Abstract Abstract #5048 Introduction: A lasting legacy of a fetus to his mother is a small number of stably persistent allogeneic cells, a phenomenon known as fetal microchimerism. We have been testing the hypothesis that fetal microchimerism is protective against cancer in the mother by participation in immune surveillance. Prior studies demonstrated that fetal microchimerism in the peripheral blood is associated with protection from breast cancer (Cancer Res, 67(19):9035-8 and PLoS ONE, 3(3):e1706). Here we examine whether the same association of fetal microchimerism extends to the tissue of interest, the breast.
 Methods: Total genomic DNA was extracted from frozen normal breast tissue adjacent to invasive disease in women with breast cancer. Control tissues were from reduction mammoplasty procedures performed for women with no prior history of invasive breast cancer. The presence of male DNA, presumably from a prior male fetus, was tested with a quantitative PCR assay for the Y chromosome gene, DYS14. Proportions of tissues harboring fetal microchimerism were compared between women with and without prior history of invasive breast cancer.
 Results: At present, tissues from 19 women with cancer and 19 without have been tested for the presence and quantity of fetal microchimerism using the DYS14 assay. Fetal microchimerism was detected less frequently in tissues from women with cancer (26%) than without (63%). The odds ratio for this protective association of fetal microchimerism with breast cancer is 0.21 (0.04-1.0 95% confidence interval; p=0.05). A trend towards larger median quantities of fetal microchimerism was noted in control tissues versus normal breast from cancer patients (0.34 versus 0 fetal genomes/million maternal cells, p>0.05).
 Conclusions: Preliminary results suggest that the negative association of breast cancer and fetal microchimerism observed previously in the blood is also reflected at the level of the tissue of interest. These results merit further investigation in the breast tissue of controls to identify the phenotype of the cells detected. Furthermore, the findings here provide further support to the overall hypothesis that fetal microchimerism may participate in immune surveillance against breast cancer in healthy women. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 5048.