Bone Marrow Endothelial Cells Take Up Blood-Borne Immune Complexes via Fcγ Receptor IIb2 in an Erythropoietin-Dependent Manner

Polyclonal regeneration of mouse bone marrow endothelial cells after irradiative conditioning

Bone marrow endothelial cells (BM-ECs) are the essential components of the BM niche and support the function of hematopoietic stem cells (HSCs). However, conditioning for HSC transplantation causes damage to the recipients' BM-ECs and may lead to transplantation-related morbidity. Here, we investigated the cellular and clonal mechanisms of BM-EC regeneration after irradiative conditioning. Using single-cell RNA sequencing, imaging, and flow cytometry, we revealed how the heterogeneous pool of BM-ECs changes during regeneration from irradiation stress. Next, we developed a single-cell in vitro clonogenic assay and demonstrated that all EC fractions hold a high potential to reenter the cell cycle and form vessel-like structures. Finally, we used Rainbow mice and a machine-learning-based model to show that the regeneration of BM-ECs after irradiation is mostly polyclonal and driven by the broad fraction of BM-ECs; however, the cell output among clones varies at later stages of regeneration.

Regulators of placental antibody transfer through a modeling lens

Infants are vulnerable to infections owing to a limited ability to mount a humoral immune response and their tolerogenic immune phenotype, which has impeded the success of newborn vaccination. Transplacental transfer of IgG from mother to fetus provides crucial protection in the first weeks of life, and maternal immunization has recently been implemented as a public health strategy to protect newborns against serious infections. Despite their early success, current maternal vaccines do not provide comparable protection across pregnancies with varying gestational lengths and placental and maternal immune features, and they do not account for the dynamic interplay between the maternal immune response and placental transfer. Moreover, progress toward the rational design of maternal vaccines has been hindered by inadequacies of existing experimental models and safety challenges of investigating longitudinal dynamics of IgG transfer in pregnant humans. Alternatively, in silico mechanistic models are a logical framework to disentangle the processes regulating placental antibody transfer. This Review synthesizes current literature through a mechanistic modeling lens to identify placental and maternal regulators of antibody transfer, their clinical covariates, and knowledge gaps to guide future research. We also describe opportunities to use integrated modeling and experimental approaches toward the rational design of vaccines against existing and emerging neonatal pathogen threats.

Bone Marrow: The Central Immune System

Bone marrow is known as the site of hematopoiesis. What is not being described in textbooks of immunology is the fact that bone marrow is not only a generative, but also an antigen-responsive, immune organ. It is also a major storage site for antigen-specific memory B and T cells. That bone marrow is a priming site for T cell responses to blood borne antigens was discovered exactly 20 years ago. This review celebrates this important discovery. The review provides a number of examples of medical relevance of bone marrow as a central immune system, including cancer, microbial infections, autoimmune reactions, and bone marrow transplantation. Bone marrow mesenchymal stem cell-derived stromal cells provide distinct bone marrow niches for stem cells and immune cells. By transmitting anti-inflammatory dampening effects, facilitating wound healing and tissue regeneration mesenchymal stem cells contribute to homeostasis of bone and other tissues. Based on the evidence presented, the review proposes that bone marrow is a multifunctional and protective immune system. In an analogy to the central nervous system, it is suggested that bone marrow be designated as the central immune system.

FcγR-Mediated Trogocytosis 2.0: Revisiting History Gives Rise to a Unifying Hypothesis

There is increasing interest in the clinical implications and immunology of trogocytosis, a process in which the receptors on acceptor cells remove and internalize cognate ligands from donor cells. We have reported that this phenomenon occurs in cancer immunotherapy, in which cells that express FcγR remove and internalize CD20 and bound mAbs from malignant B cells. This process can be generalized to include other reactions including the immune adherence phenomenon and antibody-induced immunosuppression. We discuss in detail FcγR-mediated trogocytosis and the evidence supporting a proposed predominant role for liver sinusoidal endothelial cells via the action of the inhibitory receptor FcγRIIb2. We describe experiments to test the validity of this hypothesis. The elucidation of the details of FcγR-mediated trogocytosis has the potential to allow for the development of novel therapies that can potentially block or enhance this reaction, depending upon whether the process leads to unfavorable or positive biological effects.