The use of Trastuzumab (Herceptin), a monoclonal antibody (mAb) targeting HER2/neu, results in an increased median survival in Her2 + breast cancer patients. the tumour mutational burden and the presence of tumour infiltrating lymphocytes (TILs) clearly correlate with response to trastuzumab. Here, we investigated if the immunogenicity of the transplantable rat-neu + tumour cell line (tUBo) derived from a BALB/c-NeuT primary tumour is associated with the response to anti-neu mAb therapy. We compared the TUBO tumour outgrowth and tumour infiltrating T cells in isogenic (BALB/c-NeuT) and non-isogenic (WT BALB/c) recipient mice. Furthermore, therapeutic efficacy of anti-neu mAb and the contribution of t cells were examined in both mouse strains. the outgrowth of untreated tumours was significantly better in BALB/c-NeuT than WT BALB/c mice. Moreover, tumour infiltrating T cells were more abundantly present in WT BALB/c than BALB/c-NeuT mice, showing that the TUBO tumour was more immunogenic in WT BALB/c mice. In TUBO tumour bearing WT BALB/c mice, anti-neu mAb therapy resulted in an increase of tumour infiltrating T cells and long-term survival. When T cells were depleted, this strong anti-tumour effect was reduced to an outgrowth delay. In contrast, in TUBO tumour bearing BALB/c-NeuT mice, treatment with anti-neu mAb resulted only in tumour outgrowth delay, both in the presence and absence of t cells. We concluded that in immunogenic tumours the response to anti-neu mAb therapy is enhanced by additional t cell involvement compared to the response to anti-neu mAb in non-immunogenic tumours.Overexpression of oncogenic Her2 protein occurs in 15-20% of breast cancers and is associated with highly aggressive disease. Trastuzumab, a humanized IgG1 monoclonal antibody targeting Her2, is the standard therapy for Human epidermal growth factor receptor 2 (HER2/Erbb2/neu) overexpressing breast cancer owing to its apparent efficacy in adjuvant and neoadjuvant uses 1,2 . This antibody was designed to disrupt the ligand-independent HER2-HER2 interaction resulting in rapid inhibition of pro-survival signalling pathways, leading to cell cycle arrest of the cancer cells 3,4 . The clinical success of Trastuzumab has paved the way for devising novel Her2 targeting approaches in breast cancer treatment. To date, two other therapeutic agents are available to inhibit her2 mediated signalling, a monoclonal antibody (pertuzumab) and tyrosine kinase inhibitors (lapatinib, neratinib). However, a recent clinical study has shown that Trastuzumab therapy increases the pathological complete response (pCR) in patients more than laptinib 5 . This can be attributed to the ability of Trastuzumab to engage the immune system to achieve tumor killing. Several preclinical studies have suggested that innate immune responses are essential to anti-Her2/neu mAb cancer therapies through the recruitment of Fcγ receptor (FcγR) expressing immune cells which can mediate antibody-dependent cellular cytotoxicity (ADCC) 6-8 . This is supported by the observation th...