FIG 4.

CD74 isoform Ii-2 confers to vvIBDV the ability to attach to a vvIBDV-nonpermissive cell line. (A) 293T cells (nonpermissive to vvIBDV) were transfected with a full-length chicken CD74 Ii-2 plasmid with an HA tag (or the empty vector as a negative control). Twenty-four hours after transfection, cells were incubated with vvIBDV at an MOI of 5 at 37°C for 36 h to investigate whether CD74 confers susceptibility to IBDV infection. (Top) Cells were processed for confocal analysis, using IBDV VP2 mAb and an HA tag antibody as the primary antibodies. No virus (green fluorescence) was detected in the empty vector control. (Bottom) In the CD74 Ii-2 overexpression group, CD74 (red fluorescence) accumulated on the cell membrane, colocalizing with the IBDV particles (green fluorescence). No virus was observed to enter CD74-overexpressing nonpermissive cells. CD74 could confer attachment ability but not susceptibility to IBDV infection. (B) 293T cells overexpressed HA-tagged chicken CD74 Ii-2 or the empty vector for 24 h and were then incubated with 200 μg SVPs at 4°C for 1 h. After washing with PBS 5 times, cells were processed for confocal analysis as described above. SVPs (green fluorescence) were bound to the membranes of CD74-overexpressing cells (red fluorescence) and colocalized with CD74, but no binding was observed for the empty vector group. (C and D) 293T and HeLa cells (both of which are nonpermissive to vvIBDV) were transfected with a eukaryotic expression plasmid of HA-tagged chicken CD74 Ii-2 (or the empty vector as a negative control) and maintained for 24 h under normal culture conditions. The cells were then incubated with vvIBDV at an MOI of 50 at 4°C for 1 h for the binding assay and washed with PBS 5 times to remove the unbound viruses. Cells were processed for confocal analysis using the same antibodies and procedure as the ones described above. No virus (green fluorescence) was found to bind or infect 293T (C) or HeLa (D) cells transfected with the empty vector. In contrast, IBDV (green fluorescence) colocalized with overexpressed chicken CD74 (red fluorescence) on the membrane of nonpermissive 293T (C) and HeLa (D) cells. (E and F) qPCR analysis indicating that CD74 overexpression promotes IBDV binding to 293T or HeLa cells compared with the empty vector transfection group (9.55-fold increase in 293T cells [E] and 5.07-fold increase in HeLa cells [F]; both P < 0.05). The arithmetic means and standard deviations for at least three independent experiments performed in duplicate are shown.