Delay in expression of a mammary tumor provirus is responsible for defective clonal deletion during postnatal period

A gene-encoding ligand for deletion of T cells bearing TcRVβ6 and Vβ8.1 cosegregates a new mammary tumor provirus locus, Mtv-50 in NC mice. The sequence of the open reading frame (ORF) in the 3′ long terminal repeat (LTR) of Mtv-50 was strikingly similar to those of Mtv-7, Mtv-43 and exogenous mouse mammary tumor virus (SW) with properties of minor lymphocyte stimulating antigen 1^a. Consistent with previous reports, clonal deletion of mature thymocytes bearing TcRVβ6 was defective during the early postnatal period of mice carrying Mtv-50. Appreciable levels of mRNA corresponding to common Mtv ORF and Mtv-6 ORF were expressed in the neonatal thymus, while little, if any, mRNA corresponding to Mtv-50 ORF was detected in the thymus at the early postnatal stage. Delay in expression of Mtv-50 ORF during the postnatal period may be responsible for the failure of clonal deletion of Vβ6-T cells in the early postnatal life of mice carrying Mtv-50.