Immunologic Effects of B Cell Depletion on T Cells in High-Risk Prostate Cancer.

5096 Background: High-risk prostate cancer (HR-PCa) remains one of the leading causes of cancer-related death. Previous studies reported that a high density of immunosuppressive B cells in HR-PCa resection specimens was found to correlate with biochemical failure. In this context, a single arm prospective study assessing neoadjuvant rituximab, an anti-CD20 monoclonal antibody used to deplete B cells, was previously conducted on patients with HR-PCa eligible for radical prostatectomy (“PROTUX”). Here, we performed deep TCR sequencing to elucidate the impact of B cell depletion on T cells in prostate cancer patients. Methods: A single arm study enrolled patients with HR-PCa to receive neoadjuvant rituximab prior to radical prostatectomy (NCT: 01804712). Patients were treated with rituximab 375 mg/m 2 once weekly for 4 treatments followed by prostatectomy within 14 days of the final treatment of rituximab. A matched cohort of patients with HR-PCa who underwent prostatectomy without any prior therapy were used as controls. Specimens from 14 patients were retrieved and genomic DNA was isolated using a QIAamp DNA FFPE Advanced UNG kit. TCR sequencing was performed with Adaptive Biotechnologies using the Ultradeep TCRB v4b assay. Data were then analyzed using the Adaptive immunoSEQ Analyzer. Groups were compared using a Mann-Whitney U test. Statistical analysis and graphing were done in Prism 10 and R. Results: We observed that patients treated with rituximab had higher TCRb productive clonality (0.080 vs 0.026, p = 0.03). Strikingly, the calculated number of T cells present in prostatectomy samples was significantly greater in patients receiving rituximab compared to controls (1.18 cells/ng DNA input vs 0.10 cells/ng, p = 0.002). Maximum productive frequency and the cumulative frequency of the top 10 clones were not significantly different between the two cohorts. However, we found that the T cells from patients who received rituximab had longer TCRb CDR3 regions (proportion CDR3 ≥16 amino acids of 0.232 vs 0.197, p = 0.007), suggesting a less antigen-experienced phenotype. Conclusions: Using specimens from a clinical trial assessing the use of neoadjuvant rituximab in HR-PCa, we found that B cell depletion can significantly affect T cell infiltration, clonality, and phenotype. Given the increasing appreciation for crosstalk between T cells and B cells in mediating anti-tumor immune responses, these findings suggest a context dependent role of B-cells in patients with HR-PCa. Clinical trial information: NCT01804712 .