CD8+ T cells and associated signaling pathways influence osteoclast formation and osteoblast differentiation in osteoporosisImmune Cells and Signaling Pathways Influence Bone Health in Osteoporosis

IntroductionOsteoporosis weakens the skeleton by lowering bone mass and damaging bone microarchitecture. A growing body of evidence points to disruptions in osteoimmune homeostasis as a key driver of this disease. In particular, apoptosis of bone cells and immune cells appears to be a critical contributing mechanism. Within the bone marrow, CD8+ T cells display a surprising functional duality: whether they protect bone or promote its loss depends heavily on local signals. Under healthy conditions or after mechanical loading, activated CD8+ T cells help maintain bone mass. They do so mainly by secreting interferon‑γ (IFN‑γ), which blocks osteoclast formation through interference with NF‑κB and MAPK signaling. However, this protective role is not fixed. Pathological cues such as estrogen deficiency can flip CD8+ T cells into a bone‑destroying phenotype. For example, when estrogen drops, muscle‑derived IL‑33 decreases. That drop prompts bone marrow CD8+ T cells to release large amounts of CCL5. CCL5 then binds to CCR3 on osteoclast precursors and activates the ERK pathway, thereby accelerating bone loss in postmenopausal osteoporosis. Aging and chronic inflammation add another layer of complexity. They drive the accumulation of senescent CD28- CD8+ T cells. These aged cells no longer produce enough IFN‑γ to protect bone; instead, they adopt a senescence‑associated secretory phenotype (SASP) that suppresses osteoblast differentiation and reduces mesenchymal stromal cell viability. Moreover, apoptosis of osteoblasts and osteocytes—triggered by TNF‑α, IFN‑γ, or glucocorticoids—worsens bone loss. New findings suggest that CD8+ T cells may fuel this apoptotic process through Fas/FasL interactions and the granzyme/perforin pathway. This review brings together these context‑dependent mechanisms. We place special emphasis on how hormonal shifts, metabolic changes, and inflammatory mediators converge to decide whether CD8+ T cells support skeletal integrity or drive bone resorption.MethodsWe performed a narrative review of recent studies on CD8+ T cells in osteoporosis. We searched PubMed and Web of Science using keywords such as “osteoporosis”, “osteoimmunology”, “CD8+ T cells”, “IFN‑γ”, “CCL5”, “cellular senescence”, “SASP”, “Fas/FasL”, and “granzyme”. From the retrieved articles, we selected original research (including in vitro co‑culture systems, animal models, and clinical samples) and relevant reviews. We then extracted and integrated key mechanistic evidence regarding the dual functionality of CD8+ T cells and its regulatory pathways.ResultsCD8+ T cells have two opposing functions. Under healthy conditions or mechanical loading, activated CD8+ T cells secrete IFN‑γ, which inhibits osteoclast formation by interfering with NF‑κB and MAPK signaling, thus maintaining bone mass. In contrast, when estrogen is deficient, reduced muscle‑derived IL‑33 causes bone marrow CD8+ T cells to release large amounts of CCL5. CCL5 binds to CCR3 on osteoclast precursors and activates the ERK pathway, accelerating bone loss. Aging and chronic inflammation shift the balance. Aging and chronic inflammation lead to the accumulation of senescent CD28- CD8+ T cells. These cells no longer produce sufficient protective IFN‑γ. Instead, they exhibit a senescence‑associated secretory phenotype (SASP) that suppresses osteoblast differentiation and reduces mesenchymal stromal cell viability. CD8+ T cells may promote bone cell apoptosis through multiple pathways. TNF‑α, IFN‑γ, or glucocorticoids can trigger apoptosis of osteoblasts and osteocytes. Newer evidence suggests that CD8+ T cells amplify this process via Fas/FasL interactions and the granzyme/perforin pathway. Two newly identified CD8+ T cell‑derived molecules deserve attention. Granzyme K protects against bone loss, whereas CCL5 promotes bone loss. These two molecules offer fresh opportunities for biomarkers and therapies.DiscussionOur synthesis shows that CD8+ T cells act as a double‑edged sword in osteoporosis: their functional switch depends on the integration of hormonal, metabolic, and inflammatory signals. Under normal conditions or after mechanical loading, the anti‑osteoclastogenic effect of IFN‑γ dominates. But when estrogen levels fall, the IL‑33/CCL5/CCR3/ERK axis becomes activated, turning CD8+ T cells toward a bone‑resorbing phenotype. During aging, the buildup of CD28⁻ CD8+ T cells and their SASP further weakens bone formation and worsens bone cell death through apoptotic pathways. These observations point to a more precise therapeutic strategy. Instead of broadly modulating T cells, one could specifically target pathogenic pathways—for example, by interfering with the IL 33/ST2/CCL5 cascade or by clearing immune senescent CD28⁻ subsets. Such approaches might help restore osteoimmune balance. Two recently discovered CD8+ T cell derived molecules, granzyme K and CCL5, are particularly interesting because they respectively protect against and promote bone loss. They could serve as novel biomarkers or even therapeutic targets. Future studies should examine the cell type specific regulation of these pathways in vivo and test the safety and efficacy of targeted interventions.