Normal human sera contain one or several factors cytotoxic for normal guinea pig thymocytes, and when serum is precipitated with ammonium sulphate (60 saturated) and the precipitate dissolved and dialyzed, the activity is preserved. Gel chromatography with Sephadex G-150 and Sepharose CL-6B indicated a molecular weight of approximately 900,000 daltons. The active fractions contained a high amount of IgM according to single radial immunodiffusion and two-dimensional gel electrophoresis. Qantitation of the IgM band in one-dimensional gel electrophoresis preparations by gel scanning indicated that IgM accounted for 65 of the eluted proteins in active fractions. Purified human IgM from myeloma patients eluted as the active factor during gel chromatography. Elimination of IgM from serum by affinity chromatography eliminated the cytotoxic activity. The serum could also be inactivated by heating. The mixing of IgM-depleted serum with either polyclonal IgM or heat-inactivated serum restored the activity. Thus, the cytotoxic activity is due to IgM antibodies plus a heat-labile component (presumably complement). The presence of the cytotoxic activity in autologous (guinea pig) serum was recently demonstrated. The possible functional role of these antibodies in the elimination process of a large number of cortical thymocytes is suggested.
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