Key Subsets of Immune Cells in Bone Marrow Myeloid Compartment Indicate Degree of Tumor Infiltration and Avenues for Successful Immunotherapy in Multiple Myeloma

With the onset of single-cell omics, the complete gene expression landscape of immune cells can be leveraged to differentiate distinct subsets with inherently diverse functions. By analyzing the sample-specific composition of these distinct cellular groups, we can decipher the hallmarks of healthy and fully tumor-infiltrated bone marrow microenvironments as well as compositional modulations during effective immunotherapy. Unique to our approach, our analyses leverage over 500,000 existing RNA-Seq experiments packaged in the ARCHS4 repository to strengthen the biological signal and quality of our tertiary analyses by informing us in an unbiased way about lesser known immune-typologies.

We applied this technique to our well-established immunocompetent mouse model of multiple myeloma (MM) which we rendered sensitive to IMiDs by introducing a human CRBN transgene: Vk*MYChCRBN. In multiple single cell experiments measuring effectiveness of Iberdomide in combination with Dexamethasone and an additional experiment comparing aged and untreated Vk*MYC mice with multiple myeloma (MM), key compositional patterns emerge in the murine bone marrow niche. Two distinct polarities of C1q expressing macrophages are present within murine bone marrow and spleen-one expressing Fcgr4 and present in untreated, tumor-bearing samples and a separate population elevated in Apoe, Vcam1, and MHC-II markers that can be found in healthy and tumor-bearing mice. Among splenocytes, we observed that the proportion of C1q macrophages correlates strongly with tumor burden and that BM and spleen samples in the process of tumor debulking via effective immunotherapy dramatically reduce relative frequencies of C1q macrophages. We hypothesize that the C1q macrophages without MHC-II features represent a tumor-associated macrophage (TAM) phenotype that is MM dependent and promotes tumor progression within the BM niche.¹ Additional analyses of murine splenocytes have revealed that effective immunotherapy via an IAP antagonist induces the presence of immature or migratory dendritic cells (high expressers of Ccl5 and likely key immune recruiters) and that tumor-bearing splenocytes typically maintain large segments of immature neutrophils (high expressers of Ngp and Camp) without other neutrophils in more advanced stages of development²-while both immature and mature neutrophils are found in healthy controls.

When comparing the BM composition of an aged, tumor-bearing mouse with that of a young, healthy mouse, key differences emerge. While multiple subsets of progenitors in murine BM niche can be identified via single cell, a unique population of cells over-expressing Cxcl12 that resemble Lepr+ mesenchymal stem cells from existing experiments (GSM3676085) are nearly absent in the young mouse, highly represented in the aged mouse, and may be a critical in creating a BM niche that supports tumor expansion.³ Curiously, a sizeable population of basophils is evident in the young, non-tumor bearing mouse but not found in the adult, tumor-bearing mouse. In addition, both types of C1q macrophages have expanded frequencies in the aged mouse. In total, single-cell analyses provide a snapshot of the compositional landscape of immune cell types thus illustrating hallmarks of tumor-infiltrated, healthy, and therapy modulated BM niches, providing potential new therapeutic targets.

References

1. Wang, B. et al. Transition of tumor-associated macrophages from MHC class II hi to MHC class II low mediates tumor progression in mice. BMC Immunol.12, 43 (2011).

2. Grieshaber-Bouyer, R. et al. The neutrotime transcriptional signature defines a single continuum of neutrophils across biological compartments. Nature Communications vol. 12 (2021).

3. Cuiffo, B. G. & Karnoub, A. E. Mesenchymal stem cells in tumor development. Cell Adhesion & Migration vol. 6 220-230 (2012).

Chesi:Pfizer, Novartis.: Consultancy; Abcuro, Palleon Pharmaceuticals, Pi Therapeutics.: Patents & Royalties: Genetically engineered mouse model of myeloma.. Bergsagel:GSK: Consultancy; Pfizer: Consultancy; Novartis: Consultancy; Janssen: Consultancy; Oncopeptides: Consultancy.