Chronic inflammatory diseases are a major health problem, whose incidence increases with advancing age. Many of these diseases are associated with bone loss due to the disruption of bone homeostasis. Inflammatory bone damage relies on several factors including: (i) complex interactions between activated immune cells and bone cell progenitors; (ii) immune cell trafficking from inflammatory sites to the bone marrow and (iii) on the modifications they induce in the bone marrow micro-environment, affecting its integrity. Recently, this bone damage has also been associated with modifications in the gut microbiota that increase inflammatory responses and the production of inflammatory cytokines including those affecting the bone homeostasis. Osteoimmunology is the conceptual framework that studies the interplay between the immune and the skeletal system. It emerged 20 years ago as a means for proposing an integrated vision to decipher these interactions in health and disease.

Bone-forming osteoblasts and bone-resorbing osteoclasts regulate each other to maintain bone homeostasis. But above all, they also interact with bone marrow-derived hematopoietic cells, including HSCs and their progenitors, memory lymphocytes and other immune cells to preserve hematopoiesis and immune memory. Over these 20 years, advances in osteoimmunology research have greatly contributed to the understanding of the coordinated development of bone and the hematopoietic system. This research has also contributed to deciphering the mechanisms involved in inflammation and bone destruction in rheumatic diseases and other chronic inflammatory diseases such periodontitis, inflammatory bowel diseases, psoriasis, as well as osteoporosis, chronic infections and bone cancer and metastasis.

As part of these major advances is the identification of the essential role of immune cells and their products in (i) the control of bone cell differentiation; (ii) the participation of bone cells in regulating hematopoietic niches; (iii) the immunomodulatory functions of bone cells; (iv) the heterogeneity of osteoblasts and osteoclasts under physiological conditions and in chronic inflammatory diseases and (v) the implication of gut microbiota in controlling bone homeostasis via immune modulation.

In this Research Topic, we aim to gather a series of articles that contribute to the major achievements in Osteoimmunology from an immunological perspective. We welcome the submission of Review, Focused Review, Mini-Review, Original Research, Protocol, Method, Perspective and Opinion articles with a major immunological focus that cover the following topics within the scope of Frontiers in Immunology:

1. Osteoclast and osteoblast origin and their diversity in inflammatory conditions.
2. Immunological mechanisms underlying the interplay between rheumatic diseases and bone.
3. Role of immune cells in other inflammatory diseases that affect bone homeostasis.
4. The role of adaptive and innate immune cells in modulating osteoblast and osteoclast differentiation.
5. Role of DAMPs / Alarmins, TLR signaling and cytokines in bone homeostasis.
6. Bone cells and bone marrow niches for hematopoietic and cancer cells.
7. The interplay between microbiota and osteoimmunology.
8. Chronic infection and its effects on bone cell differentiation and activity.
9. Osteoimmunology from a clinical perspective.

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