ABSTRACT B cell precursor acute lymphoblastic leukemia (BCP-ALL), a malignancy that arises from mutant pre-B cells, is the most common childhood leukemia but also occurs in adults. While effective in up to 90% of patients, current chemo- and targeted therapies for BCP-ALL temporarily destroy the humoral (antibody-producing) arm of the immune system and cause toxicities with long term consequences. The goal of this R21 grant proposal is to engage in conceptual research regarding the utility of the pre-B cell receptor (pre-BCR) as a unique therapeutic target for the treatment of BCP-ALL. The pre-BCR comprises an immunoglobulin (Ig) heavy chain and a surrogate light chain (SLC) together with the signaling molecules Ig alpha and Ig beta. The SLC is composed of two noncovalently-linked polypeptides, VpreB and l5. In normal tissues, the pre-BCR is expressed only in pre- B cells but not in more mature B cells nor in any other tissues. Importantly, our preliminary data suggest that VpreB and l5 are expressed in >90% of BCP-ALL. We therefore generated high avidity monoclonal antibodies (mAbs) specific for VpreB and l5 and found that the mAbs are internalized into BCP-ALL cells expressing the pre-BCR. Internalization is an important prerequisite for the antibody component of an antibody-drug conjugate (ADC). Our central hypothesis is that attacking BCP-ALL with an ADC specific for VpreB or l5 will eliminate the leukemia cells while sparing the mature humoral immune system to combat infection. Our goal is to obtain proof- of-concept data that first, confirms internalization of VpreB mAbs in several BCP-ALL cell lines and allows selection of a single, most efficiently internalizing lead mAb (Specific Aim 1); second, show at the protein level the widespread expression of the pre-BCR in 35 BCP-ALL patient leukemia samples (Specific Aim 2); and third, demonstrate the ability of our lead VpreB ADC to kill established and patient-derived BCP-ALL cell lines in vitro, ex vivo, and in a patient-derived xenograft model (Specific Aim 3). We hope that this research will support the development of a new BCP-ALL therapeutic that will safely eliminate leukemia cells while sparing the humoral immune system and ultimately supplanting chemotherapy so that patients can achieve full remission without short- or long-term adverse consequences.