Abstract
Monocytes/macrophages, which are found in a variety of organs, maintain tissue homeostasis at a steady state and act as the first line of defence during pathogen-induced inflammation in the host. Most monocyte/macrophage lineage studies in chickens have been largely performed using cell lines, while few studies using primary cells have been conducted. In the present study, the phenotypic and functional characteristics of splenic monocyte/macrophage lineage cells during steady state and inflammatory conditions were examined. Splenic monocyte/macrophage lineage cells could be identified as MRC1loMHCIIhi and MRC1hiMHCIIlo cells based on their surface expression of MRC1 and MHCII. In the steady state, MRC1loMHCIIhi cells were more frequently found among MRC1+ cells. MRC1loMHCIIhi cells expressed a higher number of antigen-presenting molecules (MHCII, MHCI, and CD80) than MRC1hiMHCIIlo cells. In contrast, MRC1hiMHCIIlo cells showed better phagocytic and CCR5-dependent migratory properties than MRC1loMHCIIhi cells. Furthermore, MRC1hiMHCIIlo cells infiltrated the spleen in vivo and then became MRC1loMHCIIhi cells. During lipopolysaccharide (LPS)-induced inflammatory conditions that were produced via intraperitoneal (i.p.) injection, the proportion and absolute number of MRC1hiMHCIIlo cells were increased in the spleen. Uniquely, inflammation induced the downregulation of MHCII expression in MRC1hiMHCIIlo cells. The major source of inflammatory cytokines (IL-1β, IL-6, and IL-12) was MRC1loMHCIIhi cells. Furthermore, MRC1hiMHCIIlo cells showed greater bactericidal activity than MRC1loMHCIIhi cells during LPS-induced inflammation. Collectively, these results suggest that two subsets of monocyte/macrophage lineage cells exist in the chicken spleen that have functional differences.
Highlights
Monocytes/macrophages, which comprise the majority of mononuclear phagocytes, are derived from bone marrow precursors [1]
The blood was diluted in phosphate-buffered saline (PBS) at a 1:1 ratio, and peripheral blood mononuclear cells (PBMCs) were isolated by density gradient centrifugation for 20 min at 400 × g without braking using Ficoll-Paque (Sigma-Aldrich, St Louis, MO, USA) and washed with PBS containing 5% fetal bovine serum (FBS)
Two subsets of splenic mannose receptor C-type 1 (MRC1)+ cells showed different expression patterns for MHCII and MRC1 To investigate the distribution of chicken primary monocyte/macrophage lineage cells in different organs, primary and secondary immune organs from 3-week-old chickens were collected, and their phenotypes were analysed based on the expression of MRC1, a well-known chicken monocyte/macrophage marker [11], and MHCII (Additional file 1)
Summary
Monocytes/macrophages, which comprise the majority of mononuclear phagocytes, are derived from bone marrow precursors [1]. Monocytes/macrophages are part of the innate immune system and function as the first line of defence in the host through various effector functions. They express several kinds of pattern recognition receptors (PRRs), including Toll-like receptors (TLRs) and C-type lectin receptors that recognize pathogens [6], and phagocytose and clear the pathogen by lysosomal acidification [7]. It has been reported that red pulp monocyte/ macrophage lineage cells in spleen from chicken express MHCII and show a high phagocytosis ability that is similar to that of mammalian red pulp macrophages [11]. Monocyte/macrophage lineage cells are found in chicken ellipsoids [11, 12], which are analogous to the mammalian marginal zone
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