Definition and Characterization of a Peptide Warehouse for the Patient-Individualized Peptide Vaccination Study (iVAC-L-CLL01) after First Line Therapy of CLL

The treatment of chronic lymphocytic leukemia (CLL) has substantially advanced over the last decade with chemo/immunotherapy as standard of care achieving overall response rates up to 90%. However, besides allogeneic stem cell transplantation, so far no curative treatment for CLL is available. This is mostly due to the persistence of residual CLL cells after therapy (minimal residual disease, MRD), which cause disease relapse. Thus, novel strategies such as peptide-based immunotherapy aiming to eliminate MRD are needed to achieve lasting disease remission or even cure. With this aim, we implemented a first-in-man multi-centric clinical phase II study (iVAC-L-CLL01, NCT02802943), in which patient-individualized peptide vaccination with or without lenalidomide as systemic immune modulator is applied after first-line therapy of CLL. The peptide vaccine cocktail consists of 10 tumor-associated peptides selected according to the HLA ligandome of the individual patients from a pre-manufactured "peptide warehouse". This peptide warehouse contains CLL-associated antigens, which are frequently and exclusively presented by HLA molecules on the surface of CLL cells. The peptides have been selected using a direct mass spectrometry based comparative analysis of the HLA ligandome of 30 CLL patients and a normal tissue ligandome database (Kowalewski et al. PNAS, 2015). The warehouse includes 35 HLA class I and 5 HLA class II peptides. The HLA class I peptides are restricted by the 7 most common HLA class I allotypes (A*01, A*02, A*03, A*24, B*07, B*08 and B*44). Prior to clinical application, the immunogenicity of the warehouse peptides was validated using different T-cell assays. We performed IFNg ELISpot assays using PBMC obtained from CLL patients and healthy donors. Peptide-specific memory T-cell responses were detected for 20/35 (57%) HLA class I peptides (A*01, A*02, A*03 and B*08) and 5/5 (100%) HLA class II peptides in samples obtained from CLL patients. Immune responses in healthy donors could not be detected for any of the 40 warehouse peptides, indicating a strictly CLL directed T-cell response. The immunogenicity of the remaining peptides could not be approved by detection of spontaneous memory T-cell responses in CLL patients due to lack of HLA-matched CLL-patient samples. Hence, we implemented artificial APC-based in vitro priming assays using CD8⁺ T cells of healthy donors proofing the ability of the peptides to prime naïve CD8⁺ T cells. For all 15 analyzed peptides (HLA A*24, B*07 and B*44), an induction of tetramer positive CD8⁺ T cell populations with frequencies of 0.1-35.1% could be detected. Therefore, all 40 warehouse peptides could be confirmed as immunogenic T-cell epitopes

The iVAC-L-CLL01 study is recruiting since October 2016. Vaccination of the first included study patient was performed in July 2017.