Heat-killed Listeria monocytogenes and L. monocytogenes soluble antigen induce clonable CD4+ T lymphocytes with protective and chemotactic activities in vivo.

In the present study we attempted to analyze the possibility to induce in mice a T-cell response using killed Listeria monocytogenes in adjuvant. Clearly, nonviable antigen is capable of inducing protective and granuloma-forming T cells in C57BL/6 mice when emulsified in complete Freund's adjuvant. These T cells were cloned in vitro by using antigen and irradiated splenocytes, as antigen-presenting cells, and the clones were characterized in vivo. Listeria-specific T-cell clones showed protective and chemotactic activity upon adoptive transfer, although some degree of functional heterogeneity among different clones was observed. The heterogeneous in vivo functions could not be correlated with the ability of the clones to produce gamma interferon or T-cell growth factor (interleukin-2 and/or interleukin-4). It was demonstrated that an in vivo relevant fraction of listeria-specific T lymphocytes can be induced by nonviable antigen in complete Freund's adjuvant. These results show that the low immunogenicity of heat-killed bacteria is not due to the expression of specific protective T-cell epitopes only by live bacteria.