Why is HIV rarely transmitted by oral secretions? Saliva can disrupt orally shed, infected leukocytes

Background: Oral transmission of human immunodeficiency virus (HIV) by the millions of HIV-infected individuals is a rare event, even when infected bl ood and exudate is present. Saliva of viremic individuals usually contains only noninfectious components of HIV indicating virus breakdown. Objective: To determine whether unknown HIV inhibitory mechanisms may expla in the almost complete absence of infectious HIV in the saliva. Methods: Since most of the infectious HIV that is shed mucosally by asympto matic individuals is found in, produced by, and transmitted by infected mon onuclear leukocytes, we determined whether saliva, which is hypotonic, may disrupt these infected cells, thereby preventing virus multiplication and c ell-to-cell transmission of HIV. Specifically, we measured (1) whether mono nuclear leukocytes were lysed by saliva and (2) whether the lysis by saliva inhibits the multiplication of HIV and other viruses in infected leukocyte s and other cells. Results: Saliva rapidly disrupted 90% or more of blood mononuclear leukocyt es and other cultured cells. Concomitantly, there was a 10 000-fold or high er inhibition of the multiplication of HIV and surrogate viruses. Further e xperiments indicated that the cell disruption is due to the hypotonicity of saliva. Conclusions: Hypotonic disruption may be a major mechanism by which saliva kills infected mononuclear leukocytes and prevents their attachment to muco sal epithelial cells and production of infectious HIV, thereby preventing t ransmission. Implications for the known oral HIV transmission by milk and s eminal fluid, as well as potential oral transmission to contacts and health care workers, are considered. This effective salivary defense may be appli cable medically to interdict vaginal, rectal, and oral transmission of HIV by infected cells in seminal fluid or milk by the use of anticellular subst ances.