25 Expression of Novel Surface Antigens on Early Hematopoietic Cells a HANS-JÖRG BÜHRING, b,d MARTINA SEIFFERT, b THOMAS A. BOCK, b STEFAN SCHEDING, b ANDREAS THIEL, c ALEXANDER SCHEFFOLD, c LOTHAR KANZ, b AND WOLFRAM BRUGGER b b Department of Hematology and Oncology, University of Tübingen, Tübingen, Germany c Department of Immunology, German Rheuma Research Center, Berlin, Germany ABSTRACT: The purpose of this report is to demonstrate the expression of very recently identified surface antigens on CD34 + and AC133 + bone marrow (BM) cells. Coexpression analysis of AC133 and defined antigens on CD34 + BM cells revealed that the majority of the CD164 + , CD135 + , CD117 + , CD38 low , CD33 + , and CD71 low cells resides in the AC133 + population. In contrast, most of the CD10 + and CD19 + B cell progenitors and a fraction of the CD71 high pop- ulation are AC133 - , indicating that CD34 + AC133 + cells are enriched in prim- itive and myeloid progenitor cells, whereas CD34 + AC133 - cells mainly consist of B cell and late erythroid progenitors. This corresponds to the highly reduced percentage of CD10 + B cells and the absence of CD71 high erythroid progenitors on AC133 + selected BM cells. A portion of 0.2–0.7% of the AC133 + selected cells do not coexpress CD34. These cells are very small and define a uniform CD71 - , CD117 - , CD10 - , CD38 low , CD135 + , HLA-DR high , CD45 + population with unknown delineation. Four color analysis on CD34 + CD38 - BM cells re- vealed that virtually all of these primitive cells express AC133. Using an im- proved liposome-enhanced labeling technique for the staining of weakly expressed antigens, subsets of this population could be identified which express the angiopoietin receptors TIE (67.6%) and TEK (36.8%), the vascular endo- thelial growth factor receptors FLT1 (7%), FLT4 (3.2%), and KDR (10.4%), or the receptor tyrosine kinases HER-2 (15.4%) and FLT3 (CD135; 77.6%). Our results suggest that the CD34 + CD38 - population is heterogeneous with re- spect to the expression of the analyzed receptor tyrosine kinases. INTRODUCTION In an attempt to define the surface marker profile of hematopoietic stem cells, monoclonal antibodies against cell surface antigens were developed which either se- lectively recognize immature hematopoietic cells or further dissect populations within the stem cell compartment. Based on the establishment of such antibodies most of the hematopoietic stem cells can now be defined by the a These studies were supported by the Deutsche Forschungsgemeinschaft (SFB 510, project A1), by a grant from the Research Program of the University Clinic of Tübingen (fortüne; project 433), and by a grant from the Federal Ministry of Education and Research and the Interdiscipli- nary Clinical Research Center (project IIA1). d Correspondence to: Hans-Jörg Bühring, Ph.D., Department of Hematology and Oncology, University of Tübingen, FACS-Laboratory, Otfried-Müller-Str.10, 72076 Tübingen, Germany. Phone, +49-7071-298 2730; fax, +49-7071-29 2730; e-mail, hjbuehri@med.uni-tuebingen.de