Endocytosis Deficient Murine Xcl1-Fusion Vaccine Enhances Protective Antibody Responses in Mice

Molecular basis for chemokine recognition and activation of XCR1

The X-C motif chemokine receptor XCR1, which selectively binds to the chemokine XCL1, is highly expressed in conventional dendritic cells subtype 1 (cDC1s) and crucial for their activation. Modulating XCR1 signaling in cDC1s could offer novel opportunities in cancer immunotherapy and vaccine development by enhancing the antigen presentation function of cDC1s. To investigate the molecular mechanism of XCL-induced XCR1 signaling, we determined a high-resolution structure of the human XCR1 and Gi complex with an engineered form of XCL1, XCL1 CC3, by cryoelectron microscopy. Through mutagenesis and structural analysis, we elucidated the molecular details for the binding of the N-terminal segment of XCL1 CC3, which is vital for activating XCR1. The unique arrangement within the XCL1 CC3 binding site confers specificity for XCL1 in XCR1. We propose an activation mechanism for XCR1 involving structural alterations of key residues at the bottom of the XCL1 binding pocket. These detailed insights into XCL1 CC3-XCR1 interaction and XCR1 activation pave the way for developing novel XCR1-targeted therapeutics.

Neuraminidase delivered as an APC-targeted DNA vaccine induces protective antibodies against influenza

Conventional influenza vaccines focus on hemagglutinin (HA). However, antibody responses to neuraminidase (NA) have been established as an independent correlate of protection. Here, we introduced the ectodomain of NA into DNA vaccines that, as translated dimeric vaccine proteins, target antigen-presenting cells (APCs). The targeting was mediated by an single-chain variable fragment specific for major histocompatibility complex (MHC) class II, which is genetically linked to NA via a dimerization motif. A single immunization of BALB/c mice elicited strong and long-lasting NA-specific antibodies that inhibited NA enzymatic activity and reduced viral replication. Vaccine-induced NA immunity completely protected against a homologous influenza virus and out-competed NA immunity induced by a conventional inactivated virus vaccine. The protection was mainly mediated by antibodies, although NA-specific T cells also contributed. APC-targeting and antigen bivalency were crucial for vaccine efficacy. The APC-targeted vaccine was potent at low doses of DNA, indicating a dose-sparing effect. Similar results were obtained with NA vaccines that targeted different surface molecules on dendritic cells. Interestingly, the protective efficacy of NA as antigen compared favorably with HA and therefore ought to receive more attention in influenza vaccine research.

Antigen bivalency of antigen-presenting cell-targeted vaccines increases B cell responses

Antibodies are important for vaccine efficacy. Targeting antigens to antigen-presenting cells (APCs) increases antibody levels. Here, we explore the role of antigen valency in MHC class II (MHCII)-targeted vaccines delivered as DNA. We design heterodimeric proteins that carry either two identical (bivalent vaccines), or two different antigens (monovalent vaccines). Bivalent vaccines with two identical influenza hemagglutinins (HA) elicit higher amounts of anti-HA antibodies in mice than monovalent versions with two different HAs. Bivalent vaccines increase the levels of germinal center (GC) B cells and long-lived plasma cells. Only HA-bivalent vaccines completely protect mice against challenge with homologous influenza virus. Similar results are obtained with other antigens by targeting CD11c and Xcr1 on dendritic cells (DCs) or when administering the vaccine as protein with adjuvant. Bivalency probably increases B cell responses by cross-linking BCRs in readily observable DC-B cell synapses. These results are important for generating potent APC-targeted vaccines.

Intranasal delivery of a cDC1 targeted influenza vaccine with poly(I:C) enhances T cell responses and protects against influenza infection

Targeting antigens to dendritic cells represent a promising method for enhancing immune responses against specific antigens. However, many studies have focused on systemic delivery (intravenous or intraperitoneally) of targeted antigen, approaches that are not easily transferable to humans. Here we evaluate the efficacy of an influenza vaccine targeting Xcr1⁺ cDC1 administered by intranasal immunization. Intranasal delivery of antigen fused to the chemokine Xcl1, the ligand of Xcr1, resulted in specific uptake by lung CD103⁺ cDC1. Interestingly, intranasal immunization with influenza A/PR/8/34 haemagglutinin (HA) fused to Xcl1, formulated with poly(I:C), resulted in enhanced induction of antigen-specific IFNγ⁺ CD4⁺ and IFNγ⁺ CD8⁺ T cell responses in lung compared non-targeted anti-NIP-HA (αNIP-HA). Induction of antibody responses was, however, similar in Xcl1-HA and αNIP-HA immunized mice, but significantly higher than in mice immunized with monomeric HA. Both Xcl1-HA and αNIP-HA vaccines induced full protection when mice were challenged with a lethal dose of influenza PR8 virus, reflecting the strong induction of HA-specific antibodies. Our results demonstrate that i.n. delivery of Xcl1-HA is a promising vaccine strategy for enhancing T cell responses in addition to inducing strong antibody responses.

Unboxing dendritic cells: Tales of multi-faceted biology and function

Often referred to as the bridge between innate and adaptive immunity, dendritic cells (DCs) are professional antigen-presenting cells (APCs) that constitute a unique, yet complex cell system. Among other APCs, DCs display the unique property of inducing protective immune responses against invading microbes, or cancer cells, while safeguarding the proper homeostatic equilibrium of the immune system and maintaining self-tolerance. Unsurprisingly, DCs play a role in many diseases such as autoimmunity, allergy, infectious disease and cancer. This makes them attractive but challenging targets for therapeutics. Since their initial discovery, research and understanding of DC biology have flourished. We now recognize the presence of multiple subsets of DCs distributed across tissues. Recent studies of phenotype and gene expression at the single cell level have identified heterogeneity even within the same DC type, supporting the idea that DCs have evolved to greatly expand the flexibility of the immune system to react appropriately to a wide range of threats. This review is meant to serve as a quick and robust guide to understand the basic divisions of DC subsets and their role in the immune system. Between mice and humans, there are some differences in how these subsets are identified and function, and we will point out specific distinctions as necessary. Throughout the text, we are using both fundamental and therapeutic lens to describe overlaps and distinctions and what this could mean for future research and therapies.

Targeting human langerin promotes HIV-1 specific humoral immune responses

The main avenue for the development of an HIV-1 vaccine remains the induction of protective antibodies. A rationale approach is to target antigen to specific receptors on dendritic cells (DC) via fused monoclonal antibodies (mAb). In mouse and non-human primate models, targeting of skin Langerhans cells (LC) with anti-Langerin mAbs fused with HIV-1 Gag antigen drives antigen-specific humoral responses. The development of these immunization strategies in humans requires a better understanding of early immune events driven by human LC. We therefore produced anti-Langerin mAbs fused with the HIV-1 gp140z Envelope (αLC.Env). First, we show that primary skin human LC and in vitro differentiated LC induce differentiation and expansion of naïve CD4⁺ T cells into T follicular helper (Tfh) cells. Second, when human LC are pre-treated with αLC.Env, differentiated Tfh cells significantly promote the production of specific IgG by B cells. Strikingly, HIV-Env-specific Ig are secreted by HIV-specific memory B cells. Consistently, we found that receptors and cytokines involved in Tfh differentiation and B cell functions are upregulated by LC during their maturation and after targeting Langerin. Finally, we show that subcutaneous immunization of mice by αLC.Env induces germinal center (GC) reaction in draining lymph nodes with higher numbers of Tfh cells, Env-specific B cells, as well as specific IgG serum levels compared to mice immunized with the non-targeting Env antigen. Altogether, we provide evidence that human LC properly targeted may be licensed to efficiently induce Tfh cell and B cell responses in GC.

In recent years, the place of innovative vaccines based on the induction/regulation and modulation of the immune response with the aim to elicit an integrated T- and B cell immune responses against complex antigens has emerged besides “classical” vaccine vectors. Targeting antigens to dendritic cells is a vaccine technology concept supported by more than a decade of animal models and human pre-clinical experimentation. Recent investigations in animals underscored that Langerhans cells (LC) are an important target to consider for the induction of antibody responses by DC targeting vaccine approaches. Nonetheless, the development of these immunization strategies in humans remains elusive. We therefore developed and produced an HIV vaccine candidate targeting specifically LC through the Langerin receptor. We tested the ability of our vaccine candidate of targeting LC from skin explant and of inducing in vitro the differentiation of T follicular helper (Tfh) cells. Using complementary in vitro models, we demonstrated that Tfh cells induced by human LC are functional and the targeting of LC by our vaccine candidate promotes the secretion of anti-HIV IgG by memory B cells from HIV-infected individuals. In this study human LC exhibit key cellular functions able to drive potent anti-HIV-1 humoral responses providing mechanistic evidence of the Tfh- and B cell stimulating functions of primary skin targeted LC. Finally, we demonstrated in Xcr1^DTA mice the significant advantage of LC targeting for inducing Tfh and germinal center (GC)-B cells and anti-HIV-1 antibodies. Therefore, the targeting of the human Langerin receptor appears to be a promising strategy for developing efficient HIV-1 vaccine.

Targeting Antigens to Different Receptors on Conventional Type 1 Dendritic Cells Impacts the Immune Response

Targeting Ag to surface receptors on conventional type 1 dendritic cells can enhance induction of Ab and T cell responses. However, it is unclear to what extent the targeted receptor influences the resulting responses. In this study, we target Ag to Xcr1, Clec9A, or DEC-205, surface receptors that are expressed on conventional type 1 dendritic cells, and compare immune responses in BALB/c and C57BL/6 mice in vitro and in vivo after intradermal DNA vaccination. Targeting hemagglutinin from influenza A to Clec9A induced Ab responses with higher avidity that more efficiently neutralized influenza virus compared with Xcr1 and DEC-205 targeting. In contrast, targeting Xcr1 resulted in higher IFN-γ⁺CD8⁺ T cell responses in spleen and lung and stronger cytotoxicity. Both Clec9A and Xcr1 targeting induced Th1-polarized Ab responses, although the Th1 polarization of CD4⁺ T cells was more pronounced after Xcr1 targeting. Targeting DEC-205 resulted in poor Ab responses in BALB/c mice and a more mixed Th response. In an influenza challenge model, targeting either Xcr1 or Clec9A induced full and long-term protection against influenza infection, whereas only partial short-term protection was obtained when targeting DEC-205. In summary, the choice of targeting receptor, even on the same dendritic cell subpopulation, may strongly influence the resulting immune response, suggesting that different targeting strategies should be considered depending on the pathogen.

Targeting Conventional Dendritic Cells to Fine-Tune Antibody Responses

Dendritic cells (DCs) facilitate cross talk between the innate and adaptive immune system. They sense and phagocytose invading pathogens, and are not only capable of activating naïve T cells, but can also determine the polarization of T cell responses into different effector subtypes. Polarized T cells in turn have a crucial role in antibody class switching and affinity maturation, and consequently the quality of the resulting humoral immunity. Targeting vaccines to DCs thus provides a great deal of opportunities for influencing the humoral immune responses, by fine-tuning the T cell response as well as regulating antigen availability for B cells. In this review we aim to outline how different DC targeted vaccination strategies can be utilized to induce a desired humoral immune response. A range of factors, including route of vaccine administration, use of adjuvants, choice of DC subset and surface receptor to target have been reported to influence the resulting immune response and will be reviewed herein. Finally, we will discuss opportunities for designing improved vaccines and challenges with translating this knowledge into clinical or veterinary medicine.