Isolation and assay of a human plasma factor affecting human thymus-derived lymphocytes

An amino acid-containing factor has been isolated from a dialysate of defibrinated human plasma using Sephadex G-25 and Sephadex CM-25 column chromatography. This factor is capable of enhancing "avid" E-rosette formation by normal human lymphocytes, where avid E-rosettes are defined as lymphocytes binding 10 or more sheep red cells. This factor is effective at concentrations as low as 10(-12) g/ml. It resembles the porcine and murine "facteur thymique sérique" (e.g. J.-F. Bach, M. Dardenne, J.-M. Pleau, and J. Rosa, Nature (London) 266, 55, 1977) in its elution characteristics, its capacity to enhance avid E-rosette formation and, tentatively, its amino acid composition.