Ox red blood cells (ORBCs) sensitized with rat IgM antibodies formed antibody-coated red blood cell rosettes (EAR) around 9% of rat spleen cells freshly prepared at 4 °C, while an enhancement of the rosette-forming cell (RFC) frequency to 18-20% was observed after having exposed the spleen cells to a temperature shift from 4 to 37 °C. Although the temperature shift was found to increase the RFC frequency, a shedding of IgM-Fc receptors IgM-FcR into the medium was also detected (IgM-FcR-I). Regeneration of shed receptors within 5 hr has been proved, and the sheading-regeneration cycle could be repeated several times. IgM-Fc receptors detectable after the temperature shift (IgM-FcR-II) are neither shed nor detectable on freshly prepared spleen cells. This latter type of receptor is expressed only after incubating the cells at 37 °C for an optimal period of 8-10 hr. Both type I and II IgM-FcRs were detected on T lymphocytes as well as B lymphocytes; therefore they do not mark subpopulation. Both receptors are sensitive to trypsin and the activity of both requires Ca2+ ions. Shed receptors are able to inhibit EAR formation by cells carrying either IgM-FcR-I or IgM-FcR-II. They are sensitive to higher temperatures and agglutinate ORBCs sensitized by IgM antibodies in the presence of Ca2+ ions. EAR-inhibiting capacity was detected in two fractions obtained by gel chromatography of the supernatant after shedding. The elution volume of one of the active fractions corresponds to 130,000 daltons (D), and that of the other to approximately 50,000 D.
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