Occurrence of specific phytohaemagglutinin-reactive immunoglobulins in the sera of certain individuals. Relevance to the detection of cell surface antigens on phytohaemagglutinin-stimulated cells.

While investigating sera for possible antibodies to TL-like determinants, multiparous sera were selected that had specific cytotoxic reactivity against phytohaemagglutinin (PHA)-activated lymphocytes of certain individuals and were negative against unstimulated T or B cells from the same donor. The reactivity patterns were not correlated to any HLA specificities. However, in an indirect immunofluorescence assay with flow cytometry, these same sera had strong specific monomorphic reactivity to PHA-activated lymphocytes from all the individuals in the panel. Unstimulated cells remained negative. In contrast, other human sera lacked reactivity with PHA-stimulated lymphocytes. The addition of PHA to fresh lymphocytes followed by incubation at 4 degrees C for 30 min resulted in the same pattern of reactive and non-reactive sera. When incubated with PHA, different cell types, including 0- erythrocytes and a murine lymphoid cell line, reacted similarly with the sera. Using plates coated directly with PHA-E, L, and P in a cell-free ELISA system. PHA-specific sera reacted specifically with PHA-coated wells. The anti-PHA activity was removed by absorption with 0- erythrocytes coated with PHA without affecting the titre of the anti-HLA antibody in the same serum. These findings suggested that IgG molecules in certain sera react directly with residual PHA bound to the cell surface and not necessarily with molecules of cellular origin induced by exposure to PHA, complicating the search for antibodies specific for T-cell activation antigens.