Ligation of Macrophage Fcγ Receptors Recapitulates the Gene Expression Pattern of Vulnerable Human Carotid Plaques

Abstract

Stroke is a leading cause of death in the United States. As ∼60% of strokes result from carotid plaque rupture, elucidating the mechanisms that underlie vulnerability is critical for therapeutic intervention. We tested the hypothesis that stable and vulnerable human plaques differentially express genes associated with matrix degradation. Examination established that femoral, and the distal region of carotid, plaques were histologically stable while the proximal carotid plaque regions were vulnerable. Quantitative RT-PCR was used to compare expression of 22 genes among these tissues. Distal carotid and femoral gene expression was not significantly different, permitting the distal carotid segments to be used as a paired control for their corresponding proximal regions. Analysis of the paired plaques revealed differences in 16 genes that impact plaque stability: matrix metalloproteinases (MMP, higher in vulnerable), MMP modulators (inhibitors: lower, activators: higher in vulnerable), activating Fc receptors (FcγR, higher in vulnerable) and FcγR signaling molecules (higher in vulnerable). Surprisingly, the relative expression of smooth muscle cell and macrophage markers in the three plaque types was not significantly different, suggesting that macrophage distribution and/or activation state correlates with (in)stability. Immunohistochemistry revealed that macrophages and smooth muscle cells localize to distinct and non-overlapping regions in all plaques. MMP protein localized to macrophage-rich regions. In vitro, treatment of macrophages with immune complexes, but not oxidized low density lipoprotein, C-reactive protein, or TNF-α, induced a gene expression profile similar to that of the vulnerable plaques. That ligation of FcγR recapitulates the pattern of gene expression in vulnerable plaques suggests that the FcγR → macrophage activation pathway may play a greater role in human plaque vulnerability than previously appreciated.

Anti‐Inflammatory Immune Skewing Is Atheroprotective: Apoe −/− FcγRIIb −/− Mice Develop Fibrous Carotid Plaques

“…Based on our studies 9 and those of others, 13–14 we predicted that loss of FcγRIIb would exacerbate atherosclerosis. Surprisingly, the results disproved the hypothesis, with initial evidence coming from longitudinal studies (Figure 1).…”

“…Given that its expression was normalized to β‐actin, the results suggest that the percentage of macrophages is similar regardless of plaque size and is consistent with what has been reported for human carotid plaques. 9 Thus, differences in the percentage of MØ cannot explain the fibrotic nature of the DKO plaques.…”

A small molecule PAI-1 functional inhibitor attenuates neointimal hyperplasia and vascular smooth muscle cell survival by promoting PAI-1 cleavage

“…Indeed, ligation-induced neointima formation was significantly attenuated by treatment with both tiplaxtinin and CL-PAI-1. Tiplaxtinin could be a useful therapeutic option in the context of elevated uPA and PAI-1 as it promotes a substrate-like conversion of PAI-1 [36–42]. …”

Readapting the adaptive immune response – therapeutic strategies for atherosclerosis

“…There is a family of Fcγ receptors (I–IV) for IgG, FcεR (CD23) for IgE, FcμR (Toso) for IgM and the neonatal Fc receptor (FcRn), which is important in IgG recycling (Smith and Clatworthy, ). Macrophages within atherosclerotic plaques are influenced by IgG immune complexes via Fcγ receptors and vulnerable plaques exhibit enhanced levels of FcγR and downstream signalling molecules (Lennartz et al, ). FcγRIII (CD16) is an important marker of non‐classical patrolling monocytes, compared with classical inflammatory monocytes that express high levels of the toll‐like receptor 4 co‐receptor CD14.…”