Inhibition of antigen-induced T-cell proliferation by antibodies to endogenous AKR-MuLV

In vitro growth inhibition of murine leukemia cells by antibody specific for the major envelope glycoprotein (gp71) of Friend leukemia virus

An in vitro complement (C')-independent growth cytostasis model is described in which the replication of Friend leukemia virus (FLV)-induced erythroleukemia cells (of the FLC-745 cell line) is inhibited by goat serum directed against the major FLV envelope glycoprotein, gp71. The cytostatic effect is reversible, with the degree of this reversibility dependent on both the concentration and duration of exposure to immune serum. The inhibitory factor in heat-activated goat anti-FLV gp71 (delta G alpha FLV gp71) serum has been identified as virus-specific IgG antibody, and F(ab')2 fragments of this antibody are highly effective in suppressing FLC-745 cell growth. Studies with various murine leukemia and lymphoma cell lines, as well as with a panel of antisera directed against various murine oncornaviruses or viral proteins, have demonstrated that antibodies reactive with the group or type determinants of FLV gp71 are capable of mediating cytostasis. Under conditions of antibody-mediated growth inhibition of FLC-745 cells, specific modulation of gp71 expression is followed by nonspecific modulation of H-2d antigen expression. In addition, considerable cell death occurs in cytostatic cultures which is accompanied by continued division (as measured by DNA synthesis) of a portion of the cell population. Cytofluorimetric analysis of nuclei from growth-inhibited FLC-745 cells demonstrates a diminution in the frequency of cells in the G2/M phase of the cell cycle. It is suggested that antibody-mediate FLC-745 cell growth inhibition operates via a blockade of the cell cycle which prevents most cells in the population from traversing G2/M. While these blocked cells appear to be subject to slow cytolysis by a C'-independent mechanism, a portion of the cells escape this blockade and continue to replicate, thus offsetting the death of the former cells to yield a relatively constant density of viable cells for at least 72-96 h of growth inhibition. The possible relevance of this in vitro phenomenon to in vivo passive therapy against FLV-induced disease with G alpha FLV gp71 and similar antisera is briefly considered.

Mitogenic effect of anti-Friend leukemia virus antiserum on T cells

It has been previously reported that goat- and rat antisera directed against Friend leukemia virus (aFLV) are mitogenic for some B cells, but not for T cells. Here we report that activation of T cells by concanavalin A (Con A) rendered T cells responsive to the mitogenic activity of aFLV. This activity was contained in the immunoglobulin fraction and could be absorbed by purified FLV preparations. Optimal conditions for measuring the mitogenc activity of aFLV include 48 h preincubation of thymocytes with 3 micrograms/ml Con A followed by reculturing the activated thymocytes for 28 h with aFLV. The acquisition of an aFLV-responsive state was dependent on early protein synthesis during the Con A-induced activation period. aFLV did not substitute for interleukin 2 (IL2) in a costimulator assay. Evidence is presented that aFLV acted in a cocultivation assay via growth factor(s). In contrast to control cultures, aFLV-treated lymphoblasts contained, in their supernatants, IL2 activity as demonstrated by their effect on an IL2-dependent T cell line. The data suggest that aFLV acted upon activated T cells by enhancing the endogenous production and/or release of IL2.

Cancer: re-evolution of eucaryotic to procaryotic replication units--a result of genetic transposition?

We have consistently tried to present our "procaryote-hypothesis" of oncogenesis in the light of evolution. It is suggested that the replication units are the essential starting points of oncogenesis, and cancer is the result of a short and compact re-evolution from eucaryotic to procaryotic replicons. In our view, the process of re-evolution or reactivation of procaryotic replication units and/or their precursors starts with the activity of a cancerogen and proceeds by genetic rearrangements, possibly by genetic transposition. Consequently, some remarks had to be made on: (a) the evolution of self-replicative species; (b) the evolvement of procaryotes to eucaryotes; and (c) the various mobile repeated genetic elements. Moreover, we have taken into consideration the important fact that most of the mammalian genome is reversibly repressed and "that there is in these modern genes still a memory of ancient sequences" ( Eigen et al., 1981). We can only speculate on the extent of the evolutionary conservation, particularly of certain ancient nucleotide sequences related to autonomous, indefinite self-replication. Nevertheless, no real argument can be made against the simple possibility that a re-evolution of eucaryotic to procaryotic replicons lead to cancer.

Ly- and ecotropic MuLV antigens are separate entities

Murine alloantisera to lymphocyte (Ly) alloantigens also contain antibodies to murine leukaemia viruses (MuLV) in high titre. A number of such sera were extensively absorbed with ecotropic AKR murine leukaemia virus to remove contaminating anti-MuLV antibodies. The virus-absorbed and unabsorbed antisera were then examined by the radioimmune precipitation (RIP) assay for anti-MuLV antibodies, and for anti-Ly antibodies by cytotoxicity assays against specific target cells. The results presented here demonstrate that whilst virus absorption markedly reduced the anti-viral titres of the alloantisera they had no effect on the anti-Ly titres of any of them. This indicates that none of the Ly antigens which was examined cross reacts with the antigens of ecotropic AKR-MuLV and further suggests that Ly antigens are not encoded by the genes of endogenous ecotropic MuLVs. It was also demonstrated that the majority of the contaminating anti-MuLV antibodies present in anti-Ly-6.2 antiserum recognize viral envelope antigens rather than the internal structural proteins of the virus.