Enteric Immunization Induces Active Mucosal and Systemic Immunity in Neonatal Ruminants

Abstract

Neonatal animals are highly susceptible to infectious diseases, many of which gain entry through mucosal surfaces. Although immunization is a cost-effective way of controlling newborn disease, vaccination of newborn ruminants is not common. Vaccines are thought to be ineffective in newborn ruminants due to interference by maternal antibodies and a less reactive, immature immune system. These conclusions were based on observations made following the use of vaccines injected parenterally. However, recent work at VIDO indicates that oral vaccination of newborns is successful.

The overall objective of our study was to determine if enteric immunization of newborn ruminants would induce active local and systemic immune responses. Our vaccine was comprised of the Human Adenovirus 5 vector with the gene for the gD antigen of Bovine Herpes virus-1 inserted in the E3 region (HAd5-gD/E3). Our animal model was lambs in which gut-loops were surgically prepared in utero that contained either ileal or jejunal Peyer's patches. Mucosal and systemic immune responses were evaluated by determination of gD-specific antibody secreting cells (ASC), interferon gamma secreting cells (IFN-γ SC), and lymphocyte proliferation response (LPR) in the gut, lung lymph node (LN) and spleen. Specific antibodies were also evaluated in intestinal contents and in the serum.

To determine if gut-associated lymphoid tissues (GALT) was immunocompetent in ruminants we compared immune responses in newborn lambs with responses in 5-6 weeks old lambs. Similar mucosal and systemic responses were detected in the two groups of lambs with regard to ASC, IFN-γ SC, LPR and antibody in intestinal contents and in serum. Thus, GALT is immunocompetent in ruminants at birth.

To determine if maternal antibody specific for vaccine antigen interferes with induction of mucosal and systemic responses in neonatal lambs, we compared immune responses of lambs fed colostrum from immunized ewes with the responses of lambs fed colostrum from nonimmunized ewes. Similar mucosal (LPR, IFN-γ SC and ASC in gut) and systemic (LPR and IFN-γ SC in spleen) responses were found in both groups of lambs. This indicated that maternal antibody did not interfere with induction of immunity by the adenovirus vaccine vector.

To determine if enteric immunization of neonates induced long term mucosal memory in the respiratory system, groups of neonatal lambs were immunized with HAd5-gD/E3 either in gut-loops or intradermally. Secondary immunization with gD in liposomes was done intratracheally after 5 months. There were strong immune responses in the regional lymph nodes (LPR, IFN-γ SC, ASC) and in the spleen (LPR, IFN-γ SC) of both groups of lambs. Interestingly, gD-specific ASC were present in lung LN only in lambs that had a primary oral immunization.

We conclude that mucosal and systemic immunity was induced in neonates following enteric immunization with an adenovirus vector. Maternal antibody specific for the vaccine antigen did not appear to interfere with the development of the immune responses. In addition, enteric immunization induced long term memory. Oral immunization has the potential to protect against respiratory pathogens.