CD146(+) bone marrow osteoprogenitors increase in the advanced stages of primary myelofibrosis

Abstract

CD146(+) bone marrow stromal cells have been recently recognized as clonogenic osteoprogenitors able to organize a complete hematopoietic microenvironment. In this study we used immunohistochemical analysis to investigate the contribution of CD146(+) bone marrow osteoprogenitors to the stromal remodeling occurring in the different stages of primary myelofibrosis. We found that CD146(+) cells sited at the abluminal side of the bone marrow vessels and branching among hematopoietic cells significantly increased in the advanced stages of primary myelofibrosis (p<0.001), paralleling the extent of fibrosis (rho=0.916, p<0.0001) and the microvascular density (r=0.883, p<0.0001). Coherently with a mural cell function, such cells also displayed smooth-muscle actin expression. Our data providing evidence of CD146(+) cell involvement in bone marrow stromal changes occurring in primary myelofibrosis are consistent with the capability of these cells to participate in fiber deposition, angiogenesis, and bone formation. They could also represent rationale for new therapies targeting the bone marrow stroma in primary myelofibrosis.

Figure 1.

(A) Comparison between microvascular density in controls, early and advanced stage of primary myelofibrosis. Microvascular density shows a remarkable increase in primary myelofibrosis cases. (B) Comparison between CD146 expression in controls, early and advanced primary myelofibrosis cases. Advanced primary myelofibrosis cases display a noticeable increase in CD146 positive cells compared to early stage and controls, while the differences between early stage cases and controls are less remarkable. (C) The graph shows the significant association between the number of CD146 positive cells and the degree of fibrosis, graded according to the EUMNET consensus criteria. Larger dots represent a higher number of cases. (D) The graph shows a significant association between number of CD146 positive cells and the microvascular density in different phases of primary myelofibrosis. Larger dots represent a higher number of cases.

Figure 2.

A–B. Sections from a case of advanced (grade-2) primary myelofibrosis showing that subendothelial cells, branching into the surrounding hematopoietic parenchyma (violet arrows), stain positive either for CD146 (A) or smooth muscle actin (B), while endothelial cells are negative (black arrows) for both antibodies. Inset: the distinction between CD34 positive endothelial cells (blue) and CD146 positive adventitial cells (red) is highlighted by double immunostaining. (Strept-ABC, original magnification 400x). C–H. Differences in microvascular density and in the number of CD146 positive cells betwen an early (C, D, E; grade-1 fibrosis) and an advanced primary myelofibrosis (F, G, H; grade-2 fibrosis). CD146 positive subendothelial cells form a wider and denser meshwork in the advanced stage (G) of primary myelofibrosis than in the early subendothelial stage (D). The slight increase in microvascular density observed in the early stage (C) is not associated with a remarkable increase in CD146 expression (D) (C, F: CD34 immunostaining; D, G: CD146 immunostaining; E, H: Gomori’s staining; original magnification 200×).