Establishment and comparison of delayed-type hypersensitivity models in the B₆C₃F₁ mouse

The objective of these studies was to establish and compare delayed-type hypersensitivity (DTH) models, using keyhole limpet hemocyanin (KLH), sheep red blood cells (SRBC), and Candida albicans as sensitizing antigens, for their capability to assess a DTH response (utilizing footpad swelling as the endpoint) with minimal confounding factors resulting from antigen-specific antibody (Ab) production. The key elements of the DTH are the sensitization dose, time interval between sensitization and challenge [i.e. the challenge interval (CI)], and the challenge dose. Models were established by first determining the challenge dose, or the amount of antigen that produced no greater footpad swelling 24-h post-injection than the trauma induced by injection of physiological saline. Time-course studies determined the CI that produced a peak response for each antigen. Dose-response sensitization studies were conducted to determine the optimum sensitization concentration (i.e. maximum footpad swelling with minimal impact by antigen-specific Ab production). Footpad swelling decreased dose-responsively with increasing KLH sensitization concentration and corresponded to a dose-responsive increase in KLH-specific Ab levels. In the SRBC model, footpad swelling decreased at the high dose (1 x 10⁹ SRBC/mouse), and a corresponding increase in SRBC-specific Ab was observed at this dose level. A dose-responsive increase in footpad swelling was observed in the C. albicans model up to 3 x 10⁷ organisms/mouse, while antigen-specific antibody levels were not different from background (unsensitized) levels following sensitization with any concentration of C. albicans (up to 1.2 x 10⁸ organisms/mouse, the highest concentration tested). Finally, each model was evaluated for its ability to detect immunosuppression following exposure to benzo[a]pyrene (B[a]P), with the C. albicans model demonstrating greater sensitivity than the other models. These results indicate that, of the three models examined here, the C. albicans DTH model may be the most appropriate model for evaluating effects on cell-mediated immunity when conducting immunotoxicological investigations.