Loss of CD169 alters macrophage populations with subsequent effects on bone anabolism — ASN Events
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Loss of CD169 alters macrophage populations with subsequent effects on bone anabolism (#180)

Susan Millard 1 , Lena Batoon 1 , Michal Bartnikowski 2 , Nicole Bartnikowski 2 , Vaida Glatt 3 , Liza Raggatt 1 , Allison Pettit 1
  1. Mater Research Insitute, Woolloongabba, QLD, Australia
  2. Institute of Health and Biomedical Innovation (IHBI), Queensland University of Technology, Brisbane, QLD, Australia
  3. Orthopaedic Surgery, University of Texas Health Science Centre, San Antonio, TX, USA

Osteomacs, the tissue-resident macrophages in bone lining tissues, support osteoblast function during homeostasis and bone repair. We have developed enzyme-free cell isolation methods that enrich for bone lining tissues. Flow cytometry demonstrated that both endosteum and periosteum contain CD169+ macrophages and in vivo approaches confirmed that osteomacs are CD169+. CD169 is a cell adhesion molecule encoded by the Siglec-1 gene with expression restricted to specific macrophage subsets. The mouse transcriptional atlas data on www.biogps.org confirmed expression of Siglec-1 in mouse macrophages, with minimal expression in osteoclast cultures (which contain ‘contaminating’ mononuclear cells including macrophages) and no expression in osteoblasts. Siglec-1DTR/WT knock-in mice are haploinsufficient for Siglec 1 and display reduced cell surface expression of CD169 by macrophages (>50% reduction in median fluorescence intensity (MFI) of CD169 staining in bone marrow (BM) resident macrophages (F4/80+Ly6G-VCAM1+Ly6Cint)). Siglec-1 haploinsufficiency was associated with a modest 6% reduction in BM resident macrophages assessed by flow cytometry. Histomorphometric quantification of the percent of endosteal bone surface covered by osteomacs in young adult males (8wk) showed a 35% reduction in osteomac surface. Interestingly, this was associated with a concomitant 27% reduction in osteoblast surface. Siglec-1 haploinsufficiency had no impact on osteoclast surface. Siglec-1DTR/DTR mice are null for Siglec-1 which was confirmed by an absence of cell surface CD169 expression in BM resident macrophages. Furthermore the number of BM resident macrophages was reduced by 50% in these Siglec-1 null mice. Micro-computed tomography of mature adult males (16wk) revealed a 32% reduction in fractional bone volume at the distal femur compared to wild-type mice. This data demonstrates that loss of CD169 results in a reduction in tissue-resident macrophages, including osteomacs, and this is associated with a reduction in osteoblast numbers and fractional bone volume.