Susceptibility of chicken Kupffer cells to Chinese virulent infectious bursal disease virus

Abstract Infectious bursal disease (IBD) is an acute, highly contagious, and immunosuppressive avian disease caused by IBD virus (IBDV). Although the effects of IBDV on bursa of Fabricius in chickens have been well reported, the impacts of IBDV on liver after IBDV infection are still unclear. In the present study, specific pathogen free (SPF) chickens were experimentally inoculated with IBDV Chinese virulent strain BC6/85, and the cells in liver and bursa were examined by immunohistochemisrty and transmission electron microscopy (TEM). The congestion of liver tissue and fatty degeneration of hepatocytes were characteristics of microscopical changes in chicken liver at 3 days post infection (d.p.i.), whereas there were follicular lymphoid necrosis, apoptosis, depletion, as well as edema and congestion in bursa. In addition, the number of IBDV-positive cells peaked at 4 d.p.i. in bursa and at 3 d.p.i. in liver, respectively. With respect to ultrastructural pathological changes of hepatocytes, mitochondria swelled and nucleus deformed into an irregular shape or its chromatin peripherally condensed which indicated that the hepatocyte was at the early stage of apoptosis, and the electron-lucent lipid droplets in a variety of sizes were observed within cytoplasm. Kupffer cells became “swollen-like” and the electron-density of their cytoplasm was lower than that of cells in uninfected group. Liver glycogen deposits significantly declined from 2 to 5 d.p.i. and recovered strongly at 6 d.p.i. More importantly, KLU01 (macrophage marker) positive (KUL01 + ) cells were infiltrated in bursa and liver in IBDV-exposed chickens by immunoperoxidase staining. To demonstrate the correlation between IBDV and macrophages in bursa and liver, we further investigated the colocalization of viral antigens and macrophages by double immunofluorescence labeling. At 4 d.p.i., the percentage of double positive cells (IBDV positive and KUL01 + cells) accounted for 26.5 percent of the total IBDV positive cells or 57 percent of the total KUL01 + cells in bursa. In comparison, the percentage of double positive cells in liver constituted 97 percent of the total IBDV positive cells or 99 percent of the total KUL01 + cells. These results suggest that IBDV was susceptible to KUL01 + cells in liver (mainly Kupffer cells) and replicated in the KUL01 + cells. By comparison with the influence of IBDV on bursa, our findings were the first to elucidate the pathological changes in liver after IBDV infection on a microscopical and ultrastructural scale, and, especially, to gain the initial insight into the susceptibility of Kupffer cells to IBDV.