Human and murine splenic neutrophils are potent phagocytes of IgG-opsonized red blood cells

Abstract

Key Points• In steady state, where no IgGs against RBCs are present, macrophages are the primary phagocytes of RBCs.• In conditions where RBCs are IgGopsonized, neutrophils can have a major effect on RBC clearance.Red blood cell (RBC) clearance is known to occur primarily in the spleen, and is presumed to be executed by red pulp macrophages. Erythrophagocytosis in the spleen takes place as part of the homeostatic turnover of RBCs to remove old RBCs. It can be strongly promoted by immunoglobulin G (IgG) opsonization of RBCs, a condition that can occur as a consequence of autoantibody or alloantibody formation. The purpose of our study was to investigate which phagocytes are involved in IgG-mediated RBC clearance in the human spleen. We developed a highly specific in vitro assay to monitor RBC phagocytosis in total human splenocytes. Surprisingly, we have found that whereas homeostatic clearance of RBCs is primarily a task for splenic macrophages, neutrophils and, to a lesser extent, also monocytes can be a major factor in clearance of IgG-opsonized RBCs. Erythrophagocytosis by neutrophils is strongly dependent on the degree of opsonization of the RBCs. Additionally, the process is enhanced after blocking the "do not eat me" signal CD47 on the opsonized RBCs, which binds signal regulatory protein a, a myeloid inhibitory receptor that restricts phagocytosis. Moreover, RBCs isolated from autoimmune hemolytic anemia patients, opsonized by auto-IgGs, were shown to be readily phagocytosed by neutrophils. Finally, priming of neutrophils by inflammatory mediators such as tumor necrosis factor a and lipopolysaccharide further increases the magnitude of erythrophagocytosis. Collectively, our data suggest that neutrophils contribute significantly to the phagocytosis of antibody-opsonized RBCs, especially under inflammatory conditions. This indicates a hereto unanticipated contribution of neutrophils in RBC phagocytosis, especially under pathological conditions such as alloimmunization or autoimmunization.

Biocompatible coupling of therapeutic fusion proteins to human erythrocytes

“…In contrast, antibodies against RhD and RhCE failed to demonstrate significant rigidification of human RBCs, consistent with prior literature. 31 Antibodies against GPA and RhD can produce markedly different effects on different subsets of phagocytic cells, 48 and, although the authors hypothesized that copy number was critical, the current findings suggest that altered deformability may have also been contributory. The precise function of RhCE has been difficult to define 49 and a large diversity of polymorphisms has been described.…”

Red pulp macrophages in the human spleen are a distinct cell population with a unique expression of Fc-γ receptors

“…12 Possibly, neutrophils may also play a role in the uptake of IgG-opsonized blood cells because neutrophils from the human spleen were shown to be highly potent in the phagocytosis of erythrocytes opsonized with high concentrations of murine IgG antibodies. 28 However, neutrophils were not capable of phagocytozing anti-RhD opsonized erythrocytes. 28 Because we have only tested phagocytosis by red pulp macrophages with anti-RhD-opsonized erythrocytes, we cannot be sure that our findings can be extrapolated to other opsonized antigens or blood groups.…”

“…28 However, neutrophils were not capable of phagocytozing anti-RhD opsonized erythrocytes. 28 Because we have only tested phagocytosis by red pulp macrophages with anti-RhD-opsonized erythrocytes, we cannot be sure that our findings can be extrapolated to other opsonized antigens or blood groups. However, we have previously shown that different antibodies and antigens showed similar results in erythrophagocytosis by monocyte-derived macrophages.…”

Transfusion-related immunomodulation: a reappraisal

“…Interestingly, although the latter is a clear example of (antigen-specific) “immunosuppression,” it is not generally considered to be a type of TRIM. RBC transfusions also affect innate immunity, with specific effects on macrophages, dendritic cells, and granulocytes [22, 23]. Of course, because of crosstalk mechanisms, it is not surprising that effects on innate immunity also influence adaptive immune responses [22, 24].…”

“…Taken together, these outcomes could produce a TRIM effect of enhancing post-surgical or nosocomial bacterial infection rates and severity [42]. In addition, ingestion of (antibody-coated) RBCs by splenic granulocytes [23] can impair oxidative-burst function [41], thereby potentially impairing this type of protection against invading blood-borne pathogens.…”