From nasal to basal: Single-cell sequence illustrates the winding infection route for IBDV and its influence on bursal cells

Nasal cavity is the main gateway for pathogen infection although it composed of many layers of defending barriers. Pathogens strongly willing to enter and infect from nasal cavity with the dominant form aerosol or respiratory droplets. However, the underlying mechanism of virus nasal infection and transmission are still unknown. Hence, a better understanding of this mechanism may provide insight into the pathogenesis of virus. In this study, IBDV was select as a model virus to study nasal infection and transmission for it could infect chicken from head (nasal) to tail (basal) and lead to massive destruction of bursal IgM+ B-lymphocytes. Initially, we found IBDV mostly entered and infected the interior of chicken's nasal, where full of lymphoid tissue and easily for virus to transfer into blood. After passing nasal barriers, IBDV was subsequently transmit into blood and infected PBMCs. Following blood circulates, IBDV then infected bursal and hugely destroyed B-lymphocytes. However, the mechanism of how IBDV influenced the bursal cells, especially how virus destroyed B-lymphocytes were still unclear. With the help of single cells RNA sequence, we identified five vigorous clusters, including three immune cells types (B-cells: 64.70%, dendritic cells: 3.32% and T-cells: 6.33%) and two non-immune cell types (epithelial cells: 23.86% and fibroblast cells: 1.80% cells). Further analyses found that B-cells population were serious damaged, especially IgM+ B cells. However, the IgA+ B cells population hugely increased after IBDV infection. Interesting, we first demonstrated that basal cells and other non-immune cells in Bursa of Fabricius (BF) were the main target for IBDV infection and replication. Together, our study not only comprehensive elaborated the airborne IBDV and its transmission via intranasal route into the BF, but also explained its distribution in different immune and nonimmune cells and immunoglobulins rearrangement after immunosuppressive disorder on an age dependent infected organ. Overall design: scRNA-seq of whole cells from four chicken's bursal were performed to illustrate it. Chicken were grouped as: S1. Control two weeks old chick, S2. IBDV infected two weeks old chick, S3. Control three weeks old chick and S4. IBDV infected three weeks old chick.