Cells of the myeloid lineage, including suppressor cells (MDSCs), macrophages (Mφ) and dendritic cells (DCs), are very important players in organ transplant rejection and graft-versus-host disease in unmanipulated recipients.1 Myeloid-derived cells have also been involved in rodent models of tolerance induction using costimulation blockade2 and through the administration of tolerogenic cytokines.3 Cell therapy approaches using in vitro–cultured Mφ and monocyte-derived DCs in organ transplantation have been developed in recent years,1 including clinical trials, such as The ONE Study consortium (www.onestudy.org/) and efforts have been launched to standardize cell culture and phenotyping methods, such as the Action to Focus and Accelerate Cell-based Tolerance-inducing Therapies consortium (www.afactt.eu). Because myeloid-derived cells are heterogeneous and plastic in their function, it is important to better define these cells and compare their phenotype, genetic program, and function. As an example of efforts in this direction, a recent publication made side-by-side comparisons of mouse MDSCs, regulatory Mφ (Mregs) and monocyte-derived Tolerogenic-DCs (TolDCs).4 These cell types had different phenotypes, could suppress T cell proliferation in vitro, and prolong skin allograft survival through different mechanisms.
Mregs are generated from peripheral blood monocytes cultured in vitro with low doses of M-CSF in the presence of human serum and finally activated with IFN-γ.5 Human Mregs and TolDCs are the monocyte-derived cells of the panel of tolerogenic cells used in The ONE Study consortium.
With the objective of discovering novel markers of human Mregs, Riquelme et al6 describe in this issue of Transplantation that combining the generation of a monoclonal antibody and transcriptomic analyses of newly and previously generated data that dehydrogenase/reductase 9 (DHRS9) is a marker of human Mregs over other monocyte-derived Mφ, including resting Mφ, LPS + IFNγ-stimulated Mφ, IL-4-stimulated Mφ and immunoglobulin-stimulated Mφ. DHRS9 was also preferentially expressed by Mregs over other monocyte-derived cells, such as immature mo-DCs, Tol-DCs, Rapamycin-treated DCs, IL-10 conditioned DCs or PGE2-induced MDSCs.
DHRS9 is a protein with oxidoreductase activity involved in the metabolism of retinoic acid (RA), estrogens and progesterone and that has also been described as a transcription factor.7 Mregs express other enzymes to convert retinol and β carotene to RA6 and thus could potentially act as inducers of regulatory T cells, as is the case for gut macrophages8 but this demands to be analyzed. As a note of caution, administration in vivo of a naturally occurring isomer of RA resulted in differentiation of tumor suppressive myeloid cells into immunogenic ones and increased immune responses in a cancer model,9 suggesting that RA metabolism may not necessarily be a tolerogenic mechanism or at least not directly exploited by the administration of RA. Therefore, more work is needed to define whether DHRS9 is a marker of Mregs or if it also plays a role in the tolerogenic function of Mregs and if yes by what mechanism.
Mregs activated by IFN-γ or LPS increased or maintained DHRS9 expression.6 Publically available transcriptomic data of human macrophages showed that DHRS9 expression was increased by IFN-γ or dexamethasone and was decreased by several other stimuli, such as TNF-α, LPS, the TLR2 ligand Pam3CysSK4, and combinations of them with PGE2.6 It will be of interest in the future to define whether DHRS9 expression is maintained, and Mregs are resistant to maturation to at least to some inflammatory stimuli in vivo. This is important because other regulatory myeloid cells such as MDSCs have been shown to switch to an immunogenic function through inflammasome activation in a graft-versus-host disease model.10
DHRS9 was mainly expressed intracellularly and analysis by cytofluorimetry resulted in a signal that was continuously distributed as single population of cells with around half of the cells above background signal. Expression of DHRS9 was also observed by a subpopulation of human macrophages of the spleen, and it will be important in the future to know more about the distribution of DHRS9 in Mφ of other human tissues and whether these cells have tolerogenic function.
Mregs were generated by low (5 ng/mL) but not high doses (100 ng/mL) of M-CSF in the presence of human but not of fetal calf serum and IFN-γ increased their suppressive activity. DHRS9 expression was associated to these in vitro parameters of Mreg generation. This raises interesting perspectives, such as potential of IL-34, another ligand of the M-CSF receptor but with higher affinity and with tolerogenic activity,3 to induce Mregs and to induce DHRS9 expression. DHRS9 analysis should also facilitate screening of human serum-derived factors for induction of human Mregs.
Overall, the description of DHRS9 is an interesting and potentially important finding for the translation of tolerogenic cell therapies to clinical transplantation.
1. Turnquist HR, Thomson AW. Identity crisis. Transplantation
2. Conde P, Rodriguez M, van der Touw W, et al. DC-SIGN(+) macrophages control the induction of transplantation tolerance. Immunity
3. Bezie S, Picarda E, Ossart J, et al. IL-34 is a Treg-specific cytokine and mediates transplant tolerance. J Clin Invest
4. Carretero-Iglesia L, Bouchet-Delbos L, Louvet C, et al. Comparative study of the immunoregulatory capacity of in vitro generated tolerogenic dendritic cells, suppressor macrophages, and myeloid-derived suppressor cells. Transplantation
5. Hutchinson JA, Riquelme P, Sawitzki B, et al. Cutting edge: immunological consequences and trafficking of human regulatory macrophages administered to renal transplant recipients. J Immunol
6. Riquelme P, Amodio G, Macedo C, et al. DHRS9 is a stable marker of human regulatory macrophages. Transplantation
7. Markova NG, Pinkas-Sarafova A, Karaman-Jurukovska N, et al. Expression pattern and biochemical characteristics of a major epidermal retinol dehydrogenase. Mol Genet Metab Rep
8. Raverdeau M, Breen CJ, Misiak A, et al. Retinoic acid suppresses IL-17 production and pathogenic activity of γδ T cells in CNS autoimmunity. Immunol Cell Biol
9. Kusmartsev S, Cheng F, Yu B, et al. All-trans-retinoic acid eliminates immature myeloid cells from tumor-bearing mice and improves the effect of vaccination. Cancer Res
10. Koehn BH, Apostolova P, Haverkamp JM, et al. GVHD-associated, inflammasome-mediated loss of function in adoptively transferred myeloid-derived suppressor cells. Blood