The Effect of B 6 Anti - B 6 . S Mouse Serum on Washed

Properties of the late erythrocytic progenitor cells (CFU-E) 1 (1, 2) and of the earlier erythropoietic progenitor cells (BFU-E) (3) have largely been deduced from the behavior of their differentiated progeny in erythrocytic colonies or bursts, rather than from direct studies on the progenitor cells themselves. Because changes in the expression of cell surface molecules are known to occur along the differentiation pathways of several cell types (4, 5), we used an immunological approach to try to identify cell surface molecules characteristic of particular stages of differentiation in the erythrocytic lineage of mice. Such markers, if found, should make it possible to do direct studies on these progenitor cells. A scheme of reciprocal immunization was chosen to exploit the limited genetic difference between the congenic mouse strains C57BL/6Ut (B6) and B6.S/Ut (B6.S). The B6.S mouse strain had been made congenic with the B6 inbred strain (6) for the purpose of studying the genetic control of the host response to the erythroleukemiainducing Friend virus (7). The genetic difference between B6 and B6.S was later mapped (8) to the same segment of chromosome 9 as the Fv-2 gene locus, the major determinant of susceptibility or resistance to this virus (9). The B6.S mouse strain carrying the Fv-28 allele is susceptible, whereas the B6 mouse strain, with the Fv-2" allele, is resistant to Friend virus. Recently (10) it was discovered that the Fv-2 locus functions in uninfected animals as well, where it controls the proportion of erythropoietic progenitor cells BFU-E normally synthesizing DNA. The Fv-2 ~ allele in B6.S is associated with a high proportion of BFU-E in DNA synthesis, and the Fv-2" allele in B6 mice with a low proportion of BFU-E synthesizing DNA. The findings of Suzuki and Axelrad (10) established that Fv-2 or a closely linked gene had a significant influence on cell proliferation at a particular stage in erythrocytic differentiation. Thus it was hoped that our strategy of congenic immunization would generate a limited number of antibodies, among which might be found antibody directed against the Fv-2 gene * Supported by grants from the National Cancer Institute of Canada and the Medical Research Council of Canada. 1 Abbreviations used in this paper: BFU-E, early erythropoietic progenitor cell; C, complement; CFU-E, late erythrocytic progenitor cell; PBS, phosphate-buffered saline.