Different lethal effects of mitomycin C and actinomycin D during the division cycle of HeLa cells

The lethal actions of mitomycin C and actinomycin D were followed during the division cycle of HeLa cells. The cells were most susceptible to a 2 hr pulse of mitomycin C during the G(1) phase, whereas their sensitivity to actinomycin D was most pronounced in the S phase. Posttreatment of the cells with acetoxycycloheximide, a potent inhibitor of protein synthesis, increased the survival (colony-forming ability) of cells treated with mitomycin C but had very little effect on the survival of cells treated with actinomycin D. The significance of these findings is discussed.

Antigenic modulation. Loss of TL antigen from cells exposed to TL antibody. Study of the phenomenon in vitro

Antigenic modulation (the loss of TL antigens from TL+ cells exposed to TL antibody in the absence of lytic complement) has been demonstrated in vitro. An ascites leukemia, phenotype TL.1,2,3, which modulates rapidly and completely when incubated with TL antiserum in vitro, was selected for further study of the phenomenon. Over a wide range of TL antibody concentrations modulation at 37 degrees C was detectable within 10 min and was complete within approximately 1 hr. The cells were initially sensitized to C' by their contact with antibody, thereafter losing this sensitivity to C' lysis together with their sensitivity to TL antibody and C' in the cytotoxic test. The capacity of the cells to undergo modulation was abolished by actinomycin D and by iodoacetamide, and by reducing the temperature of incubation to 0 degrees C. Thus modulation apparently is an active cellular process. Antigens TL. 1,2, and 3 are all modulated by anti-TL.1,3 serum and by anti-TL.3 serum. This modulation affects all three TL components together, even when antibody to one or two of them is lacking. aAnti-TL.2 serum does not induce modulation and in fact impairs modulation by the other TL antibodies. The influence of the TL phenotype of cells upon the demonstrable content of H-2 (D region) isoantigen, first shown in cells modulated in vivo, has been observed with cells modulated in vitro. Cells undergoing modulation show a progressive increase in H-2 (D region) antigen over a period of 4 hr, with no change in H-2 antigens of the K region. Restoration of the TL+ phenotype of modulated cells after removal of antibody is less rapid than TL+ --> TL- modulation and may require several cell divisions.