CD69 and infection: According to the study by Abdolmaleki et al.23, the expression of chicken NK cell receptors such as CD69, B-Lec, NK-lysin and IFN-γ were down-regulated after infection with velogenic NDV strains, whereas chickens infected with a vaccine strain of NDV showed minor effects on both the expression of their surface receptors and the total population of CD3−/28.4+ IEL-NK cells.