Abstract 512: Leukemia-specific immunity in healthy individuals and patients post-transplant targets phosphorylated tumor antigens
Authors: De La Peña Hugo , Turner James E. , Butler Thomas , Buka Richard , Morton Laura , Penny Sarah , Millar David George , Craddock Charles F. , Pratt Guy , Engelhard Victor H. , Hunt Donald F. , Cobbold Mark 
Published: August 2012

Abstract
The major antigenic targets of endogenous tumor immunity remain unknown although many minor targets have been identified. At a proteomic level, cancer is considered a disease of deregulated cell signal pathway activation; therefore we investigated whether endogenous anti-tumor immunity targeted phosphorylated protein antigens generated through the transformation process. In previous work we identified phosphorylated peptides differentially expressed on primary leukemia samples: 58 for acute myeloid leukemia (AML) samples and 31 for chronic lymphocytic leukemia (CLL). Many of the identified phosphopeptides were derived from established leukemogenic oncoproteins such as MLL, RUNX1, c-Myc, EP300, CXCR4, SKI, GRK, CD19, CD20, NCOA1, and TLK1 among others and differentially expressed between malignant and healthy counterpart tissue (splenic B cells, bone marrow). Using ELISpot and HLA-phosphopeptide multimers, we screened 10 healthy donors, 17 patients in complete remission with underlying AML and 14 patients with CLL. Healthy donors (HD) exhibited a highly heterogeneous reactivity against this panel of phosphoantigens but all responded to at least 10/57 phosphoantigens tested. Further investigation revealed this immunity resided within the central memory CD8 T cell compartment at the same magnitude of responses as those against influenza and adenovirus. By contrast only 2/12 AML and 5/14 CLL patients demonstrated anti-phosphopeptide immunity compared to 10/10 HDs (≥2&lt; 0.01 for both). In contrast immunity against viral and mitogenic stimuli was similar between patients and HDs. Following allogeneic stem cell transplantation the majority (10/12) AML patients reconstituted anti-phosphopeptide immunity. In some cases dramatic expansions, of similar magnitude to cytomegalovirus responses, were observed approaching 1% of CD8 T cells for a single phosphopeptide epitope. In CLL, none of whom were transplanted, we found that over the study period of 12 months that among the patients with anti-phosphopeptide immunity only 1 of 5 patients required treatment compared to 4 of 9 patients requiring treatment where no anti phosphopeptide responses were seen (≥2=NS). Importantly, ex vivo expanded phosphopeptide specific T cells isolated from both healthy individuals and patients post-transplant were able to kill both HLA-matched primary leukemia samples and AML cell lines in a phosphopeptide-specific manner. These data suggest a prominent role for tumor-associated phosphopeptide antigens in the prevention and control of haematolymphoid disease in man.
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 512. doi:1538-7445.AM2012-512