Model of PRRSV invasion and spread in porcine nasal mucosa. The porcine nasal mucosa consists of a ciliated pseudostratified columnar epithelium, an underlying basement membrane and the lamina propria. Different mechanisms of invasion were observed between PRRSV strains LV and Lena. LV mainly infects CD163+Sn+ cells. The small number of Sn+ cells (5.2 cells/mm2) that are present within the epithelial cell layer does not allow LV to reach high levels of replication. Only 2.5 LV infected cells/mm2 were observed at 72 hpi, all of them located in the lamina propria. 97% of the LV-infected cells had a phenotype of CD163+Sn+ (1.). Lena showed a wider cell tropism. This strain infects both CD163+Sn+ and CD163+Sn- cells in order to migrate into the depth (2.). Lena was found to spread easily from one cell to another, forming after 48 hpi, clusters of mainly CD163+Sn- infected cells within the epithelial cell layer and CD163+Sn+ in the lamina propria. These clusters expanded at 72 hpi and CD163-Sn- cells were detected at the periphery of them (3.).