PDCD1 and systemic lupus erythematosus: We hypothesize that the mild but significant LIP potential in neonates creates a situation in which establishment of tolerance in the nascent T cell population by PD-1 is important, and the absence of PD-1-mediated control of neonatal LIP may set the stage for the lupus-like autoimmunity that manifests later in life in PD-1−/− animals.