NLRP3 in protective immunity and vaccination against respiratory infection

Keywords::

• inflammasome
• NLRP3
• respiratory
• vaccine

The WHO reports that acute respiratory infections caused by insults such as Streptococcus pneumoniae, Haemophilus influenzae and Mycobacterium tuberculosis are among the leading causes of deaths in children under 5 years of age [101]. The role of vaccines is to induce protective pathogen-specific adaptive immune responses, but this relies on the efficient and appropriate induction of innate immunity, which instructs the adaptive response. Host recognition of invading organisms is a crucial function of innate immune cells and is mediated by the sensing of conserved microbial moieties by pathogen recognition receptors.

There is increasing evidence that the nucleotide-binding domain, leucine-rich-repeat-containing protein (NLR) family play key roles in innate immune defense. In total, 23 members of this family have been identified and are expressed in many cell types, including lymphoid and epithelial cells, where they function as cytosolic sensors. These receptors consist of three distinct domains, a leucine-rich-repeat recognition domain, a nucleotide oligomerization domain required for oligomerization and an effector protein–protein interaction motif at the amino terminus. NLRs can be subdivided based on the nature of the N-terminal effector domain [1]. The subfamilies are:

• • NLRA: containing an acidic domain;

• • NLRB: containing a baculovirus inhibitor of apoptosis protein repeat (BIR) domain;

• • NLRC: containing a caspase recruitment (CARD) domain;

• • NLRP: containing a pyrin domain;

• • NLRX: containing a domain with no strong homology to the N-terminal domain of any other NLR subfamily member.

The NLRP family comprises 14 members and these are characterized by the presence of a pyrin domain. Of these, NLRP3 is the best characterized and has been implicated in responsiveness to a wide range of exogenous and endogenous stimuli. Upon cellular activation, NLRP3 forms large multiprotein complexes called inflammasomes, which result in activation of caspase-1 and the consequent processing of the proinflammatory cytokines IL-1β and IL-18. In humans, mutations in the NLRP3 gene, causing hyperactivation of the NLRP3 inflammasome, are associated with a variety of autoinflammatory disorders, such as Muckle–Wells syndrome and familial cold autoinflammatory syndrome [1].

Role of NLRP3 in protective immunity against respiratory pathogens

In recent years, the NLRP3 inflammasome has been implicated in the induction of protective immunity against a number of respiratory pathogens. The bacterium, Chlamydia pneumoniae, which is associated with atypical pneumonia, bronchitis and sinusitis, activates the NLRP3 inflammasome, resulting in enhanced IL-1β secretion from macrophages. In a mouse model of acute C. pneumoniae infection, IL-1 signaling was required for optimal neutrophil infiltration and host defense [2]. M. tuberculosis, the main causative agent of TB, induces IL-1β secretion at the site of infection and promotes NLRP3 inflammasome activation via an early secretory antigenic target (ESAT)-dependent process [3]. However, recent data indicate that host defense against chronic M. tuberculosis infection does not require NLRP3 or caspase-1 [4]. We recently showed that infection of dendritic cells with the Gram-positive bacterium, S. pneumoniae resulted in potent NLRP3-dependent IL-1β secretion and NLRP3 was required for protective immunity against respiratory infection with this bacterium [5]. In addition, Staphylococcus aureus, which can cause necrotizing pneumonia, activated NLRP3 via release of α-hemolysin [6].

Mouse models of infection with influenza A have also revealed a protective role for the NLRP3 inflammasome in respiratory viral infection. NLRP3 and caspase-1-deficient mice were more susceptible to infection than wild-type mice and this was associated with reduced cellular recruitment and cytokine production in the lungs [7]. These recent studies indicate that NLRP3 may play a central role in protection against respiratory infection with both bacterial and viral pathogens.

Role of NLRP3 in directing adaptive immunity

NLRP3 activation promotes the production of IL-1 and IL-18, which are potent stimuli for T-cell differentiation, particularly in promoting Th17 and Th1 responses. IL-1 plays a vital role in promoting the secretion of IL-17 [8], which exerts multiple effects in host defense including the induction of neutrophil-recruiting chemokines and the production of antimicrobial peptides. Hyperactivation of the NLRP3 inflammasome results in increased IL-1β secretion and enhanced Th17-cell differentiation [9].

IL-17 is required for host defense against a number of respiratory pathogens including Klebsiella pneumoniae, S. pneumoniae and Bordetella pertussis[10]. Infection of IL-17 receptor-deficient mice with K. pneumoniae resulted in reduced survival compared with wild-type animals [11], while adenylate cyclase toxin from B. pertussis promoted NLRP3-dependent IL-1 production, which was required for a protective Th17 cell response [12]. In humans, IL-17 is also thought to be important for protection against respiratory pathogens. Patients with hyper-IgE syndrome who have mutations in signal transducer and activator of transcription 3 (STAT3) and consequently have impaired Th17 differentiation are often characterized by recurrent bacterial and fungal infections, predominantly skin and lung infections caused by S. aureus[13].

In addition to the important role for IL-1 in Th17-cell induction, IL-18 is a key factor in promoting Th1 responses. IL-18 was recently shown to play a decisive role in protective immunity against M. tuberculosis and this was associated with a reduction in protective Th1 responses [14].

NLRP3 & vaccine design

The emerging importance of the NLRP3 inflammasome in mediating protection against respiratory pathogens provides a strong basis for the development of inflammasome-activating vaccines. Recent advances in the identification of antigen candidates provides great encouragement that new vaccines for Neisseria meningitidis serogroup B and other respiratory pathogens, including S. pneumoniae, may be developed in the near future. Effective adjuvants will be required to facilitate this and the known role of NLRP3 in protection against pneumococcus suggests that NLRP3-activating adjuvants may be beneficial. Particulate adjuvants including biodegradable micro- and nanoparticles are effective stimuli for NLRP3 inflammasome activation both in vitro and following injection [15]. The mechanism underlying inflammasome activation by these adjuvants has not been fully resolved, although roles for potassium efflux, lysosomal acidification and rupture, and subsequent cathepsin B release have been shown [15].

These particles have been widely used in experimental vaccine delivery to the GI and respiratory tracts due to their potential to protect antigen, enhance uptake and stimulate local and systemic immunity. However, extensive studies will be required to identify the optimal adjuvants for different routes of vaccination.

In conclusion, given the important role of NLRP3 in protection against a number of major respiratory pathogens and in producing key Th1 and Th17 cell-promoting cytokines, the development of vaccines that efficiently target this pathway is justified.

Financial & competing interests disclosure

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

No writing assistance was utilized in the production of this manuscript.