Recombinant attenuated Salmonella harboring 4-1BB ligand gene enhances cellular immunity

To transfect antigen presenting cells (APCs) with 4-1BB ligand DNA by attenuated Salmonella enterica serovar Typhimurium in vivo, and to observe the effects of ectogenous 4-1BBL on the immune functions of infected rats.

Attenuated Salmonella typhimurium (vaccine strain) carrying plasmids pIRES2-EGFP-4-1BBL was constructed and used to infect HepG2 hepatoma cells. The expression of reporter gene, green fluorescent protein (GFP) and rat 4-1BBL in the transfected cells was detected by double-immunofluorescence staining. Rats were fed with the recombinant bacteria intragastrically on three occasions in 2 weeks, and were then sacrificed. The transcription and expression of GFP and 4-1BBL genes in splenocytes were measured by RT-PCR and flow cytometry. The phenotypes of T cells in peripheral blood and splenocytes were determined by flow cytometry. The content of IFN-gamma in the cultural supernatant of splenocytes stimulated by PHA was measured by ELISA.

The recombinant bacteria harboring 4-1BBL had the same invasive abilities as the original bacteria, and it was able to deliver exogenous genes into HepG2 cells, where the GFP and 4-1BBL were successfully expressed. There were significant upregulations of CD3(+)CD8(+) T cells (P=0.018) and CD3(+)CD25(+) T cells (P=0.019) in the peripheral blood cells as well as CD3(+)CD8(+) T cells (P=0.022), and CD3(+)CD25(+) T cells (P=0.008) in splenocytes of the infected rats. The rats had more 4-1BBL expression detected in the spleen. IFN-gamma released by PHA-stimulated splenocytes increased significantly by the recombinant bacteria as compared with controls (P=0.002).

Salmonella serovar Typhimurium containing 4-1BBL can transfect target genes into antigen presenting cells in vivo, and the expression of exogenous 4-1BBL enhances cellular immunity markedly.