Mucosal vaccination of the newborn: an unrealized opportunity

Vaccination of the newborn has been viewed as a challenge for a wide variety of reasons. The immature state of the neonatal immune system is frequently cited as a reason why vaccination cannot induce protective immune responses and is the basis for vaccination protocols that recommend multiple immunizations. Studies in newborn mice established that the neonatal immune response is biased and unable to produce a full range of critical immune regulatory cytokines [1]. Assays with blood leukocytes isolated from newborn human infants also supported the hypothesis that the neonatal immune system is functionally immature [2]. Recent studies, using potent immune adjuvants, contradict previous conclusions and indicate the neonatal immune system has a much greater capacity to respond to vaccines than previously reported [3].

Neonatal vaccination has also been contraindicated due to possible interference by maternal antibody [3,4]. This has been a major issue for vaccines injected parenterally and was a concern with killed or inactivated vaccines and modified-live vaccines. Many studies reported an absence of detectable antibody responses following vaccination and this was interpreted as evidence for vaccine failure. More recent studies have analyzed T-cell responses and revealed that immunization, in the face of maternal antibody, does induce antigen-specific responses and immune memory [5]. We are, therefore, being challenged to reconsider concepts regarding neonatal immune competence and vaccination strategies for a population that is highly susceptible to infectious disease.

Our recent investigations of in utero vaccination in sheep confirmed that the fetal immune system can respond to DNA vaccination with protective immune responses and immune memory [6,7]. Furthermore, enteric immunization of newborn lambs with a viral vector revealed no significant differences in mucosal immune responses when compared with older animals [8]. These results may not be all that surprising when viewed from the perspective of the poultry industry, which routinely practices in ovo vaccination [9]. Based on our in utero and neonatal vaccination studies, we propose that a single mucosal vaccination of newborn infants or newborn animals can induce protection and eliminate many of the problems associated with injected vaccines.

Characterization of the mucosal immune system has been based primarily upon studies performed in rodents. The defining characteristics of the mucosal immune system include a significant role for immunoglobulin A (IgA) in preventing viral and bacterial attachment [10–12] and a specific role for mucosal-associated lymphoid tissues (MALTs), such as Peyer’s patches (PPs), to function as immune induction sites [13,14]. Immune effector cells generated within MALT preferentially home to mucosal surfaces throughout the body [15,16] and this selective homing is referred to as, ‘the common mucosal immune system.’ The concept of such a system explains why mucosal vaccination provides better immune protection against mucosal infections than parenteral vaccination [17]. These key characteristics of the mucosal immune system are conserved among all mammalian species.

The mouse model has, however, failed to adequately instruct us in the development, distribution and function of the neonatal mucosal immune system. MALT in the murine small intestine is not active until 5–6 weeks of age [18] and PPs are few in number and completely fail to develop in the absence of gut microflora [19]. In contrast, MALT in the upper respiratory tract and intestine of humans [20] and most domestic species displays extensive fetal and neonatal development in the complete absence of commensal microflora [21]. Furthermore, our investigations confirmed that fetal and neonatal MALT in the sheep upper respiratory tract and intestine respond to mucosal vaccination and provide both disease protection and immune memory [6–8]. These studies provided the scientific impetus for the development of the first commercial modified-live virus intranasal vaccine for newborn calves [22].

Mucosal vaccination faces numerous challenges. The rapid degradation of proteins and high transit rates have limited oral vaccine delivery, and identifying safe and effective mucosal adjuvants in clinically relevant species also remains a challenge [23]. Our research has confirmed, however, that, with appropriate vaccine-delivery systems, such as DNA vaccines or modified-live viral vectors, it is possible to induce protective immune responses in the neonate following a single vaccination [6–8]. Furthermore, these studies demonstrated that, despite transferring high levels of maternal antibody specific for the vaccine antigen, there was no interference with either mucosal or systemic immune responses [8]. Targeting vaccine delivery to either the upper respiratory tract (intranasal) or the small intestine may be critical since these are sites where there is limited transport of maternal antibody to the mucosal surface.

Disease protection is often dependent upon both humoral and cell-mediated effector mechanisms, as well as immune memory, which accelerates the immune response following infection. There has been concern that mucosal immune memory may not persist as long as systemic memory [24]. Our studies with parenteral vaccination in neonatal lambs demonstrated that both the magnitude and duration of a primary immune response contributes significantly to the duration of immune memory [25]. Therefore, the use of vaccine-delivery technologies, such as viral vectors, which amplify the quantity of vaccine antigen and the duration of antigenic stimulation, may be critical for rapid onset of disease protection and prolonged immune memory following a single mucosal vaccination. We have confirmed that a single enteric immunization of the neonate with a viral vector can induce immune memory that persists for at least 6 months [8].

The concept that the neonate is not fully immune competent has been used to argue that modified-live viral or bacterial vaccines may pose an increased risk with the potential for reversion to virulence. Our observation that the mucosal immune system is well developed in the neonate would suggest that this potential danger has been overestimated, especially if using vectors with a tropism restricted to mucosal epithelium. There may, however, be a potential risk if using viral or bacterial vectors for oral immunization of the neonate. Our analysis of intestinal MALT function in the neonate has revealed that PPs in the ileum or terminal small intestine do not function as efficient immune induction sites [26]. Furthermore, there is a possible association between ileal PPs and the development of inflammatory bowel disease in humans [27]. Therefore, further investigations are required to determine whether targeting viral or bacterial vectors to the neonatal ileal PP may interfere with the induction of protective mucosal immune responses or predispose individuals to inflammatory bowel disease.

In conclusion, mucosal surfaces provide the largest interface between our body and the environment and a primary route of entry for infectious agents. Over 90% of infectious agents utilize mucosal epithelia as portals of entry. The challenge for mucosal immune systems is to prevent invasion without disrupting key biological functions. Failure to achieve this delicate balance frequently occurs during the neonatal period, resulting in severe diarrhea or respiratory disease. Our research has clearly demonstrated that the newborn MALT in both the upper respiratory tract and intestine has the capacity to induce both protective immune responses and immune memory [6–8]. Mucosal vaccination of the newborn, using appropriate vaccine technologies, offers the opportunity to achieve immune protection of the neonate following a single vaccination. Mucosal delivery would reduce the risk of vaccine interference by maternal antibody and eliminate iatrogenic disease transmission, which can occur when using needles. Furthermore, this noninvasive route of vaccine delivery would enhance patient and client compliance by eliminating the fear associated with injected vaccines and may facilitate selfvaccination. Animal health companies have taken the lead in developing mucosal vaccines for the newborn and are providing further evidence that more appropriate animal models are required to fully evaluate the potential risks and benefits of mucosal vaccination in newborn infants.

Financial & competing interests disclosure

Research was supported by funding from the Saskatchewan Health Research Foundation, Alberta Agriculture Research Institute and Canadian Institutes for Health Research. The author has no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.