Assessing the infiltration of immune cells in the upper trachea mucosa after infectious laryngotracheitis virus (ILTV) vaccination and challenge

The types of immune cells that populate the trachea after ILTV vaccination and infection have not been assessed. The objective of this study was to quantify CD4⁺, CD8α⁺, CD8β⁺, TCRγδ⁺, and MRC1LB⁺ cells that infiltrate the trachea after vaccination with chicken embryo origin (CEO), tissue culture origin (TCO), and recombinant herpesvirus of turkey-laryngotracheitis (rHVT-LT) vaccines, and after challenge of vaccinated and non-vaccinated chickens with a virulent ILTV strain. Eye-drop vaccination with CEO, or TCO, or <i>in ovo</i> vaccination with rHVT-LT did not alter the number of CD4⁺, CD8α⁺, CD8β⁺, TCRγδ⁺, and MRC1LB⁺ cells in the trachea. After challenge, the CEO vaccinated group of chickens showed swift clearance of the challenge virus, the mucosa epithelium of the trachea remained intact, and a limited number of CD4⁺, CD8α⁺, and CD8β⁺ cells were detected in the upper trachea mucosa. The TCO and rHVT-LT vaccinated groups of chickens showed narrow viral clearance with moderate disruption of the trachea epithelial integrity, and a significant increase in CD4⁺, CD8α⁺, CD8β⁺, and TCRγδ⁺ cells infiltrated the upper trachea mucosa. Non-vaccinated challenged chickens showed high levels of viral replication, the epithelial organization of the upper trachea mucosa was heavily disrupted, and the predominant infiltrates were CD4⁺, TCRγδ⁺, and MRC1LB⁺ cells. Hence, the very robust protection provided by CEO vaccination was characterized by minimal immune cell infiltration to the trachea mucosa. In contrast, partial protection induced by the TCO and rHVT-LT vaccines requires a prolonged period of T cell expansion to overcome the established infection in the trachea mucosa.