An inducible transgenic mouse breast cancer model for the analysis of tumor antigen specific CD8+ T-cell responses

Abstract

In Simian virus 40 (SV40) transgenic BALB/c WAP-T mice tumor development and progression is driven by SV40 tumor antigens encoded by inducible transgenes. WAP-T mice constitute a well characterized mouse model for breast cancer with strong similarities to the corresponding human disease. BALB/c mice mount only a weak cellular immune response against SV40 T-antigen (T-Ag). For studying tumor antigen specific CD8⁺ T-cell responses against transgene expressing cells, we created WAP-T_NP mice, in which the transgene additionally codes for the NP_118–126-epitope contained within the nucleoprotein of lymphocytic choriomeningitis virus (LCMV), the immune-dominant T-cell epitope in BALB/c mice. We then investigated in WAP-T_NP mice the immune responses against SV40 tumor antigens and the NP-epitope within the chimeric T-Ag/NP protein (T-Ag_NP). Analysis of the immune-reactivity against T-Ag in WAP-T and of T-Ag_NP in WAP-T_NP mice revealed that, in contrast to wild type (wt) BALB/c mice, WAP-T and WAP-T_NP mice were non-reactive against T-Ag. However, like wtBALB/c mice, WAP-T as well as WAP-T_NP mice were highly reactive against the immune-dominant LCMV NP-epitope, thereby allowing the analysis of NP-epitope specific cellular immune responses in WAP-T_NP mice. LCMV infection of WAP-T_NP mice induced a strong, LCMV NP-epitope specific CD8⁺ T-cell response, which was able to specifically eliminate T-Ag_NP expressing mammary epithelial cells both prior to tumor formation (i.e. in cells of lactating mammary glands), as well as in invasive tumors. Elimination of tumor cells, however, was only transient, even after repeated LCMV infections. Further studies showed that already non-infected WAP-T_NP tumor mice contained LCMV NP-epitope specific CD8⁺ T-cells, albeit with strongly reduced, though measurable activity. Functional impairment of these ‘endogenous’ NP-epitope specific T-cells seems to be caused by expression of the programmed death-1 protein (PD1), as anti-PD1 treatment of splenocytes from WAP-T_NP tumor mice restored their activity. These characteristics are similar to those found in many tumor patients and render WAP-T_NP mice a suitable model for analyzing parameters to overcome the blockade of immune checkpoints in tumor patients.