Abstract
Background Stimulating bone regeneration is a central aim in orthopaedic and trauma surgery.
Although the replacement of bone with artificial materials like cement or apatite
helps to keep up bone stability, new bone often cannot be regenerated. Increasing
research efforts have led to the clinical application of growth factors stimulating
bone growth (e.g. bone morphogenic protein, BMP) and inhibitors preventing bone consumption
(e.g. RANKL blocking antibodies). These factors mostly concentrate on stimulating
osteoblast or preventing osteoclast activity.
Current Situation It is widely accepted that osteoblasts and osteoclasts are central players in bone
regeneration. This concept assumes that osteoblasts are responsible for bone growth
while osteoclasts cause bone consumption by secreting matrix-degrading enzymes such
as cathepsin K and matrix metalloproteinases (MMP). However, according to new research
results, bone growth or consumption are not regulated by single cell types. It is
rather the interaction of various cell types that regulates bone metabolism. While
factors secreted by osteoblasts are essential for osteoclast differentiation and activation,
factors secreted by activated osteoclasts are essential for osteoblast activity. In
addition, recent research results imply that the influence of the immune system on
bone metabolism has long been neglected. Factors secreted by macrophages or T cells
strongly influence bone growth or degradation, depending on the bone microenvironment.
Infections, sterile inflammation or tumour metastases not only affect bone cells directly,
but also influence immune cells such as T cells indirectly. Furthermore, immune cells
and bone are mechanistically regulated by similar factors such as cytokines, chemokines
and transcription factors, suggesting that the definition of bone and immune cells
has to be thought over.
Outlook Bone and the immune system are regulated by similar mechanisms. These newly identified
similarities between bone and the immune system imply that medication developed for
tumour and autoimmune patients could also be applied in bone diseases.
Key words
osteoimmunology - osteoblast - osteoclast - bone regeneration