Commitment of Decidual Haematopoietic Progenitor Cells in First Trimester Pregnancy Laszlo Szereday 1* , Eva Miko 1* , Matyas Meggyes 1 , Aliz Barakonyi 1 , Balint Farkas 2 , Akos Varnagy 2 , Jozsef Bodis 2 , Lydia Lynch 3 , Cliona O’Farrelly 4 , Julia Szekeres-Bartho 1 1 Department of Medical Microbiology and Immunology, University of Pecs, Medical School, Pecs, Hungary 2 Department of Obstetrics and Gynaecology, University of Pecs, Medical School, Pecs, Hungary 3 Beth Israel Deaconess Medical Centre, Harvard Medical School, Boston, MA, USA 4 School of Biochemistry & Immunology, Trinity College Dublin, Ireland Introduction Pregnancy is a natural model of an optimal immune regulation in a graft–host relation. The semi-allo- geneic fetus expresses antigens from paternal origin, and there is ample evidence that these antigens are recognized by the immune system of the mother. Three relatively small lymphocyte subpopulations, natural killer (NK) cells, c ⁄ d T cells and iNKT cells are significantly enriched in the decidua and play a major role in creating a favorable environment for implantation and the early development of the fetus by recognizing fetal human leukocyte antigens (HLA) presented by the extravillous trophoblast. 1–4 Little is known about the homing, proliferation and differentiation of decidual lymphocytes (DL). Besides being recruited from the periphery, local (extrathymic) lymphocyte development is also con- ceivable. The proliferation of uterine lymphocytes significantly increases during the late secretory phase of the menstrual cycle and during pregnancy. 5 Wira et al. 6 suggested that endometrial progenitor Keywords Decidua, haematopoietic progenitor cells, miscarriage, NK cells, pregnancy Correspondence Laszlo Szereday, Department of Medical Microbiology and Immunology, University of Pecs, Medical School, H-7624 Pecs, Szigeti ut 12. Hungary. E-mail: laszlo.szereday@aok.pte.hu *Contributed equally. Submitted January 28, 2011; accepted May 2, 2011. Citation Szereday L, Miko E, Meggyes M, Barakonyi A, Farkas B, Varnagy A, Bodis J, Lynch L, O’Farrelly C, Szekeres-Bartho J. Commitment of decidual haematopoietic progenitor cells in first trimester pregnancy. Am J Reprod Immunol 2011 doi:10.1111/j.1600-0897.2011.01029.x Problem The aim of this study was to investigate the phenotype and commitment of decidual haematopoietic progenitor cells (HPCs) in healthy pregnant women and in women with early miscarriage. Method of study Peripheral blood and decidual tissue from healthy and pathological preg- nant women were examined for HPCs and lymphoid progenitors using flow cytometric analysis. Results Compared with peripheral blood, we found a significant increase in decidual HPCs in both healthy pregnant women and women with spon- taneous abortion. T ⁄ NK, natural killer (NK), gamma-delta and NKT cell progenitors were identified in all peripheral blood and decidual samples. In pathologic pregnant women, the ratios of decidual T ⁄ NK and NK cell progenitors were significantly increased compared with healthy preg- nant controls. Conclusion We demonstrated decidual cells with haematopoietic progenitor cell phenotype in human decidua. Increased levels of NK progenitors in the decidua of women with early spontaneous abortion suggest a dysregula- tion of this pathway that may contribute to pregnancy failure. ORIGINAL ARTICLE American Journal of Reproductive Immunology (2011) ª 2011 John Wiley & Sons A/S 1