Abstract 509: Immunoglobulin reactivity analysis of chronic lymphocytic leukemia patients unveils Lymphocyte Cytosolic Protein 1 (LCP1) as a novel CLL exosome antigen

Abstract

Abstract Chronic lymphocytic leukemia (CLL) is incurable accumulation of malignant clonal B lymphocytes in the blood, bone marrow, lymph nodes, and spleen. A hallmark of CLL is profound immunosupression mediated by a litany of costimulatory, cytokine, and exosome signals within the microenvironment. One of the most promising recent advances in this disease has been the development of membrane targeted immunotherapeutics such as the monoclonal antibodies rituximab and alemtuzumab. However, a shortfall of most current and experimental immunotherapeutics is the reliance upon cluster of differentiation (CD) antigens that are also common to a variety of healthy tissues. Moreover, the non-obligate nature of many CD markers renders them susceptible to shedding and down- modulation in resistant CLL cases. In recent studies, we and others have identified endogenous adaptive immune responses against known CLL specific antigens such as ROR1. Curiously, this effect appears biased to tumor relevant antigens with defined malignant signaling. Using retrospective antigen identifications as proof of principle, we sought to further investigate humoral reactivity against autologous CLL membrane associated antigens to identify novel targets for immunotherapy. Mass spectroscopic analysis of CLL membrane antigens targeted by patient derived serum IgG repeatedly identified LCP1 as a lymphocyte specific target. Confirmation by immunoblot and ELISA revealed that CLL patients maintain elevated LCP1 IgG response despite an inability to respond to common vaccine antigens such as tetanus toxin, suggesting continued antigenic stimulation. Flow cytometry, RT-PCR, immunoblot, and confocal microscopic analyses revealed few differences in LCP1 protein levels, glyocoslyation, transcripts, and localization between healthy and malignant B lymphocytes. Given the potential for CLL exosomes to provide a vehicle for continued antigenic stimulation as well as a target for future therapeutics we investigated their LCP1 content. We found that LCP1 is an integral component of CLL exosomes and that production of LCP1-laden exosomes is increased upon CLL stimulation. Ongoing studies aimed at addressing the potential role of exosomal LCP1 in CLL will be discussed. Supported in part by NIH and D. Warren Brown Foundation Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 509. doi:1538-7445.AM2012-509