Relationship between tumour resistance and the in vitro and in vivo cytotoxicity elicited by Salmonella antigens in immunized mice.

Immunization of mice with the attenuated 11RX strain of Salmonella enteritidis (11RX) induces resistance to intraperitoneal (i.p.) tumour growth. Tumour resistance is much greater and lasts for a longer time following i.p. immunization than following intravenous (i.v.) immunization. This paper extends our previous observations that, after this resistance is lost, it can be recalled by a T cell-mediated reaction to an antigenic extract of the bacteria (11RX antigen) which is not protective in unimmunized mice. The duration of this sensitization to 11RX antigen was determined in mice immunized i.p. or i.v. with live 11RX by challenging them at various times after immunization with 131I (or 125I)-labelled Ehrlich ascites tumour (EAT) cells alone or mixed with 11RX antigen. In vivo killing of EAT cells was assayed by monitoring whole-body retention of radioactivity and this was correlated in the same mice with suppression of tumour growth and survival of the mice. The resistance recalled by 11RX antigen was short-lived and in vivo cytotoxic activity had subsided by 6 days after antigen injection. 11RX antigen also recalled the ability of the peritoneal cells to lyse 51Cr-labelled EAT cells in vitro and a close correlation was found between this activity and the cytotoxicity measured in vivo. The adherence properties of the cytotoxic cells and their inhibition by trypan blue indicated that they were macrophages.