Biologic and molecular genetic characteristics of a unique MCF virus that is highly leukemogenic in ecotropic virus-negative mice.

California wild mouse-derived ecotropic virus Cas-Br-M induces a spongiform encephalopathy and a wide variety of hematopoietic neoplasms on inoculation of neonatal mice. We isolated a MCF virus [Ns-6(186) MCF] from a thymic T-cell lymphoma developing in a NFS mouse inoculated with Cas-Br-M virus. Biologically cloned NS-6(186) MCF virus, in contrast to previously studied MCF viruses, was found to induce thymic or nonthymic T-cell lymphomas with high efficiency in the absence of ecotropic helper virus. Comparison of the restriction endonuclease maps derived from Cas-Br-M and NS-6(186) MCF revealed differences only in the env region, between 5.8 and 7.8 kb from the 5' end. Two biologically active molecular clones of the NS-6(186) MCF (clone 15 with two LTRs and clone 19 with 1 LTR) were studied. Although both clones exhibited similar in vitro activities, clone 15-derived virus induced only T-cell lymphomas with short latency whereas clone 19-derived virus induced a wide variety of neoplasms with a significantly longer latency. Nucleotide sequence analysis established that the U3 region of each of the two LTRs of clone 15 has a 53-bp duplication which includes "enhancer elements," but that the single LTR of clone 19 has no such duplication.