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Donor-derived immunity against tumor-associated antigens (TAAs) may exert selective antileukemic activity reprieving the allogeneic recipient from graft-versus-host disease. As TAAs are highly expressed in placental tissues we hypothesized that pregnancy could drive respective immunity in healthy individuals. Thus, we investigated the frequency and level of immune responses against clinically relevant TAAs in 114 blood donors and 44 women during their first pregnancy. Quantitative reverse-transcription polymerase chain reaction was employed to detect low levels of interferon-γ after primary peptide stimulation of CD8+ T lymphocytes. In blood donors, primary immune responses of low and/or high avidity were found against WT1 (15%), MUC1 (14%), PRAME (7%), and HER2/neu (5%) and exerted killing functions against leukemic cells. Men had higher responses than women, likely due to gonadal cancer-testis-antigen expression. Interestingly, a history of prior delivery was not associated with increased responses, whereas the strongest responses during pregnancy were found in early trimesters to disappear after delivery. This boost and loss of TAA-specific immunity suggests that virtually every donor harbors the potential to mount antileukemic immune responses in a recipient. However, in the absence of the driving target and a permissive environment, they are short-lived and thus require supplemental strategies such as vaccination or immunomodulation to facilitate their persistence. [ABSTRACT FROM AUTHOR]