Abstract P5-04-15: An immunogenic mouse model using BALB.B-MMTV-neuT breast cancers reveals spontaneous protective humoral immunity
Authors: Castro Sarah Anne , Pena Alvaro , Khazaie Khashayarsha , Pease Larry R. 
Published: March 2020

Abstract
The BALB/c-neuT mouse has been a faithful model for investigating metastatic breast cancer. Spontaneous breast tumors from BALB/c-neuT mice are readily transplantable orthotopically into syngeneic or rat neuT negative BALB/c WT hosts facilitating controlled studies of developing breast cancers. When introduced into the breast ducts of BALB/c WT hosts, lung metastases form similarly to how they appear in mice with spontaneous tumors. Tumor grafts grow out with comparable kinetics and induce similar systemic immune abnormalities in the spleens and lymph nodes of both lines of mice as seen in mice with spontaneous tumors. Factors inducing these abnormal immune cell configurations are not yet known. Immunization of BALB/c WT mice against the rat Her2 antigen can induce both T cell and antibody mediated rejection of the transplanted tumors, but these approaches have not been equally successful in syngeneic, rat neuT+ positive hosts, indicating tolerance to rat Her2 is a significant barrier to the induction of immunity against these breast tumors. The MHC encodes class I and class II antigen presenting molecules which are known to be highly polymorphic in both structure and function. We introduced the H-2b MHC haplotype into the MMTV-NeuT model to assess the influence of a different set of antigen presenting molecules on tumor biology. In BALB.B-neuT mice spontaneous tumors appeared with frequencies and in numbers similar to those seen in the BALB/c model. In anything, lung metastases are more frequent and more aggressive in BALB.B-neuT mice. Again, the tumors were readily transplantable into the breast fat pads of syngeneic BALB.B-neuT hosts which developed systemic immune abnormalities very similar to those observed in BALB/c -neuT or BALB/c WT mice. However, BALB.B WT hosts initially accepted the grafts, but subsequently rejected them after they were established. Mice that were in the process of rejecting the tumors retained normal cellularity within their spleens and lymph nodes indicating the importance of an ongoing immune response and/or tumor rejection in shaping the microenvironment in secondary lymphoid organs. Adoptive transfer of spleen cells from BALB.B WT mice which had rejected established tumor conferred resistance to the tumor grafts as long as B cells were included (P&lt;0.01), indicating active humoral immunity was important to tumor rejection. A mechanistic understanding or how antibody-mediated immunotherapy works in patients is incomplete. This new model provides an opportunity to address this question. The Her2 ecto-domain is known to be shed from tumor cells. We propose a model in which the ecto-domain of rat neuT is shed, but is insufficiently immunogenic in BALB/c (H-2d) hosts, but highly immunogenic in BALB.B (H-2b) mice. Consistent with this model is the prediction using the Immune Epitope and Data Base Resource of the presence of a potentially immunogenic IAb-restricted class II epitope within the ecto-domain of rat Her2 neuT oncogene, but no such epitope restricted by IAd or IEd was identified. This provides a compelling hypothesis explaining why grafted spontaneous tumors grow in BALB/c, but not in BALB.B WT mice. An important prediction from this model is that linkage of a potent class II epitope to the Erbb2-encoded ecto-domain may provide a basis for a potent vaccine against Her2 positive breast tumors which should be a strategy adaptable to any MHC genotype. It also will be of interest to determine whether the presence of an antibody reactive against established tumor restores the normal cellular composition of secondary lymphoid organs which have been perturbed by established breast tumors.
Citation Format: Sarah Anne Castro, Alvaro Pena, Khashayarsha Khazaie, Larry R. Pease. An immunogenic mouse model using BALB.B-MMTV-neuT breast cancers reveals spontaneous protective humoral immunity [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P5-04-15.