Expression of receptors for Fc portion of IgM (Fc.MU.R) and surface neuraminic acid on the human peripheral lymphocytes.

The purpose of this investigation is to establish a method of quantitation of sialic acid on the lymphocyte membrane surface and to clarify the importance of sialic acid in the functional manifestation of the lymphocyte receptor for the Fc portion of IgM (Fc mu . R) as well as the cell surface. The experiments yielded results leading to the following conclusions: Cell surface sialic acid can be exclusively recovered at consistent rates by treatment of lymphocytes with 0.1 IU of neuraminidase in the presence of Ca++ at 37 degrees C for 30 min. The sialic acid obtained, though in minute quantities, was accurately quantitated by the TBA method. By these procedures, the amount of sialic acid present on the cell membrane of human peripheral blood lymphocytes was determined to be 2.46 micrograms/10(7). Surface sialic acid on lymphocytes cultured at 37 degrees C after treatment with neuraminidase was resynthesized sufficiently at about 6 hr. On the cell membrane of lymphocytes deprived of Fc mu . R by treatment with neuraminidase, the receptor for IgM Fc became redetectable following incubation of the cells at 37 degrees C; the time-course of redevelopment of Fc mu . R showed a striking coincidence with that of sialic acid resynthesis. A sufficient redevelopment of Fc mu . R occurred in 6 hr of incubation of neuraminidase-treated Fc mu . R-deprived lymphocytes; the time required for sufficient redevelopment was comparable to that reported for pronase-treated lymphocytes. It is concluded from these findings that Fc mu . R deprived of sialic acid is no longer repaired while the receptor is resynthesized de novo on incubation of lymphocytes under adequate conditions.