Table 1.

Function of different innate immune cell types and their role in osteoimmunology.

Cell type	Physiological role	Role in bone biology
Macrophage	Inflammation, phagocytosis, tissue repair	M1 macrophage promotes bone resorption via osteoclastogenesis (21, 28); M2 macrophage majorly promotes bone formation by stimulating differentiation of precursor cells into mature OBs (29, 30). However, in the absence of estrogen M2 macrophages can get differentiated in OCs (31). Osteal macrophages help in efficient bone mineralization (32).
Monocyte	Inflammation	Serves as a precursor to OCs, macrophages, and DCs (33). Helps in the recruitment of immune cells to the bone remodeling sites by producing chemokines (34).
Dendritic cell	Inflammation, antigen presentation	Can transdifferentiate to osteoclasts in the inflammatory milieu (35).
Neutrophils	Inflammation, phagocytosis	Promotes bone resorption by increased expression of mRANKL (36).
Eosinophils	Inflammation, allergic response	Found to be increased in number in vitamin D deficiency (37); Source of IL-31 and IL-31 found to be associated in postmenopausal osteoporosis (38)
Mast cell	Allergic response, inflammation	Triggers osteoclastogenesis by producing pro-inflammatory mediators such as, histamine, TNF-α & IL-6 (39, 40).
NK cell	Cellular cytotoxicity, ADCC, inflammation	Promotes osteoclastogenesis by producing RANKL & MCSF (26). Coculture with monocyte in the presence of IL-15 also promotes osteoclast formation (26).
ILCs	Tissue homeostasis, regulation of innate and adaptive immunity	Different subtypes of ILCs produce various factors like RANKL, GMCSF, IL-17 which are involved in multiple bone disorders (27, 41, 42).