The Nature of the Stimulatory Cell in Human Allogeneic and Autologous MLC Reactions; Role of Isolated IgM-Bearing B Cells

Abstract

Abstract Highly purified surface IgM-bearing B cells were isolated from T cell- and monocyte-depleted mononuclear cells by their ability to rosette with purified anti-IgM antibodies covalently linked to bovine red blood cells. Surface IgM-bearing B cells from both peripheral blood and tonsil were found to be excellent stimulator cells in unidirectional MLC reactions. In the majority of cases, the surface IgM-bearing B cells had a higher stimulatory capacity in the allogeneic MLC than non-T cells lacking surface IgM. Monocytes were also demonstrated to be good stimulators in the allogeneic MLC when a low number of cells was used. At high numbers, inhibition of tritiated thymidine uptake was observed, which appeared to be partly due to the release of prostaglandins and cold thymidine. The population of non-T cells lacking surface IgM also showed stimulation that was especially strong in certain individuals. Because of the heterogeneity of this fraction, the primary cell involved could not be determined. It is of interest that this population was a more potent stimulator in the autologous MLC than was the surface IgM-bearing B cell fraction, although the latter was clearly active. Results from these studies also indicated that monocytes were relatively poor stimulators in the autologous MLC. Isolated Fab′ fragments of a rabbit anti-human Ia antiserum caused significant inhibition of both the allogeneic and autologous MLC.