Tumor-infiltrating myeloid cells induce tumor cell resistance to cytotoxic T cells in mice
Tangying Lu, Rupal Ramakrishnan, Soner Altiok, Je-In Youn, Pingyan Cheng, Esteban Celis, Vladimir Pisarev, Simon Sherman, Michael B. Sporn, Dmitry Gabrilovich
Tangying Lu, Rupal Ramakrishnan, Soner Altiok, Je-In Youn, Pingyan Cheng, Esteban Celis, Vladimir Pisarev, Simon Sherman, Michael B. Sporn, Dmitry Gabrilovich
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Research Article Immunology

Tumor-infiltrating myeloid cells induce tumor cell resistance to cytotoxic T cells in mice

Abstract

Cancer immunotherapeutic approaches induce tumor-specific immune responses, in particular CTL responses, in many patients treated. However, such approaches are clinically beneficial to only a few patients. We set out to investigate one possible explanation for the failure of CTLs to eliminate tumors, specifically, the concept that this failure is not dependent on inhibition of T cell function. In a previous study, we found that in mice, myeloid-derived suppressor cells (MDSCs) are a source of the free radical peroxynitrite (PNT). Here, we show that pre-treatment of mouse and human tumor cells with PNT or with MDSCs inhibits binding of processed peptides to tumor cell–associated MHC, and as a result, tumor cells become resistant to antigen-specific CTLs. This effect was abrogated in MDSCs treated with a PNT inhibitor. In a mouse model of tumor-associated inflammation in which the antitumor effects of antigen-specific CTLs are eradicated by expression of IL-1β in the tumor cells, we determined that therapeutic failure was not caused by more profound suppression of CTLs by IL-1β–expressing tumors than tumors not expressing this proinflammatory cytokine. Rather, therapeutic failure was a result of the presence of PNT. Clinical relevance for these data was suggested by the observation that myeloid cells were the predominant source of PNT in human lung, pancreatic, and breast cancer samples. Our data therefore suggest what we believe to be a novel mechanism of MDSC-mediated tumor cell resistance to CTLs.

Authors

Tangying Lu, Rupal Ramakrishnan, Soner Altiok, Je-In Youn, Pingyan Cheng, Esteban Celis, Vladimir Pisarev, Simon Sherman, Michael B. Sporn, Dmitry Gabrilovich

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Figure 2

PNT affects binding of the peptides to MHC class I.

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PNT affects binding of the peptides to MHC class I. (A) Pre-treatment of...
(A) Pre-treatment of target cells with PNT decreased binding of the specific peptide. EL-4 cells were treated with 0.1 mM PNT before or after loading with specific or control peptides at the indicated concentrations. Specific peptide loaded on MHC class I was detected by florescence-conjugated anti-SIINFEKL bound to H-2Kb. Typical result of 1 of 5 performed experiments is shown. (B) Effect of PNT treatment on the expression of MHC class I (H-2Kb) molecules on EL-4 tumor cells. The MFI of H-2Kb expression is shown. Data represent mean ± SEM from 3 performed experiments. (C) Effect of pre-treatment of T2 human cells with SIN-1 on the binding of HLA-A2–matching human survivin-derived peptides. “Background” indicates T2 cells incubated without peptides. Mean ± SEM of 3 experiments is shown. P < 0.05, untreated versus SIN-1–treated cells for each experimental point. (D) Binding of non-modified HLA-A2–matched human TERT-derived PVYAETKHFL and nitrated PVY(NO2)AETKHFL peptides to T2 cells. Mean ± SEM of 3 experiments is shown. P < 0.05, non-modified versus nitrated peptides for each experimental point. (E and F) PNT does not affect expression of pMHC on cells expressing single-chain H-2Kb-SIINFEKL protein. LLC (E) or B16-F10 (F) cells expressing single-chain H-2Kb-SIINFEKL were treated with PNT at indicated concentrations and then labeled with anti-NT or pMHC Abs. For each cell line, 2 experiments with the same results were performed. (G) PNT does not affect CTL killing of tumor cells expressing single-chain H-2Kb-SIINFEKL protein. Experiment was performed as described in F. LLC and LLC-H-2Kb-SIINFEKL cells were mixed at a 1:1 ratio and were untreated or pretreated with PNT, then used as targets for CTLs. Two experiments were performed, and cumulative results are shown.