Pathogenicity of avian myelocytomatosis virus MC29 in two lines of chickens

The pathogenicity of avian myelocytomatosis virus MC29 was studied in F 1 hybrids of two Prague inbred lines of chickens-CC ×IA (B 4 /B 7 genotype) and CB × IA (B 12 /B 7 genotype). A shorter latent period and a higher mortality was found in the CC x IA chickens. In most chickens of this group, subperiosteal accumulations of primitive cells on the ribs, sternum and pelvic bones were observed, which were never detected in the CB x IA chickens. Unlike the typical myelocytomatosis, however, most of these cells were without specific cytoplasmic granules which indicated that they were less differentiated than the myelocytes. Another unexpected finding was the severe leukemia which developed in most CC x IA chickens, the primitive cells in the peripheral circulation having the typical morphology of myeloblasts. Tumour growths in the proventriculus were observed for the first time in both experimental groups of chickens. Lymphoid leukosis of a short latency was found in two CB x IA chickens. Of particular interest is the demonstrated severe selective suppression of myelopoiesis in the bone marrow of two CC x IA chickens as this is quite opposite to the usual activity of virus MC29. The results presented indicate that the pathogenic and oncogenic effects of virus MC29 depend on host genetic factors and that after infection of some inbred line of chickens, the myeloid cell differentiation might be arrested at various levels, including at the myeloblast stage.