Differentiation of monocytes to macrophages induced by influenza virus-infected apoptotic cells

The effect of the culture supernatant of influenza virus (IV)-infected apoptotic and non-apoptotic cells on the differentiation of monocytes to macrophages was investigated. IV infection induced apoptotic DNA fragmentation in cultured chorion cells but not in amnion cells prepared from human foetal membrane tissue. To examine the differentiation of monocytes to macrophages, an adhesion assay was employed using the human monocytic leukaemia THP-1 cell line. THP-1 cells became adherent to a substrate by incubation with the culture supernatant of IV-infected chorion cells, but not with that of amnion cells. The spreading THP-1 cells were morphologically characteristic of macrophages and they phagocytosed latex particles. RT-PCR analysis revealed that the expression of class A scavenger receptor mRNA was induced in THP-1 cells by incubation with the culture supernatant of IV-infected chorion cells. These results suggested that monocytic THP-1 cells were morphologically and functionally differentiated to macrophages by IV-infected apoptotic cells due to a soluble factor released from the apoptotic cells.