Cells in tumor tissue

The most complex lineage of non-malignant cells infiltrating the tumor are immune cells, i.e., tumor-infiltrating lymphocytes/leukocytes (TILs). Basically, all immune subpopulations can infiltrate a tumor, regardless of whether they exert tumor-suppressive or tumor-promoting functions. While cells such as cytotoxic T cells, T helper cells, and M1 macrophages are tumor suppressive, regulatory T cells or M2 macrophages usually promote tumor growth (PMID: 24122236). The frequency of TILs is highly variable and ranges mostly between 2% and 50%. The overall TIL population can be easily detected by their ubiquitous and specific expression of CD45. In addition, dozens of TIL subpopulations can be defined by the expression of surface marker combinations.

Cancer-associated fibroblasts play a key role in the modulation of a tumor’s extracellular matrix and can be present in high frequencies inside the tumor and the tissue surrounding the tumor. The effect of transforming tissue into dense fibrotic structures is called desmoplasia and represents a common phenomenon in many carcinomas, including breast or pancreatic cancer (PMID 14581350). Besides shaping tissue architecture, fibroblasts also can play an active role in tumor growth promotion and metastasis by secretion of cytokines and pro-tumorigenic extracellular matrix molecules, for example (PMID: 19308067). Fibroblasts can cover more than 50% of the tumor area in tissue sections because of their large size and the deposition of extracellular matrix filling a significant part of the tumor space. Their actual frequency however is quite low. Fibroblasts express markers such as CD44, CD90.1 or CD90.2 (depending on the mouse strain), and vimentin. As these markers are also expressed by other lineages within the tumor, marker combinations are required for unambiguous fibroblast identification.

To assure oxygen and nutrient supply, tumors induce vascularization by endothelial cells and pericytes. However, the frequency of these cell types usually is below 4%. Endothelial cells can be detected using the markers CD31 and CD146. Specific detection also requires marker combinations such as CD45^–CD31⁺.

In general, the frequency of fibroblasts and endothelial cells is lower in mouse tumor models compared to the corresponding human tumors due to the faster tumor growth kinetics in mice.

Solid tumors are infiltrated by cells of non-tumor origin, including heterogeneous lymphocyte subpopulations, fibroblasts, and endothelial cells. The amount and composition of infiltrating cells is highly variable, which makes the analysis of primary tumor samples difficult. In addition, the culture of mouse tumor cells is frequently hampered by fibroblasts overgrowing the target cells, which can bias assays such as drug sensitivity tests.

To overcome these limitations, Miltenyi Biotec has developed the Tumor Cell Isolation Kit, mouse, which provides a fast and easy method to isolate untouched mouse tumor cells from dissociated primary tissue. This cell isolation procedure is based on the comprehensive depletion of cells of non-tumor origin such as TILs, fibroblasts, and endothelial cells based on MACS® Technology. The underlying negative selection strategy enables the isolation of tumor cell populations without knowledge of surface markers on the tumor cells. Even from tissues that initially contain only low numbers of tumor cells (<20%), purities of greater than 95% can be achieved in less than 20 minutes. Enriched tumor cells remain untouched, improving downstream analyses.

CD326 (EpCAM) MicroBeads, mouse allow for the positive selection or depletion of epithelial cells or epithelial tumor cells from single-cell suspensions of tissues expressing the mouse CD326 (EpCAM) antigen. For the enrichment of epithelial cells based on CD326 (EpCAM) MicroBeads, prior depletion of CD45⁺ cells with CD45 MicroBeads is recommended.

To facilitate the analysis of samples from xenograft models, Miltenyi Biotec offers effective methods to separate cells of mouse and human origin. For details, see the chapter Isolation of tumor cells from xenograft models under Tumor cells, human.

Immunotherapy against cancer has proven clinical efficacy and tremendous potential in multiple tumor entities. Syngeneic mouse tumor models represent excellent systems to analyze effects of immunotherapy as they possess a fully competent immune repertoire. However, amounts and composition of TILs are highly variable, which complicates the targeted analysis of subpopulations. In particular, small subpopulations might escape analysis as they can be lost in the background noise. Immunophenotyping of TILs by flow cytometry is time consuming and data processing highly work intensive. Therefore, efficient pre-enrichment of TILs is desirable to increase the sensitivity of analysis and save time and effort during flow cytometry. 

Miltenyi Biotec has established an automated workflow combining gentle tissue dissociation with specific isolation of TILs. The process preserves cell surface epitopes and thus overcomes bias in immunophenotyping, which is a frequent issue with other tissue dissociation procedures that rely on aggressive or impure enzymes. A list of preserved epitopes can be downloaded from the Related resources panel to the right.

CD45 (TIL) MicroBeads, mouse enable the positive selection of the overall CD45⁺ TIL population from suspensions derived from solid mouse tumors, e.g., CT26.WT, 4T1, and B16F10.

A scientific poster presenting TIL isolation from syngeneic mouse tumors with CD45 (TIL) MicroBeads can be downloaded from the Related resources panel to the right.

CD4/CD8 (TIL) MicroBeads are optimized for the isolation of CD4⁺ and CD8⁺ TILs from single-cell suspensions of mouse tumors. For the specific isolation of individual CD4⁺ and CD8⁺ TIL subpopulations CD4 and CD8 (TIL) MicroBeads are available.

A scientific poster presenting the isolation of CD4⁺ and CD8⁺ TILs using MACS MicroBeads can be downloaded from the Related resources panel to the right. 

The frequency of endothelial cells and pericytes in tumor tissue is usually below 4%. Therefore, pre-enrichment is necessary for most downstream assays. Depletion of CD45⁺ cells prior to endothelial cell enrichment with CD31 or CD146 (LSEC) MicroBeads allows specific detection based on markers such as CD31 and CD146.

Flow cytometry is a suitable technique for the identification and quantification of the various lineages found in mouse tumor tissue. Expression of cell surface markers on tumor cells is highly dependent on the cells’ origin. Common epithelial markers are not available, making an analysis difficult. TILs on the other hand can be easily detected by their ubiquitous and specific expression of CD45. A multitude of TIL subpopulations can be defined by the expression of surface marker combinations. Fibroblasts express CD44, CD90.1 or CD90.2 (depending on the mouse strain), and vimentin. As these markers are also expressed by other lineages within the tumor, marker combinations are required for a clear identification of fibroblasts. Endothelial cells can be detected based on the markers CD31 and CD146. However, specific detection requires marker combinations such as CD45^–CD31⁺.

Appropriate flow cytometry antibodies for immunophenotyping of tumors or the characterization of T cells from the tumor microenvironment are listed below.

A list of antibodies that are relevant for cancer research as well as a scientific poster describing a gating strategy for the analysis of TILs can be downloaded from the Related resources panel to the right. Additional details about Miltenyi Biotec antibodies and dyes for cell analysis can be found in the section  MACS Flow Cytometry – antibodies.