Target cell susceptibility to immune lysis and expression of MHC antigens are independent of position in the cell cycle

We have investigated the effects of progression through the mitotic cell cycle upon the tumor (P815) cell line's expression of H-2 antigens and susceptibility to immune cytolysis. By using a combination of DNA quantitation by Hoechst 33342 staining and fluorescence-activated cell sorter analysis we were able to obtained pure populations of G₁, S, and G₂ + M cells for study. Furthermore, by employing a fluorescence quenching technique utilizing bromodeoxyuridine substitution we were able to monitor the times required for the P815 cells to transit each of the various phases. By using mAb and cloned CTL populations with well-defined specificities we observed that neither H-2 antigen expression nor lytic susceptibility to mAb plus complement or CTL fluctuated with cell cycle progression. Each of the three major MHC loci (H-2K(d), -D(d), and L(d)) was analyzed. These results demonstrated that expression of H-2 antigens and susceptibility to immune lysis were independent of a cell's position in the cell cycle.