LAG3 and deltaretrovirus infections: They demonstrated that up-regulation of surface expression of immunoinhibitory receptors, such as programmed death 1 (PD-1), lymphocyte activation gene 3 (LAG-3), T-cell immunoglobulin and mucin domain-containing protein 3 (Tim-3) and cytotoxic T-lymphocyte antigen 4 (CTLA-4) by bovine T cells play a critical role in immune exhaustion and disease progression in the case of BLV infection, Johne’s disease (caused by Mycobacterium avium) and bovine anaplasmosis (11, 69–72).