Abstract 5508: B lymphocytes are activated during the course of sipuleucel-T treatment, leading to an antigen-specific humoral response

Abstract

Abstract Introduction: Sipuleucel-T is an autologous cellular immunotherapy FDA approved for men with asymptomatic or minimally symptomatic metastatic castrate resistant prostate cancer (mCRPC). Previous clinical data have shown that antibodies specific for the immunizing antigen, PA2024, develop within two weeks post-end of sipuleucel-T treatment. Here, we sought to determine whether B lymphocytes were activated during the course of treatment and to evaluate their phenotypic profile in two ongoing clinical studies (NEoACT and PRoACT). Materials and Methods: Sipuleucel-T is manufactured from peripheral blood mononuclear cells (PBMCs) isolated every two weeks by leukapheresis. The PBMCs are cultured with PA2024, an antigen composed of prostatic acid phosphatase (PAP) fused to granulocyte macrophage-colony stimulating factor (GM-CSF), washed, and then infused into the patient as sipuleucel-T. Each patient receives three infusions. The activation status of B lymphocytes was evaluated by multi-color flow cytometry during the manufacture of sipuleucel-T, before and after culture at each of the three treatment times. B lymphocyte subsets were identified by their expression of CD20, IgD and CD27: CD20+IgD+CD27− naive, CD20+IgD+CD27+ mature activated, and CD20+IgD−CD27+ memory. Activation of each of these subsets was determined by their expression of CD80 and CD96. Antigen-specific B lymphocyte function, in the form of antibody production, was evaluated in the serum of sipuleucel-T-treated subjects before and after treatment. Summary: In the mature activated B lymphocyte subset, an increase in the proportion of lymphocytes expressing CD80+ and CD86+ was detected following culture with PA2024, with the increase in CD86+ mature B lymphocytes being the highest after the second infusion of sipuleucel-T. An increase in mature activated B lymphocytes directly ex vivo, pre-culture, was not observed prior to the second or third infusion. Additionally, CD80 and CD86 expression was upregulated on memory B lymphocytes in response to culture with PA2024 and following the first infusion. Furthermore, PA2024- and PAP-specific antibody responses were detected and maintained 2-22 weeks after the completion of sipuleucel-T treatment.Conclusions: These data demonstrate that B lymphocyte activation is a consequence of ex vivo culture with PA2024 and precedes an antigen-specific humoral response, as detected in the patients’ serum after treatment with sipuleucel-T. Collectively, these findings provide evidence of therapy-induced humoral immune activation in men with mCRPC treated with sipuleucel-T. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5508. doi:10.1158/1538-7445.AM2011-5508