Partial activation of neonatal CD11c+ dendritic cells and induction of adult-like CD8+ cytotoxic T cell responses by synthetic microspheres

IL-12p40/IL-10 Producing preCD8α/Clec9A+ Dendritic Cells Are Induced in Neonates upon Listeria monocytogenes Infection

Infection by Listeria monocytogenes (Lm) causes serious sepsis and meningitis leading to mortality in neonates. This work explored the ability of CD11c^high lineage DCs to induce CD8⁺ T-cell immune protection against Lm in mice before 7 days of life, a period symbolized by the absence of murine IL-12p70-producing CD11c^highCD8α⁺ dendritic cells (DCs). We characterized a dominant functional Batf3-dependent precursor of CD11c^high DCs that is Clec9A⁺CD205⁺CD24⁺ but CD8α^- at 3 days of life. After Lm-OVA infection, these pre-DCs that cross-present Ag display the unique ability to produce high levels of IL-12p40 (not IL-12p70 nor IL-23), which enhances OVA-specific CD8⁺ T cell response, and regulatory IL-10 that limits OVA-specific CD8⁺ T cell response. Targeting these neonatal pre-DCs for the first time with a single treatment of anti-Clec9A-OVA antibody in combination with a DC activating agent such as poly(I:C) increased the protection against later exposure to the Lm-OVA strain. Poly(I:C) was shown to induce IL-12p40 production, but not IL-10 by neonatal pre-DCs. In conclusion, we identified a new biologically active precursor of Clec9A⁺ CD8α^- DCs, endowed with regulatory properties in early life that represents a valuable target to augment memory responses to vaccines.

Lm is a gram-positive food-borne pathogen that is the ethiological agent of listeriosis, a worldwide disease reported most frequently in developed countries. It can cause spontaneous septic abortions, fatal meningitis or encephalitis in immunocompromised and pregnant individuals. The murine model of systemic Lm infection has been demonstrated as a useful model to understand host resistance to intracellular pathogens. Neonates are highly susceptible to infections such as Lm, and display low responses to vaccines requiring IFN-γ producing T cells. In the present study, we characterized in murine neonates a precursor of conventional dendritic cells that is able to produce IL-12p40 and IL-10 cytokines and to modulate the development of the adaptive immune response, more particularly the CD8⁺ T cell response upon exposure to Lm. By targeting Lm-associated antigens to these conventional dendritic cell precursors in neonates, we succeeded to confer a partial protection to a lethal dose of Lm at the adult stage. Our study provides new insights into our understanding of the innate immune response to infections in early life and will help to design new vaccine strategies in newborns.

A liposome-based mycobacterial vaccine induces potent adult and neonatal multifunctional T cells through the exquisite targeting of dendritic cells

BACKGROUND

In the search for more potent and safer tuberculosis vaccines, CAF01 was identified as a remarkable formulation. Based on cationic liposomes and including a synthetic mycobacterial glycolipid as TLR-independent immunomodulator, it induces strong and protective T helper-1 and T helper-17 adult murine responses to Ag85B-ESAT-6, a major mycobacterial fusion protein. Here, we assessed whether these properties extend to early life and how CAF01 mediates its adjuvant properties in vivo.

METHODS/FINDINGS

Following adult or neonatal murine immunization, Ag85B-ESAT-6/CAF01 similarly reduced the post-challenge bacterial growth of M. bovis BCG, whereas no protection was observed using Alum as control. This protection was mediated by the induction of similarly strong Th1 and Th17 responses in both age groups. Multifunctional Th1 cells were already elicited after a single vaccine dose and persisted at high levels for at least 6 months even after neonatal priming. Unexpectedly, this potent adjuvanticity was not mediated by a massive targeting/activation of dendritic cells: in contrast, very few DCs in the draining lymph nodes were bearing the labeled antigen/adjuvant. The increased expression of the CD40 and CD86 activation markers was restricted to the minute portion of adjuvant-bearing DCs. However, vaccine-associated activated DCs were recovered several days after immunization.

CONCLUSION

The potent adult and neonatal adjuvanticity of CAF01 is associated in vivo with an exquisite but prolonged DC uptake and activation, fulfilling the preclinical requirements for novel tuberculosis vaccines to be used in early life.

Mucosally delivered Salmonella typhi expressing the Yersinia pestis F1 antigen elicits mucosal and systemic immunity early in life and primes the neonatal immune system for a vigorous anamnestic response to parenteral F1 boost

Neonates respond poorly to conventional vaccines. This has been attributed, in part, to the immaturity of neonatal dendritic cells that lack full capacity for Ag presentation and T cell stimulation. We engineered an attenuated Salmonella Typhi strain to express and export the F1 Ag of Y. pestis (S. Typhi(F1)) and investigated its immunogenicity early in life using a heterologous prime-boost regimen. Newborn mice primed intranasally with a single dose of S. Typhi(F1) elicited mucosal Ab- and IFN-gamma-secreting cells 1 wk after immunization. They also developed a potent and fast anamnestic response to a subsequent parenteral boost with F1-alum, which surpassed those of mice primed and boosted with S. Typhi(F1) or F1-alum. Neonatal priming with S. Typhi(F1), as opposed to priming with F1-alum, resulted in a more balanced IgG2a/IgG1 profile, enhanced avidity maturation and stimulation of B memory cells, and strong Th1-type cell-mediated immunity. S. Typhi(F1) enhanced the activation and maturation of neonatal CD11c+ dendritic cells, shown by increased expression of CD80, CD86, CD40, and MHC-II cell surface markers and production of proinflammatory cytokines IL-12, TNF-alpha, IL-6, and MCP-1. S. Typhi(F1)-stimulated neonatal DC had improved capacity for Ag presentation and T cell stimulation in vitro and induced F1-specific CD4+ and CD8+ T cell responses when adoptively transferred to newborn mice. Mucosal immunization with S. Typhi expressing a foreign Ag effectively primes the neonatal immune system for potent, fast, and broader responses to a parenteral Ag boost. Such a strategy can prevent infectious diseases, including those considered biowarfare threats, early in life.

Adult-like anti-mycobacterial T cell and in vivo dendritic cell responses following neonatal immunization with Ag85B-ESAT-6 in the IC31 adjuvant

Background

With the exception of some live vaccines, e.g. BCG, subunit vaccines formulated with “classical” adjuvants do not induce similar responses in neonates as in adults. The usual neonatal profile is characterized by lower levels of TH1-associated biomarkers. This has hampered the development of new neonatal vaccines for diseases that require early protection. Tuberculosis is one of the major targets for neonatal immunization. In this study, we assessed the immunogenicity of a novel candidate vaccine comprising a mycobacterial fusion protein, Ag85B-ESAT-6, in a neonatal murine immunization model.

Methods/Findings

The Ag85B-ESAT-6 fusion protein was formulated either with a classical alum based adjuvant or with the novel IC31® adjuvant. Following neonatal or adult immunization, 3 parameters were studied in vivo: (1) CD4⁺ T cell responses, (2) vaccine targeting/activation of dendritic cells (DC) and (3) protection in a surrogate mycobacterial challenge model. Conversely to Alum, IC31® induced in both age groups strong Th1 and Th17 responses, characterized by multifunctional T cells expressing IL-2 and TNF-α with or without IFN-γ. In the draining lymph nodes, a similarly small number of DC contained the adjuvant and/or the antigen following neonatal or adult immunization. Expression of CD40, CD80, CD86 and IL-12p40 production was focused on the minute adjuvant-bearing DC population. Again, DC targeting/activation was similar in adults and neonates. These DC/T cell responses resulted in an equivalent reduction of bacterial growth following infection with M. bovis BCG, whereas no protection was observed when Alum was used as adjuvant.

Conclusion

Neonatal immunization with the IC31®- adjuvanted Ag85B-ESAT-6 subunit vaccine elicited adult-like multifunctional protective anti-mycobacterial T cell responses through the induction of an adult pattern of in vivo DC activation.

Induction of protective immunity to Listeria monocytogenes in neonates

Neonates suffer unduly from infections and also respond suboptimally to most commonly used vaccines. However, a CD8 T cell response can be elicited in neonates if the Ag is introduced into the cytoplasm of APCs. Listeria monocytogenes (Lm) targets the cytoplasm of APC and is a strong CD8 and CD4 Th1-promoting vaccine vehicle in adult mice. We hypothesized that an attenuated strain of Lm would be safe and induce long-lasting protective immunity, even in neonates. We found that neonatal mice immunized only once with the attenuated strain DeltaactA-Lm developed robust primary and secondary CD8 and CD4 Th1 responses and were fully protected from lethal challenge with virulent wild-type Lm without the need for a booster immunization. Furthermore, DeltaactA-Lm expressing a heterologous recombinant Ag induced a strong CD8 and Th1 memory response to that Ag. Based on these data, we propose that DeltaactA-Lm or derivatives thereof might serve as a vaccine vehicle for neonatal immunization.

Protracted course of lymphocytic choriomeningitis virus WE infection in early life: induction but limited expansion of CD8+ effector T cells and absence of memory CD8+ T cells

Viral infections in human infants frequently follow a protracted course, with higher viral loads and delayed viral clearance compared to viral infections in older children. To identify the mechanisms responsible for this protracted pattern of infection, we developed an infant infection murine model using the well-characterized lymphocytic choriomeningitis virus (LCMV) WE strain in 2-week-old BALB/c mice. In contrast to adult mice, in which viral clearance occurred as expected 8 days after infection, LCMV titers persisted for several weeks after infection of infant mice. LCMV-specific effector CD8(+) T cells were elicited in infant mice and fully functional on day 7 but rapidly waned and could not be recovered from day 12 onwards. We show here that this results from the failure of LCMV-specific CD8(+) T cells to expand and the absence of protective LCMV-specific memory CD8(+) T cells. Under these early life conditions, viral control and clearance are eventually achieved only through LCMV-specific B cells that contribute to protect infant mice from early death or chronic infection.

Anthrax lethal toxin-mediated killing of human and murine dendritic cells impairs the adaptive immune response

Many pathogens have acquired strategies to combat the immune response. Bacillus anthracis interferes with host defenses by releasing anthrax lethal toxin (LT), which inactivates mitogen-activated protein kinase pathways, rendering dendritic cells (DCs) and T lymphocytes nonresponsive to immune stimulation. However, these cell types are considered resistant to killing by LT. Here we show that LT kills primary human DCs in vitro, and murine DCs in vitro and in vivo. Kinetics of LT-mediated killing of murine DCs, as well as cell death pathways induced, were dependent upon genetic background: LT triggered rapid necrosis in BALB/c-derived DCs, and slow apoptosis in C57BL/6-derived DCs. This is consistent with rapid and slow killing of LT-injected BALB/c and C57BL/6 mice, respectively. We present evidence that anthrax LT impairs adaptive immunity by specifically targeting DCs. This may represent an immune-evasion strategy of the bacterium, and contribute to anthrax disease progression. We also established that genetic background determines whether apoptosis or necrosis is induced by LT. Finally, killing of C57BL/6-derived DCs by LT mirrors that of human DCs, suggesting that C57BL/6 DCs represent a better model system for human anthrax than the prototypical BALB/c macrophages.

Dendritic cells (DCs) are specialized white blood cells that identify and present antigens to immune cells, T cells, in order to mount an immune response targeted against specific pathogens. DCs are critical to a host's defense against infection. Previous work has shown that the anthrax bacterium disables many immune cells, including DCs, through the action of a released toxin, lethal toxin. Here the authors show that lethal toxin efficiently kills both human and murine DCs. The means by which DCs were killed by the anthrax toxin were notably distinct and dependent on their genetic background. Human DCs, as well as those derived from the murine strain C57BL/6, died over the course of 72 h via activation of apoptosis, or programmed cell death. DCs from BALB/c mice, however, died rapidly of a necrotic cell death following toxin exposure. As human and C57BL/6 DCs share an identical response to anthrax toxin, C57BL/6 mice appear to provide an excellent model for human anthrax. The study's findings suggest that specific targeting of DCs by the anthrax toxin impairs the immune response of the infected host, and the authors believe that this strategy promotes spread of the bacterium and disease progression.