Characterization of Pichinde virus infection of cells of the monocytic lineage

To establish a model of viral infection of monocytes, we examined infection of human cells and cell lines of the monocytic series with the arenavirus Pichinde virus. We demonstrate for the first time that human peripheral blood monocytes are susceptible to Pichinde virus infection, as shown by immunoprecipatation of virus-specific polypeptides from infected cells, immunofluorescence analyses, and quantitation of virus production from infected cells. The human promyelocytic leukemia cell line HL60 did not support Pichinde virus replication, even if cells were induced with the phorbol ester phorbol myristate acetate (PMA) to differentiate to monocytes. However, the human promonocytic leukemia cell line THP-1 did support Pichinde virus replication. Replication depended on exposure of the cells to PMA. We examined the nature of the effect of PMA in the induction of THP-1 cells to support Pichinde virus replication. We found that 5 min of exposure of THP-1 cells to PMA is sufficient to support virus growth and that PMA-treated THP-1 cells remain susceptible to infection up to 4 days after the initial PMA treatment. We also showed that infection of PMA-treated THP-1 cells is mediated through protein kinase C (PKC). H7, a PKC inhibitor, was able to block both PMA-induced differentiation and Pichinde virus infection of THP-1 cells. The synthetic diacylglycerol and PKC agonist, diC8, was able to stimulate THP-1 cells to support virus growth, albeit to lower levels than PMA. Dactinomycin abrogated the ability of virus to replicate and suggested a requirement for host cell transcription. The PMA effect did not appear to relate to receptor modulation. These results suggest that PMA-induced susceptibility to Pichinde virus infection occurs at a point later than the initial binding and penetration stages and that infection depends on the activation or differentiation state of the cell.