Effect of pregnancy-specific β₁-glycoprotein on the expression of arginase-1 and indolamine-2,3-dioxygenase by myeloid-derived suppressor cells

Myeloid-derived suppressor cells (MDSC) - a population of immature cells of myeloid origin with inhibitory functions, mainly related to T lymphocytes. Normally, MDSC account for less than 1% of leukocytes in peripheral blood. The number of these cells increases during healthy pregnancy. However, MDSC have been shown to play a critical role in the maintenance of tumor growth and in autoimmune diseases.

Because MDSC are now considered important regulators of immunity, finding ways to manipulate their functions is important for the development of therapies for malignant and autoimmune diseases, as well as for pregnancy pathologies and post-transplant complications. The immunosuppressive mechanisms of these cells are mediated by their expression of the surface molecules CD73, ADAM17, PD -L1, the enzymes arginase 1 (Arg 1), inducible nitric oxide synthase (iNOS), and indoleamine 2,3-dioxygenase (IDO), reactive oxygen species, and the production of the anti-inflammatory cytokines IL -10 and TGF-β1.

Pregnancy-specific β1-glycoprotein (PSG) is a pregnancy glycoprotein that has immunomodulatory effects on natural (dendritic cells and macrophages) and adaptive (T cells) immunity cells. At the same time, the effect of PSG on MDSC has not been investigated so far. Since this glycoprotein has promising pharmacological applications, it is necessary to study not only the native variant of PSG but also its recombinant form.

Since the main function of MDSC is immunosuppression, the aim of our work was to evaluate one of its mechanisms, namely the intracellular expression of amino acid degradation enzymes Arg1 and IDO under the influence of native and recombinant PSG in vitro.

MDSC differentiation was performed from CD11b+ cells isolated from peripheral blood of healthy volunteers. Cells were cultured for 7 days with stepwise addition of GM-CSF, IL -1β, and LPS. Native (n) (1, 10, and 100 μg/mL) and recombinant (r) (1 and 10 μg/mL) PSG was added to the cultures three days before the end of incubation. The percentage of MDSCs (Lin- HLA-DR -CD11b+CD33+) intracellularly expressing Arg1 and IDO was determined by flow cytometry.

It was found that nPSG and rPSG did not alter the amount of Arg1-expressing MDSCs at all concentrations examined. However, at a concentration of 10 μg/mL, both types of proteins caused a statistically significant increase in the percentage of cells expressing IDO.

We have already established that nPSG and rPSG affect MDSC differentiation by increasing the proportion of these cells belonging to the monocytic subpopulation. However, now we can say that PSG, in addition, enhances the suppressive function of the studied cells.

The obtained data are novel and open perspectives for targeting myeloid suppressor cells to improve cellular technologies in science and medicine.