Further studies of the adjuvant properties of homologous IgG split products: mode of action of F(ab')2 and related fragments.

Peptic bivalent and monovalent Fab' fragments as well as F(ab')2 fragment from a purified rabbit anti-DNP antibody were equally effective in enhancement of the immune response to SRBC. In contrast, the papain Fab fragment of normal IgG possessed a considerably lower enhancing activity. These data indicate that the effector site(s) controlling the adjuvant activity of F(ab')2 and related fragments is structurally distinct from the antibody active site(s) within the fragment molecule, and is located in C-terminal region of the heavy chain Fd fragment. When F(ab')2 fragment was injected into rabbits together with the proteinase inhibitor Trasylol it failed to enhance the immune response to SRBC. Furthermore, Trasylol interferes with the ability of F(ab')2 or Fab' fragment to activate the complement system both in vivo and in vitro. This, together with the observation that F(ab')2 fragments enhance the response to T-dependent antigens (SRBC and bovine gamma globulin) but fail to augment the response to LPS, suggests that these fragments may operate via activation of complement and thereby influence indirectly the reactivity of complement receptor B cells.