Expression of membrane‐bound IgM and HLA antigens on lymphoblastoid cells in different stages of the cell cycle

The expression of membrane‐bound IgM and HLA on human lymphoid cells in different stages of the cell cycle was determined by cytophotometric techniques. Asynchronously proliferating cells were pulse labeled with [3H]thymidine. IgM and HLA were then stained directly and indirectly with fluorescein‐conjugated antibodies and quantitated on the single cell level by microspectrofluorometry. The same individual cells were analyzed further by dry mass and Feulgen DNA determinations using microinterferometry and microspectrophotometry, respectively. Cells in the S phase were subsequently identified by [3H]thymidine autoradiography. The amounts of membrane‐bound IgM and HLA were both found to increase continously throughout interphase. Thus, in absolute terms, the IgM and HLA content was maximal in G2, minimal in the G1 and intermediate in the S phase. However, since the total proteins (dry mass) increased in the same manner, the IgM (and HLA)/total protein ration remained constant during the cell cycle. For geometric reasons, the total cell surface area increases less than the cell volume (mass) during interphase. This might indicate that the density of IgM and HLA on the membrane is greater in the G2 than in the G1 phase.