A permanent L-cell variant cell line (LC1) was isolated by the growth of the parent L-cell line (L) in the presence of a cytostimulatory dose (1:200) of rabbit anti-L-cell antiserum (AL) for 9 mo. LC1 differed from L in many aspects: (a) it was larger (1,533 mm3 vs. 1,284 mm3), (b) it grew faster (1.5- to 2-fold), (c) it grew in aggregated fashion, (d) its growth was no longer stimulated by AL, (e) it was almost completely resistant to high concentrations of AL in the presence of complement (C), (f) its original membrane antigens (immunogenic for AL) were redistributed in sparse and patchy clumps as noted by fluorescence microscopy, (g) it contained about 65% of the total original 125I-AL membrane-binding sites (1.4 X 10(7)/cell vs. 2.2 X 10(7)/cell), (h) its AL-binding sites displayed a lower average affinity constant (K = 0.9 X 10(5) M-1 vs. 2.8 X 10(5) M-1), (i) it contained a smaller proportion of high affinity (K greater than 10(6) M-1) binding sites (13% vs 21%), and (j) LC1 was fully immunogenic in that it was readily killed by homologous antiserum (ALC1) and C, whereas L was not similarly affected by ALC1 indicating that LC1 contained new membrane antigens not present on L. Another variant (LC2) was produced by growth of LC1 in a 10-fold higher dose (1:20) of AL (cytotoxic for L) for 1 mo. LC2 was even more resistant to AL in the presence of C, contained 0.84 X 10(7) AL-binding sites/cell with an average affinity constant of 1 X 10(5) M-1 (unchanged from LC1), and was less susceptible than LC1 to lysis in the presence of ALC1 and C. These findings confirm and extend our previous in vitro and in vivo observations dealing with the direct stimulation effects of antibody on tumor cell metabolism and suggest that immunostimulation may be a mechanism of tumor escape from immune control in vivo possibly by immunoselection and antigenic modulation as proposed by other investigators.

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