DISTINCT EFFECTS OF RECOMBINANT CHOLERA-TOXIN-B SUBUNIT AND HOLOTOXINON DIFFERENT STAGES OF CLASS-II MHC ANTIGEN-PROCESSING AND PRESENTATION BY MACROPHAGES

Cholera toxin (CT) is a potent mucosal adjuvant with enhancing effects on Ag presentation, although the mechanisms of its adjuvanticity rema in poorly understood, Using an in vitro Ag presentation assay, we foun d CT and recombinant B subunit (rCTB) to have distinct effects on diff erent stages of processing and class II MHC (MHC-II)-restricted presen tation of hen egg lysozyme (HEL). CT treatment of macrophages resulted in enhanced presentation of soluble HEL(48-61) peptide to 3A9 T hybri doma cells, However, CT had inhibitory effects on intracellular proces sing of soluble native Ag, Thus, CT inhibited presentation when added prior to HEL, whereas presentation was enhanced when CT was added afte r HEL exposure and the generation of peptide-MHC-II complexes, Pretrea tment of macrophages with CT also markedly inhibited phagocytic proces sing of a Crl-HEL fusion protein (containing the HEL(48-61) epitope) e xpressed in intact bacteria (Escherichia coh HB101.Crl-HEL or Salmonel la typhimurium 14028s.Crl-HEL), whereas addition of CT to macrophages after a 2-h incubation with the bacteria again enhanced presentation, CT produced little effect on overall uptake and catabolism of radiolab eled HEL or HB101.Crl-HEL. In contrast to the holotoxin, purified rCTB subunit did not inhibit intracellular processing of soluble or bacter ial Ag, although it similarly enhanced the presentation of surface HEL -(48-61)-I-A(k) complexes to 3A9 cells; These data suggest that the in hibitory effects of CT on Ag processing are mediated by the A subunit.