Complement activation by human lymphocytes from different lymphoid organs: role of sialic acid and lack of...

A study was made of the complement activation (CA) capacity of human lymphocytes from lymphoid organs. Whereas thymocytes did not show complement membrane fluorescence (CMF) following incubation in normal homologous serum (NHS), a mean of 18 and 34%, respectively, of tonsil and spleen cells were positive for antihuman C3 fluorescent serum. Isolated T cells did not show CA capacity, while 62% of a purified tonsil B population possessed this capacity. The CMF was abolished by adding EDTA and when the incubation was performed in C1q and factor-D-depleted serum. Addition of EGTA, supplemented with MgCl2 to NHS failed to abolish the fluorescence, indicating that the alternative pathway was involved in the phenomenon. Removal of sialic acid with neuraminidase increased the percentage of cells showing CMF. Since sialic acid significantly contributes to the extent of net negative surface charge, we have studied whether this parameter influences the CA capacity. Tonsil B cells were separated in fractions with different electrophoretic mobility by preparative cell electrophoresis, and similar percentages of CMF-positive cells were found in all fractions. Therefore, it is likely that the capacity to activate complement is unrelated to the membrane electrical charge.