Chimeric maternal cells with tissue-specific antigen expression and morphology are common in infant tissues

Abstract

Maternal microchimerism (MMc) has been purported to play a role in the pathogenesis of autoimmunity, but how a small number of foreign cells could contribute to chronic, systemic inflammation has not been explained. Reports of peripheral blood cells differentiating into tissue-specific cell types may shed light on the problem in that chimeric maternal cells could act as target cells within tissues. We investigated MMc in tissues from 7 male infants. Female cells, presumed maternal, were characterized by simultaneous immunohistochemistry and fluorescence in situ hybridization for X- and Y-chromosomes. Maternal cells constituted 0.017% to 1.9% of parenchymal cells and were found in all infants in liver, pancreas, lung, kidney, bladder, skin, and spleen. Maternal cells were differentiated: maternal hepatocytes in liver, renal tubular cells in kidney, and beta-islet cells in pancreas. Maternal cells were not found in areas of tissue injury or inflammatory infiltrate. Maternal hematopoietic cells were found only in hearts from patients with neonatal lupus. Thus, differentiated maternal cells are present in multiple tissue types and occur independently of inflammation or tissue injury. Loss of tolerance to maternal parenchymal cells could lead to organ-specific "auto" inflammatory disease and elimination of maternal cells in areas of inflammation.

Figure 1. Quantification of maternal cells in organs from seven infants

Maternal cells were identified by FISH for X- and Y-chromosomes. Female (presumed maternal) nuclei with two red X-chromosomes were identified among the infant’s male nuclei with one red X-chromosome and one green Y-chromosome. At least 2,000 nuclei with two clearly-defined sex chromosomes were examined per slide. Each bar represents one tissue section on an individual slide. X- axes refer to subject number; Y-axes, MMc as percentage of host cells.

Figure 2. Maternal cells characterized in infant tissues

Immunohistochemistry was performed with antibodies to tissue-specific proteins followed by FISH for X- and Y-chromosomes. Arrows designate female (presumed maternal) nuclei with two red X-chromosomes among the infant’s male nuclei, each with one red X-chromosome and one green Y-chromosome. A, B. One maternal hepatocyte expressed cytokeratin (brown) throughout the cytoplasm surrounding the maternal nucleus incorporated into the hepatic tissue structure (20X, inset 100X). C, D. Maternal epithelial cell in the lung identified by cytokeratin expression (brown, arrows) but not the endothelial cell marker Von Willebrand Factor (blue-grey stain, white arrowhead). (40X) E, F. Maternal myocardial cell identified by expression of sarcomeric α-actin (brown). The maternal cell does not express CD45 (blue-grey), which serves as an additional internal control, confirming that the maternal cell is not a hematopoietic cell overlying a myocardial cell. (40X) G, H. Maternal cell in the myocardium that does not express CD45 or sarcomeric α-actin. (40X)

Figure 3. Maternal cells characterized in infant tissues

Maternal cells were identified as per methods described in Figure 2. A. A maternal (female) cell in a cross-section of a renal tubule was identified by FISH. (40X) B, C. Immunohistochemistry was performed with antibodies to cytokeratin (brown) and CD45 (blue-gray). A representative maternal cell stained with antibodies to cytokeratin (brown) and was incorporated into a renal tubule (40X, inset 100X). C, D. Maternal β-islet cell in the pancreas. Immunohistochemistry was performed with antibodies to insulin (brown), and glucagon (grey). Insulin expression is seen throughout the tissue and in the cytoplasm surrounding the maternal nucleus (arrowhead, 40X).