Immunostimulatory Effects Triggered by Enterococcus faecalis CECT7121 Probiotic Strain Involve Activation of Dendritic Cells and Interferon-Gamma Production.

Probiotics can modulate the immune system, conferring beneficial effects on the host. Understanding how these microorganisms contribute to improve the health status is still a challenge. Previously, we have demonstrated that Enterococcus faecalis CECT7121 implants itself and persists in the murine gastrointestinal tract, and enhances and skews the profile of cytokines towards the Th1 phenotype in several biological models. Given the importance of dendritic cells (DCs) in the orchestration of immunity, the aim of this work was to elucidate the influence of E. faecalis CECT7121 on DCs and the outcome of the immune responses. In this work we show that E. faecalis CECT7121 induces a strong dose-dependent activation of DCs and secretion of high levels of IL-12, IL-6, TNFα, and IL-10. This stimulation is dependent on TLR signaling, and skews the activation of T cells towards the production of IFNγ. The influence of this activation in the establishment of Th responses in vivo shows the accumulation of specific IFNγ-producing cells. Our findings indicate that the activation exerted by E. faecalis CECT7121 on DCs and its consequence on the cellular adaptive immune response may have broad therapeutic implications in immunomodulation.

In vivo targeting of human DC-SIGN drastically enhances CD8⁺ T-cell-mediated protective immunity.

Vaccination is one of the oldest yet still most effective methods to prevent infectious diseases. However, eradication of intracellular pathogens and treatment of certain diseases like cancer requiring efficient cytotoxic immune responses remain a medical challenge. In mice, a successful approach to induce strong cytotoxic CD8⁺ T-cell (CTL) reactions is to target antigens to DCs using specific antibodies against surface receptors in combination with adjuvants. A major drawback for translating this strategy into one for the clinic is the lack of analogous targets in human DCs. DC-SIGN (DC-specific-ICAM3-grabbing-nonintegrin/CD209) is a C-type lectin receptor with potent endocytic capacity and a highly restricted expression on human immature DCs. Therefore, DC-SIGN represents an ideal candidate for DC targeting. Using transgenic mice that express human DC-SIGN under the control of the murine CD11c promoter (hSIGN mice), we explored the efficacy of anti-DC-SIGN antibodies to target antigens to DCs and induce protective immune responses in vivo. We show that anti-DC-SIGN antibodies conjugated to OVA induced strong and persistent antigen-specific CD4⁺ and CD8⁺ T-cell responses, which efficiently protected from infection with OVA-expressing Listeria monocytogenes. Thus, we propose DC targeting via DC-SIGN as a promising strategy for novel vaccination protocols against intracellular pathogens.

Targeting DC-SIGN via its neck region leads to prolonged antigen residence in early endosomes, delayed lysosomal degradation, and cross-presentation.

Targeting antigens to dendritic cell (DC)-specific receptors, such as DC-SIGN, induces potent T cell-mediated immune responses. DC-SIGN is a transmembrane C-type lectin receptor with a long extracellular neck region and a carbohydrate recognition domain (CRD). Thus far, only antibodies binding the CRD have been used to target antigens to DC-SIGN. We evaluated the endocytic pathway triggered by antineck antibodies as well as their intracellular routing and ability to induce CD8(+) T-cell activation. In contrast to anti-CRD antibodies, antineck antibodies induced a clathrin-independent mode of DC-SIGN internalization, as demonstrated by the lack of colocalization with clathrin and the observation that silencing clathrin did not affect antibody internalization in human DCs. Interestingly, we observed that anti-neck and anti-CRD antibodies were differentially routed within DCs. Whereas anti-CRD antibodies were mainly routed to late endosomal compartments, anti-neck antibodies remained associated with early endosomal compartments positive for EEA-1 and MHC class I for up to 2 hours after internalization. Finally, cross-presentation of protein antigen conjugated to antineck antibodies was approximately 1000-fold more effective than nonconjugated antigen. Our studies demonstrate that anti-neck antibodies trigger a distinct mode of DC-SIGN internalization that shows potential for targeted vaccination strategies.