Studies of immunity and bacterial invasiveness in mice given a recombinant Salmonella vector encoding murine interleukin-6

Interleukin-6 (IL-6) was expressed in Salmonella typhimurium in an attempt to increase the mucosal immune response against the bacterium. Murine IL-6 was PCR amplified from cDNA, cloned, sequenced, and found to be functionally active when expressed in S. typhimurium BRD509, the Delta aroA Delta aroD vaccine strain. Expression of murine IL-6 did not appear to adversely affect the growth of salmonellae, as the construct was retained in the absence of antibiotic selection and the growth rate was unaffected compared with that of the parent strain in vitro. However, IL-6 expression led to a significant reduction in bacterial invasiveness in vitro and in vivo. Splenocytes and small intestinal lamina propria lymphocytes mere isolated from mice orally immunized with BRD509 expressing IL-6 (pKK233-2/IL-6), and the number of antibody-secreting cells was determined by the ELISPOT technique. No differences were observed between mice immunized with BRD509(pKK233-2/IL-6) and those immunized with BRD509(pKK233-2) with respect to the antibody subclass-specific responses elicited despite the markedly reduced invasiveness of the former. Serum antibody responses were also examined by a kinetic enzyme-linked immunosorbent assay (ELISA), and equivalent levels of antibody response were detected in mice given BRD509(pKK233-2/IL-6) and those given BRD509(pKK233-2). The humoral immune response against bacterial lipopolysaccharides was also examined in transgenic IL-6-deficient mice given oral inocula of BRD509. Equivalent numbers of antibody-secreting cells (ELISPOTs) were observed in the spleens and laminae propriae of both IL-6-deficient (-/-) mice and control (+/+) mice harboring an intact IL-6 gene, whereas small, yet significant differences in the serum immunoglobulin A ELISA titers were observed. These data suggest that the immunoglobulin A response against Salmonella lipopolysaccharides is largely IL-6 independent.