RBC alloimmunization remains a common clinical problem in transfusion medicine. Alloantibodies are responsible for hemolytic transfusion reactions; leading to substantial morbidity and occasional mortality. For those patients unfortunate enough to make multiple alloantibodies, provision of compatible RBCs can be both time and resource intensive. In some cases, this can result in an inability to locate an otherwise life-saving therapy. Despite their clinical importance, our understanding of how long-lived memory RBC alloantibody responses are generated by the weak stimulus of RBC transfusion remains limited. Given that memory lymphocytes share functional attributes typically reserved for stem cells such as homeostatic self-renewal, we and others have hypothesized that they have reactivated a portion of the hematopoietic stem cell genetic program. In support of this idea, we published a common transcriptional signature shared between memory B cells, memory CD8+ T cells and hematopoietic stem cell. We have gone on to show that the transcription factor Pou6f1 is selectively upregulated in memory B and CD8+ T cells relative to shorter-lived cells. Pou6f1 is a member of the Pou-domain family of transcription factors and is a paralog of Pou5f1 (aka Oct4). Oct4 serves as one of the master regulators of embryonic stem cell self-renewal; establishing a stable, self-reinforcing genetic circuit that directs active transcription of self-renewal genes while maintaining differentiation specific transcription factor in an off, but poised state. We believe Pou6f1 functions in memory T and B cells in a manner similar to Oct4 in embryonic stem cells. The central hypothesis is that Pou6f1 directs memory antibody responses to RBC alloimmunization. This hypothesis leads to a set of predictions that we will test in vivo in a series of genetic and biochemical experiments combining Pou6f1 floxed mice we have recently generated with both the k and HOD RBC alloimmunization models. Aim 1 is to test the prediction that Pou6f1 expression in both T and B cells is required for the generation of long-lived, boostable RBC alloantibody responses. Aim 2 is to test the prediction that continued Pou6f1 expression in memory T and B lymphocytes is required to maintain recall responses to RBC alloimmunization, and that Pou6f1 is required to maintain responsiveness of antigen specific memory lymphocytes to homeostatic cytokine signals. In the fullness of time, we anticipate that identification of the molecular circuitry driving RBC antibody production will provide therapeutic and diagnostic targets for further clinical development.