Morphologic changes in the murine leukemia L5178Y cells treated with antibodies in the absence of complement activity

An investigation into the mechanism ofL-asparaginase resistance in L5178Y murine leukemia cells

Resistance of leukemia cells toL-asparaginase is presumed to be due to increased expression of asparagine synthetase activity by resistant cells, so they are no longer dependent on an exogenous source ofL-asparagine for growth. The mechanism by which cells acquire the ability for increased enzyme expression, however, has not been clearly defined. Evidence presented here indicates that genomic alterations in the form of translocations, gene amplification, or increased P-glycoprotein expression, do not account for the phenotypic transformation fromL-asparaginase sensitivity toL-asparaginase resistance. Instead, both sensitive and resistant L5178Y cells contain immunoreactive material detected by Western blotting with an antiserum prepared against bovine pancreatic asparagine synthetase. This suggests that the mechanism of resistance might involve modification of asparagine synthetase inL-asparaginase-resistant cells by an as-yet-unidentified mechanism or by inhibition of enzyme activity in theL-asparaginase-sensitive cells.

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.