Bone Marrow Stroma-Dependent Modulation of CD45R Isoform Expression on Abelson Virus Transformed Pre-B Cells

Abstract

The CD45 glycoprotein family exhibits cell-lineage-associated structural heterogeneity which is due, in part, to alternative pre-mRNA splicing. The Abelson murine leukemia (A-MuLV) preferentially transforms immature B cells that express a B-cell-specific high molecular weight CD45 isoform, called B220. However, we observed that A-MuLV-transformed cell lines are often B220^- while maintaining high levels of “pan” CD45 expression. In vitro transformation of murine bone marrow revealed that the stromal microenvironment over which A-MuLV-transformed lymphoblasts are grown affected the B220 phenotype of the pre-B cells. Over a period of a few weeks, B220⁺ populations grown over a clonal stromal cell line gradually became B220^-. However, the transition from a B220⁺ to B220^- phenotype was dependent on the lot of fetal calf serum used. In contrast, cells grown over a heterogeneous bone marrow stroma maintained B220⁺ expression for long periods of time. The appearance of B220^- cells in clonal B220⁺ populations indicated that the change in phenotype resulted in part from modulation of B220 expression. B220^- B-cell lines did not express the high molecular weight CD45 RNA species indicating that the B220^- phenotype was due to alternative pre-mRNA splicing. Finally, the shift from B220⁺ to B220^- was not accompanied by changes in the stage of development of the cultures. These observations demonstrate that expression of B220 is not required for the continued proliferation of Abelson-transformed pre-B cells and is regulated by unknown environmental factors.