Atherosclerosis, the underlying cause of the majority of cardiovascular diseases (CVD), is a lipid driven, inflammatory disease of the large arteries. Although the development of lipid lowering drugs, in particular HMG-CoA reductase inhibitors (statins), has helped to lower the incidence of cardiovascular disease (CVD), a substantial part of the population still suffers from CVD notwithstanding optimal lipid-modulating therapy. Hence, additional strategies to decrease the persistent residual risk of CVD in the population are needed.
In conjunction with hyperlipidemia, the interaction between the different immune cells, and the (subsequent) secretion of immune-regulatory and activating cytokines and chemokines determines the progression of atherosclerosis. Therefore, defining mediators of the inflammatory aspect of atherosclerosis, and the identification of novel drug targets that block atherosclerosis associated inflammation is a widely recognized scientific goal with great therapeutic implications.
During atherogenesis, complex interactions between immune cells of the innate and adaptive immune system occur. Co-stimulatory molecules play a central role in fine-tuning immunological reactions.
In the late 90s,we showed that CD40L and CD40 are expressed on plaque cell types and play a pivotal role in atherogenesis. Since then, my laboratory has been focusing on how immune-modulation can reduce atherosclerosis, and especially how inhibition of CD40(L)-signaling can be developed into a therapy for atherosclerosis. CD40 does not have intrinsic signal capabilities but needs adaptor molecules, the TNF-receptor associated factors (TRAFs), to exert signaling. Using mice specifically lacking CD40-TRAF6 or CD40-TRAF2/3/5 interactions, I showed that mice deficient in CD40-TRAF6, and not CD40-TRAF2/3/5 were protected against atherosclerosis.
Although CD40(TRAF6) inhibition is powerful in reducing atherosclerosis, we do not yet understand how. To understand the mechanism how inhibition of CD40(-TRAF6) interactions halts atherosclerosis we investigated the effect of CD40 inhibition and CD40-TRAF6 inhibition in different cell types. Because CD40-CD40L interactions are predominantly known to play a role in the adaptive immune system, and play a pivotal role in antigen-presentation, T cell proliferation and differentiation, and Ig isotype switching in B-cells, we were surprised to find that CD40 activation in the macrophage, a cell of the innate immune system, seems to drive atherosclerosis.
The main objective of this grant was to define the role of macrophage CD40 in atherosclerosis and develop and establish small molecule inhibitors directed against components of the CD40-signaling pathway in macrophages.
The project contained 3 aims:
1.Biology:Identification of CD40 signaling and regulation of its effectors in macrophages
2.Disease: Define the role of macrophage CD40 signaling in atherosclerosis and other inflammatory diseases
3.Targeting - Development of novel SMI directed against CD40-signaling intermediates in the macrophage.
The main findings of this project are 1) that CD40-TRAF6 interactions indeed drive monocyte recruitment and macrophage migration. Although we found that CD40 can also bind to TNIK, TNIK was not essential for propagating CD40-TRAF6 signals, which we found to be driven predominantly by canonical NFkB pathways; 2) that macrophage CD40-signaling is involved in both acute, and chronic inflammatory diseases. We found that besides the classic T-cell-DC CD40L-CD40 interactions, macrophage CD40 (and particularly macrophage associated CD40-TRAF6 interactions was crucial in driving atherosclerosis, the metabolic syndrome, multiple sclerosis, cardiac hypertrophy and sepsis; 3) that blocking macrophage CD40-TRAF6 signaling with our in house developed small molecule inhibitor is effective and safe in ameliorating chronic inflammatory diseases (especially atherosclerosis), without inducing immune-suppressive side effects.
