Cholera Toxin B-Subunit Gene Enhances Mucosal Immunoglobulin A, Th1-Type, and CD8⁺Cytotoxic Responses When Coadministered Intradermally with a DNA Vaccine

Abstract

A plasmid vector encoding the cholera toxin B subunit (pCtB) was evaluated as an intradermal genetic adjuvant for a model DNA vaccine expressing the human papillomavirus type 16 L1 capsid gene (p16L1) in mice. p16L1 was coadministered with plasmid pCtB or commercial polypeptide CtB as a positive control. Coadministration of pCtB induced a significant increment of specific anti-L1 immunoglobulin A (IgA) antibodies in cervical secretions (P < 0.05) and fecal extracts (P < 0.005). Additionally, coadministration of pCtB enhanced the production of interleukin-2 and gamma interferon by spleen cells but did not affect the production of interleukin-4, suggesting a Th1-type helper response. Furthermore, improved CD8 ؉ T-cell-mediated cytotoxic activity was observed in mice vaccinated with the DNA vaccine with pCtB as an adjuvant. This adjuvant effect was comparable to that induced by the CtB polypeptide. These results indicate that intradermal coadministration of pCtB is an adequate means to enhance the mucosa-, Th1-, and CD8؉ -mediated cytotoxic responses induced by a DNA vaccine.Cholera toxin (CT), the enterotoxin produced by Vibrio cholerae, is a potent immunoadjuvant (26). CT is composed of two structurally and functionally different subunits, the toxic A subunit (CtA) and the cell-binding B subunit (CtB). Biologically active CtB assembles into pentamers and binds with a high affinity to the cellular receptor GM 1 ganglioside (35), which is expressed by a wide range of nucleated cells, including epithelial cells, lymphocytes, and antigen-presenting cells (APCs). CtB is recognized as a mucosal adjuvant itself (12). When CtB is applied simultaneously with heterologous antigens, it induces the stimulation of a mucosal response to the admixed antigen (15).The adjuvant capacity of CT for orally administered antigens has long been recognized (26). More recently, CtB has been used to enhance immune responses to antigens delivered by novel immunization routes (16,20). In particular, application of CtB onto mouse skin has been proved to induce potent humoral and cellular responses against the coadministered antigen (1).The use of DNA vaccines represents a novel strategy for the induction of specific mucosal immune responses. Like traditional vaccines, the ability of DNA vaccines to elicit mucosal responses can potentially be improved by the use of adjuvants. Coadministration of CT has been used to increase the specific immunoglobulin G (IgG) (8) and mucosal IgA (24) responses mediated by DNA vaccines. However, the use of CtB alone as an adjuvant for DNA vaccines is still limited. In addition, use of the CT-coding sequences as genetic adjuvants for DNA vaccines has recently been proposed as an innovative approach that has produced promising results (2) and that deserves further investigation.Plasmid DNA encoding suitable antigens can readily and economically be constructed and produced in large quantities. The final product retains an intrinsic characteristic of DNA: stability. This is of particular interest for vaccines meant...

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