Macrophage imbalance (M1 vs. M2) and upregulation of mast cells in wall of ruptured human cerebral aneurysms: preliminary results

Abstract

Background: M1 and M2 cells are two major subsets of human macrophages that exert opposite effects on the inflammatory response. This study aims to investigate the role of macrophage M1/M2 imbalance and mast cells in the progression of human cerebral aneurysms to rupture.

Methods: Ten patients with cerebral aneurysms (five ruptured and five unruptured) underwent microsurgical clipping. During the procedure, a segment of the aneurysm dome was resected and immunostained with monoclonal antibodies for M1 cells (anti-HLA DR), M2 cells (anti-CD 163), and mast cells (anti-tryptase clone AA). A segment of the superficial temporal artery (STA) was also removed and immunostained with monoclonal antibodies for M1, M2, and mast cells.

Results: All ten aneurysm tissues stained positive for M1, M2, and mast cells. M1 and M2 cells were present in equal proportions in unruptured aneurysms. This contrasted with a marked predominance of M1 over M2 cells in ruptured aneurysms (p = 0.045). Mast cells were also prominently upregulated in ruptured aneurysms (p = 0.001). Few M1 and M2 cells were present in STA samples.

Conclusions: M1/M2 macrophages and mast cells are found in human cerebral aneurysms; however, M1 and mast cell expression seems to markedly increase in ruptured aneurysms. These findings suggest that macrophage M1/M2 imbalance and upregulation of mast cells may have a role in the progression of cerebral aneurysms to rupture.

Figure 1

Immunostaining of monoclonal antibodies with anti-HLA DR for M1 and anti-CD 163 for M2 cells. In ruptured aneurysms, there is a clear predominance of M1 over M2 cells (1A-1B). In unruptured aneurysms, M1 cell count is almost the same as M2 (2A-2B). The expression of M1 and M2 cells is almost similar in STA specimen, but significantly lower than in ruptured and unruptured aneurysms (3A-3B).

Figure 2

Immunostaining of monoclonal antibodies for anti-mast cells tryptase (clone AA1). There are more mast cells visible in ruptured aneurysms than in unruptured aneurysms.

Figure 3

Semiquantitative grading for M1, M2, and mast cells in ruptured and unruptured aneurysms. The number of M1 and M2 cells in unruptured aneurysms is almost equal, with significantly more M1 cells noted in ruptured aneurysms. Clearly more mast cells are present in ruptured aneurysms compared to unruptured aneurysms.

Figure 4

M1 vs. M2 balance. Schematic diagram illustrating our hypothesis that a balance between M1 and M2 cells leads to stable aneurysm and that imbalance between M1 and M2 cells with increased population of M1 cells and upregulation of mast cells predisposes the aneurysm to rupture.

Figure 5

Polarized monocytes and cytokine production. Schematic diagram demonstrating two populations of monocytes (CD14^high CD16^low, CD14low CD16^high) and their polarization to two subtypes of macrophages (M1 and M2) with their cell-specific cytokine production.

Figure 6

Mast cell – macrophage interaction. Schematic diagram demonstrating the interaction of mast cells with macrophages (M1 cells) and their cytokine production implicated in aneurysm formation and rupture.