Abstract 5203: Naturally occurring colorectal cancer in rhesus macaques as model for human cancer immunotherapy: a first report on pembrolizumab ± radiotherapy
Authors: Deycmar Simon , Schaaf George W. , Johnson Brendan J. , Bourland J. Daniel , Ververs James D. , Whitlow Christopher T. , Cline J. Mark 
Published: April 2023

Introduction: Rhesus macaques share extensive similarities with humans including immune cell populations, regulatory mechanisms, and targets for cancer immunotherapy but also naturally develop cancers at similar life stages. Hence, treating rhesus macaques with spontaneous tumors bears enormous potential as model for human cancer immunotherapy.

Material and Methods: We identified 2 female rhesus macaques, 23.6y (CRC_1) and 20.3y (CRC_2) old with naturally occurring colorectal cancer by imaging and clinical assessments. We collected a pre-treatment tumor biopsy in CRC_1 prior to treatment with 8 Gy intensity-modulated radiotherapy (IMRT) to the proximal portion of the tumor and 2 mg/kg i.v. pembrolizumab. The second candidate (CRC_2) was not tumor biopsy-eligible due to anemia and was treated with pembrolizumab alone. We collected blood and peripheral lymph nodes (LNs) pre- and on-treatment and complete tissue collection was performed at the 3w time point including tumor-adjacent mesenteric LNs. Follow-up assessments include IHC (e.g. CD3, CD20, IBA1), flow cytometry, cytokine assessment in plasma, and transcriptomics of LN and tumor samples.

Results and Discussions: Imaging revealed focal thickening of the proximal colonic wall with disrupted architecture in CRC_1 and focal colonic thickening and an enlarged adjacent LN in CRC_2. Both tumors displayed mismatch repair-deficiency (MLH1 & PMS2 loss) and a drastic reduction in infiltrating CD3 cells in neoplastic areas compared to adjacent healthy colon. On treatment, tumor-associated LNs demonstrated a mild increase in CD20+ cells in the medullary sinuses of both patients and increased follicle numbers with normal follicular architecture in reference to peripheral LNs. In both patients there was an increased number of tertiary lymphoid structures (TLS) surrounding the tumors, predominately populated with CD20+ cells and with Ki67+ germinal centers, profoundly so in CRC_2. Moreover, IBA1+ cells abundantly infiltrated both tumors but showed a higher aggregation within the stromal elements of the neoplasms. A gradient of histologic differentiation was evident in CRC_1 and coincided with a reduction of TLS, IBA1+ cell abundance, and CD3+ cell infiltration in those areas which were poorly differentiated. Within the irradiated tumor of CRC_1, there were abundant apoptotic cells which stained positively with cleaved caspase 3 at the 3w time point. Circulating blood counts as preliminary peripheral read-out revealed a reduction in circulating lymphocytes in both patients beginning at 3d (CRC_1) resp. 6d (CRC_2) and a sharp increase in monocytes at 3d in unirradiated patient CRC_2.

Conclusion: The immunological and tumoral response to pembrolizumab and radiotherapy matches the clinical response in humans and enables an unprecedented examination of the peripheral and local lymph node response.

Citation Format: Simon Deycmar, George W. Schaaf, Brendan J. Johnson, J. Daniel Bourland, James D. Ververs, Christopher T. Whitlow, J. Mark Cline. Naturally occurring colorectal cancer in rhesus macaques as model for human cancer immunotherapy: a first report on pembrolizumab ± radiotherapy. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5203.