Virus spread is reduced in the presence of specific a 1antitrypsins Next, we wanted to know whether the observed inhibition of LASV GP processing correlates with the ability of the different a 1antitrypsin variants to inhibit virus spread. To investigate this, we established a 96-well plate assay in which infected cells are immunostained with True Blue substrate as described in Materials and Methods. Virus spread can be monitored by the appearance of characteristic comet-shaped foci, showing that the virus progeny is carried over the cell monolayer, while prevention of virus spread results in limited radial growth, due to infection of only neighbouring cells. This assay allows rapid screening of potential inhibitors [50] . Effect of S1P Inhibition on LASV Replication www.plosntds.org type cells (Fig. 5A, upper panel) . In contrast, virus spread was significantly diminished in cells expressing a 1 -AT specific for S1P (Fig. 5A, lower panel) . These data indicate that S1P-adapted a 1antitrypsins have the potential to specifically inhibit the processing of LASV GP, which in turn is required for efficient virus spread. It should be noted that the infectious foci observed in a 1 -AT RRIL expressing cells were larger compared to SRD-12B cells in which virtually no virus spread of VSVDG/LASVGP was observed, resulting in only single infected cells (Fig. 5A) . Although similar inhibition values were observed by means of immunoblot quantification (Fig. 3) , a few remaining non-detectable cleavage events may count for this limited cell-to-cell spread in a 1 -AT RRIL expressing cells. Cells expressing the furin-adapted a 1 -AT variant RVKR did not prevent virus spread. At first glance, we rather observed an enhancement of infectivity compared to noninduced cells, which might be due to an increase in the LASV cellular receptor a-dystroglycan on the cell surface [56] .