Recombinant antigen-enterotoxin A2/B chimeric mucosal immunogens differentially enhance antibody responses and B7-dependent costimulation of CD4(+) Tcells

The ADP-ribosylating enterotoxins, cholera toxin (CT) and the Escherichia c oli heat-labile toxin (LT-IIa), have been shown to enhance mucosal and syst emic antibody (Ab) responses to coadministered antigens. The purpose of the present study was to compare the ability of the nontoxic A2/B subunits of these toxins, which have distinct targeting properties, to augment the immu nogenicity of a genetically coupled protein antigen. Structurally similar c himeric proteins were generated by genetically replacing the toxic Al subun it of CT or LT-IIa with the saliva-binding region (SBR) from the streptococ cal adhesin AgI/II. Intranasal immunization of BALB/c mice with either chim eric protein induced significantly higher plasma and mucosal anti-SEE immun oglobulin A (IgA) and IgG Ab responses than SBR alone. Moreover, compared t o SBR-LT-IIaA2/B, SBR-CTA2/B elicited significantly higher levels of plasma IgG1 and salivary IgA anti-SEE Ah responses. Ex vivo and in vitro experime nts revealed that SBR-CTA2/B selectively up-regulated B7-2 expression on mu rine B cells isolated from both the nasal associated lymphoid tissue, cervi cal lymph nodes, and spleen. In contrast, SBR-LT-IIaA2/B had little effect on B7-1 or B7-2 expression on B220(+), CD11b(+), or CD11c(+) cells. Analysi s of the functional costimulatory activity of SBR-CTA2/B-treated B cells re vealed a significant enhancement in anti-CD3-stimulated CD4(+) T-cell proli ferative responses, and this proliferation was significantly reduced by tre atment with anti-B7-2 but not with anti-B7-1 or isotype control Abs. Thus, SBR-CTA2/B and SBR-LT-IIaA2/B exhibit distinct patterns of antibody respons es associated with differential effects on B7-2 expression and subsequent c ostimulatory effects on CD4(+) T cells.