Innate immunity exploits biochemical weapons in different tissues of the human body and has its main alarm system switch in the “inflammasome,” a multiprotein complex made of pattern recognition receptors and caspase-1, which processes pro-interleukin (IL)-1 into its mature form, IL-1β. All activated innate immune cells display their antibacterial activity through the inflammasome-mediated production of powerful inflammatory cytokines, such as IL-1β, to counteract microbial threats [1,2]. The discovery of monogenic defects in the innate immunity has clarified that both inflammasome activity and IL-1 release are altered in a cluster of rare diseases marked by recurrent inflammatory symptoms affecting skin, serosal membranes, joints, gut, central nervous system and other tissues, in which IL-1β represents the main driver of inflammation [3,4]. The word “autoinflammatory” in ‘autoinflammatory diseases’ (ADs) describes the “apparent” spontaneous burst of inflammation, as infectious triggers, autoreactive T lymphocytes and/or specific autoantibodies are absent [5]. ADs are caused by impaired production of pro-inflammatory cytokines, with IL-1β playing a seminal role, along with a delayed shut-down of the immune response, leading to recurrent episodes of fever, and in some cases, inflammation limited to specific organs [6]. These diseases can be categorized in hereditary monogenic and multifactorial polygenic disorders encompassing many protean conditions such as periodic fever/aphthous stomatitis/pharyngitis/cervical adenopathy (PFAPA) syndrome, which is still the most mysterious cause of idiopathic recurrent fevers in childhood [7]. All these disorders represent a diagnostic challenge because their symptoms are nonspecific and sometimes refer to infectious or malignant diseases. Although many classification criteria have been developed to help diagnose rare monogenic ADs, patients with recurrent fevers and inflammatory symptoms experience a general delay in identification of the disease, which can lead to further morbidities and dreadful complications, such as AA-amyloidosis [8,9]. The repertoire of ADs was expanded to include inflammatory diseases such as Behçet’s disease, gout and idiopathic recurrent pericarditis, all of which have autoinflammatory-mediated mechanisms and a presumed polygenic basis [10]. Identification of the causative genes has also allowed for the confirmation of the clinical diagnosis in familial Mediterranean fever (FMF); tumor necrosis factor receptor-associated periodic syndrome (TRAPS); cryopyrin-associated periodic syndrome (CAPS), which includes familial cold-induced autoinflammatory syndrome (FCAS), Muckle-Wells syndrome (MWS) and chronic infantile neurological cutaneous articular (CINCA) syndrome; mevalonate kinase deficiency (MKD); pyogenic diseases including pyogenic arthritis/pyoderma gangrenosum/acne (PAPA) syndrome, Majeed syndrome (MS) and deficiency of the IL-1 receptor antagonist (DIRA); Blau syndrome (BS); OTULIN-related autoinflammatory syndrome (ORAS) and proteasome-associated autoinflammatory syndromes (PRAAS). A summary of the monogenic ADs is listed in Table 1. Genes associated with these diseases have been sequentially identified since 1997 onwards, and with the exception of MKD, the majority of them encode for proteins involved in the inflammasome architecture or in programmed cell death [11]. In particular, inflammasomes, structured with a central sensor protein, an adaptor protein and CASPASE-1, modulate IL-1β release and work as platforms to protect the human body from the outnumber of pathogenic organisms [12].

Autoinflammatory diseases of unknown etiology with a presumed either polygenic or multifactorial origin have been recognized in adult people, including Behçet’s disease, idiopathic recurrent pericarditis, crystal-induced arthropathies and adult-onset Still’s disease, but also in children, including systemic juvenile idiopathic arthritis (sJIA), Kawasaki disease (KD) and PFAPA syndrome. These conditions are largely characterized by dysregulation of the innate immune network and upregulation of inflammasome-associated genes [76]. The whole IL-1 cytokine family is abundantly involved in polygenic/multifactorial ADs, and different randomized placebo-controlled clinical trials have confirmed the efficacy of IL-1 inhibitors in their management [77]. Symptoms of such complex heterogeneous diseases, including recurrent fevers, synovitis and serositis, may overlap with monogenic ADs; in addition, some well-known life-threatening complications as macrophage activation syndrome or reactive amyloidosis might occur [78,79,80]. In particular, sJIA, part of the group of childhood arthritides, has peculiar characteristics derived from uncontrolled activation of phagocytes and hypersecretion of both IL-1 and IL-6, differently from other forms of juvenile idiopathic arthritis. Indeed, sJIA is marked by extra-articular signs that include spiking fevers, rash, hepatosplenomegaly, generalized lymphadenopathy and serositis [81]. Table 7 defines sJIA according to the PRINTO classification criteria [82].

Another febrile disorder with a supposed autoinflammatory basis is KD, an acute self-limiting vasculitis of unknown etiology, usually affecting children younger than 5 years, particularly those of Asian descent. This condition has a typical monophasic course characterized by unremitting high fever combined with a constellation of nonspecific skin, orofacial or cervical signs: the risk of coronary artery damage in 1/4 of untreated patients makes KD the most common cause of acquired heart disease for children living in the developed world [83,84]. Despite more than 5 decades of enquiries, the underlying mechanisms provoking coronary artery suffering in KD remain unknown: some scientific gaps include genetic predisposition to KD, dysregulated activation of autoinflammatory pathways and tendency to recur in a minority of cases. A series of studies indicate that the primum movens may be an abnormal immune response to different infectious agents, causing both endothelial cell malfunction and turbulent inflammatory cascade in a genetically-predisposed child [85]. Treatment with intravenous immunoglobulin during the first 10 days of disease decreases the risk of developing coronary artery aneurysms by five-fold, but non-responders are those with the higher risk of developing heart complications [86,87]. The multi-systemic inflammatory syndrome seen in children with ‘coronavirus disease 2019′ has been described to partially overlap with KD, but the upregulation of autoinflammation-related genes and hypersecretion of IL-1α, IL-6 and TNF are typical of KD [88]. Early identification of KD patients refractory to immunoglobulin might allow a more intensive treatment to prevent coronary artery abnormalities: for instance, Koné-Paut et al. recently found that intravenous immunoglobulin-resistant KD patients could be successfully treated with IL-1 blockade (i.e., anakinra) to obtain temperature normalization and overall improvement of vasculitis-related manifestations [89].

The most frequent cause of recurrent fevers in children younger than 5 years remains PFAPA syndrome, defined by febrile attacks having “clockwork” periodicity with stereotyped signs affecting the oral cavity and neck alternated by periods of whole well-being: this autoinflammatory disease displays a negative impact on child’s and parents’ quality of life, though its outcome is generally favorable due to remission after an unpredictable period of months or years [90]. There are limited studies focusing on the cyclic nature of PFAPA symptom recurrence, though clock-related genes and their interaction with different immunologic activities have been proven [91]. PFAPA clinical picture overlaps with several other causes of recurring fevers in the pediatric population, such as recurrent tonsillitis, Behçet’s disease and mostly cyclic neutropenia, an ultra-rare hereditary condition diagnosed via demonstration of periodic oscillations of the neutrophil count every 21 days [92]. Adults with de novo PFAPA syndrome or with reappearance of PFAPA symptoms after a first disease resolution have been increasingly reported, but they have a less strict phenotype [93,94]. IL-1-mediated PFAPA pathogenesis suggests that the syndrome could be framed as a rhythmic self-limited dysregulation of innate immunity, disrupting the commensal oral ecosystem and specifically the microbial community in the tonsils [95]. The genetic susceptibility of PFAPA syndrome is yet to prove, although many overlapping symptoms with monogenic hereditary ADs, its dominant recurrency in about 10% of PFAPA patients, hyperexpression of inflammasome-associated genes during febrile flares, and therapeutic efficacy of IL-1 blockers suggest a potential genetic origin [96]. Recently, Sangiorgi et al. studied a small population of familial and sporadic cases of PFAPA-like patients identifying variants in the ALPK1 gene [97]; a specific missense mutation (T237M) in this same gene has been also related to a new autosomal dominant autoinflammatory condition called ROSAH syndrome, characterized by retinal dystrophy, optic nerve edema, splenomegaly, anhidrosis and migraine [98]. More studies are obviously needed to confirm the role of such rare ALPK1 variants in larger cohorts of PFAPA patients. Table 8 shows the definition of PFAPA syndrome in childhood according to Marshall’s criteria [99], criteria set off for adult patients [100], and according to Eurofever/PRINTO classification criteria [50].