Unlocking myeloma's spatial world at the micrometer scale Free

In this issue of Blood, Yip et al¹ apply spatial transcriptomics to bone marrow (BM) trephine biopsies to investigate the interplay between tumor-cell subpopulations and the tumor microenvironment (TME) in precursor stages and newly-diagnosed multiple myeloma (MM). Their study yields 2 major findings. First, spatial transcriptomics is feasible on BM trephines when optimized sample processing methods are used. Second, and most intriguingly, they provide evidence that distinct plasma-cell subpopulations can locally shape the TME. For example, plasma-cell subpopulations with an inflammatory phenotype were enriched in regions with a high T-cell density. Notably, the differences in cellular composition between distinct intratumoral neighborhoods were more pronounced than those observed between precursor stages and MM, with no consistent TME changes across disease stages. As outlined by the authors, these findings challenge the prevailing assumption that universal BM-wide changes in the TME are the primary drivers of disease progression in plasma-cell dyscrasias. Instead, they underscore the need to account for tumor-intrinsic factors when interpreting alterations within the TME.

Recent advances in spatial analysis technologies are, in our view, highly promising and will enable detailed investigations of the tumor within its native TME.² This includes understanding how treatment shapes subclonal architecture and how alterations in the TME contribute to disease progression and resistance. However, when examining heterogeneity at the micrometer scale, it is important to acknowledge that multiple additional layers of complexity exist in MM (see figure). For example, heterogeneity at the centimeter scale, which is typically revealed by whole-body functional imaging, is not captured by a BM biopsy that samples BM-confined cells from a single, randomly selected site at the iliac crest. In precursor stages, where diffuse plasma-cell infiltration is the predominant growth pattern,³ such a biopsy likely provides a representative snapshot of the diseased ecosystem. In contrast, focal lesions, which are centimeter-sized accumulations of plasma cells, are present in ∼80% of patients with symptomatic MM and are usually associated with osteolytic bone destruction.³ As the disease progresses, MM cells can become increasingly independent of the BM and grow as soft tissue masses adjacent to bone, commonly referred to as breakout lesions or paramedullary disease.⁴ In more advanced stages, tumor growth can occur entirely outside the BM in other organs, a manifestation known as extramedullary disease (EMD).⁴ 

Using spatial transcriptomic techniques on breakout lesions obtained during orthopedic surgeries, as well as on punch biopsies of EMD, we recently identified a distinct TME within these masses.^5,6 Compared to paired BM samples, the TME in these lesions showed marked alterations in the composition and cellular states of stromal cells, natural killer cells, and macrophages, along with expanded populations of putative tumor-reactive T cells. Consistent with the findings reported by Yip et al, the distribution of these T cells within breakout lesions was not uniform but instead correlated with the spatial location of distinct tumor subclones, supporting the notion of coevolution between MM cells and their immediate microenvironment.⁶ Together, accounting for heterogeneity on the centimeter scale is essential when studying disease progression, as key MM-defining events may not be captured in biopsies from the iliac crest alone.

Intrapatient heterogeneity is just one hallmark of MM. The other is interpatient heterogeneity, which is arguably a meter-scale phenomenon. Whole-genome sequencing studies have identified up to 12 distinct molecular subgroups of MM patients.⁷ Building on this, our evolving understanding of the TME, as illustrated in the current study by Yip et al, will likely reveal additional clinically relevant patient subgroups in the future, as well as providing deeper insights into tumor-TME interactions. On an even larger scale, there are clear geographical differences in MM incidence, with the highest rates reported in the United States and the lowest in Asia. This kilometer-scale heterogeneity remains poorly understood but is the focus of ongoing genome-wide association studies.⁸ 

Ultimately, despite this multilayered complexity, we must recognize that key events in MM evolution, such as the formation of focal lesions or treatment-resistant disease, are often driven by single cells. Individual MM cells can persist in minimal residual disease–negative patients for years, even decades, in a dormant state before reactivating, expanding, and causing relapse.⁹ Understanding why a particular MM-cell outcompetes previously dominant subclones, evades immune surveillance, or survives treatment remains one of the central challenges in the field. Notably, individual cells can differ in their antigen expression profiles. For example, using super-resolution microscopy, heterogeneity in CD19 expression levels has been demonstrated in MM,¹⁰ highlighting expression of individual molecules (the nanometer scale) as yet another dimension in the complex landscape of MM heterogeneity.

In summary, Yip et al present 1 of the first spatial transcriptomic analyses in MM, revealing a diverse and spatially patchy ecosystem within the BM that appears to be shaped primarily by the tumor’s subclonal architecture. We anticipate that this work will pave the way for many more spatial studies, employing complementary technologies to explore heterogeneity across both micro- and macro-scales. Collectively, these efforts promise to advance a more holistic understanding of MM.

Conflict-of-interest disclosure: L.R. declares consultancy for Johnson & Johnson, Amgen, GlaxoSmithKline (GSK), Pfizer, Bristol Myers Squibb (BMS), and Sanofi; honoraria from Johnson & Johnson, GSK, Pfizer, BMS, and Sanofi; and research funding from Skyline Dx and BMS. N.W. declares honoraria from Johnson & Johnson, GSK, and Sanofi and research funding from Johnson & Johnson, Sanofi, and BMS.