Tumor-infiltrating lymphocytes

Tumor-infiltrating lymphocytes, also tumour infiltrating lymphocytes, are white blood cells that have left the bloodstream and migrated into a tumor. They are mononuclear immune cells, a mix of different types of cells (i.e., T cells, B cells, NK cells, macrophages) in variable proportions, T cells being the most abundant cells.

They can often be found in the stroma and within the tumour itself.

TILs are implicated in killing tumor cells. The presence of lymphocytes in tumors is often associated with better clinical outcomes.

When TILs are present the lymphocytes are found between the tumor cells; cells in the stroma surrounding the tumor cells do not count. It should be noted that histologic definitions for TILs vary.

CD3 has been used to detect lymphocytes in tumor samples. Tumor immune infiltration can also be determined using gene expression methods like Micro array or RNA Sequencing. Detection of gene expression specific for different kind of immune cell populations can then be used to determine the degree of lymphocyte infiltration as has been shown in breast cancer. An active immune environment within the tumor often indicates a better prognosis as can be determined by the Immunological constant of rejection.

In colorectal cancer, they are associated with microsatellite instability cancers, as may be seen in Lynch syndrome.

TILs are needed for checkpoint inhibitor therapy to work in GI cancers.

They are an important prognostic factor in melanoma and higher levels being associated with a better outcome.

They are key to an experimental autologous cell therapy (Contego) for metastatic melanoma.

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