Validation of Stereotyped Immunoglobulin Heavy Chain CDR3 Sequences As Candidate Antigens for Immunotherapy of CLL

Abstract 1775

The immunoglobulin gene repertoire in CLL is remarkably restricted with greater than 30% of cases carrying quasi-identical (stereotyped)heavy complementarity-determining region 3 (VH CDR3) sequences. Indeed, cases can be clustered into different subsets based on shared, subset-biased motifs within the clonotypic VH CDR3s, with, notably, only a handful of subsets accounting for almost 10% of all CLL. VH CDR3 stereotypes are more frequent in cases with unmutated IGHV genes (U-CLL) who are associated with adverse prognosis. In principle, VH CDR3 stereotypy might allow to exploit these IG motifs as candidate Tumor Associated Antigens (TAA) for targeted immunotherapy of CLL. The aim of our study was to validate as potential TAA subset-specific IG motifs from major CLL subsets, focusing especially on subsets #1 and #2 that are the largest overall and both associated with aggressive clinical course.

We have so far identified, by in silico analysis, 1–3 long peptides (15-mer) encompassing the VH CDR3 protein regions of subsets #1, 2, 4, 6, 8, 10 with (i) high binding score to MHC class II molecules and (ii) also containing minimal HLA class I-specific epitopes (HLA-A2, -A3, -A24, DR1, DR7, DR13 that are most frequent in the Caucasian population).

Blood lymphocytes from 18 CLL patients were collected and phenotyped by flow cytometry with appropriate antibodies to assess the expression of stimulatory, co-stimulatory and negative regulatory molecules on both T and B cells. In addition, HLA typing of CLL patients was performed to select patients expressing the aforementioned HLA molecules. Overall, 13/18 patients matched the defined HLA class I and/or class II molecules.

Negatively purified T cells from 11 CLL patients expressing HLA-A2 and/or DR13 have been then stimulated in vitro with the synthesized peptides of the specific stereotype (subset #1 and 2) in the presence of culture medium containing 5% of human serum plus IL-2 (20 IU/ml) and IL-15 (10 ng/ml). These T lymphocytes were then weekly stimulated with autologous irradiated antigen presenting cells (APC; monocytes, B cells, etc.) pulsed with the peptides. Starting from the third week of culture, the specific recognition of CDR3-derived TAAs and of tumor cells (autologous CLL cells) by the T cell cultures has been assessed by in vitro functional assays (ELISPOT assay).

We were able to isolate CDR3- (subsets #1 and #2) and tumor-specific T cells from 5/11 CLL patients. In addition, in 4 selected patients the Ag- and tumor specific T lymphocytes have been expanded in vitro by Rapid Expansion Protocol (REP), based on the stimulation of T cells with allogeneic irradiated PBMCs from healthy donors plus OKT3 and high doses of IL-2. Using this protocol we were able to obtain large numbers (2–10 ×10⁹) of anti-CDR3 T cells in all 4 cases tested, thereby, in principle, achieving the potential to use this protocol for expanding sufficient cells for clinical applications. Interestingly, post-REP T cell cultures showed enrichment (85–90%) of CD3⁺CD8⁺ T cells and down-modulation of negative regulatory molecules, such as CTLA-4, as compared to pre-REP in vitro stimulated T cells. These cells could be expanded in vitro for up to 6 weeks without any decay in proliferation.

Taken together, these results indicate that stereotyped VH CDR3 peptide sequences can represent candidate antigens to elicit T cell-mediated anti-CLL responses, especially in poor prognosis cases, where therapeutic innovation is more urgently needed. After validation of this protocol in a larger series, our results may provide the proof of principle for the design of new immunotherapy protocols for CLL, including both active vaccination and adoptive cell therapy.