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Here, we present a method using permeable membrane supports to facilitate the study of non-contact paracrine signaling used by tumor cells to suppress the immune response. The system is amenable to studying the role of tumor-secreted factors in dampening macrophage activation.
Tumor-derived paracrine signaling is an overlooked component of local immunosuppression and can lead to a permissive environment for continued cancer growth and metastasis. Paracrine signals can involve cell-cell contact between different cell types, such as PD-L1 expressed on the surface of tumors interacting directly with PD-1 on the surface of T cells, or the secretion of ligands by a tumor cell to affect an immune cell. Here we describe a co-culture method to interrogate the effects of tumor-secreted ligands on immune cell (macrophage) activation. This straightforward procedure utilizes commercially available 0.4 µm polycarbonate membrane permeable supports and standard tissue culture plates. In the process described, macrophages are cultured in the upper chamber and tumor cells in the lower chamber. The presence of the 0.4 µm barrier allows for the study of intercellular signaling without the confounding variable of physical contact, because the two cell types share the same medium and exposure to paracrine ligands. This approach can be combined with others, such as genetic alterations of the macrophage (e.g., isolation from genetic knock-out mice) or tumor (e.g., CRISPR-mediated alterations) to study the role of specific secreted factors and receptors. The approach also lends itself to standard molecular biological analyses such as quantitative reverse transcription polymerase chain reaction (qRT-PCR) or Western blot analysis, without the need for flow sorting to separate the two cell populations. Enzyme-linked immunosorbent assays (ELISAs) can similarly be utilized to measure secreted ligands to better understand the dynamic interaction of cell signaling in the multiple cell type context. Duration of co-culture can also be varied for the study of temporally regulated events. This co-culture method is a robust tool that facilitates the study of tumor-secreted signals in the immune context.
Recent studies have focused on the ability of cancer cells to avoid detection by immune cells, suppress local immune activation, or to produce a tolerogenic tumor-permissive milieu in the tumor microenvironment. Two broad classes of tumor and immune cell interactions have been described that facilitate these effects: contact-mediated interactions or tumor-secreted ligands. One of the most well-studied and clinically tractable mechanisms of contact-mediated immune inhibition utilized by tumors is the expression of PD-L1, which interacts with PD-1 on T cells to inhibit their activation and function1,2. In response to interferon-gamma (IFNγ), which is expressed by a number of activated immune cells, tumor cells can increase expression of PD-L1 to induce exhaustion of PD-1–expressing activated T cells, thereby preventing them from effectively eradicating tumor cells3. The use of antibodies to block the interaction between PD-L1 and PD-1 is currently used to treat multiple cancer types in humans4. In light of this clinical success and others, the identification and targeting of tumor-derived immunosuppressive mechanisms has received increasing attention.
Beyond suppression of adaptive immunity, tumors are also known to secrete factors that suppress the pro-inflammatory responses of innate immune cells. Tumor-derived or tumor-induced secretions, including IL-6, IL-10, VEGF, IL-23, and colony stimulating factor (CSF-1), have been shown to inhibit antitumor responses of natural killer (NK) cells, granulocytes, and dendritic cells in the tumor microenvironment5,6,7. Tumor cells can also secrete factors that skew the recruitment and differentiation of myeloid-derived cells in the tumor microenvironment to promote suppression of T cell activation8,9.
One type of innate immune cell that has a profound effect on tumor progression is the macrophage. For many years, the presence of tumor-associated macrophages (TAMs) has been used as a negative prognostic of patient survival10. The concept that immunosuppressive TAMs dampen immune cell-mediated clearance of tumors was introduced more than 40 years ago11. More recently, it has been shown that the macrophage pro-inflammatory response can be downregulated while a pro-tumor phenotype can be induced in the tumor microenvironment. These immunosuppressive macrophages can contribute to a tolerogenic response, driving tumor progression and resistance to chemo- and immunotherapy12. Given that macrophages are often one of the most abundant leukocytes with the tumor, restoration of their tumor-specific immune activity represents a potential target for anticancer therapeutics13.
While contact-mediated interactions between tumor cells and macrophages can be modeled through direct coculture, the use of permeable membrane supports can elucidate which tumor-secreted factors are immunomodulatory without the potentially confounding influence of tumor-immune cell-cell contact. Using somewhat similar methods, others have demonstrated the potential of identifying secreted factors in microglia/neuronal interactions14 as well as tumor cells with mesothelial cells15. We have also successfully used this co-culture technique to characterize the role of a tumor-secreted protein, Pros1, as a suppressor of pro-inflammatory gene expression after the stimulation of peritoneal macrophages with LPS and interferon-gamma16. Here we describe a straightforward methodology that can be used to interrogate how tumor-secreted factors can affect macrophage activation.
All procedures related to the harvest and use of murine peritoneal macrophages were conducted at the University of North Carolina at Chapel Hill (UNC) and were approved by the UNC Institutional Animal Care and Use Committee (IACUC).
1. Macrophage culture
NOTE: This procedure can utilize primary peritoneal macrophages (described in detail below), bone marrow-derived macrophages, or macrophage cell lines such as J774 (ATCC) or RAW264 (ATCC).
2. Coculture of tumor cells with macrophages
3. Treatment of co-cultured cells
4. Downstream analysis of co-cultured cells
To determine the effect of tumor-secreted ligands on macrophage polarization, the procedure described was utilized. Peritoneal macrophages cultured in the absence of tumor cells were used as negative (untreated = far left) and positive (IFNγ and LPS stimulated = 2nd from left) controls (Figure 2A). Alternatively, peritoneal macrophages were co-cultured with B16F10 melanoma tumor cells (ATCC) (Figure 1A). Immediately after plating, cells were eith...
The co-culture assay presented here is a modification of previously established assays that allows for the study of tumor-secreted factors on immune cell activation. While cell-cell contact is known to induce changes in immune activity, the ability of tumor-secreted ligands to modulate immune activation is less well understood. We describe a method which, unlike direct co-culture, can be used to interrogate how tumor-derived secreted factors impact immune cell activation without the confounding nature of contact-mediated...
The authors have nothing to disclose.
Eric Ubil was funded, in part, by the American Cancer Society Postdoctoral Fellowship (128770-PF-15-216-01-LIB). The work was supported by a grant from the NIH (R01-CA205398) and a Breast Cancer Research Foundation award (BCRF-18-041) to HSE.
Name | Company | Catalog Number | Comments |
B16-F10 | ATCC | ATCC CRL-6475 | |
cDNA synthesis kit | Promega | A3500 | |
DMEM/F12 media | ThermoFisher Scientific- Gibco | 11320033 | |
Fetal Bovine Serum | Millipore | TMS-013-B | |
J774A.1 | ATCC | ATCC TIB-67 | |
Lipopolysaccharides from Escherichia coli O111:B4 | Sigma-Aldrich | L5293-2ML | |
Murine M-CSF | Prospec | CYT-439 | |
Penicillin-Streptomycin (10,000 U/mL) | ThermoFisher Scientific- Gibco | 15140122 | |
Pros1 ELISA | MyBioSource | MBS2886720 | |
RAW264.7 | ATCC | ATCC TIB-71 | |
Recombinant Mouse IFNγ | BioLegend | 575302 | |
Sensimix SYBR Low-ROX kit | Bioline | QT625-05 | |
Transwell permeable supports | Fisher Scientific | 07-200-170 | |
Trypsin-EDTA | ThermoFisher Scientific- Gibco | 25200056 |
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