Regulated Delayed Expression of rfaH in an Attenuated Salmonella enterica Serovar Typhimurium Vaccine Enhances Immunogenicity of Outer Membrane Proteins and a Heterologous Antigen

RfaH is a transcriptional antiterminator that reduces the polarity of long operons encoding secreted and surface-associated cell components of Salmonella enterica serovar Typhimurium, including O antigen and lipopolysaccharide core sugars. A Delta rfaH mutant strain is attenuated in mice (50% lethal dose [LD50], > 10(8) CFU). To examine the potential for using rfaH in conjunction with other attenuating mutations, we designed a series of strains in which we replaced the native rfaH promoter with the tightly regulated arabinose-dependent araC P-BAD promoter so that rfaH expression was dependent on exogenously supplied arabinose provided during in vitro growth. Following colonization of host lymphoid tissues, where arabinose was not available, the P-BAD promoter was no longer active and rfaH was not expressed. In the absence of RfaH, O antigen and core sugars were not synthesized. We constructed three mutant strains that expressed different levels of RfaH by altering the ribosome-binding sequence and start codon. One mutation, Delta P-rfaH178, was introduced into the attenuated vaccine strain chi 9241 (Delta pabA Delta pabB Delta asdA) expressing the pneumococcal surface protein PspA from an Asd(+) balanced-lethal plasmid. Mice immunized with this strain and boosted 4 weeks later induced higher levels of serum immunoglobulin G specific for PspA and for outer membrane proteins from other enteric bacteria than either an isogenic Delta rfaH derivative or the isogenic RfaH(+) parent. Eight weeks after primary oral immunization, mice were challenged with 200 LD50 of virulent Streptococcus pneumoniae WU2. Immunization with Delta P-rfaH178 mutant strains led to increased levels of protection compared to that of the parent chi 9241 and of a Delta rfaH derivative of chi 9241.