Porcine reproductive and respiratory syndrome virus (PRRSV) has a specific cell tropism for differentiated macrophages, such as porcine alveolar macrophages (PAMs). We analyzed the expression of CD163 on PAMs and macrophages derived from CD14 positive blood monocytes (MDMs), in correlation with PRRSV replication. By flow cytometry analysis, we showed that the levels of CD163 expression correlated well with the overall level of PRRSV replication. We further examined the effects of modulators of macrophage function, including 12-O-tetradecanoylphorbol-13-acetate (TPA), lipopolysaccharide (LPS), and interleukin (IL)-10 on the expression of CD163 and PRRSV replication. Pre-treatment of PAMs or MDMs with TPA or LPS resulted in decreased expression of CD163 and reduction in PRRSV replication. On the contrary, the incubation of CD14 positive monocytes with IL-10 during differentiation into MDMs resulted in up-regulated expression of CD163 with a corresponding increase in PRRSV infection. These data indicate that the expression of CD163 on macrophages in different microenvironments, in vivo, may determine the replication efficiency and subsequent pathogenecity of PRRSV.