PROJECT SUMMARY/ABSTRACT B cell-mediated humoral responses are associated with immune protection in multiple cancers. Cellular adaptive immunity progresses in coordination with humoral responses to achieve maximum effectiveness and long-term memory. In this project, we will investigate the spontaneous influence of tumor-derived antibodies, as well as their immunotherapeutic potential, through two different and novel mechanisms of anti-tumor immunity. First, we hypothesized that IgA antibodies with the transcytosis capacity could target intracellular antigens. Second, naturally produced anti-PD-1 antibodies contribute to support the immune pressure spontaneously exerted by T and B cells against malignant progression but impair the effectiveness of immune-checkpoint-blockade therapy (ICB). Both the hypothesis is based on our preliminary findings of tumor-derived IgA transcytoses in endosomes through binding with polymeric immunoglobulin receptor (pIgR) in human ovarian cancer cells, suggesting that intracellular oncodrivers could be targeted using IgA, and spontaneously produced PD-1-specific IgA and IgG antibodies in human ovarian cancer bed. Dimeric IgA gets internalized and transcytoses all the way through ovarian cancer cells upon interaction with pIgR. Considering the ability of IgA to target intracellular viral antigens at endosomal compartments, we will utilize our dimeric IgA antibodies specific for the KRASG12D mutation to track its internalization and interaction with mutated vs. wild-type (WT) KRAS, as well as its subcellular localization and therapeutic potential. We predict that mutations in intracellular oncodrivers dependent on endosomal recycling (i.e., KRAS) could be effectively targeted with antigen-specific dimeric IgA. By defining the efficacy and specificity of neutralizing intracellular mutated KRAS with IgA in humanized animal models, we could pave the way for novel immunotherapies effectively targeting essential and yet undruggable mechanisms. On the other hand, by defining the frequency and functional relevance of spontaneously generated anti-PD-1 circulating antibodies in ovarian cancer patients, we will determine the prognostic relevance of these auto-antibodies. Further, our concept is that spontaneously produced anti-PD-1 antibodies contribute to support the immune pressure spontaneously exerted by T and B cells against cancer progression but impair the effectiveness of ICB therapy, which will have obvious implications for developing personalized immunotherapeutic interventions to maximize effectiveness in the right set of patients. In summary, we will offer a basis for novel tailored immunotherapies against ovarian cancer, based on a better understanding of its unique immunobiology, by combining clinical samples with relevant preclinical tumor models.