Delineation of IgM-receptor bearing human T and B lymphocytes using a direct plaque forming cell (PFC) assay.

Based on the observation that binding of IgM cytophilic antibodies to lymphocytes is temperature dependent, a direct plaque forming cell (PFC) assay was developed to detect IgM-receptor bearing human peripheral blood T and B lymphocytes. Lymphocytes were passively sensitized with IgM anti-SRBC molecules at 4 degrees C, added to SRBC monolayers then incubated at 37 degrees C with guinea pig complement to develop the plaques. The PFC assay has methodological advantages over rosetting methods which demonstrate IgM receptors, and under certain conditions is more sensitive than these rosette techniques. A mean of 17% of freshly isolated uncultured lymphocytes, enriched for B cells, formed direct plaques while a mean of 3% of T-enriched preparations formed direct plaques. However, if the lymphocytes were preincubated with vibrio cholerae neuraminidase (VCN) these figures increased to 46% and 35% respectively. The specificity of plaque formation by VCN-treated lymphocytes was established. SRBC sensitized with a F(ab')2 preparation of an IgG anti-SRBC reagent failed to bind to VCN-treated lymphocytes, inclusion of IgM, but not other Ig molecules in the test medium, inhibited plaque formation, and, most important, plaque formation by T and B cells was inhibited by F(c)5 mu but not by Fab mu fragments. These results indicate that T and B lymphocytes express IgM-class specific membrane receptors, that these receptors may be hidden on normal lymphocytes but are revealed by treatment with VCN and that the IgM receptor on VCN-treated lymphocytes is F(c)mu specific. These findings are discussed briefly with regard to other and partly contradictory data obtained after overnight in vitro lymphocyte culture. As demonstrated by direct PFC assay, the B cell IgM receptor is trypsin sensitive.