Azathioprine-induced lymphocytic neoplasms of NZB mice lack ecotropic murine leukaemia virus.

NZB mice injected intramuscularly throughout a 6-month period with the immunosuppressant azathioprine (Imuran) developed lymphocytic lymphomas 6--7 months after treatment was initiated. These malignancies were quite distinct from the reticulum-cell neoplasia which occurs spontaneously in the strain, and were readily transplantable to NZB or histocompatible BALB/c recipients. Xenotropic, but not ecotropic murine leukaemia virus (MuLV) was detected in leukaemic tissues of some donor and recipient NZBs when tested in vitro by co-cultivation with permissive cell lines, genome rescue, XC and viral polymerase assays. Virus filtrates prepared from donor leukaemic tissues were non-pathogenic when injected into newborn C3H mice. These results are evidence against a mandatory ecotropic MuLV genome in lymphocytic neoplasia.