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 TcRV beta 6 and V beta 8.1 cosegregates a new mammary tumor provirus locust 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 propertie s of miner lymphocyte stimulating antigen 1(a). Consistent with previo us reports, clonal deletion of mature thymocytes bearing TcRV beta 6 w as 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 p ostnatal stage. Delay in expression of Mtv-50 ORF during the postnatal period may be responsible for the failure of clonal deletion of V bet a b-T cells in the early postnatal life of mice carrying Mtv-50.