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Immune cells surveil aberrantly sialylated O-glycans on megakaryocytes to regulate platelet count

ABSTRACT: Immune thrombocytopenia (ITP) is a common platelet disorder in pediatric patients. Pediatric and adult ITP have been associated with sialic acid alterations, but the pathophysiology of ITP remains elusive, and ITP is often a diagnosis of exclusion. Our analysis of pediatric ITP plasma samples showed increased anti-Thomsen-Friedenreich antigen (TF-antigen) antibody representation, suggesting increased exposure of the typically sialylated and cryptic TF-antigen in these patients. The O-glycan sialyltransferase St3gal1 add sialic acid specifically on the TF-antigen. To understand if TF-antigen exposure associates with thrombocytopenia, we generated a mouse model with targeted deletion of St3gal1 in megakaryocytes (MK) (St3gal1MK-/-). TF-antigen exposure was restricted to MKs and resulted in thrombocytopenia. Deletion of Jak3 in St3gal1MK-/- mice normalized platelet counts implicating involvement of immune cells. Interferon-producing Siglec H-positive bone marrow (BM) immune cells engaged with O-glycan sialic acid moieties to regulate type I interferon (IFN-I) secretion and platelet release (thrombopoiesis), as evidenced by partially normalized platelet count following and inhibition of interferon and Siglec H receptors. Single cell RNAseq determined that TF-antigen exposure by MKs primed St3gal1MK-/- BM immune cells to release IFN-I. Single cell RNAseq further revealed a new population of immune cells with a plasmacytoid dendritic cell (pDC)-like signature and concomitant upregulation of immunoglobulin re-arrangement gene transcripts Igkc and Ighm, suggesting additional immune regulatory mechanisms. Thus, aberrant TF-antigen moieties, often found in pathological conditions, regulate immune cells and thrombopoiesis in the BM, leading to reduced platelet count.

ORGANISM(S): Mus musculus

PROVIDER: GSE179067 | GEO | 2021/06/29

REPOSITORIES: GEO

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Project description:Rituximab (RTX) is widely used as a first-line therapeutic strategy in B cell-mediated autoimmune diseases. However, a large proportion of patients either do not respond to the treatment or relapse during B cell reconstitution. Primary immune thrombocytopenia (ITP) is a prototypic B cell mediated autoimmune disease in which pathogenic antibodies directed against the platelet membrane glycoprotein IIb-IIIa (GPIIbIIIa) lead to platelet destruction by macrophages in the spleen. In ITP, RTX-mediated B-cell depletion leads to an immediate clinical response in 50% of patients but 80% of patients will subsequently relapse in the next following months. Patients failing to respond or relapsing after RTX treatment are splenectomized when alternative therapies are inefficient or unavailable. Therapeutic splenectomy, resulting in a durable platelet response in 60-70 % of ITP patients, offers a unique access to study the autoimmune response in the spleen. To understand the cellular basis of disease's relapse in secondary lymphoid organs in humans, memory B cells were sorted and analyzed by scRNAseq from the spleen of three different groups of splenectomized ITP patients: 1) Patients that achieved a complete response after a course of RTX and relapsed during B-cell reconstitution (hereafter referred to as “RTX relapse” patients), 2) patients with primary failure of RTX, i.e. who did not respond to treatment at the time of B-cell depletion (“RTX failure” patients), 3) patients with active ITP that were not treated with RTX (“ITP” patients). All were compared with sorted splenic memory B cells from organ donors with no immune disease who died from stroke or head trauma, hereafter referred to as healthy donors (“HD” patients). We also included in this analysis splenic memory B cells from children with sickle cell disease that underwent splenectomy and are known to have a high frequency of B cell activation secondary to classical childhood infections and vaccinations, reflected by an increased number of GC. Single-cell mRNA sequencing was performed according to an adapted version of the SORT-seq protocol (Muraro et al., 2016, Cell Systems 3, 385–394), with cDNA libraries generation, sequencing and reads alignment performed at Single Cell Discoveries (Utrecht, Netherlands).

Project description:The hematopoietic system gives rise to a heterogeneous population of terminally differentiated cells that reside in the bone marrow (BM) to fulfill their roles in immunity, blood clotting and tissue oxygenation. Hematopoietic stem and progenitor cells are at the apex of a hierarchically organized maturation cascade constantly replenishing the pool of differentiated cells to maintain blood cell homeostasis. The bone marrow microenvironment is functionally compartmentalized by heterogeneous niche cells that provide physical and soluble signals to spatio-temporally organize hematopoiesis. Megakaryocytes (MKs) are niche cells of hematopoietic origin that support hematopoietic stem cell (HSCs) homeostasis and generate circulating platelets. Platelet production involves the release of MK fragments while their cell body is entirely consumed in a process termed thrombopoiesis. Consequently, replenishment of fragmented MKs from MK progenitors (termed megakaryopoiesis) is required to ensure sufficient platelet production, but also to maintain MK homeostasis within the HSC niche. Here, we used intravital imaging of the megakaryocytic lineage to identify the spatio-temporal patterns of megakaryopoiesis during steady state and thrombocytopenia. We show that MK consumption during thrombopoiesis is compensated by immediate and local proliferation of MK progenitors. MK homeostasis is controlled by plasmacytoid dendritic cells (pDCs) that scan the BM and secrete Interferon alpha (INFa) upon detection of exhausted megakaryocytes to trigger megakaryopoiesis. We identified local pDC-derived INFa release as a physiological inflammatory stimulus of the bone marrow niche that synchronizes megakaryopoiesis and thrombopoiesis to maintain homeostasis of MKs and platelets in steady state and under stress. Viral infection with SARS-CoV-2 can manipulate pDC-driven MK proliferation leading to inappropriate megakaryopoiesis, which has been associated with thrombotic complications during COVID-19.

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Immune cells surveil aberrantly sialylated O-glycans on megakaryocytes to regulate platelet count

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