Progressing Moloney sarcoma virus (M-MSV) induced tumors: effect of host's immunomanipulation.

We have observed that mice that were selectively 'deprived' of T lymphocytes by adult thymectomy, lethal-irradiation and bone-marrow reconstitution, are highly susceptible to M-MSV oncogenesis throughout all their life time, and develop progressively growing tumors in all instances. However, transfer of even low doses (0.25 x 106) of non-immune syngeneic lymphoid cells brings about tumor prevention or regression in 50% of M-MSV-injected deprived mice. The protective effect is long-lasting since comparable results are obtained when deprived mice are injected with graded numbers of cells 60 days or 1 day before M-MSV inoculation. M-MSV tumor prevention is exclusively due to the T-cell population since T-cell-depleted spleen cells from tumor regressor donor mice have no protective effect when transferred into M-MSV-injected deprived mice. We have also observed that M-MSV immune T cells can cause regression even in deprived mice bearing large tumors at the moment of cell transfer.