Cell cycle related heterogeneity of Ia antigen expression on a murine B lymphoma cell line:analysis by flow cytometry.

In this investigation we have used quantitative flow cytometry to study the expression of I region antigens on an established B lymphoma cell line, WEHI-231. Although a majority of the WEHI-231 cells do not react with antisera against the Ia.7 (I-E/C) or Ia.8 (IaA) antigens, a distinct subpopulation of cells, comprising 20 to 30% of the total population, reacts strongly with these alloantisera. Several mechanisms were proposed to explain this intratumor Ia antigen heterogeneity: 1) the effect of volume (surface area) heterogeneity on antigen expression; 2) the existence of a stable variant of Ia+ cells in the cell line; 3) cell cycle regulation of Ia antigen expression. The first possibility was excluded on the basis of FCM analysis, which failed to detect any relationship between cell volume and fluorescence intensity of anti-Ia stained cells. Additionally, the rapid reappearance of Ia antigen heterogeneity within Ia- and Ia+ lines, sorted by the fluorescence-activated cell sorter, argued against the presence of a stable variant population of Ia+ cells. Rather, our data suggest that cell cycle related events are responsible for the Ia antigen heterogeneity. The Ia.7+ sorted cells were shown to be significantly enriched in the G0/G1 phase of the growth cycle. In contrast, it was the Ia.8- cells that were enriched in this phase of the cell cycle. These results suggest that the I-A and I-E/C region gene products are independently regulated in this cell line and imply that similar controls may possibly influence Ia antigen expression on normal B lymphocytes.