MEMBRANE UNPACKING AND THE RAPID DISPOSAL OF APOPTOTIC CELLS

There is evidence that macrophages can recognize phosphatidylserine (P S) on the surface of apoptotic thymocytes, where PS exposure relates t o looser packing ('unpacking') of the polar headgroups of the outer la mina, detectable by lipophilic dyes (Fadok, V.A., Voelker, D.R., Campb ell, P.A., Cohen, J.J., Bratton, D.L. and Henson, P.M. (1992) J. Immun ol. 148, 2207). We have shown that membrane unpacking also occurs in B cells, where this event actually precedes DNA cleavage (Mower, D.A. J r., Peckham, D.W., Illera, V.A., Fishbaugh, J.K., Stunt, L.L. and Ashm an, R.F. (1994) J. Immunol. 152, 4832). This paper demonstrates that t he time interval between membrane unpacking (detected as merocyanine 5 40 binding) and DNA cleavage (detected by flow cytometry of propidium iodide stained nuclei) also occurs in both T cells and thymocytes. The tight coupling of these two apparently distinct events is emphasized by their co-regulation by a variety of agents which accelerate or inhi bit apoptosis. One hypothesis to explain the very low numbers of free apoptotic cells seen in vivo is that macrophages can recognize cells w ith unpacked membranes and destroy them before they cleave their DNA. In support of this hypothesis, we demonstrated that parenteral cyclohe ximide triggers a wave of apoptosis in the spleen detected by merocyan ine 540 as well as by hypodiploid nuclei. Significantly, both paramete rs returned from peak values at 2 h virtually to normal by 4 h, testif ying to the existence of a rapid disposal mechanism in vivo for cells with unpacked membranes as well as hypodiploid nuclei.