Effective adjuvants for the induction of antigen-specific delayed-type hypersensitivity

Vaccines utilizing poorly immunogenic subunit antigens are dependent upon adjuvants to drive the appropriate T cell responses. In an effort to determine the ability of several adjuvants to promote cell-mediated immunity (CMI), we assessed delayed-type hypersensitivity (DTH) in mice inoculated with heat-killed Listeria monocytogenes (HKLM) vaccines. The vaccines were formulated as oil-in-water emulsions containing one or more of the following bacterial-derived immunostimulators: MPL immunostimulant, a monophosphoryl lipid A preparation, synthetic trehalose dicorynomycolate (TDCM) and Mycobacterium phlei cell wall skeleton (CWS). Oil-in-water emulsions containing HKLM without adjuvants did not induce DTH responsiveness in mice. The incorporation of TDCM, or MPL plus TDCM and/or CWS to the formulation enabled the HKLM vaccine to stimulate CMI characterized by DTH responsiveness. Following antigen challenge the resulting increases in footpad thickness ranged from 15-20% and were comparable to the DTH driven by complete Freund's adjuvant. Adjuvants composed of MPL/TDCM and MPL/TDCM/CWS induced responses equivalent to those measured in mice immunized with viable L. monocytogenes, and the responses remained at these levels for at least 2 months. Furthermore, in vivo depletion of CD4+ T cells, but not CD8+ T cells, abrogated the induction and expression of DTH, indicating that the response is mediated by CD4+ T cells.