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Selective Uptake of Indocyanine Green by Reticulocytes in Circulation

¹From the Wellman Laboratories of Photomedicine, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts.

PURPOSE. Hyperfluorescent cells labeled with indocyanine green (ICG) have been observed in retinal and choroidal circulation using scanning laser ophthalmoscopy. It has been suggested that ICG labels leukocytes and that ICG can be used to track leukocyte movement in vivo. The purpose of this study is to identify the cell population that takes up ICG and to study their trafficking pattern in vivo by confocal fluorescence microscopy.

METHODS. ICG was injected into the mouse tail vein, and images were taken by in vivo confocal microscopy. The trafficking pattern of ICG-labeled cells was compared with that of rhodamine 6G-labeled leukocytes. In vitro labeling of human blood cells with antibodies against cell lineage markers and with DNA stains was further used to identify the ICG-labeled cells. Antibodies against the following cell surface markers were used: CD45 (leukocytes), CD3 (T lymphocytes), CD19 (B lymphocytes), CD16 (Fc receptor), glycophorin A (erythroid lineage cells), and CD71 (transferrin receptor).

RESULTS. The ICG-labeled cells were made up of two blood cell populations with distinct levels of ICG uptake. The strongly ICG-labeled cells did not roll on dermal vascular endothelium in vivo, in contrast to rhodamine 6G–labeled leukocytes. They were identified as reticulocytes because antibody staining showed that they were CD 45^-, glycophorin A⁺ and CD 71⁺. The weakly ICG-labeled cells were identified as neutrophils because they were CD45⁺, CD16⁺, CD3^-, and CD19^-.

CONCLUSIONS. ICG strongly labels reticulocytes and weakly labels neutrophils. To the authors’ knowledge, this is the first report of selective staining of reticulocytes by ICG.