Selective inhibition of mesenchymal and hematopoietic RANKL

Receptor activator of NF-κB ligand (RANKL) is not only an essential cytokine for osteoclast differentiation, activation, and survival, but may also be involved in the pathogenesis of acute myocardial infarction (MI) and heart failure. On the basis of the fundamental role of the RANKL/osteoprotegerin (OPG) system in bone resorption, human monoclonal antibodies against human RANKL (hRANKL) such as denosumab and AMG161, the IgG1 version of denosumab, have been developed. Furthermore, hRANKL knock-in (KI) mice were recently generated which carry the human instead of the murine exon 5 in their RANKL gene. This chimeric RANKL protein is capable of inducing bone resorption in mice, while being fully inhibited by denosumab or AMG161.

 Furthermore, we provided compelling evidence that only hematopoietic and endothelial, but not stromal precursors, engraft in host bone marrow after transplantation with unfractionated bone marrow (Ödorfer KI et al, JCMM 2011). The project is based on the idea that, together with an anti-hRANKL antibody such as AMG161, hRANKL-KI mice could be used as a powerful tool to separate between the pathophysiological role of RANKL derived from mesenchymal and hematopoietic/endothelial sources. To reach this goal, wild-type and hRANKL-KI mice are lethally irradiated, and transplanted with bone marrow from hRANKL-KI and wild-type donors, respectively. In irradiated wild-type mice reconstituted with bone marrow from homozygous hRANKL-KI donors, all hematopoietic and some endothelial cells produce chimeric, humanized RANKL, while all mesenchymal cells produce murine RANKL. Thus, because AMG161 blocks human but not murine RANKL, AMG161 will block only chimeric RANKL derived from hematopoietic/endothelial sources in this model. Vice versa, when homozygous hRANKL-KI mice are reconstituted with bone marrow from wild-type mice, only humanized RANKL derived from mesenchymal sources will be blocked.

Together with hRANKL-KI mice as a control for the effects of total RANKL inhibition, such a system could provide unprecedented insight into the pathophysiology of diseases in which RANKL signaling plays a role.

Although this model could be exploited in a very broad fashion, we are currently focused on osteoimmunology and myocardial infarction. We aim to answer the following questions: 1) What is the relative contribution of RANKL derived from mesenchymal or immune/endothelial cells in the pathophysiology of sex steroid deficiency-induced bone loss? 2) Does total RANKL inhibition or selective inhibition of hematopoietic/endothelial and mesenchymal cell-derived RANKL have a beneficial effect on the healing of myocardial lesions? Osteoporosis and myocardial infarction are leading causes of disability and death in modern societies with high percentages of elderly which is why we believe that this project may have important implications for our health care systems.

This project is performed in collaboration with AMGEN.

Related Papers:
Hematopoietic bone marrow cells participate in endothelial, but not epithelial or mesenchymal cell renewal in adult rats.
Odörfer KI, Egerbacher M, Unger NJ, Weber K, Jamnig A, Lepperdinger G, Kleiter M, Sandgren EP, Erben RG.
J Cell Mol Med. 2011 Oct;15(10):2232-44. doi: 10.1111/j.1582-4934.2010.01216 1

MSC culture reveals lack of MSC engraftment in reconstituted rats. Histochemical staining after heat pre-treatment of cultured MSC isolated from wt F344 rats (A), from lethally irradiated wt F344 rats reconstituted with unfractionated BM of ALPP-tg co-isogeneic donors, 4 weeks after bone marrow transplantation (B), or from ALPP-tg F344 rats (C). ALPP staining was absent in wt (A) and BMT MSC (B), whereas MSC of ALPP-tg donors showed strong ALPP expression (C).


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