(OA-29) In Situ Visualisation of BCMA Bispecific Antibody and the Myeloma Immune Microenvironment

Despite therapeutic advances, multiple myeloma remains difficult to cure. Novel T cell redirecting therapies such as CAR-T cells and bispecific antibodies have shown promising efficacy in the relapse setting, however durable remissions have yet to be achieved. The mechanisms of resistance and the dynamics of immune cell interactions of T cell redirecting therapies within the bone marrow (BM) remain poorly characterised. Here, we used in situ imaging of the BM in living mice (intravital imaging) combined with spatial transcriptomics to characterise immune cell interactions with myeloma in the context of BCMA bispecific antibody.



Methods:

We used a traceable myeloma cell line derived from the spontaneous Vk*MYC mouse model (Vk14451-GFP) that recapitulates human disease. To visualise the immune response, Vk14451 cells were transplanted into reporter mice where CD8 T cells were labelled with fluorescent tomato protein (E8I-Cre tomato). >Five weeks post-transplant, mice were treated with 1 to 4 weekly doses of CD3-BCMA or CD3-KLH/control bispecific antibody and T cell responses tracked using multi-day intravital imaging of the calvarium BM. To investigate transcriptional responses of T cells and myeloma in the BM, long bones were prepared from cohorts of mice and analysed by spatial transcriptomics.



Results:

Direct visualisation of T cells in situ without therapeutic intervention revealed CD8 T cells were excluded from areas of myeloma infiltration. Furthermore, CD8 T cells that gained entry to tumour had significantly altered behaviour compared to T cells in healthy bone marrow. After treatment with bispecific antibody, significant changes in the biology and spatial distribution of CD8 T cells were observed. Interestingly, the immunosuppressive myeloma microenvironment had no effect on the ability of CD8 T cells to engage with myeloma cells within tumour foci and undergo rapid expansion. However, we did observe altered T cell persistence cells with significant in situ death detected within the myeloma foci. In cohorts of mice that received repeated dosing of bispecific antibody, we observed loss of tumour control. In these mice, we still observed access of T cells to the tumour foci. However, we did not observe sustained interactions and active killing of MM cells suggesting development of a tumour agnostic state of T cells. Additionally, we profiled the expression of 475 genes with subcellular spatial transcriptomics to characterise the molecular state of T cells in situ. Using this approach, we could detect significant changes in T cell phenotype both spatially and temporally within long bones of mice treated with bispecific antibody.



Conclusions:

This dynamic analysis of immune cell interactions with myeloma within the BM microenvironment suggests that targeting persistence of activated T cells may play a key role in the therapeutic efficacy of T cell redirecting therapies.