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Although human eosinophils express low concentrations of CD4, the capacity of mature, non-replicating eosinophils to be infected with human immunodeficiency virus-1 (HIV-1) has not been established. Using peripheral blood eosinophils isolated free of contaminating lymphocytes and mononuclear leukocytes, we evaluated eosinophil infection with HIV-1. Eosinophils could be infected with strains of HIV-1 as evidenced by HIV-induced cytolytic effects, progressive release of p24 antigen in cultures of infected eosinophils, recovery of HIV from infected eosinophils by co-cultivation, and detection of HIV-1 gag viral DNA from infected eosinophils by polymerase chain reaction (PCR) amplification. Greater p24 antigen release from infected eosinophils was elicited by the phorbol ester, PMA; and eosinophil killing by HIV-1 was enhanced by the cytokine GM-CSF. By light and electron microscopy, HIV-infected eosinophils demonstrated apoptosis and necrosis. Apoptotic subdiploid nuclear staining was detected by flow cytometric analyses of propidium iodide-stained nuclei from HIV-infected eosinophils, and DNA isolated from HIV-infected eosinophils showed both nucleosomal fragmentation and diffuse degradation. Thus, mature eosinophils, non-replicating terminally differentiated leukocytes, can be infected with HIV-1. HIV-1 expression in eosinophils is promoted by increased granulocyte-macrophage colony-stimulating factor (GM-CSF) and can cause eosinophils to undergo death due to apoptosis and necrosis.