Tumor-infiltrating lymphocytes, also tumour infiltrating lymphocytes, are white blood cells that have left the bloodstream and migrated into a tumor. They include T cells and B cells and are part of the larger category of ‘tumor-infiltrating immune cells’ which consist of both mononuclear and polymorphonuclear immune cells, (i.e., T cells, B cells, natural killer cells, macrophages, neutrophils, dendritic cells, mast cells, eosinophils, basophils, etc.) in variable proportions. Their abundance varies about tumor type and stage and in some cases relate to disease prognosis 
Tumor-infiltrating immune cells can often be found in the stroma and within the tumour itself.
Detection and characteristics
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.
Associations with cancer prognosis
Use in an autologous cell therapy
Use as an adoptive cell transfer therapy
The use of TILs as an adoptive cell transfer therapy to treat cancer was pioneered by Dr. Steven Rosenberg at the National Cancer Institute. Autologous lymphocytes are isolated from patients’ tumors and cultured to large numbers of cells in vitro. Prior to TIL treatment, patients are given nonmyeloablative chemotherapy to deplete native lymphocytes that can inhibit the response. Once lymphodepletion is completed, patients are infused with TILs in combination with interleukin 2 (IL-2). Lion Biotechnologies is developing adoptive cell transfer with TILs as a cancer therapy.
Clinical trials have used TILs to treat patients with metastatic melanoma. Tumor reduction of 50% or more was observed in about half of patients. Some patients experienced complete responses with no detectable tumor remaining years after treatment.
For other cancers
Under investigation are the use of TILs to treat other tumors, including lung, ovarian, bladder, and breast.
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