Antibody‐mediated infection of macrophages and macrophage‐like cell lines with 17D‐yellow fever virus

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Abstract
The 17D vaccine strain of yellow fever virus (17D‐YF) produces a safe human arboviral infection that can provide antisera of well‐defined specificity under chronologically defined conditions. We studied 17D‐YF growth in human peripheral blood macrophages and in two continuous Fc receptor‐bearing, macrophage‐like cell lines, P388D of mouse origin and U937 of human origin. Cells were infected with virus in the presence or absence of antibody to 17D‐YF and to two related flaviviruses, St. Louis encephalitis (SLE) and dengue 2 (D2V). The virus 17D‐YF grew in the three cell types when infection was established without antibody; viral yields were increased by addition of antibody to 17D‐YF, SLE, and D2V. Increased titers of virus were accompanied by an increased number of infected cells by immunofluorescent assay. Enhancing activity was present in the IgG but not the IgM fractions of immune sera. Infection without cytopathic effect was observed in U937.