Summary Mucosal IgA secreted by local plasma cells (PCs) is a critical component of mucosal immunity. Although IgA class switching can occur at mucosal sites, high-affinity PCs are optimally generated in germinal centers (GCs) in a T cell-dependent fashion. However, the mechanism of how CD4+ helper T cells induce mucosal-homing IgA-PCs remains unclear. We show here that TGFβ1 and IL-21, produced by follicular helper T cells (TFH), synergize to generate abundant IgA-plasmablasts (PBs). In the presence of IL-21, TGFβ1 promotes naive B cell proliferation and differentiation, and it overrides IL-21-induced IgG class switching in favor of IgA. Furthermore, in combination with IL-21, TGFβ1 downregulates CXCR5 while upregulating CCR10 on PBs, enabling their exit from GCs and migration towards local mucosa. This is supported by the presence of CCR10+IgA+PBs in tonsil GCs. These findings show that TFH contribute to mucosal IgA. Thus, mucosal vaccines should aim to induce robust TFH responses.

Monocytes exposed to serum from SLE patients promote B cell differentiation to IgG and IgA plasmablasts dependent on BAFF and IL-10 or APRIL, respectively.

Dectin-1, a C-type lectin recognizing fungal and mycobacterial pathogens, can deliver intracellular signals that activate dendritic cells (DCs), resulting in initiation of immune responses and expansion of Th17 CD4+ T cell responses. In this paper, we studied the roles of human Dectin-1 (hDectin-1) expressed on DCs in the induction and activation of Ag-specific CD8+ T cell responses. We first generated an agonistic anti–hDectin-1 mAb, which recognizes the hDectin-1 Glu143-Ile162 region. It bound to in vitro monocyte-derived DCs and to in vivo CD1c+CD1a+ dermal DCs but not to epidermal Langerhans cells. Anti–hDectin-1–mediated DC activation resulted in upregulation of costimulatory molecules and secretion of multiple cytokines and chemokines in a Syk-dependent manner. DCs activated with the anti–hDectin-1 mAb could significantly enhance both neo and foreign Ag-specific CD8+ T cell responses by promoting both the expansion of CD8+ T cells and their functional activities. We further demonstrated that delivering Ags to DCs via hDectin-1 using anti–hDectin-1-Ag conjugates resulted in potent Ag-specific CD8+ T cell responses. Thus, hDectin-1 expressed on DCs can contribute to the induction and activation of cellular immunity against intracellular pathogens, such as mycobacteria, that are recognized by DCs via Dectin-1. Vaccines based on delivering Ags to DCs with an agonistic anti–hDectin-1 mAb could elicit CD8+ T cell-mediated immunity.

Targeting antigens directly to DCs through anti-DC receptor antibody fused to antigen proteins is a promising approach to vaccine development. However, not all antigens can be expressed as a recombinant antibody directly fused to a protein antigen. Here, we show that non-covalent assembly of antibody - antigen complexes, mediated by interaction between dockerin and cohesin domains from cellulose-degrading bacteria, can greatly expand the range of antigens for this DC-targeting vaccine technology. Recombinant antibodies with a dockerin domain fused to the antibody heavy chain C-terminus are efficiently secreted by mammalian cells, while many antigens not secreted as antibody fusion proteins are readily expressed as cohesin directly fused to antigen either via secretion from mammalian cells, or as soluble cytoplasmic E. coli products. These form very stable and homogeneous complexes with antibody fused to dockerin. In vitro, these complexes can efficiently bind to human DC receptors followed by presentation to antigen-specific CD4+ and CD8+ T cells. Low doses of the HA1 subunit of Influenza hemagglutinin conjugated through this means to anti-Langerin antibodies elicited Flu HA1-specific antibody and T cell responses in mice. Thus, the non-covalent assembly of antibody and antigen through dockerin and cohesin interaction provides a useful modular strategy for developing and testing prototype vaccines for eliciting antigen-specific T and B cell responses, particularly when direct antibody fusions to antigen cannot be expressed.

The mechanisms by which microbial vaccines interact with human APCs remain elusive. Herein, we describe the transcriptional programs induced in human DCs by pathogens, innate receptor ligands and vaccines. Exposure of DCs to influenza, Salmonella enterica and Staphylococcus aureus allows us to build a modular framework containing 204 transcript clusters. We use this framework to characterize the responses of human monocytes, monocyte-derived DCs and blood DC subsets to 13 vaccines. Different vaccines induce distinct transcriptional programs based on pathogen type, adjuvant formulation and APC targeted. Fluzone®, Pneumovax® and Gardasil® respectively activate monocyte-derived DCs, monocytes and CD1c+ blood DCs, highlighting APC specialization in response to vaccines. Finally, the blood signatures from individuals vaccinated with Fluzone or infected with influenza reveal a signature of adaptive immunity activation following vaccination and symptomatic infections, but not asymptomatic infections. These data, offered with a web interface, may guide the development of improved vaccines.