Osteoclasts and foreign body giant cells (FBGCs) are multinucleated cells derived from monocytes, but they have distinct functions. Osteoclasts resorb bone while FBGCs form in response to foreign material. Regarding bone implants, osteoclasts are responsible for implant integration, but also for bone resorption associated to implant loosening, while FBGCs play a role in the immune response to the foreign material. Little is known about which proteins in the local environment fine-tune the multinucleation of osteoclasts or FBGCs. One candidate is Activin A (ActA). It has been shown to induce larger, more active osteoclasts, but its effect on FBGC differentiation is unknown.

We investigated the effect of ActA on the differentiation of osteoclasts and FBGCs from human CD14-positive monocytes. The number of multinucleated cells and the cell area was measured. qPCR was performed to assess the effect of ActA on gene expression. ActA's influence on osteoclast and FBGC formation was studied on plastic, bone and hydroxyapatite coated Titanium discs (ALD-HA).

ActA induced fewer, but bigger and more active osteoclasts on plastic and bone. In contrast, ActA did not have an effect on FBGC number. On ALD-HA, ActA reduced the number of FBGCs, but did not influence osteoclast numbers. qPCR showed that ActA upregulated the expression of several genes such as TRAcP, CIITA, OLR1, RHOBTB1 and ALK4, but mainly in osteoclasts. These results show that ActA has a different effect on osteoclasts compared to FBGCs. This difference could be caused by a difference in the expression in the canonical ActA receptor ALK4.