Abstract 1092: Trogocytosis of HER2 overexpressing human breast cancer cell lines may induce its dormancy without depending on HER family signal
Authors: Aratake Junichi , Suzuki Eiji , Uehiro Natsue , Nishie Mariko , Kawaguchi Kosuke , Sato Fumiaki , Toi Masakazu 
Published: May 2015

Abstract
We and others showed that HER2 loss of the target cancer cell was caused by shaving of HER2 by CD14+ cells (known as trogocytosis) when trastuzumab mediated antibody dependent cellular cytotoxicity (ADCC) was introduced in the HER2 positive breast cancer cells. In the current study, we report that loss of HER2 and HER3 of SK-BR-3 and BT-474 (both HER2 overexpressing human breast cancer cell lines) is caused via uptake of the receptors by CD14+ cells without trastuzumab administration. This cell-cell contact phenomenon is quite similar to the situation of the interaction between circulating tumor cell (CTC) and immune cells in the blood stream. Interestingly, the cancer cells in contact with immune cells (trogocytosis occurs) exhibited a long term dormancy in vitro. Therefore this in vitro trogocytosis concept may explain how cancer cell changes their proliferation ability into dormancy in tumor microenvironment interacting with immune cells.
Briefly, SK-BR-3 or BT-474 cells were co-cultured with PBMCs for 60 to 90-min with various Effector immune cell / Target cancer cell (E/T) ratio as trogocytosis assay. The cell suspension of 90-min incubation was applied to flow cytometry to evaluate the expression level of HER1, HER2, and HER3. The cell suspension of 60-min incubation was applied to MACS cell separation kit (Miltenyi Biotec) and separated to cancer cells and immune cells. Gene expression profile of the separated cancer cells was tested by DNA microarray and the candidate up-regulated and down-regulated shedding of HER family related genes were re-confirmed by qRT-PCR thereafter. The separated cancer cells and intact SK-BR-3 were cultured and compared their proliferation ability.
Higher E/T ratio induced higher loss of HER2 and HER3 on both SK-BR-3 and BT-474 while certain amount of HER2 and HER3 were still expressed on the cancer cells with an optimized E/T ratio. There was no HER1 loss on the cancer cells after trogocytosis. SK-BR-3 that underwent trogocytosis (SK-T) showed extremely low proliferation ability as compared to the intact cancer cells. The candidate genes that might be related to proteolytic shedding of HER family tested by DNA microarray were ADAM-10, -15, and -17 and MMP-1, -3, and -28. However, such different expression profile was not confirmed when the cells were tested by qRT-PCR.
These findings suggested that loss of HER2 and HER3 of the cancer cells during contact with immune cells might not be affected by proteolytic activity change of ADAM or MMP. Although it is reported that the target cells after trogocytosis survive without apoptosis, our study revealed that trogocytosis with higher E/T ratio induced incomplete apoptosis of target cancer cell, which result in the cancer cell dormancy. Because the cancer cells that underwent trogocytosis became quiescent even if some extent of the receptors were present, factors other than HER family that affect on the dormancy still remain to be identified.
Citation Format: Junichi Aratake, Eiji Suzuki, Natsue Uehiro, Mariko Nishie, Kosuke Kawaguchi, Fumiaki Sato, Masakazu Toi. Trogocytosis of HER2 overexpressing human breast cancer cell lines may induce its dormancy without depending on HER family signal. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1092. doi:10.1158/1538-7445.AM2014-1092