The interaction between inflammatory molecules and the bone system is defined as “osteoimmunology” [31]. In this context, receptor activator of NF-kB (RANK) and RANKL, though initially identified as mediators of T cell activation, were subsequently found to play a pivotal role in osteoclastogenesis in physiological and pathological conditions.

RANKL is a transmembrane protein mainly expressed by osteoblasts, periosteal cells, and osteocytes, whereas its receptor, RANK, is expressed by osteoclasts and preosteoclasts. After binding to RANK, RANKL stimulates the formation and activity of osteoclasts, which are responsible for bone resorption. However, osteoblasts produce an additional soluble protein, osteoprotegerin (OPG), which acts as a “decoy receptor”, preventing the binding of RANKL to RANK and, consequently, osteoclast activation. Under physiological conditions, the ratio of RANKL to OPG is balanced, and bone resorption is counterbalanced by bone deposition. Aging, estrogen deficiency, and systemic glucocorticoid exposure can alter this ratio in favor of RANKL, causing a progressive reduction of BMD that may result in the clinical condition of osteoporosis [32]. The RANK-RANKL system has been extensively studied in the context of osteoporosis related to chronic inflammatory disease. In fact, rheumatic diseases such as rheumatoid arthritis (RA) and ankylosing spondylitis are frequently associated with bone loss, both localized and systemic, and are characterized by an increased risk of osteoporotic fractures in all age groups [33] (Fig. 1).

The RANK-RANKL-OPG system. Bone metabolism is the result of the balance between bone resorption by osteoclasts and bone formation by osteoblasts. RANK is a transmembrane protein expressed by osteoclast precursors that derive from the colony-forming unit for macrophages (CFU-M). The binding of RANKL, expressed by osteoblasts, stimulates the formation and the activity of the osteoclasts, favoring bone resorption. OPG, an additional soluble protein produced by osteoblasts, prevents the binding between RANK and RANKL, counterbalancing bone resorption. Immune cells such as activated T cells and inflammatory cytokines act synergistically promoting the RANK-RANKL pathway. SARS-Cov-2 could have an additive effect promoting the cytokine storm which promotes bone resorption, but a direct effect of the virus on osteoclastic activation cannot be excluded. From a pharmacological point of view, estrogens and tocilizumab can reduce inflammatory cytokine levels (specially IL-6), whereas denosumab directly inhibits the RANK-RANKL interaction and they all represent potential protective agents against bone resorption

In a recent review, De Martinis et al. [34] have summarized the main mechanism involved in the activation of osteoclastogenesis by inflammatory phenomena. They stressed that infection, trauma, and injuries induce the production of endogenous signaling mediators of inflammatory response named “alarmins”. Alarmins are released by mesenchymal cells and act as chemotactic factors and pattern recognition receptors, allowing the innate immune cells to be alerted to tissue damage. By recalling the cells of the immune system, these molecules initiate tissue repair processes but, at the same time, are also responsible for the bone resorption that occurs during acute inflammatory phenomena. Among the most important molecules involved, the authors mention IL-1, receptor for advanced glycation end products (RAGE), and high mobility group box 1 protein (HMGB1) protein (Fig. 2).

Proposed mechanism of alarmins. Modified from Fig. 1 in De Martinis et al. [22]

IL-1 is a proinflammatory cytokine, also known as “osteoclast activating factor”, that increases osteoclast vitality and resorptive activity by an independent and RANKL-dependent mechanism. In fact, osteoclasts express on their surface the specific IL-1 receptor (IL1-R), whose expression is further increased by RANKL, which directly promotes their resorptive activity through the same intracellular pathway, RANK-RANKL. Moreover, as in the case of other inflammatory cytokines such as IL-18 and TNF-α, IL-1 upregulates RANKL production by T cells and increases PGE2 synthesis in fibroblasts, with a consequent additional activation of osteoclastogenesis. RAGE is a multiligand receptor belonging to the immunoglobulin receptor superfamily, whose circulating levels correlate with the risk of osteoporosis. Higher levels of RAGE are common in chronic conditions such as diabetes, cancer, cardiovascular disease, and neurodegeneration. RAGE is expressed by mesenchymal cells (osteocytes, osteoblasts, and osteoclasts) and guides their growth and development. It binds endogenous factors, like advanced glycation end products (AGE) and HMGB1 protein, which are released in case of stress or cell death, and activates the downstream signaling pathways (AP-1, NFAT, NF-κB, STAT3, and CREB) involved in osteoclastogenesis, acting synergistically with the RANK-RANKL system. Notably, even reduced levels of RAGE have a negative effect on bone metabolism: it has been suggested that its overexpression in inflammatory conditions could have a role in inflammation resolution and tissue repair, whereas insufficient RAGE expression induces suppression of PPAR-α and its cofactor PGC1-α, leading to a proinflammatory state and to BMD decrease. Finally, HMGB1, a nuclear protein secreted by macrophages after RANKL binding, is required for osteoclast formation and TNF-α expression. It acts as an alarmin in several tissues, and it binds RAGE and toll-like receptors (TLR) of immune cells, amplifying the inflammatory process. At the bone level, HMGB1 increases the expression of RANKL, IL-6, and TNF-α by stromal osteoblastogenic cells of the bone marrow and acts as a chemotactic stimulus for mature osteoclasts [28]. Moreover, RANKL is directly expressed by CD4+ T cells, which activate osteoclasts and stimulate other RANKL-expressing cells such as monocytes/macrophages, promoting bone catabolism [35].

Inflammatory molecules are also physiologically produced during aging, in a process called “inflammaging” (inflammation plus aging). Circulating levels of TNF-α, IL-6, and CRP progressively increase with aging and are overexpressed in chronic conditions such as atherosclerosis, diabetes mellitus, and chronic obstructive pulmonary disease. As in the case of rheumatic diseases, elderly subjects with chronic disease show a higher prevalence of osteoporosis. In vitro studies have suggested that IL-1, IL-6, and TNF-α could have a role in reducing BMD in these patients, although the in vivo data are conflicting. The recent OsteoLaus study has found no correlations between circulating inflammatory molecules and bone metabolism parameters in postmenopausal women after exclusion of acute inflammatory conditions [36].

The discovery of the RANK-RANKL pathway has led to the development of denosumab, a fully human antibody that binds to and inhibits RANKL, thus preventing bone resorption, which has been shown to reduce the risk of vertebral, nonvertebral, and hip fractures in postmenopausal women with osteoporosis [37]. In patients with RA, denosumab given subcutaneously for 6–12 months inhibited the occurrence and progression of bone erosions [38, 39], even though this effect raised doubts about a possible immunosuppressive effect.

These considerations suggest that during the current pandemic, denosumab could be the best therapeutic choice to preserve bone from the adverse effect of SARS-CoV-2 infection, correcting the RANK-RANKL system imbalance due to the massive cytokine release and to high-dose steroid treatment.

Currently, the mainstay of COVID-19 pharmacological treatment is represented by antiviral drugs, whose action is directed toward structural elements of the virus, and by immune system modulators such as glucocorticoids and antirheumatic drugs, which dampen the excess of cytokines produced in response to disease [40]. Interestingly, a side effect of antirheumatic drugs could be bone loss prevention. In RA patients, hydroxychloroquine has been shown to prevent bone resorption (measured by β-CTx levels) after 3 and 6 months of treatment [33]. Above all, tocilizumab, a monoclonal antibody that binds to IL-6 receptor and which is currently one of the most promising drugs for COVID-19 treatment [40], has long been approved for use in RA patients, where it reduces bone turnover and improves BMD [33].