[HIV].

In Japan, recently, an increasing number of heterosexual contacts have resulted in HIV infection and warns of an explosive outbreak in a few years. Both the United States and Thailand experienced such explosive outbreaks in the past, through contacts with a large number of intravenous drug users. As the results of a survey in Japan showed a low HIV positive rate among intravenous drug users, an explosive outbreak might be avoided in the future. Since recognition of AIDS in 1981, many valuable new observations have been made. HIV can infect a variety of human cells lacking the CD4 molecule, such as liver cells and some nerve cells. Some human T cells show differential susceptibility despite high expression of the CD4 molecule. Biological properties such as high replication and syncytium inducing isolates are associated with a decrease in CD4 cells. The precursor frequency of HIV-1 expressing cells was about 1/1000 and that of HIV-1 DNA was at least 1/100 CD4 cells in AIDS patients. Soluble CD4 analog binds up gp120 with an affinity and specificity comparable to intact CD4 and is capable of neutralizing the infectiousness of HIV-1. The resistance of HIV-1 to soluble CD4 neutralization is largely determined by the sequence of the V3 loop in gp120. Protection of chimpanzees from infection by HIV-1 after vaccination with recombinant gp120 and prevention of HIV infection by CD4 immunoadhesin were confirmed. Unexplained CD4 T-lymphocyte depletion without evidence of HIV infection has been noted since 1989 and epidemiologic and laboratory investigations are in progress.