Conditioned medium from macrophages, osteoclasts and treated osteoclasts all selleck inhibitor significantly increased CD69 expression on γδ T cells to a similar extent (Fig. 3). This was in contrast to our findings with CD4+ T cells, since conditioned medium from macrophages or untreated osteoclasts consistently failed to induce upregulation of CD69 on CD4+ T cells. However, conditioned medium from treated osteoclasts did induce a significant increase in CD69 expression on CD4+ T cells. Taken

together, these results indicate that γδ T cell activation by macrophages or osteoclasts is mediated by soluble factors and does not fundamentally require cell–cell contact. However, the stimulatory effect of osteoclasts on CD4+ T cells requires co-culture conditions, suggesting that cell–cell interactions play an important role in this process. TNFα is a potent stimulator of T cell activation and is capable of co-stimulatory effects on T cell survival [23] and [24]. We therefore investigated whether macrophages and osteoclasts were triggering γδ T cell activation Proteasome inhibitor via production of TNFα. Using a neutralising anti-TNFα antibody we observed that the stimulatory effect of macrophage- and osteoclast-derived

conditioned medium on CD69 expression by γδ T cells was significantly reduced versus the isotype control (Fig. 4). There was also a trend for TNFα neutralisation to diminish the stimulatory effects of treated

osteoclast-derived conditioned medium but this was not statistically significant versus the isotype control. While the stimulatory effect of conditioned medium on γδ T cell activation was attenuated by anti-TNFα treatment, also it was not abolished entirely, indicating that other stimulatory factors are present in osteoclast-derived conditioned medium that trigger γδ T cell activation. Following our observation that osteoclasts induce γδ T cell activation we then sought to determine whether these stimulatory effects of osteoclasts could trigger proliferative responses in γδ T cells. Using CFSE-labelled γδ and CD4+ T cells in co-cultures with autologous osteoclasts, we observed no proliferative effects of autologous osteoclasts on unstimulated γδ T cells or CD4+ T cells (Fig. 5A). However, activation of γδ T cells with IL-2 (which induced marked upregulation of CD69 on γδ T cells — Fig. 3A) resulted in extensive proliferation of γδ T cells, and this proliferative effect was further enhanced by co-culture with osteoclasts (Fig. 5A). In contrast to this, CD4+ T cells did not exhibit any proliferative responses to IL-2 alone or in co-culture with osteoclasts. This suggests that unstimulated osteoclasts provide co-stimulatory signals that augment IL-2-induced γδ T cell proliferation, but such co-stimulatory signals do not confer responsiveness of CD4+ T cells to IL-2 stimulation.