CD274 and neoplasm: Increased PD-1, PD-L1 expression, and PD-L1 T-cells in mouse mammary tissues during normal postpartum involution. Increased expression of CD8 +T cells expressing co-inhibitory receptors PD-1 and Lag-3 in PPBC mice models. Reduced tumor growth in postpartum mice using PD-1 targeted therapies. Reduced lymphatic vessel frequency using PD-1 therapies.