Antibody response to Moloney type C virus-induced tumours.

Abstract

Animals infected with RNA tumour viruses show an immune response to new virus-induced antigens present on the tumour cell surface. In tumours induced by murine leukaemia virus (MuLV), serologically detected cell membrane antigens have been placed into two major groups; the Gross cell-surface antigen(s), GCSA and GIX, are associated with Gross leukaemia virus infection1 and the FMR antigen (s), associated with infection by Friend, Moloney or Rauscher type C viruses2. There is good evidence that some of these antigens are related to the structural proteins of the virus. The GIX antigen is found on the viral envelope protein, gp70 (refs 3–5) and the GCSA antigen is on the p30 protein which is present in the membrane as part of the glycosylated ‘gag’ polyprotein(s)6–8. It is not certain whether the FMR antigen(s) is found on membrane-located viral structural precursors or polyproteins, or whether it is present on a tumour-associated protein. Several antigens of this latter type have been described. For example, the Moloney leukaemia virus-induced cell-surface antigen (MCSA) is thought to be a non-viral protein associated with ‘gag’ products, p30, p15 and p12 on the membranes of Moloney lymphoma cells9,10. MCSA and the antigens on cells infected by Abelson MuLV11 may be very specific cellular antigens associated with Moloney MuLV infection. There has also been much interest in whether the src gene product is present in the membrane of sarcoma virus-induced tumours, although it is thought not to be present on the external membrane surface12. A more controversial point is whether there are other virus-induced cell-surface antigens (CSA) which may serve as targets for an effective immune response to viral tumours13. We have previously found14 that serum from BALB/c mice with regressed Moloney virus-induced fibrosarcomas contained specificities only for Moloney viral proteins and not for any tumour-associated protein like MCSA. We have now identified the viral proteins with which the Moloney MuSV antisera react. Using a syngeneic system of immunisation with freshly induced Moloney sarcoma tumours, we show that the antibody response is directed only at viral structural antigens and that the major part of the response is to antigens on p30 protein probably present as part of a precursor polyprotein on the tumour cell surface.