Critically, NK cytotoxicity was clearly inhibited by TGF‐β and the expression of activating receptor NKp46 was downregulated in neoplastic lymph nodes, suggesting that inactivated NK cells enhance tumor formation during BLV‐infection by inducing TGF‐β secretion from CD4+CD25+ Foxp3+ T cells. Here, TGFB1 is linked to deltaretrovirus infections.