5046 Background: Sip-T is an FDA-approved autologous cellular immunotherapy for patients with asymptomatic or minimally symptomatic metastatic CRPC (mCRPC), manufactured from peripheral blood mononuclear cells (PBMCs) cultured with the immunogen PA2024 (prostatic acid phosphatase [PAP] conjugated to granulocyte macrophage colony-stimulating factor). Treatment with sip-T induces cellular and humoral immune responses to both PA2024 and PAP (Antonarakis, ASCO 2015; Petrylak, ECC 2015; Small Clin Cancer Res 2015). To further elucidate the mechanism of sip-T–induced immune responses, we evaluated proliferative and cytolytic characteristics of PA2024- and PAP-specific T cells. Methods: Antigen-specific proliferation of CD4 (T helper [TH]) and CD8 (CTL) T cells was quantified using flow cytometry. To assess CTL lytic activity, cell-surface CD107a expression, indicating lysosomal fusion with the cell membrane and expulsion of lytic enzymes, was measured after stimulation with PAP or PA2024. CTL lytic activity was assessed in 13 patients (from 3 sip-T trials [NCT01431391, NCT01981122, and NCT01487863] in hormone-sensitive PC and mCRPC patients) and 1 healthy donor (control). Results: In sip-T–treated patients, a significant increase in proliferation of both CTLs and TH cells was detected in response to PA2024 (p < 0.05) and PAP (p < 0.10) at week 6 compared with baseline. Increased PA2024-specific proliferation was maintained in TH cells through week 26 (p < 0.05). CTL CD107a expression increased in sip-T patient samples stimulated with PAP and PA2024 but not in the healthy donor control. Preliminary analyses suggest that patients with longer overall survival (OS) may have greater CTL CD107a activity. Conclusions: Sip-T treatment was associated with well-maintained antigen-specific cellular responses, including robust CTL responses, demonstrating sip-T induces antigen-specific tumor lysis and providing a biologic basis for OS benefit in mCRPC patients. Ongoing work will examine whether patients with longer OS display greater CTL activity. Clinical trial information: NCT01431391; NCT01981122; NCT01487863.