. Glutaraldehyde-inactivated cells and cell-wall fractions of were studied for their capacity to induce or inhibit the in-vitro proliferation of human peripheral blood mononuclear cells (PBMC), as measured by H-thymidine incorporation. Both the intact cells (CA) and a phosphorylated gluco-mannanprotein complex of the cell wall (GMP), in μg doses, were strong inducers of PBMC proliferation, with a peak of activity at 6-9 days of culture and varying with the PBMC donor. A significant but much lower proliferation was observed on exposure of PBMC to a low-protein (<3% by weight) mannan component (M) of the cell wall. Both a hot-alkali extracted mannan-protein complex (M-alk), comparable to GMP in crude chemical composition, and an alkali-insoluble cell-wall glucan (GG) were inactive. None of the fractions induced a lymphoproliferation of umbilical cord blood cells and all fractions, except GG, were equally effective in binding human anti- antibodies as shown by a sensitive ELISA-inhibition assay. Moreover, a monoclonal antibody against the class II determinant of the HLA complex inhibited PBMC proliferation irrespective of the antigen used. Taken together, the data shows that in inducing lymphoproliferation, fractions act as specific antigens rather than as non-specific mitogens. Use of intact cells and chemically-defined cell-wall components appears preferable to use of undefined antigenic mixtures as stimulators of PBMC proliferation.


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