We read with great interest the article by Wang et al.1 demonstrating evidence of blood chimerism of donor origin after liver transplantation potentially as a result of donor liver-derived hematopoietic stem cells (HSCs). The adult liver harbors progenitor cells that enable hematopoiesis.2, 3 Our group identified liver-derived CD34+ cells with hematopoietic potential, in agreement with earlier published work,2 which we presented at the American Association for the Study of Liver Diseases (AASLD) annual meeting in 2008.
The field of human HSC biology has progressed considerably since the discovery that the majority of HSCs are found within the CD34+ compartment.4 To date, cells with the marker profile Lin−CD34+CD38−CD90+CD45RA− satisfy the most stringent criteria for HSC appellation.2 Wang et al.1 report surprisingly high levels of HSCs within donor livers based on the antigenic profile Lin−CD34+CD38−CD90+. However, besides omitting CD45RA, the authors fail to include a stringent gate in the side scatter (SSC) versus forward scatter (FSC) dotplot or a viability dye, which can safeguard against including debris and dead cells that autofluoresce and nonspecifically bind antibodies, a common event following tissue digestion. We have previously detected CD34+ cells in the preservation fluid (perfusates) of human liver grafts.5 Upon further investigation of the perfusates, we can detect rare Lin−CD34+CD38−CD90+CD45RA− based on a stringent gating and antigenic criterion4 (mean 2.2 × 10−5% ± 0.8 × 10−5 standard error of the mean [SEM], n = 8), as shown in Fig. 1. We also were surprised at the poor level of human hematopoietic chimerism observed after engraftment in immunocompromised mice, which may be related to the authors' use of both impure Lin−CD45+ or CD45+ liver cells and the genetic strain of the recipient mouse.
In conclusion, we agree that the human (donor) liver contains a subset of rare HSCs. However, we disagree that the level of HSCs are comparable to that found in human cord blood,4 which to date, is the richest source.