Prolongation of acquired cellular resistance to Listeria monocytogenes

Intracutaneous immunization of mice with 10⁵ or 10⁶ viable listeria resulted in acquired cellular resistance (ACR) of short duration (7 days) and in delayed-type hypersensitivity (DH) lasting at least 27 days. The ACR was partially non-specific, as 50% of the mice were also protected against a lethal challenge with Salmonella enteritidis. The specific element of the ACR could be transferred by non-adherent spleen cells from immune mice to normal recipient mice. Such transfer was not possible with adherent spleen cells from immune mice or with spleen cells from normal mice.

Two systems of multiple immunizations to extend the period during which mice were protected against a challenge with 50 LD₅₀ listeria were used. In the first system, mice were immunized with 10⁶ viable listeria and subsequently challenged with 50 LD₅₀ (= 10⁷) viable listeria. Mice surviving the challenge were actually boosted at the challenge injection for ACR. In the second system mice were immunized and boosted with 10⁸ killed listeria mixed with the adjuvant dimethyl dioctadecyl ammonium bromide (DDA). In the former system after each booster injection with viable listeria the interval during which the mice were protected doubled and reached a maximum of 31 days. In the latter system all intervals between two booster injections were equally long and never exceeded 28 days.

In both systems the existence of immunological memory was suggested. The difference in results obtained after immunization with viable listeria and killed listeria mixed with DDA are discussed.