Antibody-independent complement activation by a subpopulation of mitogen-induced human T lymphoblasts

A study was made of the complement activation (CA) capacity of human T lymphoblasts obtained in vitro by PHA stimulation of highly purified T cells. The CA was studied by immunofluorescent detection of the C3 fragments deposited onto the cell membrane following incubation in autologous normal human serum (NHS). Whereas resting T cells did not stain, a mean of 12% of the blast cells were positive for monospecific anti-human C3 fluorescent serum. The possible mechanism through which C3 was deposited on the cell membrane may be a consequence of CA, because complement membrane fluorescence (CMF) was abolished by chelation of divalent cations with EDTA, complement inactivation by heating or incubation at low temperature. This reaction appears to occur in the absence of antibodies, because similar results were found when human agammaglobulinaemic serum was used as the complement source. The addition of ethyleneglycolbis-(aminoethylether)-tetraacetate (EGTA), supplemented with MgCl2 to the NHS failed to abolish the fluorescence, indicating that the alternative pathway was involved in the phenomenon. It is likely that the capacity to activate complement may distinguish a functional subpopulation of human T lymphoblasts.