Fc gamma receptor mediated modulation of osteoclastogenesis

Doonan, James Joseph (2014) Fc gamma receptor mediated modulation of osteoclastogenesis. PhD thesis, University of Glasgow.

Abstract

Osteoporosis is a condition that results from substantially weakened bone, increasing an individual’s risk of fracture. Post-menopausal osteoporosis is the most common form of the condition, affecting 30% of post-menopausal women over the age of 50. Following the menopause, female oestrogen levels decline and this perturbs bone homeostasis by promoting an environment that is biased towards bone erosion. Osteoclasts are the cells responsible for eroding bone and are normally inhibited by oestrogen. However, the decline in oestrogen production results in increased osteoclast differentiation and activity. This rapidly decreases the bone mineral density and results in fracture-prone bone. Osteoclasts are derived from mononuclear myeloid progenitors found in the blood and bone marrow, which fuse to form large multinucleated cells that reside in the bone cavity. These progenitor cells are also responsible for replenishing monocytes, macrophages and dendritic cells. One class of receptors present on the surface of these cells, which are capable of dictating a cells function, are Fcγ receptors and modulation of Fcγ receptors has been shown to inhibit the differentiation of human monocytes to osteoclasts.

This thesis investigates Fcγ receptor modulation on murine osteoclastogenesis and in order to stimulate Fcγ receptors, both IgG and IgG complexes were used. IgG complexes were generated using Staphylococcus aureus Protein A (SpA) in combination with IgG to form SpA-IgG complexes (SIC). We show that IgG and SIC are capable of engaging with Fcγ receptors resulting in the inhibition of osteoclast differentiation. Furthermore, both IgG and SIC inhibit the transcription of mRNA essential for the fusion of progenitors and enzymes for the erosion of bone matrix. Therefore, IgG and SIC are capable of inhibiting murine osteoclastogenesis.

The murine model of osteoporosis was used to further investigate the ability of SIC to inhibit murine osteoclast differentiation. Previous studies have shown that when SpA is administered in vivo it is capable of binding circulating IgG to form SIC. We used this property to test the ability of SpA to bind to the surface of monocytes. SpA was found to bind with highest affinity to blood Ly6Chigh monocytes, which are known to differentiate in vitro to OCs. IgG and SIC were also able to inhibit the in vitro osteoclastogenesis of Ly6Chigh monocytes. It was hypothesised that SpA would co-opt IgG and inhibit the in vivo differentiation of progenitors to osteoclasts in the ovariectomy model of osteoporosis. To generate this animal model the ovaries were removed from the mice in order to simulate the menopause and induce bone loss. To assess the percentage of bone present after ovariectomy, we used micro-computer tomography and discovered that SpA was unable to prevent bone loss associated with ovariectomy. Therefore, SpA can bind to the surface of osteoclast progenitors but is unable to inhibit bone loss in the model of osteoporosis.
In addition to studying the role of Fcγ receptor modulation of osteoclastogenesis, the role of Bcl-3 (a negative regulator of NF-κB) in osteoclast differentiation and bone remodelling was also investigated. NF-κB is an essential signalling molecule and transcription factor involved in osteoclast differentiation. Previous research has shown that in the absence of Bcl-3 (Bcl-3-/-) aberrant cytokine responses to LPS and TNF- occur. Therefore, RANKL stimulation of WT and Bcl-3-/- osteoclast precursors was done to determine whether Bcl 3 /- animals responded aberrantly to RANKL. WT and Bcl-3-/- animals were able to generate in vitro osteoclasts, which were phenotypically and transcriptionally similar. However, comparison of in vivo osteoclast progenitors revealed that Bcl-3-/- animals had reduced CD115+ osteoclast progenitors compared to WT animals. Examination of the trabecular bone present in the proximal tibia revealed that Bcl-3-/- animals had a higher percentage of bone present that WT controls. Therefore, Bcl-3 does not effect in vitro osteoclast differentiation but further work needs to be done to understand the role of Bcl 3 in bone remodelling.

This thesis aimed to investigate whether SpA-IgG complexes or Bcl-3 could represent a novel avenue of therapeutic intervention in osteoporotic disease. In summation, SpA is able to form IgG complexes that can inhibit the differentiation of OCs in vitro; however, treatment of osteoporotic animals with SpA was unable to halt bone loss. This suggests that SpA-IgG complexes are able to modulate Fcγ receptors in vitro and skew progenitors from differentiation into osteoclasts but cannot overcome the prevailing pro-osteoclastogenic environment that results from ovariectomy. The presence of osteoclast progenitors was also shown to be partially dependent on Bcl-3 and as such Bcl-3 may be a novel target for therapeutic agents to target osteoclast progenitors in diseases like osteoporosis. However, the role of Bcl-3 in bone remodelling requires further investigation.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Bcl-3, Fc gamma receptor, NF-kappaB, Osteoclast, Osteoporosis, SpA
• Subjects: Q Science > QR Microbiology > QR180 Immunology
• Colleges/Schools: College of Medical Veterinary and Life Sciences > School of Infection & Immunity > Immunology & Infection
• Supervisor's Name: Goodyear, Dr. Carl
• Date of Award: 2014
• Depositing User: Mr James Doonan
• Unique ID: glathesis:2014-5579
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 09 Oct 2014 14:30
• Last Modified: 14 Oct 2014 10:47
• URI: https://theses.gla.ac.uk/id/eprint/5579