Apolipoprotein is responsible for neutralization of xenotropic type C virus by mouse serum.

We have shown that the circulating lipoproteins of the mouse contain a potent inhibitor of infectivity of the xenotropic type C virus. This virus neutralization does not involve immunoglobulins or complement. After fractionation of the lipoproteins on the basis of particle size, flotation properties, and electrostatic charge, virus neutralizing activity is found primarily in the triglyceride-rich lipoproteins (predominantly the chylomicrons) and in the HDL(2) subfraction of the high density lipoproteins. In fasted animals, activity resides chiefly in the high density lipoproteins. Neutralization titers increase strikingly during alimentary lipemia in both the lipoproteins of the rho < 1.006 g/cm(3) fraction and the high density lipoproteins. Increased activity persists in the high density lipoproteins after the lipemia recedes. Virus neutralizing activity is completely eliminated in all fractions by antiserum against high density lipoproteins and, in the triglyceride-rich fractions, by antiserum to murine apolipoprotein B. Complete removal of lipids markedly reduces the neutralizing activity of both classes of lipoproteins. Apolipoproteins delipidated with tetramethylurea retain some activity, which is enhanced by binding to a phospholipid-stabilized triglyceride emulsion and which is abolished by proteolytic digestion. We have demonstrated in vitro transfer of activity between high density and very low density lipoproteins of the mouse. These data indicate that xenotropic virus neutralization by normal mouse serum depends upon a protein that transfers among lipoprotein particles in a fashion analogous to the C apolipoproteins of other mammalian species.