T-cell Aggregation in Antigen-presenting Pockets Promotes Tumor Killing

While CD8+ T-cell infiltration and activation can predict successful response to immunotherapy, the interactions facilitating local tumor cell killing remain unclear. In this study, Ludin, Stirtz, and colleagues used long-term in vivo imaging of zebrafish to evaluate T-cell dynamics in endogenous melanomas. In all tumors analyzed, CD8+ cells were found to localize to pockets devoid of, yet surrounded by, melanoma cells on the tumor surface. These ∼50-μm regions, termed cancer regions of antigen presentation and T-cell engagement and retention (CRATER), formed at the perivascular area and displayed significantly higher CD8+ T-cell density compared to the rest of the tumor. CD8+ T cells remained in CRATERs for extended periods, contacting either blood vessels or melanoma cells. Elevated expression of b2m, a component of MHC class I, was identified in CRATERs. Melanoma-specific b2m depletion decreased CD8+ T-cell enrichment in CRATERs, indicating that CD8+ T cells recognize and engage tumor cells within these regions. Treatment with CpG oligodeoxynucleotides, which function as TLR agonists, inhibited tumor growth and increased CRATER size, tumor coverage, and CD8+ T-cell density, which was abolished by melanoma b2m knockout or T-cell depletion. CpG oligodeoxynucleotide treatment elevated both IFNγ expression and direct contact between CD8+ T cells and apoptotic melanoma cells within CRATERs. CRATERS were also identified in primary and metastatic human melanoma samples, with melanoma cells expressing high levels of HLA-A, an MHC class I molecule, and dendritic cells presenting high levels of MHC class II molecules and melanoma antigens. Moreover, CRATERs showed elevated PD-L1 expression. In patient samples obtained post-immune checkpoint blockade (ICB) therapy, higher CRATER density was detected in those who became responders compared to those who became non-responders. Moreover, CRATER density increased with ICB treatment within individual responding patients, linking CRATERs to clinical outcome despite high PD-L1 expression. Altogether, this study identifies CRATERs as physical regions that facilitate antitumor immunity and possible biomarkers of response to immunotherapy.Ludin A, Stirtz GL, Tal A, Nirmal AJ, Pfaff KL, Manos M, et al. CRATER tumor niches facilitate CD8+T cell engagement and correspond with immunotherapy success. Cell 2025;188:6720–36.Note: Research Watch is written by Cancer Discovery editorial staff. Readers are encouraged to consult the original articles for full details. For more Research Watch, visit Cancer Discovery online at https://aacrjournals.org/cdnews.