They found LIF present in all of the placentas, but levels of the protein were highest in HIV-positive women who gave birth to uninfected infants. The researchers also treated placental cultures with several strains of HIV and found that when they added minute quantities of LIF to the cultures, HIV replication was blocked.