Transforming growth factor-β: transforming plaque to stability.

Lievens and colleagues report an important finding that helps to elucidate the role of transforming growth factor b (TGFb)b in atherosclerotic plaque formation. TGFb is a widely expressed cytokine produced by smooth muscle cells, endothelial cells, monocytes, macrophages, and T cells. Clinical studies suggest that TGFb is antiatherosclerotic, citing a negative correlation between plasma TGFb concentration and the extent of atherosclerosis. –7 In mouse models of atherosclerosis, inhibition of TGFb signalling causes the formation of unstable plaques. This plaque phenotype is probably caused by two consequences of inhibiting TGFb: first, blockade of TGFb’s role in promoting plaque fibrosis would make the plaques less stable, and, secondly, the absence of TGFb signalling results in increased numbers of pro-inflammatory T cells that populate atherosclerotic lesions. 12 Because dendritic cells (DCs) express the TGFb receptor (TGFbR) and have a central role in regulating immune responses, studying the potential role of the TGFbR in DCs and how it might influence T-cell activation and modulate atherosclerosis is an area of great interest. To study the effect of TGFb signal inhibition on DCs and its eventual effect on atherosclerosis development, Lievens et al. backcrossed Apoe – /– mice with mice whose TGFb-b signalling in DCs is disrupted by a dominant negative TGFbRII transgene (that encodes the extracellular and transmembrane, but not intracellular, regions of the TGFb type II receptor) inserted into the CD11c promoter. CD11c is expressed on most mouse DCs and some macrophages, and the CD11c promoter, although not DC specific, drives expression in many DCs. Previous publications using similar constructs have reported the transgene’s presence in CD11c-expressing DCs and natural killer (NK) cells, but not in T cells, NKT cells, or B cells. These CD11c-dnTGFbRII (CD11cDNR) mice were kept on chow diet and euthanized at 20 weeks of age. In both Apoe – CD11cDNR and Apoe – /– mice, most CD11c+ cells in atherosclerotic lesions were either DCs or macrophagederived foam cells (CD11c+CD11b2). In spleen and lymph nodes, there were slightly more pro-inflammatory CD8CD4 DCs in Apoe – CD11cDNR mice, which produced more tumour necrosis factor a (TNFa a) and interleukin-12 (IL-12) after lipopolysaccharide (LPS) stimulation than in control Apoe – /– mice. More importantly, bone marrow-derived CD11c+ cells from Apoe – CD11cDNR mice strongly induced T-cell proliferation, along with interferon g (IFNg g) and IL-4 production, when co-cultured with CD4+ T cells from wild-type mice previously stimulated with anti-CD3/ CD28. Similarly, when splenic DCs from Apoe – CD11cDNR mice were stimulated with anti-CD3/CD28, there was a significant increase in the production IFNg, IL-4, IL-17, and IL-10 in supernatants compared with their Apoe – /– counterparts. When examining lymphoid organs, Apoe – CD11cDNR mice had fewer naı̈ve T cells (CD44CD62L), and more effector memory T cells (CD44CD62L) within both the CD4+ and CD8+ T-cell compartments. These findings suggest that TGFb signal inhibition in DCs leads to interactions with T cells that are biased towards the maturation of an inflammatory repertoire of T cells. Consistent with this finding is the observation that Apoe – CD11cDNR mice exhibited a doubling of atherosclerotic plaque area in the aortic root, as well as nearly three times as many CD3+ cells in plaque when compared with Apoe – /– controls. Both CD4+ and CD8+ T cells were found in greater number, without a difference in the number of regulatory T cells. When examining the macrophage compartment, disruption of TGFb signalling in CD11c+ cells did not affect macrophage phenotype or the number of MOMA-2+ cells within the plaque lesions, or alter the M1/M2 balance, but immunohistochemistry revealed a more advanced stage of plaque in Apoe – CD11cDNR mice, with decreased collagen content, and decreased a-smooth muscle actin staining. Matrix metalloproteinase-9 (MMP-9) expression was also