Immunosuppressive factor(s) in sera from patients with acute viral hepatitis B [serum inhibition factor(s) (SIF)] which functioned like an antiactivator of lymphocytes were further characterized and purified. The active moiety could be separated from immunoglobulins and other serum proteins by means of gel filtration, anion exchange, and affinity chromatography. The major SIF activity always copurified with albumin. Affinity chromatography with Cibacron blue agarose matrix followed by elution with 2 M NaCl proved an optimal procedure to obtain SIF-positive albumin fractions. The SIF moiety could be dissociated from albumin by use of 5 M NaCl or 6 M urea and was separated from protein by sequential molecular filtration and G-10 gel filtration indicating a low molecular weight substance. SIF activity of lower degree could also be detected in albumin-containing fractions derived from normal sera and exhibited similar biochemical properties as the factor which was isolated from patients' sera.
The purified SIF fractions could not be stained with various protein dyes after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The active moiety was partially extractable with chloroformrmethanol indicating a lipophilic nature. Common fatty acids or bile acids were excluded as causative factors by gas chromatographic-mass spectrometric and radioimmunologic analyses.
These data suggest that the SIF effect is caused by an albumin-associated low molecular weight lipid or lipophilic peptide. SIF may be physiological immunoregulatory products of the immune system which are probably produced in response to a viral antigenic stimulus.