Murine Typhus Toxin: Studies on Identification of the Neutralizing Factor Present in Normal Human Serum

Abstract

Studies were made on the isolation and identification of the Rickettsia typhi toxin-neutralizing factor (TNF) previously demonstrated in normal human serum. By means of various methods of separating serum proteins, such as filtration on Sephadex G-200, dextran precipitation, hydroxyapatite chromatography, and ultracentrifugation, TNF was found to be closely associated with purified serum β-lipoprotein, although no serological relationship with this protein was demonstrated. Lipase as well as trypsin digestion of purified preparations of β-lipoprotein destroyed the TN activity. No evidence was obtained for an association of TNF with the immunoglobulins or with any serum protein other than β-lipoprotein. Further studies revealed that (i) serum specimens with TN titers of 1:1024 and others with titers of 1:8 or less contained the same concentration of β-lipoprotein; (ii) purified preparations of β-lipoprotein isolated from TNF positive and negative sera, and which had the same protein concentration, differed as much as 250-fold in TN titer; and (iii) the TN activity of a serum could be removed by absorption with antiserum to β-lipoprotein from a positive donor, but not with antiserum to β-lipoprotein from a negative donor.