Potent Immunosuppression by Oxidized Cholesterol
Authors: Humphries Gillian M. K. , McConnell Harden M. 
Published: January 2023

Abstract
Sepharose 4B to which ε-dinitrophenyl lysine has been covalently attached behaves as an in vitro T-independent antigen when cultured for 4 days with mouse spleen cells. Almost total inhibition of the plaque-forming cell (PFC) response is obtained when liposomes composed of dimyristoylphosphatidylcholine (DMPC) and 0.05 μmole of a particular sample of recrystallized but aged cholesterol are cultured with 107 viable spleen cells; no inhibition is apparent when liposomes composed only of DMPC are used. After purification on Sephadex LH 20 with methylene dichloride, the cholesterol is no longer inhibitory. Fractionation by various forms of chromatography, together with biologic assays, has led to the conclusion that the major inhibitory component in the impure cholesterol is 25-hydroperoxycholesterol, an oxidation product of cholesterol whose Rf on silica gel, in several solvent systems, is very close to that of cholesterol. The peroxide can be reduced metabolically, or by the action of NaBH4, to give 25-hydroxycholesterol, a compound that is at least as immunosuppressive as its precursor. It is estimated that 25-hydroperoxycholesterol is present in the impure cholesterol sample at a level slightly less than 1%.
Liposomes composed of DMPC:purified cholesterol:25-hydroxycholesterol (molor ratio 100:100:1) have been used to study the phenomenon in a better defined system. The biologic activity of such liposomes is essentially identical to that of liposomes containing DMPC:impure cholesterol (molar ratio 1:1). The liposomes are highly inhibitory when added in the first 24 hr of culture, but the system becomes rapidly less susceptible to inhibition during the next 24 hr. By the 3rd day of culture, which is when a rapid onset of measurable PFC becomes apparent, the cells are no longer susceptible to inhibition by 25-hydroxycholesterol.
Almost total inhibition of the PFC response is obtained at a concentration of 25-hydroxycholesterol, which would be expressed as 5 × 10-7 M if the compound were in monomolecular dispersion (i.e., 0.5 nanomoles/1.0 ml culture volume). At this concentration there are 3 × 107 molecules of 25-hydroxycholesterol per viable cell originally cultured and the viability of the cells at the time of assay is not significantly different from that of controls lacking liposomes. Suppression of the in vitro primary response to sheep red blood cells (SRBC) by hydrocortisone occurs at about the same minimum concentration of that steroid.
It is well known that 25-hydroxycholesterol is a potent inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase activity and hence inhibits cholesterol synthesis from acetate and other precursors of mevalonate in the biosynthetic pathway; however, mevalonate does not abrogate the inhibition of the PFC response by liposomes containing 25-hydroxycholesterol. In the absence of liposomes, the PFC response to dinitrophenylated beads is significantly and reproductibly greater in the presence of mevalonate than it is in its absence. This suggests that the cells are capable of assimilating the exogenous mevalonate. If this is true, a hypothesis that PFC suppression by 25-hydroxycholesterol is mediated by its action on HMG CoA reductase activity per se is not tenable.