nature publishing group ARTICLES Neonatal mucosal immunization with a non-living, non-genetically modified Lactococcus lactis vaccine carrier induces systemic and local Th1-type immunity and protects against lethal bacterial infection K R a m i r e z 1,4 , Y D i t a m o 1,4 , L R o d r i g u e z 1 , W L P i c k i n g 2 , M L v a n R o o s m a l e n 3 , K L e e n h o u t s 3 a n d M F P a s e t t i 1 Safe and effective immunization of newborns and infants can significantly reduce childhood mortality, yet conventional vaccines have been largely unsuccessful in stimulating the neonatal immune system. We explored the capacity of a novel mucosal antigen delivery system consisting of non-living, non-genetically modified Lactococcus lactis particles, designated as Gram-positive enhancer matrix (GEM), to induce immune responses in the neonatal setting. Yersinia pestis LcrV, used as model protective antigen, was displayed on the GEM particles. Newborn mice immunized intranasally with GEM-LcrV developed LcrV-specific antibodies, Th1-type cell-mediated immunity, and were protected against lethal Y. pestis (plague) infection. The GEM particles activated and enhanced the maturation of neonatal dendritic cells (DCs) both in vivo and in vitro . These DCs showed increased capacities for secretion of proinflammatory and Th1-cell polarizing cytokines, antigen presentation and stimulation of CD4 + and CD8 + T cells. These data show that mucosal immunization with L. lactis GEM particles carrying vaccine antigens represents a promising approach to prevent infectious diseases early in life. INTRODUCTION due to intrinsic deficiencies of the neonatal immune system, One of the major challenges in vaccinology is the development which has a fully constituted T-cell repertoire and is capable of of safe and effective vaccines that can protect newborns and responding to antigens, but mainly to the presence of imma- infants against infectious diseases. Conventional vaccines are ture or “ inexperienced ” immune cells, particularly dendritic notoriously poorly immunogenic during the first months of cells (DCs), which have a limited capacity for antigen presenta- life, inducing immune responses that are short-lived and heav- tion and stimulation of naive T cells. 1,3 – 5 The field of neonatal ily Th2 biased. Th1-type cell-mediated immunity is modest or vaccinology has experienced unprecedented progress in recent absent, and the combination of these factors heightens the risk years, and the literature supports the assertion that newborns are of intracellular bacterial infections. 1 – 3 Even though routine indeed capable of mounting potent adaptive immunity, includ- immunization of human infants starts at 6 to 12 weeks of age, ing adult-like Th1-type immune responses to vaccine antigens, several booster doses are needed to achieve protective immunity. provided that these antigens are administered with the appropri- Mounting evidence indicates that these poor responses are not ate stimulatory signals. 1 – 3,5 – 7 1 Department of Pediatrics, Center for Vaccine Development, University of Maryland School of Medicine , Baltimore , Maryland , USA . 2 Department of Microbiology and Molecular Genetics, Oklahoma State University, Stillwater, Oklahoma , USA . 3 Mucosis BV, Groningen , The Netherlands . Correspondence: MF Pasetti ( [email protected]) 4 These authors contributed equally to the study. Received 27 July 2009; accepted 16 October 2009; published online 18 November 2009. doi:10.1038/ mi.2009.131 MucosalImmunology | VOLUME 3 NUMBER 2 | MARCH 2010 159 ARTICLES In this study, we examined the possibility of priming the neo- activate and enhance the functional capacity of neonatal DCs. natal immune system through mucosal immunization using a The expression of activation and maturation cell-surface mark- novel antigen delivery system consisting of non-living, non- ers CD80, CD86, CD40, and major histocompatibility com- genetically modified cell wall particles derived from Lactococcus plex (MHC)-class II (I-Ad) was measured on bone marrow lactis . These particles, referred to as Gram-positive enhancer (BM)-derived CD11c + cells from newborn (7-day-old) mice matrix (GEM) particles, are produced by the heat-acid treat- stimulated with L. lactis GEM particles or mock-stimulated ment of freshly grown L. lactis , a process that removes the DNA cells (Figure 1a ). To determine the strength of the activation of and most of the bacterial proteins, but leaves the peptidoglycan GEM-stimulated neonatal DCs in comparison with that of adult (PGN) envelope intact.8,9 Multiple antigens can be displayed DCs, BM-derived CD11c + cells from 6- to 8-week-old mice on the particle surface using recombinant protein and affinity were included in all experiments. All markers were upregulated technology: recombinant fusion proteins containing a vaccine in neonatal and adult DCs after GEM stimulation, compared antigen and a PGN protein anchor domain that binds with high with the mock-treated DCs ( Figure 1a ). Neonatal GEM- affinity to the bacterial PGN are produced in a suitable host cell, exposed DCs exhibited a noticeable increase in the expression purified, and then combined with naked GEM particles. The of CD86, while both CD86 and MHC-II were the markers most recombinant proteins bind tightly to the GEMs, creating anti- abundantly expressed on adult GEM-stimulated DCs. A sum- gen-displaying particles. A major advantage of the GEM parti- mary of the increases in the expression of cell-surface markers cles for neonatal immunization is their safety profile. L. lactis is a in both neonatal and adult DC exposed to the L. lactis GEMs non-pathogenic Gram-positive, lactic acid bacterium, generally or to Escherichia coli lipopolysaccharide (LPS) (used as positive recognized as safe (GRAS) and widely used in dairy products. control) is shown in Table 1 . It is noted that the upregulation of Probiotics have been safely given to newborns, 10 young children MHC-II and costimulatory molecules in both neonatal and adult in day care,11 and even critically ill children. 12 Unlike recom- GEM-stimulated DCs was remarkably similar to that induced binant live organisms, including attenuated pathogens, the by LPS under the same experimental conditions. L. lactis GEM particles do not contain DNA, i.e., there is no risk of potential reversion to a virulent form. The composition of the DC stimulated with L. lactis GEMs secreted proinflammatory GEMs also contributes to their immune-stimulating properties. cytokines As spherical particles, the GEMs can be efficiently taken up by To assess the functional capacity of the GEM-activated DCs, M cells in the epithelium above the mucosal lymphoid follicles, we examined their ability to secrete proinflammatory, Th1-pro- and the transported antigens can be delivered directly to the moting and regulatory cytokines, as these are all critical signals underlying DCs in mucosal inductive sites. Furthermore, the required for the development of an effective adaptive immune PGN envelope is a potent stimulator of innate immunity.13 response. The levels of interleukin (IL)-12p70, tumor necrosis We used Yersinia pestis LcrV as a model vaccine antigen to factor (TNF)- , IL-10, IL-6, IFN- , and MCP-1 were measured demonstrate the feasibility of successful early life immuniza- in culture supernatants from neonatal and adult BM-derived tion using the GEM platform technology. The immunogenicity CD11c + cells treated with L. lactis GEMs or LPS, or from and protective efficacy of L. lactis GEM particles displaying mock-treated cells. For all of the cytokines tested, both neona- Y. pestis LcrV was investigated in a neonatal mouse model. We tal and adult GEM-stimulated DCs had an increased capacity showed, for the first time, that intranasal immunization of new- for cytokine production as compared with mock-treated cells born mice with GEM-LcrV elicits a potent mucosal and systemic ( Figure 1b ). The levels of cytokines produced by both adult and immunity that protects against lethal systemic plague infection. neonatal mouse DCs stimulated with the L. lactis GEM parti- We also demonstrated that the L. lactis GEM particles enhance cles were comparable, suggesting that neonatal GEM-activated the maturation of neonatal CD11c + DC, and that these cells DC can reach a degree of functional activity similar to that of have increased capacity for secretion of proinflammatory and adult mature DCs. Interestingly, the fold-increases for TNF- Th1-type-promoting cytokines and can stimulate antigen-spe- and IL-6 were much higher in DC from the newborns regard- cific interferon (IFN)- -secreting CD4 + T cells. Furthermore, less of whether they were stimulated with the GEMs or LPS we showed that the GEM particles were taken up by DCs from ( Table 1 ). human newborns and that these cells also acquired a mature phenotype such that they were able to stimulate human T cells. GEM-exposed mature DCs had reduced capacity for antigen Together, these results indicate that mucosally delivered antigen- uptake displaying L. lactis GEM particles represent a highly promising During the process of maturation, DCs relent their endo- vaccine approach for immunization early in life. cytic and phagocytic activity as they become more efficient antigen-presenting cells. 14 Accordingly, we examined the RESULTS capacity of GEM-stimulated DCs to incorporate fluorescein L. lactis GEM particles induced maturation of neonatal and isothiocyanate (FITC)-dextran as a measure of antigen adult mouse DCs uptake. Both