Distinct effects of recombinant cholera toxin B subunit and holotoxin on 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 remain poorly understood. Using an in vitro Ag presentation assay, we found CT and recombinant B subunit (rCTB) to have distinct effects on different stages of processing and class II MHC (MHC-II)-restricted presentation of hen egg lysozyme (HEL). CT treatment of macrophages resulted in enhanced presentation of soluble HEL(48-61) peptide to3A9 hybridoma cells. However, CT had inhibitory effects on intracellular processing of soluble native Ag. Thus, CT inhibited presentation when added prior to HEL, whereas presentation was enhanced when CT was added after HEL exposure and the generation of peptide-MHC-II complexes. Pretreatment of macrophages with CT also markedly inhibited phagocytic processing of a Crl-HEL fusion protein (containing the HEL(48-61) epitope) expressed in intact bacteria (Escherichia coli HB101.Crl-HEL or Salmonella 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 radiolabeled HEL or HB101.Crl-HEL. In contrast to the holotoxin, purified rCTB subunit did not inhibit intracellular processing of soluble or bacterial Ag, although it similarly enhanced the presentation of surface HEL-(48-61)-I-Ak complexes to 3A9 cells. These data suggest that the inhibitory effects of CT on Ag processing are mediated by the A subunit.