Delayed-type Hypersensitivity and Acquired Resistance to Plague in Mice Immunized with Killed Yersinia Pestis and Immunoregulators

A simple reproducible footpad assay of delayed-type hypersensitivity (DTH) to Yersinia pestis was studied in mice immunized with 5 x 10(7) heat killed (HK) Y. pestis alone or in conjunction with Freund's complete adjuvant (FCA), cyclophosphamide (CY), BCG, or both CY and BCG. The footpad reaction elicited with 5 x 10(6) HK Y. pestis was shown to peak at 24 hr, was transferable with immune T lymphocytes but not with serum and had the classical DTH histology, The highest level of DTH occurred 6 or 8 days after immunization, according to the immunomodulator used, and immunized mice were therefore challenged with viable Y. pestis at this time. No correlation was found between DTH level and resistance of mice; only groups immunized under the potentiating effect of BCG used alone or with CY were significantly protected against Y. pestis. Control groups pretreated with BCG or BCG pretreated and immunized with a large dose of an unrelated gram negative bacterium were as susceptible to Y. pestis infection as normal mice. To promote this protective immunity, BCG and HK Y. pestis, must be injected in an area that drains to a common lymph node. No antibody to Y. pestis was found in immunized protected mice pretreated with BCG and CY and a very low antibody titre was found in immunized mice pretreated only with BCG. This serum was unable to confer resistance on recipient mice but immune cell transfer decreased the number of Y. pestis counted in the spleens of recipients 36 hr after challenge.