PROJECT SUMMARY Early interventions for high-risk B-cell malignancies, including diffuse large B cell lymphoma (DLBCL), Burkitt's Lymphoma (BL), and B-cell acute lymphoblastic leukemia (B-ALL), remain an urgent clinical need. Development of such interventions requires a deep understanding of the molecular mechanisms underlying the evolution, i.e., initiation, establishment, and sustenance, of these malignancies. B-cell malignancies are initiated >5-fold more frequently in patients suffering from refractory autoimmune B-lymphoproliferative disorders such as systemic lupus erythematosus (SLE), making SLE a relevant disease model to study initiation of B-cell neoplasms. The hormone prolactin (PRL) is known to exacerbate the symptoms of SLE, enhance survival of lymphoid cells, and promote the expression of the protooncogenes MYC and BCL2 in these cells. Whether PRL contributes to evolution of B-cell malignancies was unknown. PRL receptors (PRLRs) are type I cytokine receptors that have long (LF), intermediate (IF, only in humans) and short (SF) isoforms generated by alternative splicing. Increased expression of the LF/IF relative to the SF PRLRs on cells leads to cell proliferation and survival, whereas increased expression of SFs relative to LF/IF inhibits proliferation, promotes differentiation, and induces apoptosis. We hypothesized that PRL, by signaling specifically through the pro-proliferative and anti-apoptotic LF/IFPRLR, promotes the malignant transformation of B cells, and establishes and sustains the growth of overt B-cell malignancies. To test our hypothesis, we measured changes in B cells in vivo in SLE- and DLBCL/BL- prone mouse models and in vitro in human B-cell malignancies after specifically knocking down expression of the LF/IFPRLR using a non-toxic splice modulating oligomer (SMO). The LFPRLR SMO prevents the synthesis of the LFPRLR in mice and the LF/IFPRLR in humans without affecting the SFPRLRs. Knockdown of LFPRLR reduced the numbers of pathogenic B-cell subsets in SLE- and DLBCL/BL-prone mice and lowered the risk of B-cell transformation in SLE-prone mice by downregulating expression of the activation-induced cytidine deaminase (AID) enzyme, whose overexpression we previously showed, drives the evolution of B-cell neoplasms. We found that overt human B-cell neoplasms aberrantly express autocrine PRL and sometimes only the LF/IFPRLR. Knockdown of LF/IFPRLR in overt B-cell malignancies reduced cell viability, downstream STAT3 activation, and expression of MYC and BCL2. Our preliminary findings warrant detailed studies of molecular pathways underlying the disturbances in B cells downstream of LF/IFPRLR in SLE-prone mice that are vulnerable to transformation (Aim 1), in mice with pre-malignant B-cell clones prone to overt B-cell malignancies (Aim 2), and in overt human B-cell neoplasms (Aim 3). Our research will solidify isoform-specific suppression of the production of LF/IFPRLR as a therapeutic strategy in SLE that concurrently...