We propose to use a feline model of retrovirus-induced aplastic anemia to determine the specific retroviral genetic sequences responsible for and the mechanisms of hemopoietic stem cell damage by retroviruses. We will use a molecularly cloned prototype anemogenic isolate of feline leukemia virus (FeLV-C-Sarma) which in vivo induces fatal aplastic anemia within 4 weeks after inoculation of specific-pathogen-free cats and in vitro produces rapid depletion of feline bone marrow erythroid (burstforming-BFU- e) progenitor cells. To identify retroviral sequences and proteins responsible for aplasia induction we will use the cloned sequenced genomes of FeLV- C and a prototype non-anemogenic minimally pathogenic FeLV-subgroup A isolate (61E) to continue our approach of preparing reciprocal intragenic viral chimeras focusing on coding regions of FeLV-C extracellular glycoprotein (gp70) gene. The viral constructs will be tested for erythroid progenitor cytopathic activity first in vitro with a BFU-e depletion assay and then in vivo by inoculation of kittens to deduce the specific genetic sequences which cause aplastic anemia. Directed by our positive data obtained thus far with these strategies, we will concentrate initially on a 886-base pair region including the N-terminus of the extracellular glycoprotein gene of FeLV-C. To investigate the role of the FeLV-C env glcoprotein in the specificity and mechanism of erythroid progenitor cell ablation, we will purify gp70 from FeLV-C, and from a chimera containing the minimal FeLV-C pathogenic determinants inserted in an FeLV-61E backbone. We will then examine the selective binding of gp70 to bone marrow cells and fibroblasts and to identify and clone the receptors for FeLV-C study and their role of this initial virus/cell interaction in the genesis of erythroid aplasia.