Does HER-2/neu antigen loss in metastatic breast tumors occur under immune pressure?

Dear Sir

Tumor immune editing has been demonstrated in the pre-clinical studies which underscore counter-regulatory function of anti-tumor immune responses in both tumor rejection and tumor escape or recurrence. We were greatly interested in the following article by Santinelli et al.¹ in this journal on discrepancy between primary breast tumors and their metastatic lesions on the status of HER-2/neu expression. Their work clearly demonstrates that a fraction of patients with HER-2/neu positive primary tumors appeared to lose HER-2/neu expression in their distant metastatic lesions. However, there was a concordance in HER-2/neu expression between primary tumors and metachronous lymph node metastasis. Similar observations were made by Lower et al.² where 382 patients with metastatic breast cancer were studied. A fraction of patients with HER-2/neu positive primary lesions also had HER-2/neu negative metastatic lesions. These observations suggest that breast tumors are not stable in the expression of HER-2/neu in vivo. Therefore, it is important to understand the mechanisms by which tumors lose or gain HER-2/neu expression in vivo. Our pre-clinical results complement their findings by demonstrating that HER-2/neu antigen loss occurred under the immune pressure in vivo and in vitro³. Our studies underscored the role of HER-2/neu-specific IFN-γ-secreting T cells in the induction of HER-2/neu antigen loss or down-regulation, which facilitated emergence of an invasive HER-2/neu negative tumor variant. Interestingly, microarray analysis showed that expression of IFN-γ Rα in the relapsed tumor lesion, which had lost HER-2/neu expression under immune pressure, was higher than that in the regressing tumor lesion.⁴ We also showed that presence of the inflammatory cytokine, IL-6, would up-regulate expression of IFN-γ Rα in human breast tumor line, SKBR3, rendering it more susceptible to the IFN-γ-induced down-regulation of HER-2/neu expression (Fig. 1). Based on these pre-clinical studies and other reports emphasizing on counter-regulatory role of IFN-γ in tumor rejection and antigen loss^5–7, and the fact that breast tumors are heterogeneous in the expression of receptors, we suggest that IFN-γ Rα positive tumor clones had lost HER-2/neu expression in the presence of HER-2/neu-specific immune responses, IFN-γ-producing T cells in particular, and became metastatic. On the other hand, IFN-γ Rα negative tumor clones continued to express HER-2/neu antigen in primary tumor lesions. Availability of tumor samples from the patients studied by Santinelli et al.¹ would allow them to determine whether such discordance between primary and metastatic breast tumors in the expression of HER-2/neu may be correlated with the status of IFN-γ Rα expression in the tumors. Further studies are required to determine whether expression of HER-2/neu in primary tumors and loss of this antigen in metastatic lesions may also be associated with the pre-existing HER-2/neu-specific T cell responses in these patients.

Figure 1. IL-6 renders SKBR3 tumors sensitive to the IFN-γ-induced down-regulation of HER-2/neu.

The HER-2/neu positive human SKBR3 cells were cultured in the presence or absence of IFN-γ (50 ng/ml) for 5 days (A), or cultured with IL-6 for 16 hrs and then cultured in the presence or absence of IFN-γ for 5 days (B). Expression of HER-2/neu was evaluated using flow cytometry analysis on the indicated cells (A–B) and RT-PCR analysis of mRNA extracted from the IL-6-pre-treated SKBR3 cells that were cultured in the presence or absence of IFN-γ for 5 days (C).