We are halfway through our blog series, in which we map key analytes to the biospecimens where they can be found. After exploring genomic variants and cell-free DNA in earlier posts, we take an in-depth look at tumor-infiltrating lymphocytes (TILs). 

TILs are immune cells that infiltrate and reside within the tumor microenvironment (TME), most commonly including T-cell populations such as CD8+ cytotoxic T cells, CD4+ helper T cells, and regulatory T cells that contribute to anti-tumor immune responses (Figure 1). Because TIL abundance, phenotype, and spatial distribution can reflect the state of the anti-tumor immune response, TILs are widely studied as biomarkers of immunotherapy response1,2. 

They are also clinically relevant both as targets for evaluating immunomodulatory therapies, such as immune checkpoint inhibitors, and as therapeutic starting material in adoptive cell therapies, including the FDA-approved TIL therapy lifileucel (brand name Amtagvi) for the treatment of advanced melanoma2.