Engraftment of marrow-derived epithelial cells: the role of fusion

Abstract

Contribution of transplanted bone marrow has, in many models, led to the appearance of marrow-derived epithelial cells in a variety of organs, including the lung. Following the initial descriptions of these cells, many questions remain about the mechanisms by which bone marrow adopts an epithelial phenotype in the murine lung. Data from other epithelial lineages, such as those of the kidney and colon, suggest that one mechanism is fusion of transplanted marrow with host pneumocytes. This process appears to require severe damage and may not be the only mechanism by which mature lung epithelia can derive from marrow. This article discusses the processes leading to the appearance of marrow-derived pneumocytes and highlights the therapeutic potential of bone marrow to fuse with or differentiate into epithelial cells of the lung.

Figure 1.

(A) Alveolar tissue from a female recipient of male marrow stained for thyroid transcription factor 1 (TTF-1) (green) and Y chromosome (red), and counterstained with 4′,6-Diamidino-2-phenylindole (DAPI) (blue). The apparent colocalization of TTF-1 and Y gives the appearance of two marrow-derived type II pneumocytes. However, as B demonstrates, addition of CD45 staining shows that these cells are covered by two blood cells and cannot be considered to be donor-derived epithelial cells. This artifact is termed “overlay.” (C) Two hepatocytes isolated from the livers of female FAH-null recipients of wild-type male marrow that have undergone fluorescence in situ hybridization (FISH) for both the X and Y chromosomes. In this image, the characteristic autofluorescence of the liver cell is shown in yellow, the Y chromosome in red, the X chromosome in green, and the nucleus is counterstained with DAPI (blue). The cell on the left contains three X chromosomes and one Y chromosome, indicating that it arose through a fusion event. The cell on the right contains one X and one Y, indicating either differentiation or fusion followed by reductive division and loss of one set of sex chromosomes.

Figure 2.

Simplified schematic of fusion events leading to GFP expression. When Z/EG marrow is transplanted into an ubiquitously expressing Cre mouse, differentiation and fusion can be determined based on reporter gene expression. In the case of a differentiation event, the Z/EG construct remains intact and lacZ is expressed (top right, blue). In the case of a fusion event, the Cre recombinase is introduced into a Z/EG-containing cell and loxP sites are excised, leading to activation of the GFP reporter transgene (bottom right, green).