Artificial Tertiary Lymphoid Structures: Exploring Mesenchymal Stromal Cells as a Platform for Immune Niche Formation.

Constructing artificial tertiary lymphoid structures (TLSs) opens new avenues for advancing cancer immunotherapy and personalized medicine by creating controllable immune niches. Mesenchymal stromal cells (MSCs) offer an ideal stromal source for such constructs, given their potent immunomodulatory abilities and accessibility. In this study, we explored the potential of adipose-derived MSCs to adopt TLS-supportive phenotypes and facilitate lymphocyte organization. Single-cell RNA sequencing revealed a distinct subpopulation of MSCs expressing key fibroblastic reticular cell (FRC)-associated markers, including IL-7, PDPN, and IL-15, though lacking follicular dendritic cell (FDC) markers. TNF-α stimulation, but not LTα2β1, further enhanced FRC marker expression (IL-7, PDPN, and ICAM1). Notably, in 3D spheroid co-culture with lymphocytes, MSCs upregulated additional FRC markers, specifically CCL21. Upon implantation into adipose tissue, MSC-lymphocyte organoids maintained structural integrity and showed extensive T-cell infiltration and partial vascularization after 15 days in vivo, although organized B-cell follicles and FDC markers were still lacking. These findings highlight MSCs' intrinsic ability to adopt an FRC-like phenotype that supports T-cell and HEV organization, suggesting that further optimization, including genetic modification, may be needed to achieve an FDC phenotype and replicate the full architectural and functional complexity of TLSs.