ROLE OF SERUM IN INHIBITION OF CULTURED LYMPH-NODE CELLS BY LYSOPHOSPHATIDYLCHOLIN

Lysophosphatidylcholine, a key intermediate of phospholipid metabolism, has detergent and highly cytolytic properties. It is produced in plasma and various tissues and approximately 200 (mu)M is usually present in human and rabbit serum. Therefore, uncoupled from in vivo homeostasis, cell cultures containing serum are under potentially cytolytic conditions. The present work deals with the role of serum in regulating the cytolytic actions of lysophosphatidylcholine in vitro.;Addition of lysophosphatidlycholine (5 (mu)M) to cultures containing preheated (66(DEGREES)C, 20min) rabbit serum greatly decreased radioactive labelling of autologous lymph-node cells with {('3)H}uridine (the term "inhibition" is used to describe this decrease.); there was much less inhibition in cultures containing unheated serum. Thus, the heating appeared to inactivate some serum activity(ies) which protected cells against lysophosphatidylcholine.;The metabolism of lysophosphatidylcholine by unheated serum was too low to ascribe the protecting activity of the serum solely to 66(DEGREES)C-labile enzyme activities. In unheated serum, exogenous lysophosphatidylcholine was principally bound to serum albumin. On heating the serum, the albumin lost its binding capacity and the majority of the lysophosphatidylcholine was transferred to lipoproteins, in particular to high density lipoprotein. However, these lysophosphatidylcholine-containing lipoproteins isolated by flotation in salt-density gradients, were not themselves inhibitory. Inhibitory activity was recovered from lipoprotein-depleted fractions. For the latter to be inhibitory, it was necessary to heat the serum prior to the addition of the lysophosphatidylcholine. When lysophosphatidylcholine was mixed with unheated serum, in spite of the subsequent heating of the serum-lysophosphatidylcholine mixture, the inhibition was greatly diminished and no inhibitory activity was recovered from salt-density gradients.;The inhibitory effect of lysophosphatidylcholine added to cultures containing preheated serum was diminished by the addition of albumin, but not by serum lipoproteins. These results suggested that the capacity of albumin to act as a growth factor for cultured lymphocytes, is due to its ability to sequester lysophosphatides which would normally be generated during cultures by serum and by cells.