Immunological enhancement by mouse isoantibodies: the importance of complement fixation.

Balb/c anti-E.L.4 serum was separated into seven fractions of increasing electrophoretic mobility by gradient elution from a column of DEAE-cellulose. The first four fractions, containing the bulk of the immunoglobulins, were tested for their effects on the growth of E.L.4 in Balb/c mice. The first fraction, which contained most of the cytotoxic activity , produced inhibition of growth, while the third fraction produced enhancement. The activities of F(ab′) and Fab′ fragments from A strain anti-E.L.4 serum were also studied. Both preparations were found to have lost cytotoxic activity on E.L.4 cells , but to be capable of blocking the effect of undigested IgG. F(ab′) retained haemagglutinating activity on C57B1 red cells, whereas Fab′ did not. Fab′ was able to block the haemagglutinating effect of undigested IgG and F(ab′). Both F(ab′) and Fab′ could produce enhancement of the growth of E.L.4 in A strain mice. It is suggested that non-complement-fixing antibodies are critical in the production of enhancement.