Characterization of erythroid cell-derived natural suppressor activity

Phenotypic Alterations in Erythroid Nucleated Cells of Spleen and Bone Marrow in Acute Hypoxia

Hypoxia leads to metabolic changes at the cellular, tissue, and organismal levels. The molecular mechanisms for controlling physiological changes during hypoxia have not yet been fully studied. Erythroid cells are essential for adjusting the rate of erythropoiesis and can influence the development and differentiation of immune cells under normal and pathological conditions. We simulated high-altitude hypoxia conditions for mice and assessed the content of erythroid nucleated cells in the spleen and bone marrow under the existing microenvironment. For a pure population of CD71+ erythroid cells, we assessed the production of cytokines and the expression of genes that regulate the immune response. Our findings show changes in the cellular composition of the bone marrow and spleen during hypoxia, as well as changes in the composition of the erythroid cell subpopulations during acute hypoxic exposure in the form of a decrease in orthochromatophilic erythroid cells that are ready for rapid enucleation and the accumulation of their precursors. Cytokine production normally differs only between organs; this effect persists during hypoxia. In the bone marrow, during hypoxia, genes of the C-lectin pathway are activated. Thus, hypoxia triggers the activation of various adaptive and compensatory mechanisms in order to limit inflammatory processes and modify metabolism.

Murine Bone Marrow Erythroid Cells Have Two Branches of Differentiation Defined by the Presence of CD45 and a Different Immune Transcriptome Than Fetal Liver Erythroid Cells

Mouse erythropoiesis is a multifaceted process involving the intricate interplay of proliferation, differentiation, and maturation of erythroid cells, leading to significant changes in their transcriptomic and proteomic profiles. While the immunoregulatory role of murine erythroid cells has been recognized historically, modern investigative techniques have been sparingly applied to decipher their functions. To address this gap, our study sought to comprehensively characterize mouse erythroid cells through contemporary transcriptomic and proteomic approaches. By evaluating CD71 and Ter-119 as sorting markers for murine erythroid cells and employing bulk NanoString transcriptomics, we discerned distinctive gene expression profiles between bone marrow and fetal liver-derived erythroid cells. Additionally, leveraging flow cytometry, we assessed the surface expression of CD44, CD45, CD71, and Ter-119 on normal and phenylhydrazine-induced hemolytic anemia mouse bone marrow and splenic erythroid cells. Key findings emerged: firstly, the utilization of CD71 for cell sorting yielded comparatively impure erythroid cell populations compared to Ter-119; secondly, discernible differences in immunoregulatory molecule expression were evident between erythroid cells from mouse bone marrow and fetal liver; thirdly, two discrete branches of mouse erythropoiesis were identified based on CD45 expression: CD45-negative and CD45-positive, which had been altered differently in response to phenylhydrazine. Our deductions underscore (1) Ter-119's superiority over CD71 as a murine erythroid cell sorting marker, (2) the potential of erythroid cells in murine antimicrobial immunity, and (3) the importance of investigating CD45-positive and CD45-negative murine erythroid cells separately and in further detail in future studies.

Murine Placental Erythroid Cells Are Mainly Represented by CD45+ Immunosuppressive Erythroid Cells and Secrete CXCL1, CCL2, CCL3 and CCL4 Chemokines

Erythroid cells are emerging players in immunological regulation that have recently been shown to play a crucial role in fetomaternal tolerance in mice. In this work, we set ourselves the goal of discovering additional information about the molecular mechanisms of this process. We used flow cytometry to study placental erythroid cells' composition and BioPlex for the secretome profiling of 23 cytokines at E12.5 and E19.5 in both allogeneic and syngeneic pregnancies. We found that (1) placental erythroid cells are mainly represented by CD45⁺ erythroid cells; (2) the secretomes of CD71⁺ placental erythroid cells differ from the ones in syngeneic pregnancy; (3) CCL2, CCL3, CCL4 and CXCL1 chemokines were secreted on each day of embryonic development and in both types of pregnancy studied. We believe that these chemokines lure placental immune cells towards erythroid cells so that erythroid cells can induce anergy in those immune cells via cell-bound ligands such as PD-L1, enzymes such as ARG1, and secreted factors such as TGFβ-1.

Production of immunoregulatory molecules by induced erythroblasts at various stages of cell differentiation

   Introduction. Bone marrow erythroblasts produce a wide range of cytokines with opposite biological effects. This may be due to a change in the spectrum of production of immunoregulatory mediators during differentiation and small qualitative and quantitative differences in the spectrum of cytokines produced at each stage of differentiation, which may be important for the regulation of hemo- and immunopoiesis.   The aim. To study the spectrum of production of mediators by erythroblasts at different stages of differentiation.   Methods. Erythroblasts were obtained from CD34+ bone marrow cells of healthy donors in the presence of recombinant cytokines. Phenotype assessment was performed using flow cytometry for erythroid (CD45, CD71, CD235a, CD44) and lymphoid markers (CD3, CD4, CD8, CD16, CD19). Blocking of erythroblast differentiation at different stages was carried out using specific blocking monoclonal antibodies to melanocortin receptors (MCR) of types 1, 2 and 5. Cytokine analysis in conditioned erythroblast media was performed using the Bio-Plex Pro Human Cytokine 48-Plex Screening Panel (Bio-Rad Laboratories, USA). Cytokine production was analyzed using the CytokineExplore online tool.   Results. The resulting erythroblasts are divided into positive and negative populations according to the CD45 marker, carry markers of erythroid cells CD71, CD235a and do not express linear markers of lymphoid cells. In type 1 MCR blockage, polychromatophilic erythroblasts predominate, in type 2 MCR blockage, basophilic erythroblasts predominate, and in type 5 MCR blockage, orthochromatophilic erythroblasts accumulate. According to the production of cytokines, it was shown that when using any of the blocking antibodies, we obtain cells that differ qualitatively and quantitatively in a number of mediators from the initial population of induced erythroblasts.   Conclusion. Thus, we have shown qualitative and quantitative differences in the production of mediators by erythroblasts depending on the stage of differentiation, which can lead to different regulatory effects.

Immune Transcriptome Study of Human Nucleated Erythroid Cells from Different Tissues by Single-Cell RNA-Sequencing

Nucleated erythroid cells (NECs) are the precursors of erythrocytes. They can be found in various hematopoietic tissues or in the blood. Recently, they have been shown to be active players in immunosuppression through the synthesis of arginase-2 and reactive oxygen species. In this work, we studied NECs in adult bone marrow, umbilical cord blood, and foetal liver parenchyma using single-cell RNA sequencing and found that: (1) all studied NECs expressed the same set of genes, which was enriched in "GO biological process" immunity-related terms; (2) early and late NECs had differential expression of the genes associated with immunosuppression, cell cycle progression, apoptosis, and glycolysis; (3) NECs from different tissues of origin had differential expression of the genes associated with immunosuppression.

Immune Transcriptome and Secretome Differ between Human CD71+ Erythroid Cells from Adult Bone Marrow and Fetal Liver Parenchyma

CD71⁺ erythroid cells (CECs) were only known as erythrocyte progenitors not so long ago. In present times, however, they have been shown to be active players in immune regulation, especially in immunosuppression by the means of ROS, arginase-1 and arginase-2 production. Here, we uncover organ-of-origin differences in cytokine gene expression using NanoString and protein production using Bio-Plex between CECs from healthy human adult bone marrow and from human fetal liver parenchyma. Namely, healthy human adult bone marrow CECs both expressed and produced IFN-a, IL-1b, IL-8, IL-18 and MIF mRNA and protein, while human fetal liver parenchyma CECs expressed and produced IFN-a, IL15, IL18 and TNF-b mRNA and protein. We also detected TLR2 and TLR9 gene expression in both varieties of CECs and TLR1 and NOD2 gene expression in human fetal liver parenchyma CECs only. These observations suggest that there might be undiscovered roles in immune response for CECs.

Human Fetal Liver Parenchyma CD71+ Cells Have AIRE and Tissue-Specific Antigen Gene Expression

Autoimmune regulator (AIRE) is a multifunctional protein that is capable of inducing tissue-specific antigens’ (TSAs) gene expression, a key event in the induction of self-tolerance, that is usually expressed and functions in the thymus. However, its expression has been detected outside the thymus and cells expressing the gene have been named extra-thymic AIRE expressing cells (eTACs). Here, we discuss the finding of AIRE and TSAs gene expression in CD71+ cells from human fetal liver parenchyma, which are mostly represented by CD71+ erythroid cells.

The role of CD71+ erythroid cells in the regulation of the immune response

Complex regulation of the immune response is necessary to support effective defense of an organism against hostile invaders and to maintain tolerance to harmless microorganisms and autoantigens. Recent studies revealed previously unappreciated roles of CD71⁺ erythroid cells (CECs) in regulation of the immune response. CECs physiologically reside in the bone marrow where erythropoiesis takes place. Under stress conditions, CECs are enriched in some organs outside of the bone marrow as a result of extramedullary erythropoiesis. However, the role of CECs goes well beyond the production of erythrocytes. In neonates, increased numbers of CECs contribute to their vulnerability to infectious diseases. On the other side, neonatal CECs suppress activation of immune cells in response to abrupt colonization with commensal microorganisms after delivery. CECs are also enriched in the peripheral blood of pregnant women as well as in the placenta and are responsible for the regulation of feto-maternal tolerance. In patients with cancer, anemia leads to increased frequency of CECs in the peripheral blood contributing to diminished antiviral and antibacterial immunity, as well as to accelerated cancer progression. Moreover, recent studies revealed the role of CECs in HIV and SARS-CoV-2 infections. CECs use a full arsenal of mechanisms to regulate immune response. These cells suppress proinflammatory responses of myeloid cells and T-cell proliferation by the depletion of ʟ-arginine by arginase. Moreover, CECs produce reactive oxygen species to decrease T-cell proliferation. CECs also secrete cytokines, including transforming growth factor β (TGF-β), which promotes T-cell differentiation into regulatory T-cells. Here, we comprehensively describe the role of CECs in orchestrating immune response and indicate some therapeutic approaches that might be used to regulate their effector functions in the treatment of human conditions.

CD71+VISTA+ erythroid cells promote the development and function of regulatory T cells through TGF-β

Cell-surface transferrin receptor (CD71⁺) erythroid cells are abundant in newborns with immunomodulatory properties. Here, we show that neonatal CD71⁺ erythroid cells express significant levels of V-domain Immunoglobulin (Ig) Suppressor of T Cell Activation (VISTA) and, via constitutive production of transforming growth factor (TGF)- β, play a pivotal role in promotion of naïve CD4⁺ T cells into regulatory T cells (Tregs). Interestingly, we discovered that CD71⁺VISTA⁺ erythroid cells produce significantly higher levels of TGF-β compared to CD71⁺VISTA⁻ erythroid cells and CD71⁺ erythroid cells from the VISTA knock-out (KO) mice. As a result, CD71⁺VISTA⁺ erythroid cells—compared to CD71⁺VISTA⁻ and CD71⁺ erythroid cells from the VISTA KO mice—significantly exceed promotion of naïve CD4⁺ T cells into induced Tregs (iTreg) via TGF-β in vitro. However, depletion of CD71⁺ erythroid cells had no significant effects on the frequency of Tregs in vivo. Surprisingly, we observed that the remaining and/or newly generated CD71⁺ erythroid cells following anti-CD71 antibody administration exhibit a different gene expression profile, evidenced by the up-regulation of VISTA, TGF-β1, TGF-β2, and program death ligand-1 (PDL-1), which may account as a compensatory mechanism for the maintenance of Treg population. We also observed that iTreg development by CD71⁺ erythroid cells is mediated through the inhibition of key signaling molecules phosphorylated protein kinase B (phospho-Akt) and phosphorylated mechanistic target of rapamycin (phospho-mTOR). Finally, we found that elimination of Tregs using forkhead box P3 (FOXP3)-diptheria toxin receptor (DTR) mice resulted in a significant expansion in the frequency of CD71⁺ erythroid cells in vivo. Collectively, these studies provide a novel, to our knowledge, insight into the cross-talk between CD71⁺ erythroid cells and Tregs in newborns. Our results highlight the biological role of CD71⁺ erythroid cells in the neonatal period and possibly beyond.

The primary role of the red blood cells is to transport oxygen, but we know relatively little about the other functions they perform. Following maturation, red blood cells exit the bone marrow and enter blood circulation. Their immature counterparts are normally absent or in very low frequency in the blood of healthy adults. However, we showed previously that immature red blood cells are abundant in the spleens of neonatal mice and in human umbilical cord blood and that these cells possess immunological properties. In this report, we studied a subset of neonatal immature red blood cells that express a protein called V-domain Immunoglobulin (Ig) Suppressor of T Cell Activation (VISTA) on their surface. We found that the presence of VISTA enables the cells to repeatedly produce the regulatory cytokine TGF-β. TGF-β induces a subset of naïve lymphocytes—the CD4⁺ T cells—and converts them into regulatory T cells, also known as Tregs. Tregs modulate and suppress other immune cells. Our studies provide novel insights, to our knowledge, into the immunological role of immature red blood cells in newborns.

New insight into an old concept: role of immature erythroid cells in immune pathogenesis of neonatal infection

Newborns are exceedingly susceptible to infection. However, very little is known about what governs the immunological differences seen in early life that result in extreme vulnerability to infection, nor how this changes during infancy. Herein, I provide evidence that the reduced ability to mount a protective immune response to pathogens is not due to an inherent immaturity of neonatal immune cells but instead the functions of these immune cells are actively suppressed by CD71⁺ erythroid cells. Furthermore, the role of CD71⁺ erythroid cells in host defense against infection is examined. CD71⁺ erythroid cells are enriched in newborns and have distinctive immunosuppressive properties that leave them vulnerable to infection. Moreover, immature erythroid cells possess exclusive immunomodulatory properties and may play a role in immune ontogeny. In addition to these distinct features, CD71⁺ erythroid cells impact digestive health by preventing excessive inflammation following the sudden transition from a sterile in utero setting to excessive colonization with commensals in the external environment. Ongoing research in identifying the beneficial and/or detrimental effects of immature erythrocytes on immune responses may serve to enhance protective newborn immune responses to infection and enable better vaccination strategies for the young to be designed.

Cytokine-producing activity of bone marrow erythrokaryocytes and its regulation under normal conditions

We showed that inhibition of cytokine production by bone marrow adherent cells and their soluble products is a mechanism of regulation of cytokine-producing activity of bone marrow erythrokaryocytes under normal conditions.

Erythroid cells in suppressing leukemia cell growth

This paper indicates that murine nucleated erythroid cells (EC) are able to reduce, in a dose-dependent manner, the proliferation of both L1210 lymphoma and P815 mastocytoma cells and that the leukemia cell growth inhibitory activity of unseparated bone marrow (BM) cells may be markedly augmented by their short-term culturing with erythropoietin (Epo). These results raise the intriguing possibility to utilize erythropoesis-stimulating, therapeutic strategies with the purpose of inhibiting leukemia cell growth in the body.

Erythroid cells in immunoregulation: characterization of a novel suppressor factor

Nucleated erythroid cells (EC) have been previously reported to possess a potent natural suppressor (NS) activity for B-cell responses. In this study, we demonstrate that murine EC are able to reduce not only lipopolysaccharide (LPS)-driven B-cell proliferation, but also proliferative and cytotoxic T-cell responses generated in a primary allogeneic mixed lymphocyte culture (MLC); and that a soluble low molecular weight factor may be involved in such EC-derived immunoregulation. In addition, the erythroid cell-derived suppressor factor (ESF) was found to be capable of effectively reducing the allergen-driven proliferation of peripheral blood mononuclear cells (PBMC) isolated from allergic patients. From the data presented herein, it appears that ESF is heat-stable (80 degrees C for 20 min) and has molecular weight (MW) lower or close to 0.5 kDa. ESF activity is resistant to both enzyme (trypsin plus chymotrypsin) proteolysis and action of the enzymes such as lipase and phospholipase C. On the other hand, ESF is effectively inactivated by neuraminidase treatment, suggesting the presence in its structure of sialic residue(s). The neuraminidase-sensitive, ESF-like activity is readily detected in the medium conditioned with normal mouse bone marrow (BM) cells. On fractionation of low MW erythroid products on a reversed-phase C16 column in a linear acetonitrile gradient (5-95%), ESF activity is detected in the first peak alone with the shortest time of its retention by the column. The results suggest that (1) by producing ESF, EC may regulate both B- and T-cell-mediated immune processes and (2) based on its physicochemical and biological characteristics, ESF can be distinguished from each of earlier characterised suppressor mediators of bone marrow origin.

Transplantation treatment of spinal cord injury patients

The minimally manipulated cells from fetal nervous and hemopoietic tissues (gestational age 16-22 weeks) were subarachnoidally implanted into 15 patients (18-52 years old) with severe consequences of traumatic spinal cord injury (SCI) at cervical or thoracic spine level. The times after SCI were from 1 month to 6 years. Each patient underwent from one to four cell transplantations (CT) with various time intervals. In 11 of 15 cases, CT was combined with an operative partial disruption of a connective tissue cyst and with implantation into a spinal cord lesion of a spinal cord fragment together with olfactory ensheathing cells. Before CT the patients showed complete motor and sensory function disorder consistent with a grade A of SCI according to Frankel classification. With CT treatment, six patients improved their neurological status from A to C grade of SCI, exhibiting incomplete restoration of both motor and sensory function. The status of other five CT-treated patients became consistent with SCI grade B and was characterized by appearance of contracting activity in some muscles and incomplete restoration of sensitivity. The remaining four patients did not exhibit any clinical improvements. No serious complications of CT were noted. The results suggest a clinical relevance of the CT-based approach to treating severe consequences of SCI.

Early dissemination rates of Friend murine leukaemia virus variants correlate with late pathogenesis

FIS-2, a less oncogenic, immunosuppressive variant of the Friend murine leukaemia virus (F-MuLV), was used to explore whether the differences in biological features were related to early virus dissemination rates or sites of replication. We found that erythroblasts were the primary target cells for both F-MuLV and FIS-2, while B- and T-cells were infected later in the infection. Although FIS-2 replicated to similar titres as F-MuLV, we observed a delay in peak viraemia titre and in the number of virus-positive cells in bone marrow and spleen. Studies including the chimeric viruses RE3 (FIS-2LTR with a F-MuLV background) and RE4 (F-MuLV LTR with a FIS-2 background) indicated that the delay in dissemination was due to mutations in FIS-2 LTR. The kinetics for early virus replication correlated with previously reported mean latency time for virus-induced erythroleukaemia in mice inoculated as newborns and with the onset of immunosuppression in adult mice. In addition, F-MuLV-induced late immunosuppression coincided with signs of erythroleukaemia and persistent viraemia. FIS-2 induced a more moderate late immunosuppression without persistent viraemia or signs of erythroleukaemia. Overall, our findings indicated that early viral replication is a prognostic factor in murine retrovirus-induced pathogenesis.