Skip to main content
NCBI home page
NIHPA Author Manuscripts logoLink to NIHPA Author Manuscripts
. Author manuscript; available in PMC: 2019 Aug 1.
Published in final edited form as: Curr Opin Pharmacol. 2018 Jun 8;41:128–136. doi: 10.1016/j.coph.2018.05.012

Exploiting vita-PAMPs in vaccines

J Magarian Blander 1,2,3,4,5,*, Gaetan Barbet 1,2
PMCID: PMC6110613  NIHMSID: NIHMS974107  PMID: 29890457

Abstract

Live attenuated vaccines elicit stronger protective immunity than dead vaccines. Distinct PAMPs designated as vita-PAMPs signify microbial viability to innate immune cells. Two vita-PAMPs have been characterized: cyclic-di-adenosine-monophosphate (c-di-AMP) and prokaryotic mRNA. c-di-AMP produced by live Gram-positive bacteria elicits augmented production of STING-dependent type-I interferon, whereas prokaryotic mRNA from live bacteria is detected by TLR8 enabling discrimination of live from dead bacteria. Bacterial mRNA from live Gram-negative bacteria triggers a heightened type-I interferon and NLRP3 inflammasome response. By mobilizing unique viability-associated innate responses, vita-PAMPs mobilize adaptive immunity that best elicits protection, including follicular T helper cell and antibody responses. Here, we review the molecular mechanisms that confer the unique adjuvanticity of vita-PAMPs and discuss their applications in vaccine design.

Keywords: Vita-PAMP, TLR8, inflammasome, type I interferon, monocyte, phagocyte, vaccine, follicular T helper cell, germinal center, antibody response

Introduction

Vaccination is one of the most effective strategies to prevent infectious diseases and pandemics. It involves administration of a live attenuated or dead pathogen, or component thereof, to generate strong and specific immunity that protects against subsequent encounter with that pathogen. Historically, live attenuated pathogens have been used to successfully vaccinate populations, often with a single dose, leading to lasting protective immunity and disease control. The first vaccine involved inoculation with live cowpox to prevent smallpox infection [1]. Other live attenuated vaccines have followed suit including an oral typhoid vaccine consisting of a live attenuated strain of Salmonella typhi T21A, a polio virus vaccine (OPV), a rotavirus vaccine (RotaShield, ROTARIX, RIT4237 bovine), a Shigella flexneri 2a SC602 vaccine, a live attenuated intranasal influenza vaccine, a live attenuated yellow fever vaccine (YF-17D), as well as vaccines for chickenpox (Varicella), Rabies and tuberculosis (BCG) [2,3].

Despite efficacy, live vaccines face many challenges. In developed countries, fears of vaccination and vaccine refusal, particularly those vaccines based on live attenuated microorganisms, have resulted in poor vaccine coverage and gaps in protection of the general public [4–7]. The preservation and delivery of live vaccines require infrastructure often not available in developing countries. More recent vaccine strategies consist of dead pathogens, novel peptides, and recombinant proteins. While these are more stable and have fewer side effects, they are less effective and generally fail to convey long-lasting immunity requiring multiple boosters [8]. The exact nature of the cellular and molecular mechanisms by which broad-based immunity is elicited is elusive [9], and vaccine design has mostly followed Pasteur’s principles: “isolate, inactivate, and inject” [3,10].

Adjuvants increase the immunogenicity of vaccines. Despite the variety of compounds with adjuvant properties, only a handful of adjuvants have been licensed for human use including aluminum salts (used in DTaP vaccines, the pneumococcal conjugate vaccine and hepatitis B vaccines), MF59 (an oil-in-water emulsion of squalene oil used in the influenza vaccine Fluad for the elderly), CpG 1018 (used in the hepatitis B vaccine Heplisav-B), GlaxoSmithKline Biologicals AS03 (an oil-in-water emulsion of D,L-α-tocopherol and squalene with polysorbate 80 emulsifier used in an H5N1 influenza vaccine), AS04 (an aluminum hydroxide with monophosphoryl lipid A (MPL) combination used in the cervical cancer vaccine Cervarix), and AS01B (used in the shingles vaccine Shingrix, comprised of MPL, a purified fat-like substance, and QS-21 from the bark of the Quillaja Saponaria tree) [11]. There is a constant need for new adjuvants for safer protective vaccines. A comprehensive understanding of the molecular basis underlying the efficacy of live vaccines will guide the design of new adjuvants that specifically target the relevant immune pathways and induce optimal long-lived protective immunity.

Vita-PAMPs: Molecular signatures of microbial viability

The innate immune system can not only discriminate self from non-self via recognition of pathogen associated molecular patterns (PAMPs) [12,13], but it can also discriminate between live and dead microorganisms through a distinct set of PAMPs called vita-PAMPs associated uniquely with live microorganisms [14,15]. Vita-PAMPs are detected in combination with classic PAMPs such as lipopolysaccharide (LPS) and DNA, which are shared between live and dead microorganisms [14,15]. Detection of PAMPs in the absence of vita-PAMPs signifies a dead microbe, whereas detection of a vita-PAMP alongside a PAMP signifies live microbes and mediates a heightened immune response not warranted to dead microorganisms [15]. The activity of virulence factors associated with live pathogens elicits a vigorous immune response, but continued expression of virulence factors becomes unnecessary once a pathogen has gained access to sterile tissues. Targeting microbial viability per se independently of virulence factors is a far better strategy for the immune system to eliminate the infectious threat and protect tissue integrity. Two vita-PAMPs, bacterial mRNA in the context of a response to live Gram-negative bacteria [14,16,17], and c-di-AMP in the context of a response to live Gram-positive bacteria [18] have been identified so far. A broad array of vita-PAMPs likely exists, and much like PAMPs, vita-PAMPs may be unique to different classes of microorganisms according to the specific cellular processes conducted by each class.

Vita-PAMPs mobilize distinct cell-autonomous innate immune responses

Detection of live avirulent bacteria by antigen presenting cells (APC), macrophages or dendritic cells (DC), elicits augmented levels of type-I interferon (IFN) production compared to dead bacteria, and this is dependent on bacterial mRNA for Gram-negative bacteria and cyclic-di-adenosine monophosphate (c-di-AMP) for Gram-positive bacteria [14,18] (Figure 1). Murine APC also secrete IL-1β in response to avirulent live but not dead Gram-negative bacteria, but they do not do so to avirulent live Gram-positive bacteria [14,18] (Figure 1). Similarly, detection of live avirulent Gram-negative Escherichia coli by human CD14+CD16− monocytes elicits a distinct transcriptional response compared to killed E. coli, including IL12B and TNF [17]. An IFN-inducible gene, IFIT2 is differentially transcribed in response to live E. coli, and so are several genes positively regulated by IFN-γ [17]. IL-1β, IL-12 and TNFα are produced by human monocytes uniquely in response to live but not dead E. coli, Bacillus subtilis and Mycobacterium bovis BCG, as well as killed E. coli supplemented with bacterial RNA or CL075, an agonist for the single-stranded RNA receptors Toll-like receptor (TLR)7 and TLR8 [17].

Figure 1. The molecular and cellular events subsequent to vita-PAMP recognition.

Figure 1

Open in a new tab

In mice, mononuclear phagocytes respond to microbial viability. Live Gram-positive and Gram-negative bacteria contain vita-PAMPs, such as bacterial mRNA and c-di-AMP, respectively, which serve as the molecular signatures of bacterial viability. Both vita-PAMPs induce type-I interferon through the Toll-like receptor adaptor TRIF for Gram-negative bacteria and through the stimulator of interferon genes (STING) and the kinase TBK1 for Gram-positive bacteria. In the response to phagocytosed Gram-negative bacteria, bacterial mRNA which gains access to the cytosol activates the NLRP3 inflammasome (1a) and IFNAR activation by TRIF-dependent IFN-β (1b) licenses NLRP3 inflammasome activation by promoting Caspase-11 expression (2) and interaction with caspase-1 to mediate IL-1β cleavage and secretion (3). In the response to phagocytosed Gram-positive bacteria, c-di-AMP gains access to STING on endoplasmic reticulum(ER) membranes to induce ER stress and subsequent ER-stress induced autophagy to orchestrate translocation of STING to autophagosomal membranes, TBK1 activation, and induction of type-I IFN production.

Dissection of the innate signaling pathways mobilized in APC upon detection of vita-PAMPs during phagocytosis of bacteria has provided a mechanistic basis for the differential production of inflammatory cytokines to live versus dead bacteria. During the innate response of murine APC to Gram-positive bacteria, direct detection of c-di-AMP by the receptor stimulator of interferon genes (STING) culminates in a TBK1 and IRF3 dependent type-I IFN response [18]. STING detection of c-di-AMP triggers an elaborate cell-autonomous stress response that begins with rapid phosphorylation of effectors of the endoplasmic reticulum (ER) stress response, PERK and IRE-1α, and subsequent inactivation of the mechanistic target of rapamycin mTORC1 [18]. These events in turn precipitate autophagy of the ER (reticulophagy or ER-phagy), which translocates STING from the ER to autophagosomes as a prerequisite to its ability to initiate type-I IFN signaling [18] (Figure 1). On the other hand, murine APC detection of bacterial mRNA released during phagosomal degradation of internalized live Gram-negative bacteria elicits an augmented IRF3-mediated IFN-β response dependent on the TLR adaptor TIR-domain-containing adapter-inducing interferon-β (TRIF) [14] (Figure 1). Despite the differences in the innate signaling pathways mobilized downstream of live Gram-positive versus Gram-negative bacteria, detection of vita-PAMPs optimizes protection against infection in mice [14,16,18].

Innate recognition of bacterial RNA varies according to the species and cells studied [19]. Human monocytes sense bacterial RNA through endosomal TLR8 while both human and murine plasmacytoid DC do so through TLR7 [19–23]. TLR8 is recruited to bacteria-carrying phagosomes [20,22], and senses bacterial RNA during infection with Group B Streptococci or Borellia burgdorferi to induce IL-6, IL-12 and IFN-β [20,21]. Human monocytes uniquely respond to live E. coli and bacterial RNA through TLR8 and MyD88 with higher levels of IL-12 and TNF-α production than to killed E. coli (Figure 1) [17]. This pathway is conserved in porcine cells [17], but not functional in murine cells where TLR8 is suggested to be non-responsive [24]. MyD88 is also involved in sensing bacterial RNA by murine bone marrow-derived macrophages and DC, but in this case, Unc93B1, which delivers nucleic acid sensing TLRs from the ER to endolysosomes, and TLR13 have been implicated [19]. Both mouse TLR13 and human TLR8 are sensors for bacterial 23S ribosomal RNA [19,22]. Whether Unc93B1 and TLR13 can specifically sense microbial viability has not been tested.

In mice and humans, detection of live but not dead Gram-negative bacteria also elicits IL-1β secretion [14,17,18] (Figure 1). In murine macrophages and DC, this is dependent on detection of bacterial mRNA and activation of the NLRP3 inflammasome irrespective of virulence factor expression [14]. This feature is unique to Gram-negative and not Gram-positive bacteria [18], and is likely dependent on concomitant cytosolic detection of the Gram-negative PAMP LPS [25], rather than inherent differences between RNA from Gram-positive versus Gram-negative bacteria [18,19]. While MyD88 is important for the transcriptional induction of Il1b and Nlrp3, murine NLRP3 inflammasome activation and IL-1β secretion by live Gram-negative bacteria and bacterial mRNA is dependent on TRIF [14]. TRIF-dependent IFN-β licenses NLRP3 inflammasome activation through transcriptional induction of pro-caspase-11 [25]. The human NLRP3 inflammasome has also been reported to sense bacterial RNA including ribosomal, messenger, transfer, and small RNA [26], but its involvement in the ability of human APC to sense microbial viability has not been tested.

It is worthwhile noting here the pre-clinical success of mRNA vaccines for cancer and infectious disease [27]. Although mRNA vaccines were initially designed to induce expression and presentation of antigens in APC, they often contain double-stranded RNA contaminants, formed during in vitro transcription, that contribute to their adjuvanticity by eliciting type-I IFN production [28]. Recent vaccine formulations have enhanced the adjuvanticity of these mRNA vaccines to induce robust antigen specific protective antibody and cytotoxic T cell responses [29]. A number of ongoing clinical trials for Rabies, influenza virus, and Zika virus use lipid nanoparticles as a delivery tool, and notably interim findings from the mRNA vaccine against H10N8 and H7N9 influenza viruses report antibody titers above the expected protective threshold [29].

Bacterial viability and bacterial RNA elicit TFH responses in mice and humans

Characterization of the innate response to vita-PAMPs has set the stage for investigating their use as unique adjuvants that could augment the performance of dead vaccines to the levels achieved by equivalent live vaccines. As proof of principle, addition of bacterial RNA to a dead vaccine comprised of killed E. coli elevated serum titers of class-switched antibodies to those induced by a counterpart live vaccine [14]. Identification of the vita-PAMP as well as the viability-associated innate pathways and effector cytokines involved enabled a targeted and systematic evaluation of the impact on the ensuing adaptive immune response [16,17]. This led to the discovery that innate detection of bacterial RNA as the vita-PAMP signifying microbial viability is a particularly robust signal for differentiation of both murine and human follicular T helper (TFH) cells [16,17]. A comparison of three E. coli vaccines in mice – live, dead and a vita-PAMP-supplemented dead vaccine (vita-vaccine) – showed that only the live and vita vaccines induced a TFH response [16]. These vaccines also activated most subsets of B cells including B1, marginal zone and the predominant follicular B2 cells. Vaccination of domestic pigs with the live but not dead form of an attenuated Salmonella vaccine increased the frequency of TFH cells. Human monocytes also promoted TFH cell differentiation in culture after stimulation with live but not dead E. coli as well as the TLR8 agonists CL075 and R848 [17]. The TFH response in mice and pigs correlated with augmented germinal center formation, IgG class-switching, and frequencies of plasma cells in response to the live vaccine [16,17], and in mice to the vita-vaccine as well [16]. Similarly, human TFH cells induced by E. coli-stimulated monocytes promoted plasma cell differentiation of co-cultured B cells and production of IgG [17]. These studies collectively show that prokaryotic RNA as a proxy of bacterial viability comprises a physiological innate trigger for TFH differentiation in both mice and humans (Figure 2). They provide a mechanistic basis for the characteristic ability of live vaccines to elicit higher serum levels of class-switched antibodies [16,17]. They also identify a much-needed pathway for the rational design of vaccines that succeed in eliciting robust TFH responses, which in turn directly promote development of the antibody response.

Figure 2. Cellular activation after sensing the vita-PAMP bacterial mRNA.

Figure 2

Open in a new tab

Bacterial mRNA recognition by monocytes increases follicular T helper cell differentiation, enhances IgG antibody response through higher follicular T helper cells and higher germinal center formation for mouse, human, and porcine (not shown) models. In mice, TFH cell differentiation by a live vaccine comprised of Gram-negative bacteria or a dead counterpart vaccine supplemented with bacterial mRNA is dependent on TRIF signaling and downstream cytokines IFN-β and IL-1β. Both cytokines act directly on mouse T cells to promote full TFH differentiation. In the human model, activation of the MyD88-dependent TLR8 receptor on monocytes induces the expression of IL-12, the main cytokine promoting human TFH cell differentiation, and induces TFH differentiation of co-cultured activated CD4 T cells. Human IL-1β is also produced in response to live bacteria by a yet to be explored inflammasome pathway and has additive effects on TFH differentiation. Despite the different cytokine requirements for TFH cell differentiation, detection of bacterial viability by either mouse or human monocytes serves as the physiological trigger for TFH cell differentiation and subsequent B cell class-switching and IgG production.

Whether the vita-PAMP c-di-AMP similarly augments TFH cell differentiation remains to be assessed. Nonetheless, cyclic-di-nucleotides (CDNs) such as c-di-AMP, c-di-GMP, and c-GAMP all serve as STING agonists and have been highly successful in inducing potent antibody and T cell responses in experimental models of vaccination [30]. Their potent adjuvanticity may stem from their ability to relay microbial viability to the innate immune system. Research and development surrounding STING agonists is presently a top priority for pharmaceutical companies, many of which have several cancer immunotherapy clinical trials already underway using synthetic CDN STING agonists alone or in combination with the checkpoint blockade inhibitor anti-PD1.

How vita-PAMPs elicit TFH cell differentiation in mice and humans

TFH cells have common and divergent requirements for differentiation in mice versus humans [31]. Detection of the vita-PAMP bacterial mRNA by either human or murine APC fulfills the criteria for instructing TFH differentiation in both species [16,17]. In both mouse and human APC, PAMPs shared by live and dead bacteria elicit similar levels of certain cytokines such as IL-6. However, live and not dead bacteria uniquely induce IL-1β production by human monocytes [17], and while dead bacteria can elicit IL-12 production [17,32], the levels produced in response to live bacteria are significantly higher than those to dead bacteria [17]. IL-12, and to a lesser extent IL-23, is the best inducer of ICOS, CXCR5, Bcl-6 and IL-21 expression by activated human CD4 T cells [32–34]. TGF-β promotes human TFH differentiation by either IL-12 or IL-23, and the additional presence of IL-1β, secreted uniquely in response to live bacteria [17], further drives commitment towards the TFH fate [35]. TLR8-MyD88 signaling augments IL-12 production in response to live bacteria [17]. For both human and porcine APC, the TLR8 agonists bacterial RNA, CL075 and R848 trigger the highest levels of IL-12 and TFH differentiation, while silencing monocyte TLR8 and MyD88 expression has the opposite effect [17]. TFH differentiation was unaffected by the presence or absence of IL-6, IL-27, or IFN-β in human monocyte-CD4 T cell co-cultures, and neutralization of IL-1β partially inhibited TFH cell differentiation [17], consistent with its promoter role in human TFH differentiation [35]. Notably, hypermorphic TLR8 polymorphism in humans is associated with improved BCG vaccine-mediated protection [17], and IL-12 receptor β1 deficiency is associated with a lower number of circulating TFH cells and lymph node germinal centers, as well as lower avidity, albeit normal levels, of circulating antibodies [34]. Collectively, these results point to TLR8 agonists as potential TFH cell skewing adjuvants.

In mice, IL-12 induces both Il21 and Bcl6 via the transcription factor STAT4 prior to CD4 T cell commitment to either TH1 or TFH cell fates [36]. The cytokines which play dominant roles in skewing towards mouse TFH cell differentiation are the STAT3 activating cytokines, IL-6, IL-21 and IL-27 [37,38], although TFH differentiation was reduced but not abrogated in the combined absence of IL-6 and IL-21 during an acute infection with lymphocytic choriomeningitis virus (LCMV) [39]. Additional cytokines such as IL-1β may be important. LCMV and other viruses such as influenza A virus, elicit inflammasome activation and IL-1β production in mice [40–42]. IL-1β and IL-1R1 are also essential in the early immune response to Mycobacterium tuberculosis and Candida albicans [43]. IL-1β is secreted in response to bacterial viability [14,17], which also triggers TFH cell differentiation in both mice and humans [16,17]. Linking the highly inflammatory and tightly regulated production of IL-1β to an adaptive TFH response that best drives neutralizing antibody production would be beneficial to the host. Indeed, dissection of the mechanisms behind the higher antibody titers elicited by live vaccines has supported a role for IL-1β as a critical mediator of TFH differentiation.

Because of the central role of TRIF in orchestrating the innate response to live Gram-negative bacteria and bacterial mRNA in mice, vaccine-induced TFH responses are critically dependent on TRIF and also rely on IRF3-mediated IFN-β and caspases 1 and 11 dependent IL-1β production. Patrolling CX3CR1+CCR2− monocytes isolated from the spleen produce IL-1β within a few hours of injecting live E. coli, and these monocytes instruct the TFH response [16]. IFN-β and IL-1β elicited by live bacteria and bacterial RNA play non-redundant roles in promoting TFH cell differentiation [16]. In mice, IL-1β induces expression of the TFH-specific transcription factor Bcl6 and TFH-specific marker CXCR5 through IL-1R1 on T cells [16], and its injection during immunization significantly increases TFH cell generation [16,44]. On the other hand, TRIF-dependent IFN-β plays two distinct roles in TFH differentiation; to mediate inflammasome activation and innate IL-1β secretion through type I interferon receptor (IFNAR) engagement on APC [25,45], and to promote TFH production of IL-21 through IFNAR engagement on T cells [16] (Figure 2). Notably, type IIFN induces STAT1 binding to the Bcl6 locus in activated CD4 T cell cultures to sustain its expression but fails to induce IL-21 production on its own unless combined with IL-6 [46]. In the murine response to vaccination when cytokines such as IL-12 and IL-6 are also present, the specific and hierarchical induction of IFN-β and IL-1β in response to a live or vita vaccine sets the stage for optimal TFH differentiation. TRIF-dependent IFN-β licenses NLRP3 inflammasome and caspase-1 activation by upregulating and activating caspase-11 through IFN-β/IFNAR signaling [25]. As such, IL-1β cleavage is downstream of and dependent on IFN-β production and availability. Addition of IL-1β, whose secretion by human monocytes is also significantly higher in response to live bacteria [17], to the combination of IL-12, TGF-β, and IL-6 in activated human CD4 T cell cultures, yields the highest expression of the master TFH transcription factor BCL-6 [35].

RIG-I-dependent type-I IFN produced by dermal human CD14+ DC in response to Dengue virus, which initiates infection through a mosquito bite, induces IL-27 expression through IFNAR signaling, and as such enables the CD14+ DC to instruct TFH differentiation [47]. Interestingly, Dengue virus RNA replication, a proxy for viral viability, is necessary for production of both type-I IFN and IL-27 and is a critical determinant of the formation of IL-21-producing CXCR5+PD1+BCL6+ human TFH cells [47]. Through IFNAR signaling on both mouse and human APC, type-I IFNs thus play an important role in instructing TFH differentiation by controlling the secretion of additional key cytokines such as IL-1β and IL-27 that act directly on CD4 T cells to drive their differentiation into TFH cells [16,47,48]. By specifically promoting TFH differentiation at different steps during the response to microbial viability, both the IRF3/IFN-β and inflammasome/IL-1β pathways impact long-term protection, germinal center formation and IgG class-switched antibody responses in vaccinated mice [16], consistent with observations in other models [45,49]. The inflammasomes and IL-1R1 signals are also necessary for eliciting mucosal IgA and systemic IgG responses during respiratory infection with influenza A virus [50].

The unique adjuvant properties of the vita-PAMP bacterial mRNA stem from its ability to induce both type-I IFN and inflammasome pathways. The contribution of inflammasome activation and IL-1β secretion to the TFH cell response is in line with previous observations in mice including dependence of the potent humoral response elicited by aluminum-containing (alum) adjuvants on NLRP3 inflammasome activation [51,52], the enhancement of antibody production through IL-1 administration during immunization [53–55], and the significant impairment of antibody production noted in IL-1α/β-deficient mice [56,57]. The attribution of alum adjuvanticity to the NLRP3 inflammasome has been contested, and the oil-in-water emulsion MF59 also works independently of NLRP3 [58–60]. Nonetheless, many particulate adjuvants that induce potent humoral responses are IL-1β inducers by both mouse and human APC [43]. The collective experimental evidence, at least so far in mice, supports the use of inflammasome activating adjuvants in vaccine design [43]. Whether NLRP3 inflammasome activation also promotes TFH responses in humans remains to be formally tested.

CONCLUDING REMARKS

Continued research is necessary to characterize more molecules with vita-PAMP properties as well as to better understand the mechanisms through which microbial viability signals are communicated to different cells of the immune system. For example, different APC have been reported to elicit TFH and antibody responses [16,61,62]. Thus, depending on the route of infection and the immunological context of the tissue, different requirements might be necessary to elicit a potent immune response. Elucidating the cellular and molecular events which lead to the robust immunity and long-lived memory that a live vaccine affords will guide the design of new adjuvants. Vita-PAMPs hold the promise of such adjuvants. Their incorporation into already existing killed and subunit vaccines would recapitulate the efficacy of live attenuated vaccines. The unparalleled performance expected of vita-vaccines would lay in their ability to effectively trigger the same immune pathways that live vaccines would mobilize but without their associated safety risks.

HIGHLIGHTS.

  • vita-PAMPs are specific PAMPs expressed only by live microbes

  • vita-PAMPs induce type I IFNs and/or inflammasome activation

  • Bacterial viability and bacterial RNA elicit follicular T helper cell and antibody responses

  • vita-PAMP supplemented (dead) vaccines induce better protection

Acknowledgments

Our work on vita-PAMPs has been supported by the US National Institutes of Health (NIH) grants AI095245, AI127658 and AI080959, the Searle Scholars Award, and the Burroughs Wellcome Fund to J.M.B. G.B. was supported by the Crohn’s and Colitis Foundation. J.M.B. and her laboratory were supported by NIH grants AI123284, AI073899, DK072201, and DK111862.

Footnotes

Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Conflict of interest statement

J.M.B has a patent related to bacterial RNA: PCT/US2012/047087 “Use of Bacterial RNA or Structural Motifs thereof as adjuvants for vaccines”.

References and recommended reading

• Of special interest

•• Of outstanding interest

  • 1.Nabel GJ. Designing tomorrow's vaccines. N Engl J Med. 2013;368:551–560. doi: 10.1056/NEJMra1204186. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 2.Pugachev KV, Guirakhoo F, Monath TP. New developments in flavivirus vaccines with special attention to yellow fever. Curr Opin Infect Dis. 2005;18:387–394. doi: 10.1097/01.qco.0000178823.28585.ad. [DOI] [PubMed] [Google Scholar]
  • 3.Rappuoli R. Bridging the knowledge gaps in vaccine design. Nat Biotechnol. 2007;25:1361–1366. doi: 10.1038/nbt1207-1361. [DOI] [PubMed] [Google Scholar]
  • 4.Amanna I, Slifka MK. Public fear of vaccination: separating fact from fiction. Viral Immunol. 2005;18:307–315. doi: 10.1089/vim.2005.18.307. [DOI] [PubMed] [Google Scholar]
  • 5.Federman RS. Understanding vaccines: a public imperative. Yale J Biol Med. 2014;87:417–422. [PMC free article] [PubMed] [Google Scholar]
  • 6.Kluger J. Who's afraid of a little vaccine? Time. 2014;184:40–43. [PubMed] [Google Scholar]
  • 7.Lantos JD, Jackson MA, Opel DJ, Marcuse EK, Myers AL, Connelly BL. Controversies in vaccine mandates. Curr Probl Pediatr Adolesc Health Care. 2010;40:38–58. doi: 10.1016/j.cppeds.2010.01.003. [DOI] [PubMed] [Google Scholar]
  • 8.Hilleman MR. Vaccines in historic evolution and perspective: a narrative of vaccine discoveries. Vaccine. 2000;18:1436–1447. doi: 10.1016/s0264-410x(99)00434-x. [DOI] [PubMed] [Google Scholar]
  • 9.Pulendran B, Ahmed R. Translating innate immunity into immunological memory: implications for vaccine development. Cell. 2006;124:849–863. doi: 10.1016/j.cell.2006.02.019. [DOI] [PubMed] [Google Scholar]
  • 10.Ulmer JB, Valley U, Rappuoli R. Vaccine manufacturing: challenges and solutions. Nat Biotechnol. 2006;24:1377–1383. doi: 10.1038/nbt1261. [DOI] [PubMed] [Google Scholar]
  • 11.Apostolico Jde S, Lunardelli VA, Coirada FC, Boscardin SB, Rosa DS. Adjuvants: Classification, Modus Operandi, and Licensing. J Immunol Res. 2016;2016:1459394. doi: 10.1155/2016/1459394. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 12.Janeway CA., Jr Approaching the asymptote? Evolution and revolution in immunology. Cold Spring Harb Symp Quant Biol. 1989;54(Pt 1):1–13. doi: 10.1101/sqb.1989.054.01.003. [DOI] [PubMed] [Google Scholar]
  • 13.Medzhitov R. Approaching the asymptote: 20 years later. Immunity. 2009;30:766–775. doi: 10.1016/j.immuni.2009.06.004. [DOI] [PubMed] [Google Scholar]
  • 14••.Sander LE, Davis MJ, Boekschoten MV, Amsen D, Dascher CC, Ryffel B, Swanson JA, Muller M, Blander JM. Detection of prokaryotic mRNA signifies microbial viability and promotes immunity. Nature. 2011;474:385–389. doi: 10.1038/nature10072. This work showed that the innate immune system can discriminate between live and dead bacteria and independently of the expression of virulence factors Vita-PAMPs are introduced here as a distinct set of PAMPs that are associated with live bacteria. Bacterial mRNA is defined as a vita-PAMP and the unique innate pathways involved in its recognition within the context of live Gram-negative bacteria are delineated. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 15••.Blander JM, Sander LE. Beyond pattern recognition: five immune checkpoints for scaling the microbial threat. Nat Rev Immunol. 2012;12:215–225. doi: 10.1038/nri3167. This review puts forth the idea that microbial viability is one checkpoint that the innate immune system uses to fine tune the response to the level of threat encountered. Other checkpoints are presented including the activity of virulence factors, the features of invasiveness, and the tissue microenvironment where the microbe is encountered. The idea is supported by a review of the experimental evidence surrpounding each checkpoint. [DOI] [PubMed] [Google Scholar]
  • 16••.Barbet G, Sander LE, Geswell M, Leonardi I, Cerutti A, Iliev I, Blander JM. Sensing Microbial Viability through Bacterial RNA Augments T Follicular Helper Cell and Antibody Responses. Immunity. 2018;48:584–598 e585. doi: 10.1016/j.immuni.2018.02.015. Bacterial viability is identified as a physiological trigger for the generation of a follicular T helper (TFH) response in mice. TRIF-dependent IRF3 and inflammasome pathways orchestrate the TFH response to a live vaccine comprised of live Gram-negative bacteria and a killed vaccine supplemented with the vita-PAMP, bacterial RNA. CX3CR1+CCR2− monocytes are shown to secrete IL-1β in response to live Gram-negative bacteria and drive the TFH response. IFN-β -dependent secretion of IL-1β engages T cell IL-1R1 to mediate TFH differentiation while IFN-β also augments TFH IL-21 production. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 17••.Ugolini M, Gerhard J, Burkert S, Jensen KJ, Georg P, Ebner F, Volkers SM, Thada S, Dietert K, Bauer L, et al. Recognition of microbial viability via TLR8 drives TFH cell differentiation and vaccine responses. Nat Immunol. 2018;19:386–396. doi: 10.1038/s41590-018-0068-4. Bacterial viability is identified as a potent trigger for human TFH differentiation. The bacterial RNA receptor TLR8 is identified as a sensor of live bacteria in human and porcine monocytes. TLR8-MyD88 engagement by the vita-PAMP bacterial RNA or TLR8 agonists induces specific cytokines that favor human TFH differentiation. In domestic pigs, this pathway promotes robust TFH and antibody responses. In humans with a hypermorphic TLR8 polymorphism, the pathway is associated with protective BCG vaccine responses. [DOI] [PubMed] [Google Scholar]
  • 18••.Moretti J, Roy S, Bozec D, Martinez J, Chapman JR, Ueberheide B, Lamming DW, Chen ZJ, Horng T, Yeretssian G, et al. STING Senses Microbial Viability to Orchestrate Stress-Mediated Autophagy of the Endoplasmic Reticulum. Cell. 2017;171:809–823 e813. doi: 10.1016/j.cell.2017.09.034. The bacterial STING ligand c-di-AMP, produced by live Gram-positive bacteria, is identified as a vita-PAMP. The work shows that live Gram-positive bacteria cause a STING-mediated ER stress, mTORC1 inactivation and ER-phagy, dependent on c-di-AMP. ER-phagy controls the unique augmented interferon response to live Gram-positive bacteria. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 19•.Eigenbrod T, Dalpke AH. Bacterial RNA: An Underestimated Stimulus for Innate Immune Responses. J Immunol. 2015;195:411–418. doi: 10.4049/jimmunol.1500530. This is a comprehensive review of the receptors and signaling pathways that mediate the innate immune response to bacterial RNA discusses the physiologivcal relevance of detecting bacterial RNA during infection as well as the RNA modifications that modulate the innate response to bacterial RNA. For additional important work related to the innate recognition of bacterial RNA, see references 20–24. [DOI] [PubMed] [Google Scholar]
  • 20.Cervantes JL, La Vake CJ, Weinerman B, Luu S, O'Connell C, Verardi PH, Salazar JC. Human TLR8 is activated upon recognition of Borrelia burgdorferi RNA in the phagosome of human monocytes. J Leukoc Biol. 2013;94:1231–1241. doi: 10.1189/jlb.0413206. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 21.Eigenbrod T, Pelka K, Latz E, Kreikemeyer B, Dalpke AH. TLR8 Senses Bacterial RNA in Human Monocytes and Plays a Nonredundant Role for Recognition of Streptococcus pyogenes. J Immunol. 2015;195:1092–1099. doi: 10.4049/jimmunol.1403173. [DOI] [PubMed] [Google Scholar]
  • 22.Kruger A, Oldenburg M, Chebrolu C, Beisser D, Kolter J, Sigmund AM, Steinmann J, Schafer S, Hochrein H, Rahmann S, et al. Human TLR8 senses UR/URR motifs in bacterial and mitochondrial RNA. EMBO Rep. 2015;16:1656–1663. doi: 10.15252/embr.201540861. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 23.Bergstrom B, Aune MH, Awuh JA, Kojen JF, Blix KJ, Ryan L, Flo TH, Mollnes TE, Espevik T, Stenvik J. TLR8 Senses Staphylococcus aureus RNA in Human Primary Monocytes and Macrophages and Induces IFN-beta Production via a TAK1-IKKbeta-IRF5 Signaling Pathway. J Immunol. 2015;195:1100–1111. doi: 10.4049/jimmunol.1403176. [DOI] [PubMed] [Google Scholar]
  • 24.Liu J, Xu C, Hsu LC, Luo Y, Xiang R, Chuang TH. A five-amino-acid motif in the undefined region of the TLR8 ectodomain is required for species-specific ligand recognition. Mol Immunol. 2010;47:1083–1090. doi: 10.1016/j.molimm.2009.11.003. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 25••.Blander JM. A long-awaited merger of the pathways mediating host defence and programmed cell death. Nat Rev Immunol. 2014;14:601–618. doi: 10.1038/nri3720. This review discusses several important studies that have contributed to understanding the canonical and non-canonical pathways of inflammasome activation within the context of a response to Gram-negative bacteria, with a focus on the roles of LPS and bacterial RNA. [DOI] [PubMed] [Google Scholar]
  • 26.Sha W, Mitoma H, Hanabuchi S, Bao M, Weng L, Sugimoto N, Liu Y, Zhang Z, Zhong J, Sun B, et al. Human NLRP3 inflammasome senses multiple types of bacterial RNAs. Proc Natl Acad Sci U S A. 2014;111:16059–16064. doi: 10.1073/pnas.1412487111. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 27.Pardi N, Hogan MJ, Porter FW, Weissman D. mRNA vaccines - a new era in vaccinology. Nat Rev Drug Discov. 2018 doi: 10.1038/nrd.2017.243. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 28.Weissman D. mRNA transcript therapy. Expert Rev Vaccines. 2015;14:265–281. doi: 10.1586/14760584.2015.973859. [DOI] [PubMed] [Google Scholar]
  • 29.Iavarone C, O'Hagan DT, Yu D, Delahaye NF, Ulmer JB. Mechanism of action of mRNA-based vaccines. Expert Rev Vaccines. 2017;16:871–881. doi: 10.1080/14760584.2017.1355245. [DOI] [PubMed] [Google Scholar]
  • 30•.Dubensky TW, Jr, Kanne DB, Leong ML. Rationale, progress and development of vaccines utilizing STING-activating cyclic dinucleotide adjuvants. Ther Adv Vaccines. 2013;1:131–143. doi: 10.1177/2051013613501988. This review examines the fundamental mechanisms underlying the innate response to cyclic-dinucleotides and their use as vaccine adjuvants in preclinical settings. It also accurately predicts the current intensity of interest in the use of cyclic-dinucleotides and STING agonists for preventive vaccination and immunotherapies in humans. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 31••.Schmitt N, Ueno H. Regulation of human helper T cell subset differentiation by cytokines. Curr Opin Immunol. 2015;34:130–136. doi: 10.1016/j.coi.2015.03.007. This review delves into the roles of cytokines in the differentiation of human TFH cells, and delineates the differences in the developmental pathways of TFH cells between mice and humans. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 32••.Schmitt N, Morita R, Bourdery L, Bentebibel SE, Zurawski SM, Banchereau J, Ueno H. Human dendritic cells induce the differentiation of interleukin-21-producing T follicular helper-like cells through interleukin-12. Immunity. 2009;31:158–169. doi: 10.1016/j.immuni.2009.04.016. Ref. [33] along with Ref. [32] identify IL-12 as the major driver of human TFH cell development. As such, these studies reveal a key differential requirement in human versus mouse TFH cell differentiation. Ref. [32] also shows that DC activated by bacteria (albeit killed bacteria) induce IL-21-producing CD4 T cells through IL-12. A comparison to activation with live bacteria, which would trigger higher IL-12 levels, was not made. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 33••.Ma CS, Suryani S, Avery DT, Chan A, Nanan R, Santner-Nanan B, Deenick EK, Tangye SG. Early commitment of naive human CD4(+) T cells to the T follicular helper (T(FH)) cell lineage is induced by IL-12. Immunol Cell Biol. 2009;87:590–600. doi: 10.1038/icb.2009.64. See annotation to Ref. [32]. [DOI] [PubMed] [Google Scholar]
  • 34.Schmitt N, Bustamante J, Bourdery L, Bentebibel SE, Boisson-Dupuis S, Hamlin F, Tran MV, Blankenship D, Pascual V, Savino DA, et al. IL-12 receptor beta1 deficiency alters in vivo T follicular helper cell response in humans. Blood. 2013;121:3375–3385. doi: 10.1182/blood-2012-08-448902. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 35•.Schmitt N, Liu Y, Bentebibel SE, Munagala I, Bourdery L, Venuprasad K, Banchereau J, Ueno H. The cytokine TGF-beta co-opts signaling via STAT3-STAT4 to promote the differentiation of human TFH cells. Nat Immunol. 2014;15:856–865. doi: 10.1038/ni.2947. TGF-β plus IL-12 or TGF-β plus IL-23 are shown to promote human TFH cell differentiation. This work is of special interest here because it investigates the contribution of IL-1β to human TFH cell differentiation, and reports that IL-1β synergizes with TGF-β and IL-12 or IL-23 to increase the expression of TFH-specific molecules including CXCR5, ICOS, IL-21, and BCL-6. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 36•.Nakayamada S, Kanno Y, Takahashi H, Jankovic D, Lu KT, Johnson TA, Sun HW, Vahedi G, Hakim O, Handon R, et al. Early Th1 cell differentiation is marked by a Tfh cell-like transition. Immunity. 2011;35:919–931. doi: 10.1016/j.immuni.2011.11.012. This work shows that IL-12 through STAT4 regulates multiple genes involved in the mouse TFH program during the induction phase of an immune response when transitional CD4 T cells share TH1 and TFH features. During infection of mice with Toxoplasma gondii the STAT-4 induced TH1-specific transcription factor T-bet constrains TFH cell expansion and consequent germinal center and antibody responses. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 37.Crotty S. Follicular helper CD4 T cells (TFH) Annu Rev Immunol. 2011;29:621–663. doi: 10.1146/annurev-immunol-031210-101400. [DOI] [PubMed] [Google Scholar]
  • 38.Sweet RA, Lee SK, Vinuesa CG. Developing connections amongst key cytokines and dysregulated germinal centers in autoimmunity. Curr Opin Immunol. 2012;24:658–664. doi: 10.1016/j.coi.2012.10.003. [DOI] [PubMed] [Google Scholar]
  • 39.Eto D, Lao C, DiToro D, Barnett B, Escobar TC, Kageyama R, Yusuf I, Crotty S. IL-21 and IL-6 are critical for different aspects of B cell immunity and redundantly induce optimal follicular helper CD4 T cell (Tfh) differentiation. PLoS One. 2011;6:e17739. doi: 10.1371/journal.pone.0017739. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 40.Masters SL, Gerlic M, Metcalf D, Preston S, Pellegrini M, O'Donnell JA, McArthur K, Baldwin TM, Chevrier S, Nowell CJ, et al. NLRP1 inflammasome activation induces pyroptosis of hematopoietic progenitor cells. Immunity. 2012;37:1009–1023. doi: 10.1016/j.immuni.2012.08.027. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 41.Kuriakose T, Kanneganti TD. Regulation and functions of NLRP3 inflammasome during influenza virus infection. Mol Immunol. 2017;86:56–64. doi: 10.1016/j.molimm.2017.01.023. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 42.Yamazaki T, Ichinohe T. Inflammasomes in antiviral immunity: clues for influenza vaccine development. Clin Exp Vaccine Res. 2014;3:5–11. doi: 10.7774/cevr.2014.3.1.5. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 43••.Harris J, Sharp FA, Lavelle EC. The role of inflammasomes in the immunostimulatory effects of particulate vaccine adjuvants. Eur J Immunol. 2010;40:634–638. doi: 10.1002/eji.200940172. This viewpoint article reviews the experimental evidence around the requirements of NLRP3 in adjuvant-induced adaptive immunity including the induction of an antibody response. [DOI] [PubMed] [Google Scholar]
  • 44.Ritvo PG, Churlaud G, Quiniou V, Florez L, Brimaud F, Fourcade G, Mariotti-Ferrandiz E, Klatzmann D. Tfr cells lack IL-2Ralpha but express decoy IL-1R2 and IL-1Ra and suppress the IL-1-dependent activation of Tfh cells. Sci Immunol. 2017;2 doi: 10.1126/sciimmunol.aan0368. [DOI] [PubMed] [Google Scholar]
  • 45•.Cucak H, Yrlid U, Reizis B, Kalinke U, Johansson-Lindbom B. Type I interferon signaling in dendritic cells stimulates the development of lymph-node-resident T follicular helper cells. Immunity. 2009;31:491–501. doi: 10.1016/j.immuni.2009.07.005. This work shows the importance of IFNAR signaling in dendritic cells in mouse TFH cell differentiation. [DOI] [PubMed] [Google Scholar]
  • 46.Nakayamada S, Poholek AC, Lu KT, Takahashi H, Kato M, Iwata S, Hirahara K, Cannons JL, Schwartzberg PL, Vahedi G, et al. Type I IFN induces binding of STAT1 to Bcl6: divergent roles of STAT family transcription factors in the T follicular helper cell genetic program. J Immunol. 2014;192:2156–2166. doi: 10.4049/jimmunol.1300675. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 47••.Sprokholt JK, Kaptein TM, van Hamme JL, Overmars RJ, Gringhuis SI, Geijtenbeek TBH. RIG-I-like receptor activation by dengue virus drives follicular T helper cell formation and antibody production. PLoS Pathog. 2017;13:e1006738. doi: 10.1371/journal.ppat.1006738. This work shows that type-I IFN-induced expression of IL-27 by Dengue virus infected human DC is critical for the instruction of human TFH cells, contrasting with direct stimulation of naive human CD4 T cells with type-I IFN, which inhibits their differentiation into TFH cells. Another relevant finding to the discussion here is that viral replication is necessary to activate RIG-I and MDA5 and to induce type-I IFN responses. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 48.Batten M, Ramamoorthi N, Kljavin NM, Ma CS, Cox JH, Dengler HS, Danilenko DM, Caplazi P, Wong M, Fulcher DA, et al. IL-27 supports germinal center function by enhancing IL-21 production and the function of T follicular helper cells. J Exp Med. 2010;207:2895–2906. doi: 10.1084/jem.20100064. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 49.Le Bon A, Schiavoni G, D'Agostino G, Gresser I, Belardelli F, Tough DF. Type i interferons potently enhance humoral immunity and can promote isotype switching by stimulating dendritic cells in vivo. Immunity. 2001;14:461–470. doi: 10.1016/s1074-7613(01)00126-1. [DOI] [PubMed] [Google Scholar]
  • 50•.Ichinohe T, Lee HK, Ogura Y, Flavell R, Iwasaki A. Inflammasome recognition of influenza virus is essential for adaptive immune responses. J Exp Med. 2009;206:79–87. doi: 10.1084/jem.20081667. In mice, inflammasome activation in response to Influenza virus infection is shown to be essential for mobilizing an effective adaptive immune response, including the ability to mount a protective IgA response. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 51.Eisenbarth SC, Colegio OR, O'Connor W, Sutterwala FS, Flavell RA. Crucial role for the Nalp3 inflammasome in the immunostimulatory properties of aluminium adjuvants. Nature. 2008;453:1122–1126. doi: 10.1038/nature06939. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 52.Li H, Willingham SB, Ting JP, Re F. Cutting edge: inflammasome activation by alum and alum's adjuvant effect are mediated by NLRP3. J Immunol. 2008;181:17–21. doi: 10.4049/jimmunol.181.1.17. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 53.Nencioni L, Villa L, Tagliabue A, Antoni G, Presentini R, Perin F, Silvestri S, Boraschi D. In vivo immunostimulating activity of the 163-171 peptide of human IL-1 beta. J Immunol. 1987;139:800–804. [PubMed] [Google Scholar]
  • 54.Reed SG, Pihl DL, Conlon PJ, Grabstein KH. IL-1 as adjuvant. Role of T cells in the augmentation of specific antibody production by recombinant human IL-1 alpha. J Immunol. 1989;142:3129–3133. [PubMed] [Google Scholar]
  • 55.Staruch MJ, Wood DD. The adjuvanticity of interleukin 1 in vivo. J Immunol. 1983;130:2191–2194. [PubMed] [Google Scholar]
  • 56.Nakae S, Asano M, Horai R, Sakaguchi N, Iwakura Y. IL-1 enhances T cell-dependent antibody production through induction of CD40 ligand and OX40 on T cells. J Immunol. 2001;167:90–97. doi: 10.4049/jimmunol.167.1.90. [DOI] [PubMed] [Google Scholar]
  • 57.Nakae S, Asano M, Horai R, Iwakura Y. Interleukin-1 beta, but not interleukin-1 alpha, is required for T-cell-dependent antibody production. Immunology. 2001;104:402–409. doi: 10.1046/j.1365-2567.2001.01337.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 58.Franchi L, Nunez G. The Nlrp3 inflammasome is critical for aluminium hydroxide-mediated IL-1beta secretion but dispensable for adjuvant activity. Eur J Immunol. 2008;38:2085–2089. doi: 10.1002/eji.200838549. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 59.McKee AS, Munks MW, MacLeod MK, Fleenor CJ, Van Rooijen N, Kappler JW, Marrack P. Alum induces innate immune responses through macrophage and mast cell sensors, but these sensors are not required for alum to act as an adjuvant for specific immunity. J Immunol. 2009;183:4403–4414. doi: 10.4049/jimmunol.0900164. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 60.Seubert A, Calabro S, Santini L, Galli B, Genovese A, Valentini S, Aprea S, Colaprico A, D'Oro U, Giuliani MM, et al. Adjuvanticity of the oil-in-water emulsion MF59 is independent of Nlrp3 inflammasome but requires the adaptor protein MyD88. Proc Natl Acad Sci U S A. 2011;108:11169–11174. doi: 10.1073/pnas.1107941108. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 61•.Krishnaswamy JK, Gowthaman U, Zhang B, Mattsson J, Szeponik L, Liu D, Wu R, White T, Calabro S, Xu L, et al. Migratory CD11b(+) conventional dendritic cells induce T follicular helper cell-dependent antibody responses. Sci Immunol. 2017;2 doi: 10.1126/sciimmunol.aam9169. This work identifies a migratory population of lung and skin dendritic cells that conveys the antigen to the lymph node to promote TFH cell differentiation during intranasal immunization. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 62.Chakarov S, Fazilleau N. Monocyte-derived dendritic cells promote T follicular helper cell differentiation. EMBO Mol Med. 2014;6:590–603. doi: 10.1002/emmm.201403841. [DOI] [PMC free article] [PubMed] [Google Scholar]

RESOURCES