CD4 and deltaretrovirus infections: Although the mechanisms behind increasing CD4+CD25highFoxp3+ T cell numbers during BLV infection remains unknown, the HIV envelope protein gp120 reportedly inhibited apoptosis of Treg after binding CD40 44, and the nuclear HTLV‐I protein Tax induced aberrant expression of IL‐2, Bcl‐XL, and CD40 in virus infected T cells 45, 46.