Frequency of Marrow Infiltrating PD-1 Expressing CD4 Effectors Following Autologous Stem Cell Transplant (ASCT) for Multiple Myeloma Is Prognostic for Clinical Outcome

Introduction: High dose therapy and autologous stem cell transplant (ASCT) remain standard first line therapy for young fit patients with multiple myeloma (MM). Beyond cytoreduction, ASCT may also alter the bone marrow (BM) immune environment to augment anti-tumour immune responses. A more precise understanding of the immune microenvironment post ASCT and its relationship with clinical outcomes may identify biomarkers to refine prognostication and provide opportunity for therapeutic intervention. Aim: We aimed to define T cell subsets and expression of co-activating and co-inhibitory proteins in the BM early post ASCT, and explore associations with clinical outcomes. Methods: BM samples were obtained at 100 days (D100) post ASCT from 61 MM patients, and 15 healthy donors (HD). BM mononuclear cells (MNCs) were stained for surface (CD3, CD4, CD8, PD-1, LAG-3, and ICOS) and intracellular markers (GzmB, Ki-67, CTLA-4, and FoxP3). Data were acquired on a BD LSR Fortessa and analysed using FlowJo. Progression free survival (PFS) was defined from ASCT as per IMWG criteria, and patient groups were compared using Mann-Whitney U test. Adverse risk genetics was defined as t(4;14), t(14;16) or del(17p). Results: Median age of patients at ASCT was 58yrs (36-71), 70.5% were male. Median follow up was 17 months (3-54). At diagnosis, 36.1% were ISS stage I, 8.2% had extramedullary disease, and 14.7% had adverse risk genetics. As induction therapy, 96.7% received a proteasome inhibitor, 45.9% an immunomodulatory drug, 54.1% both, and 21.3% received post ASCT maintenance/consolidation. Median PFS was 24 months. At D100, 24.6% had achieved CR, 47.5% VGPR, 21.3% PR, 4.9% SD and none had PD. Improved PFS was found in patients with deeper response (CR/VGPR vs. rest, p=0.003), and with ISS stage I vs II/III (p=0.013). There was a trend for improved PFS with standard risk genetics (p=0.088 cf high risk). MM BM contained lower frequencies of CD3 and CD4 T cells compared to HD (CD3, 28% of live MNCs, vs 41.2%, p=0.012; CD4, 4.1% vs 14.0%, p