Failure to cleave measles virus fusion protein in lymphoid cells. A possible mechanism for viral persistence in lymphocytes

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Abstract
The host-directed cleavage of measles virus fusion protein in infected lymphoid cells was studied to understand the mechanism of viral persistence in lymphoid cells in vivo. Several lymphoblastoid cell lines were infected with measles virus and the viral glycoproteins expressed on the cell surface were radiolabeled and analyzed for cleavage of fusion (F0) to F1 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Daudi and Ramos human lymphoblastoid cells were deficient in their ability to cleave measles virus F0 and correspondingly produced low titers of infectious measles virus, Daudi cells being more defective than Ramos cells. Other human lymphoblastoid cells studied, Victor, Raji, Wi-L2, RPMI 8866 and Seraphine, cleaved the F0 and made significantly more infectious virus. Despite their defect in cleaving F0, Daudi cells were able to assemble and release (noninfectious) measles virus particles into the fluid phase. The deficit in Daudi cells was corrected by fusing infected Daudi cells with cleavage-competent cells such as Victor or Raji. The cleavage event performed by competent cells could be mimicked at the plasma membrane by treating infected Daudi cells with trypsin, implicating the role of a plasma membrane enzyme in cleaving F0 to F1 during measles virus infection. Hence, lymphoid cells deficient in the plasma membrane enzyme required to cleave F0 are permissive for measles virus, maintain viral gene products, produce mostly noninfectious virus and fail to place the biologic activity F1 protein on their surfaces.