Donor parity no longer a barrier for female-to-male hematopoietic stem cell transplantation.

Allogeneic hematopoietic stem cell transplantation (HSCT) is a widely applied treatment for disorders mainly involving the hematopoietic system. The success of this treatment depends on many different patient- and donor-specific factors. Based on higher CD34+ yields and superior clinical outcomes associated with the use of male donors, males are generally seen as the preferred HSCT donor. In addition, female donors are notorious for bearing memory type lymphocytes induced by previous pregnancies; such alloimmune cells may provoke unwanted immune reactions such as graft-vs.-host disease in transplant recipients. Consequently, many transplant centers try to avoid parous donors, particularly when searching the best unrelated donor for a male patient. We recently showed that parous women with female offspring have an anti-male directed tolerogenic immune status comparable to that of nulliparous donors. As discussed in this article addendum, the sex of the donor's offspring combined with the presence of HY-specific T regulator cells are possibly better selection criteria than parity status per se.