Thymoproteasome-Expressing Mesenchymal Stromal Cells Confer Protective Anti-Tumor Immunity via Cross-Priming of Endogenous Dendritic Cells.

Jean-Pierre Bikorimana,Abed El-Hakim El-Kadiry,Moutih Rafei,Natasha Salame,Nehme El-Hachem,Riam Shammaa,Jamilah Abusarah

doi:10.3389/fimmu.2020.596303

Abstract

Proteasomes are complex macromolecular structures existing in various forms to regulate a myriad of cellular processes. Besides degrading unwanted or misfolded proteins (proteostasis), distinct immune functions were ascribed for the immunoproteasome and thymoproteasome (TPr) complexes. For instance, antigen degradation during ongoing immune responses mainly relies on immunoproteasome activity, whereas intrathymic CD8 T-cell development requires peptide generation by the TPr complex. Despite these substantial differences, the functional contribution of the TPr to peripheral T-cell immunity remains ill-defined. We herein explored whether the use of mesenchymal stromal cells (MSCs) engineered to exhibit altered proteasomal activity through de novo expression of the TPr complex can be exploited as a novel anti-cancer vaccine capable of triggering potent CD8 T-cell activation. Phenotypic and molecular characterization of MSC-TPr revealed a substantial decrease in MHCI (H2-Kb and H2-Dd) expression along with elevated secretion of various chemokines (CCL2, CCL9, CXCL1, LIX, and CX3CL1). In parallel, transcriptomic analysis pinpointed the limited ability of MSC-TPr to present endogenous antigens, which is consistent with their low expression levels of the peptide-loading proteins TAP, CALR, and PDAI3. Nevertheless, MSC-TPr cross-presented peptides derived from captured soluble proteins. When tested for their protective capacity, MSC-TPr triggered modest anti-tumoral responses despite efficient generation of effector memory CD4 and CD8 T cells. In contrast, clodronate administration prior to vaccination dramatically enhanced the MSC-TPr-induced anti-tumoral immunity clearly highlighting a refractory role mediated by phagocytic cells. Thus, our data elute to a DC cross-priming-dependant pathway in mediating the therapeutic effect of MSC-TPr.

Highlights

The proteasomal machinery was believed to strictly coordinate proteolysis via the degradation of aberrant cytoplasmic proteins [1]
Liposomes and liposome-Clodronate were purchased from Liposoma Research (Amersterdam, The Netherlands)
Expression of the TPr by MSCs (Figure 1E) did not induce MHCII (I-Ab), CD80, CD86 or PD-L1 expression (Figure S2), nor caused unusual alterations in their responsiveness to IFN-gamma treatment (Figure S3). When analyzed for their secretome profile, MSC-TPr expressed higher levels of VEGF (Figure 2A), in addition to CCL2, CCL9, CXCL1, LIX, and CX3CL1 while CXCL12 was decreased (Figures 2B, C). These results indicate that TPr expression in MSCs alters their MHCI expression and secretion of various soluble immune mediators

Summary

Introduction

The proteasomal machinery was believed to strictly coordinate proteolysis via the degradation of aberrant cytoplasmic proteins [1]. Its role is vital for positive selection of CD8 thymocytes as mice deficient in b5t exhibit aberrant CD8 Tcell development (CD4 T cells are not affected) despite normal thymic architecture and MHCI expression on cTECs [7, 8]. The latter observations in b5t-deficient mice led to the conclusion that the unique and poor chymotrypsin-like activity of the TPr provides a set of MHCI-associated peptides completely distinct from those generated by the CPr or IPr complexes and dedicated to CD8 T-cell development [7, 8]. Additional studies are required to investigate whether molding of peripheral CD8 T-cell immunity can be mediated by the TPr

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Conclusion