Human bone marrow stromal cells simultaneously support B and T/NK lineage development from human haematopoietic progenitors: a principal role for flt3 ligand in lymphopoiesis

A bifurcation concept for B-lymphoid/plasmacytoid dendritic cells with largely fluctuating transcriptome dynamics

Hematopoiesis was considered a hierarchical stepwise process but was revised to a continuous process following single-cell RNA sequencing. However, the uncertainty or fluctuation of single-cell transcriptome dynamics during differentiation was not considered, and the dendritic cell (DC) pathway in the lymphoid context remains unclear. Here, we identify human B-plasmacytoid DC (pDC) bifurcation as large fluctuating transcriptome dynamics in the putative B/NK progenitor region by dry and wet methods. By converting splicing kinetics into diffusion dynamics in a deep generative model, our original computational methodology reveals strong fluctuation at B/pDC bifurcation in IL-7Rα⁺ regions, and LFA-1 fluctuates positively in the pDC direction at the bifurcation. These expectancies are validated by the presence of B/pDC progenitors in the IL-7Rα⁺ fraction and preferential expression of LFA-1 in pDC-biased progenitors with a niche-like culture system. We provide a model of fluctuation-based differentiation, which reconciles continuous and discrete models and is applicable to other developmental systems.

Regulation of murine B lymphopoiesis by stromal cells

B lymphocytes are crucial for the body's humoral immune response, secreting antibodies generated against foreign antigens to fight infection. Adult murine B lymphopoiesis is initiated in the bone marrow and additional maturation occurs in the spleen. In both these organs, B lymphopoiesis involves interactions with numerous different non-hematopoietic cells, also known as stromal or microenvironment cells, which provide migratory, maturation, and survival signals. A variety of conditional knockout and transgenic mouse models have been used to identify the roles of distinct microenvironment cell types in the regulation of B lymphopoiesis. These studies have revealed that mesenchymal lineage cells and endothelial cells comprise the non-hematopoietic microenvironment cell types that support B lymphopoiesis in the bone marrow. In the spleen, various types of stromal cells and endothelial cells contribute to B lymphocyte maturation. More recently, comprehensive single cell RNA-seq studies have also been used to identify clusters of stromal cell types in the bone marrow and spleen, which will aid in further identifying key regulators of B lymphopoiesis. Here, we review the different types of microenvironment cells and key extrinsic regulators that are known to be involved in the regulation of murine B lymphopoiesis in the bone marrow and spleen.

Nonpermissive bone marrow environment impairs early B-cell development in common variable immunodeficiency

Common variable immunodeficiency (CVID) is a disease characterized by increased susceptibility to infections, hypogammaglobulinemia, and immune dysregulation. Although CVID is thought to be a disorder of the peripheral B-cell compartment, in 25% of patients, early B-cell development in the bone marrow is impaired. Because poor B-cell reconstitution after hematopoietic stem cell transplantation has been observed, we hypothesized that in some patients the bone marrow environment is not permissive to B-cell development. Studying the differentiation dynamics of bone marrow-derived CD34+ cells into immature B cells in vitro allowed us to distinguish patients with B-cell intrinsic defects and patients with a nonpermissive bone marrow environment. In the former, immature B cells did not develop and in the latter CD34+ cells differentiated into immature cells in vitro, but less efficiently in vivo. In a further group of patients, the uncommitted precursors were unable to support the constant development of B cells in vitro, indicating a possible low frequency or exhaustion of the precursor population. Hematopoietic stem cell transplantation would result in normal B-cell repopulation in case of intrinsic B-cell defect, but in defective B-cell repopulation in a nonpermissive environment. Our study points to the importance of the bone marrow niche in the pathogenesis of CVID.

Impact of bone marrow-derived signals on NK cell development and functional maturation

Natural killer (NK) cells are cytotoxic members of type I innate lymphocytes (ILC1) with a prominent role in anti-tumor and anti-viral immune responses. Despite the increasing insight into NK cell biology, the steps and stages leading to mature circulating NK cells require further investigation. Natural killer cell development and functional maturation are complex and multi-stage processes that occur predominantly in the bone marrow (BM) and originate from haematopoietic stem cells CD34⁺ (HSC). Within the BM, NK cell precursor (NKP) and NK cell development intermediates reside in specialized niches that are characterized by particular cellular components that provide signals required for their maturation. These signals consist of soluble factors or direct cellular-contact interactions mediated by cytokines and growth factors with complementary, as well as overlapping roles in distinct developmental steps. Emerging evidence highlights the plasticity of the early phase of NK cell development, and the capacity of different signal combinations to redirect precursor lineage commitment through other innate cell populations. Here, we summarize the role of signals known to guide NK cell differentiation with a particular focus on the cytokines and the receptor/ligand pairs playing a critical role in these processes. A comprehensive understanding of the mechanisms underlying NK cell development will elucidate their roles in pathological conditions and will improve protocols for NK cell therapeutic application.

CXCL12-CXCR4 Axis Is Required for Contact-Mediated Human B Lymphoid and Plasmacytoid Dendritic Cell Differentiation but Not T Lymphoid Generation

We investigated the involvement of CXCL12-CXCR4 interactions in human lymphohematopoiesis by coculture with telomerized human stromal cells. CXCR4 expression was low in CD34⁺CD38^-CD45RA^-CD10^-CD7^-CD19^- immature hematopoietic stem/precursor cells (HSPCs) but higher in CD34⁺CD38^-CD45RA⁺CD10⁺CD7^+/-CD19^- early lymphoid precursors and even higher in CD34⁺CD38⁺CD45RA⁺CD10⁺CD7^-CD19⁺ pro-B cells. Inhibition of the effect of stromal cell-produced CXCL12 by an anti-CXCR4-blocking Ab suppressed the generation of CD45RA⁺CD10^-CD7⁺CD19^- early T lymphoid precursors (ETPs) and CD45RA⁺CD10⁺CD7^-CD19^+/- B lymphoid precursors on stromal cells, but it did not affect the generation of ETPs in conditioned medium of stromal cell cultures. Replating assays showed that contact with stromal cells was critical for HSPC-derived CD45RA⁺CD10⁺CD7^-CD19^- B lineage-biased precursors to differentiate into CD19⁺ pro-B cells, which was suppressed by the anti-CXCR4 Ab. Conversely, HSPC-derived ETPs possessed T and B lymphoid and monocytic differentiation potential; stromal cell contact was not required for their growth but rather promoted B lymphoid differentiation. The anti-CXCR4 Ab did not affect the growth of ETPs in conditioned medium, but it suppressed their B lymphoid differentiation on stromal cells. CD14^-CD11c^-HLA-DR⁺CD123^highCD303⁺ plasmacytoid dendritic cells developed from HSPCs and ETPs exclusively in contact with stromal cells, which was suppressed by the anti-CXCR4 Ab. These data indicate that CXCL12 plays an essential role in stromal cell contact-mediated B lymphoid and plasmacytoid dendritic cell differentiation from immature hematopoietic and early T lymphoid precursors with a multilineage differentiation potential, but it does not participate in contact-independent generation of early T lymphoid precursors.

[Basophilic granulocytes and autoimmune diseases. Can basophilic granulocytes modulate B-cell functions in systemic lupus erythematosus?]

BACKGROUND

B-cells play an important role in the pathogenesis of systemic lupus erythematosus (SLE); however, many other cell types are also involved in disease development. In a murine lupus model it was demonstrated that basophils are indispensable for the development of lupus symptoms.

AIM

This study investigated whether there is evidence for a relevant interaction between B-cells and basophils under physiological and pathological conditions.

MATERIAL AND METHODS

A selective review of the literature was performed and some preliminary data about the interaction of basophils and B-cells are reported in this article. For the experiments, isolated B-cells were cultured in vitro in the presence or absence of basophils and B-cell survival, proliferation, plasma cell development and antibody production were determined.

RESULTS

Data from the literature show that there is evidence for an interaction between basophils and B-cells in a murine model. Our investigations confirmed that human basophils also support the survival and proliferation of B-cells. Furthermore, plasma cell differentiation and antibody production, most importantly IgG secretion, are enhanced. First experimental ex vivo analyses of basophils from SLE patients demonstrate that these cells exhibit a higher activation level compared to basophils from healthy controls.

DISCUSSION

In summary, previously published data and our own data demonstrate that there is an interaction between human basophils and B-cells. A better understanding of the role of basophils in the pathogenesis of SLE could lead to the development of new therapeutic strategies.

Early B lymphocyte development: Similarities and differences in human and mouse

B lymphocytes differentiate from hematopoietic stem cells through a series of distinct stages. Early B cell development proceeds in bone marrow until immature B cells migrate out to secondary lymphoid tissues, such as a spleen and lymph nodes, after completion of immunoglobulin heavy and light chain rearrangement. Although the information about the regulation by numerous factors, including signaling molecules, transcription factors, epigenetic changes and the microenvironment, could provide the clinical application, our knowledge on human B lymphopoiesis is limited. However, with great methodological advances, significant progress for understanding B lymphopoiesis both in human and mouse has been made. In this review, we summarize the experimental models for studies about human adult B lymphopoiesis, and the role of microenvironment and signaling molecules, such as cytokines, transforming growth factor-β superfamily, Wnt family and Notch family, with point-by-point comparison between human and mouse.

Cerebrospinal Flt3 ligand correlates to tau protein levels in primary Sjögren's syndrome

OBJECTIVES

Primary Sjögren's syndrome (pSS) is an autoimmune disease affecting the exocrine glands and internal organs including the central nervous system (CNS). The fms-related tyrosine kinase 3 ligand (Flt3L) is a maturation factor essential for brain homeostasis. Blood levels of Flt3L are increased in inflammatory diseases including the inflamed salivary glands in pSS. The present study evaluated the role of Flt3L in the CNS of patients with pSS and in two non-autoimmune conditions, fibromyalgia (FM) and Alzheimer's disease (AD).

METHOD

Levels of Flt3L were measured in cerebrospinal fluid (CSF) and serum of patients with pSS (n = 15), FM (n = 29), and AD (n = 39) and related to CNS symptoms and to markers of inflammation and degeneration.

RESULTS

Levels of CSF Flt3L in pSS and AD were significantly lower than in FM (p = 0.005 and p = 0.0003, respectively). Flt3L in pSS correlated to tau proteins [total tau (T-tau), r = 0.679; phosphorylated tau (P-tau), r = 0.646] and to a marker for microglia activation, monocyte chemoattractant protein 1 (MCP-1). Similar correlations were present in FM and AD patients. One-third of pSS patients had low levels of CSF Flt3L. This group had decreased levels of amyloid precursor protein metabolites (Aβ40 and Aβ42) in CSF, which was not seen in FM patients.

CONCLUSIONS

This study shows a strong correlation between CSF Flt3L and tau proteins in pSS patients suggesting ongoing degradation/remodelling in the CNS. In pSS patients, low levels of Flt3L were linked to changes in amyloid turnover and may represent processes similar to those in AD.

Manipulation of human early T lymphopoiesis by coculture on human bone marrow stromal cells: potential utility for adoptive immunotherapy

T cell precursors are an attractive target for adoptive immunotherapy. We examined the regulation of human early T lymphopoiesis by human bone marrow stromal cells to explore in vitro manipulation of human T cell precursors in a human-only coculture system. The generation of CD7(+)CD56(-)cyCD3(-) proT cells from human hematopoietic progenitors on telomerized human bone marrow stromal cells was enhanced by stem cell factor, flt3 ligand, and thrombopoietin, but these stimulatory effects were suppressed by interleukin 3. Expression of Notch ligands Delta-1 and -4 on stromal cells additively promoted T cell differentiation into the CD7(+)cyCD3(+) pre-T cell stage, while cell growth was strongly inhibited. By combining these coculture systems, we found that initial coculture with telomerized stromal cells in the presence of stem cell factor, flt3 ligand, and thrombopoietin, followed by coculture on Delta-1- and -4-coexpressing stromal cells led to a higher percentage and number of pre-T cells. Adoptive immunotherapy using peripheral blood T cells transduced with a tumor antigen-specific T cell receptor (TCR) is a promising strategy but has several limitations, such as the risk of forming a chimeric TCR with the endogenous TCR. We demonstrated that incubation of TCR-transduced hematopoietic progenitors with the combination of coculture systems gave rise to CD7(+)TCR(+)CD3(+)CD1a(-) T cell precursors that rapidly proliferated and differentiated under the culture condition to induce mature T cell differentiation. These data show the regulatory mechanism of early T lymphopoiesis on human stromal cells and the potential utility of engineered human stromal cells to manipulate early T cell development for clinical application.