Humoral Immunity to Allogeneic Immunoproteasome-Expressing Mesenchymal Stromal Cells Requires Efferocytosis by Endogenous Phagocytes

Abstract

The extensive use of mesenchymal stromal cells (MSCs) over the last decade has revolutionized modern medicine. From the delivery of pharmacological proteins to regenerative medicine and immune modulation, these cells have proven to be highly pleiotropic and responsive to their surrounding environment. Nevertheless, their role in promoting inflammation has been fairly limited by the questionable use of interferon-gamma, as this approach has also been proven to enhance the cells’ immune-suppressive abilities. Alternatively, we have previously shown that de novo expression of the immunoproteasome (IPr) complex instills potent antigen cross-presentation capabilities in MSCs. Interestingly, these cells were found to express the major histocompatibility class (MHC) II protein, which prompted us to investigate their ability to stimulate humoral immunity. Using a series of in vivo studies, we found that administration of allogeneic ovalbumin (OVA)-pulsed MSC-IPr cells elicits a moderate antibody titer, which was further enhanced by the combined use of pro-inflammatory cytokines. The generated antibodies were functional as they blocked CD4 T-cell activation following their co-culture with OVA-pulsed MSC-IPr and mitigated E.G7 tumor growth in vivo. The therapeutic potency of MSC-IPr was, however, dependent on efferocytosis, as phagocyte depletion prior to vaccination abrogated MSC-IPr-induced humoral responses while promoting their survival in the host. In contrast, antibody-mediated neutralization of CD47, a potent “do not eat me signal”, enhanced antibody titer levels. These observations highlight the major role played by myeloid cells in supporting antibody production by MSC-IPr and suggest that the immune outcome is dictated by a net balance between efferocytosis-stimulating and -inhibiting signals.

The CIt protocol: A blueprint to potentiate the immunogenicity of immunoproteasome-reprogrammed mesenchymal stromal cells

“… 32 , 33 Although this efferocytosis process is believed to be mainly driven by cell surface phosphatidylserine, MSCs and IRMs can also express counter-acting “don’t eat me” signals such as CD47, which can quench efferocytosis by binding to signal regulatory protein alpha (SIRPα) on the surface of myeloid cells. 34 We thus exploited this approach to transfer all IRM-containing immunogenic peptides/antigens to resident phagocytes as a mean to ensure effective T-cell stimulation in case infused IRMs fail to survive long enough to prime a CTL response. As expected, pre-treating IRMs with anti-CD47 antibodies prior to their infusion ( Figure 5 A) impacts anti-tumoral immunity (red versus blue lines - Figures 5 B and 5C).…”

LSD1 Inhibition Enhances the Immunogenicity of Mesenchymal Stromal Cells by Eliciting a dsRNA Stress Response

“…Second, TC does not reproduce the exact effects triggered by UM171a, such as ROS production [ 15 ]. This is particularly important, as mitochondrial-derived ROS are sought to play crucial roles in driving antigen cross-presentation [ 5 , 7 , 8 , 19 , 20 , 21 ]. More specifically, ROS production protects endocytosed antigens from extensive degradation mediated via pH-sensitive proteases activated during endosome maturation [ 5 , 7 , 8 , 19 , 20 , 21 ].…”

“…This is particularly important, as mitochondrial-derived ROS are sought to play crucial roles in driving antigen cross-presentation [ 5 , 7 , 8 , 19 , 20 , 21 ]. More specifically, ROS production protects endocytosed antigens from extensive degradation mediated via pH-sensitive proteases activated during endosome maturation [ 5 , 7 , 8 , 19 , 20 , 21 ]. Second, ROS can trigger lipid peroxidation, allowing for the release of captured antigens to the cytosol, where they can be efficiently processed by the proteasomal complex, and perhaps follow a sorting pathway that is specific to endosomal recycling to the cell surface [ 8 , 22 ].…”

“…MSCs can, however, be reprogrammed to behave as antigen presenting cells (APCs) that are capable of capturing and cross-presenting extracellular antigens [ 5 , 6 ]. These new functions can be instilled through the de novo expression of specific proteasomal subunits, or in response to pharmacological stimulation [ 5 , 6 , 7 , 8 ]. The latter strategy is particularly interesting as it entails the use of small molecules other than interferon (IFN)γ to elicit a selective rather than wider activation of gene expression [ 5 , 7 , 8 ].…”

“…These new functions can be instilled through the de novo expression of specific proteasomal subunits, or in response to pharmacological stimulation [ 5 , 6 , 7 , 8 ]. The latter strategy is particularly interesting as it entails the use of small molecules other than interferon (IFN)γ to elicit a selective rather than wider activation of gene expression [ 5 , 7 , 8 ]. An example of such an approach uses UM171a, a pyrimido-indole derivative that was originally identified for its ability to ex vivo stimulate hematopoietic stem cell (HSC) expansion [ 5 , 9 , 10 , 11 , 12 , 13 , 14 , 15 ].…”