P12.03 Heterotypic CD8 T cell clusters isolated from clinical samples are distinct and enriched for antitumor activity

Background An increasing body of evidence suggests that in addition to the type, density, and state of immune cells in the tumor microenvironment (TME), also their proximity to cancer cells influences immunotherapy outcome. For example, favorable responses to immune checkpoint inhibitors in melanoma are associated with higher densities of CD8+ tumor-infiltrating lymphocytes (TIL) within 20 μm distance of melanoma cells. This notion is in line with the understanding that upon specific antigen recognition, cytotoxic T cells physically engage with their target cells through their TCRs followed by immunological synapse formation. Indeed, structural and functional avidity of cytotoxic CD8+ T cells correlates strongly with their activity against cancer cells. Together, these observations point to the importance of direct interactions between cytotoxic T cells and tumor cells in the TME. This led us to investigate whether tumor-specific CD8+ T cells can be isolated from clinical cancer specimens as heterotypic clusters. Materials and Methods We employed a tumor cell-T cell co-culture in vitro model, patient samples and ex vivo assays. To evaluate functional interactions between human T cells and tumor cells, we made use of a system we engineered previously, comprising melanoma cells expressing both HLA-A*02:01 and the MART-1 tumor antigen. They were challenged with CD8+ T cells from PBMCs that were retrovirally transduced with a MART-1-specific TCR. To asses these interactions in patient material, upon surgical removal tissue was cut into small fragments, digested and analyzed by (image-based) flow cytometry. Interacting (cluster) and not-interacting (singlets) T cells were isolated and expanded in vitro. To characterize tumor cell:T cell interactions single cell TCR and RNA sequencing is used, as well as ex vivo co-cultures with autologous tumor cells. Results We found that in defined co-cultures, tumor antigen-recognizing T cells were commonly enriched over non-specific T cells in heterotypic clusters with tumor cells, prompting us to investigate whether such specific clusters could be isolated also from cancer specimens. We observed that from 10/10 human melanoma metastases, we were able to isolate heterotypic clusters, comprising CD8+ T cells interacting with one or more tumor cells and/or antigen-presenting cells (APCs), which was validated by imaging flow cytometry. Upon expansion, CD8+ T cells from tumor cell clusters and APC clusters exerted on average 7.6-fold increased melanoma-killing activity over T cell singlets, which was associated with enhanced cytokine production. CD8+ T cells from clusters were enriched for tumor-reactive and exhausted gene signatures. Integration with T cell receptor (TCR)-sequencing showed increased clonality of clustered T cells, indicative of expansion upon antigen recognition. Conclusions Together, these results demonstrate that tumor-reactive CD8+ T cells are enriched in functional clusters with tumor cells and/or APCs, and that they can be isolated and expanded from clinical samples. Being often excluded in cell sorting procedures, these distinct heterotypic CD8+ T cell clusters serve as a valuable source amenable to deciphering functional tumor-immune cell interactions, while they may also be therapeutically explored. S. Ibáñez Molero: E. Ownership Interest (stock, stock options, patent or other intellectual property); Modest; P097110NL. J. Veldman: E. Ownership Interest (stock, stock options, patent or other intellectual property); Modest; P097110NL. J. J H Traets: None. A. George: None. K. Hoefakker: None. S. Pack: None. L. Tas: None. P. Alóndiga-Mérida: None. B. van den Broek: None. R. Harkes: None. M. Nieuwland: None. M. van Baalen: None. E. Mul: None. S. Tol: None. J.B.A.G. Haanen: B. Research Grant (principal investigator, collaborator or consultant and pending grants as well as grants already received); Modest; Amgen, Asher Bio, BioNTech, BMS, MSD, Novartis, Sastra Cell Therapy. E. Ownership Interest (stock, stock options, patent or other intellectual property); Modest; Neogene Tx. F. Consultant/Advisory Board; Modest; BMS, CureVac, GSK, Imcyse, Iovance Bio, Instil Bio, Immunocore, Ipsen, Merck Serono, MSD, Molecular Partners, Novartis, Pfizer, Roche/Genentech, Sanofi, Scenic, Third Rock Ventures, Achilles Tx, BioNTech US, Instil Bio, PokeAcell, T-Knife, Scenic, Neogene Therapeutics. W.J.V. Houdt: None. D.S. Peeper: B. Research Grant (principal investigator, collaborator or consultant and pending grants as well as grants already received); Modest; Oncode Institute, Dutch Cancer Society KWF. E. Ownership Interest (stock, stock options, patent or other intellectual property); Modest; P097110NL, Immagene. F. Consultant/Advisory Board; Modest; Immagene.