Postdoc University Health Network Toronto, Ontario, Canada
Introduction: The BCMA-targeting antibody drug conjugate (ADC), belantamab mafodotin (belamaf) has been studied as monotherapy and has demonstrated clinically meaningful progression free survival (PFS) (DREAMM-7 and DREAMM-8 studies) and overall survival (DREAMM-7) benefits in combination regimens. Belamaf exerts its anti-tumor activity via direct cytotoxicity, antibody-dependent cellular cytotoxicity, antibody-dependent cellular phagocytosis and immunogenic cell death. It is proposed that the later accounts for the durability of responses observed across all DREAMM studies, however the role of T cells this process is unclear. To gain further insights into the mechanisms underlying the clinical activity of belamaf we preformed longitudinal profiling of the T cell receptor (TCR) repertoire in blood and bone marrow (BM) samples derived from patients receiving belamaf in combination with pomalidomide and dexamethasone (BPd) in the ALGONQUIN study (NCT03715478).
Methods: Patients enrolled in Part 2 of the Algonquin study of BPd provided written informed consent for sample collection and correlative studies. Peripheral blood (PB) samples were collected at screening, at cycle 2 day 1 (C2D1) and every 6th cycle thereafter. BM specimens were obtained at screening, C2D1 and end of treatment (EoT). We conducted TCR capture and sequencing (CapTCR-seq) on DNA derived from PB mononuclear cells (PBMCs) and cell free DNA (cfDNA). To study the intratumoral T cell repertoires and phenotypes we used single-cell CITE-seq (cellular indexing of transcriptomes and epitopes sequencing) combined with single-cell αβ and γδ TCR sequencing applied to viably frozen BM cells from n = 11 myeloma patients.
Results: BM CITE-seq analysis did not reveal upregulation of RNA and protein expression of T cell exhaustion markers TIGIT, PD-1, CD38, LAG3 or RNA expression of TOX across T or NK cell subsets at C2D1 or EoT. Consistent with the lack of T cell exhaustion, PB TCR diversity did not decline compared to baseline overtime including at EoT. Surprisingly, we observed that high γδ TCR diversity at C2D1 in the PB was significantly associated with improved PFS. In the BM, we observed an elevated proportion of intratumoral γδ T cells in responders compared to non-responders. Using a novel machine learning algorithm developed by our group we identified tumor-reactive γδ T cells from the CITE-seq data. Responders to BPd tended to exhibit expansion of baseline tumor-reactive γδ TCRs and influx of new tumor-reactive γδ TCRs at C2D1. Non-responders were comparatively devoid of tumor-reactive γδ T cells.
Conclusions: γδ TCR diversity and the presence of tumor-reactive γδ T cell clones at C2D1 serve as a predictive biomarker for long-term responses to BPd. Immunogenic cell death caused by belamaf may result in the release of concealed tumor antigens or disruption of the tumor micro-environment prompting a γδ T cell response.
