Background Cancer patients with liver metastases have limited treatment options, especially as only 15–20% are eligible for curative-intent surgical resection.1 Unfortunately, liver metastases also seem to be poorly responsive to immune checkpoint inhibitors (ICI)].2 3 It could be that the unique immunological hallmarks of the liver, including resident macrophages and significant numbers of NK and NKT cells, create a tumor microenvironment that is best suited to alternative forms of immunotherapy that do not rely exclusively on ICI.
Methods We investigated how the presence of T, natural killer (NK), and NKT cells impact overt liver metastases using a model in which tumor cells are delivered to the liver via intraportal injection to hosts that were either wiltype, nude, or nude with NK-depletion. NK cell depletion was achieved via administration of anti-asialo GM1 antibody 2 days before tumor cell injection and for the duration of the experiment until endpoint at 6 weeks post tumor cell injection, with NK cell depletion confirmed by flow cytometry. Tumors were assessed histologically.
Results Using the portal vein model in female nulliparous mice, overt liver metastasis incidence was about 30% across 2 different mammary tumor cell lines. The incidence rose to 80–100% when tumor cells were delivered to hosts in the post-wean window (referred to as involution hosts), mirroring increased breast cancer metastasis to the liver observed in postpartum breast cancer patients.4 Conversely, when tumor cells were delivered to nude hosts, either nulliparous or involution stages, the incidence of metastases dropped to 0–10%. Importantly, tumor cells injected into the mammary gland of nude mice grew robustly with 100% take. Nude hosts lack T cells and NKT cells; however, NK cells are present. Furthermore, the liver is enriched for NK cells, whilst the mammary gland has few NK cells.5 We hypothesized that NK cells, when in the background of T- and NKT-cell depletion (i.e. nude host), restrict outgrowth of mammary tumor cells in the liver. Six weeks after portal vein injection of mammary tumor cells to nude hosts we find increased incidence of metastasis in the NK-depleted group compared to isotype control, as well as increased number of metastases per mouse.
Conclusions Our data suggest that NK cells play an important role in controlling liver metastases in nude hosts, and that NK activity in wild type hosts is insufficient to control liver metastases. Increasing NK cell cytotoxic activity could be an effective immunotherapy strategy to control liver metastases.
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