A COMPARATIVE-STUDY OF NORMAL B-CELLS AND THE EBV-POSITIVE BURKITTS-LYMPHOMA CELL-LINE, RAJI, AS ACTIVATORS OF THE COMPLEMENT-SYSTEM

Contradictory reports regarding the ability of complement receptor typ e 2 (CR2,CD21) on normal B cells to activate complement (C') via the a lternative pathway (AP), prompted us to compare the performance of hum an peripheral blood B cells and the Epstein-Barr virus-positive Burkit t's lymphoma cell line, Raji (a well characterized AP activator) by us ing flow cytometry. Measured in terms of the membrane deposition of C3 fragments per cell, Raji cells were significantly (6- to 26-fold) mor e effective as complement activators than were normal B cells. Raji ce lls were also found to express approximately four to five times as man y CR2 as normal B cells. In addition, they distinguished themselves by displaying a greater Ca2+-dependent activation, with pooled normal hu man sera (NHS) as the complement source, and by degrading unprotected C3b fragments from iC3b to C3dg/C3d at a significantly lower rate than the B cells. The Ca2+ dependency of Raji cell activation was found to be partially a result of classical pathway (CP) triggering by specifi c antibodies in the NHS, although other triggering mechanisms may also be involved. If the influence of these variations between Raji cells and normal B cells was excluded, by relating deposition of anti-C3d-re active fragments, during AP activation, to the number of CR2 expressed , the difference in performance between the two cell types was found t o be insignificant.