Obesity, leptin and host defence of Streptococcus pneumoniae: the case for more human research

Leptin signalling and immunological functions

Leptin is an adipocyte derived hormone, with cytokine like properties, whose function is mediated by binding to a leptin receptor encoded by the LEPR gene. Whilst there are six different isoforms of the leptin receptor, only the long isoform (LepRb) can completely tranduce leptin signalling [21]. LepRb is predominantly expressed in the hypothalamus and is present in all types of immune cells, involved in both innate and adaptive immunity [22].

Most of the biological functions of leptin occur via the Janus kinase 2–signal transducer and activator of transcription 3 (JAK2-STAT3) pathway (figure 1). Leptin binds to the long isoform LepRb which causes dimerisation and stimulates JAK2 autophorylation as well as phosphorylation of tyrosine residues (Tyr974, Tyr985, Tyr1077, Tyr1138) within the receptor. Leptin receptor–phosphorlyated tyrosine 1138 mediates the interaction with STAT3 which dimerise and translocate to the nucleus to activate gene transcription. Suppressor of cytokine signalling 3 (SOCS3) acts as a negative feedback signalling mechanism during prolonged stimulation of LepRb [23, 24]. Leptin induces the activation of the mitogen-activated protein kinase pathway through activation of extracellular signal-regulated kinase 1/2. This pathway is activated following JAK2 activation where Src homology-2 domain-containing protein tyrosine phosphatase-2 recruits growth factor receptor-bound protein 2 leading to activation of the signalling cascade (figure 1). Leptin further induces the phoshatidylinositol-3 kinase/protein kinase B (P13K/Akt) pathway through insulin receptor substrate 1 phosphorylation following initial JAK2 activation [21, 25] (figure 1). Both these pathways result in cell proliferation and survival.

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FIGURE 1

Leptin receptor and intracellular leptin signalling pathways. Leptin binds to LepRb causing dimerisation and phosphorylation of Janus kinase 2 (JAK2) and tyrosine residues within LepRb. Activation of JAK2 initiates three signalling pathways. 1) JAK2/signal transducer and activator of transcription 3 (STAT3) pathway: LepRb phosphorylated Tyr1138 mediates the dimerisation and translocation of STAT3 into the nucleus to activate gene transcription. Suppressor of cytokine signalling 3 (SOCS3) acts as a negative feedback mechanism during prolonged stimulation of LepRb. 2) Mitogen-activated protein kinase (MAPK) pathway: leptin induces the activation of Src homology-2 domain-containing protein tyrosine phosphatase-2 (SHP2), which recruits the adaptor protein growth factor receptor-bound protein 2 (GRB2) to initiate the RAS/RAF/MAPK kinase 1 (MEK1)/extracellular signal-regulated kinase 1/2 (ERK1/2) signalling cascade. 3) Phoshatidylinositol-3 kinase/protein kinase B (P13K/Akt) pathway: leptin mediates P13K/Akt activation via the insulin receptor substrate 1/2 (IRS1/2). These pathways cause cell proliferation and survival.

Almost all immune cells express leptin receptors. Leptin regulates both innate and adaptive immunity through the modulation of immune cell survival and proliferation. In innate immunity, leptin increases the cytotoxicity of natural killer cells and promotes the activation of granulocytes, macrophages and dendrite cells. This activation leads to the production of pro-inflammatory cytokines (e.g. tumour necrosis factor-α (TNF-α), IL-6, IL-12) which facilitate the shifting of T-cells towards type 1 T-helper cell (Th1) priming [22, 25, 26]. In adaptive immunity, leptin increases the proliferation and maturation of naïve T- and B-cells whilst decreasing the inhibitory effects of regulatory T-cells on the immune response [22, 25]. Leptin promotes pro-inflammatory Th1 cytokines rather than anti-inflammatory Th2 cytokines and facilitates Th17 responses. Lastly, leptin regulates B-cell development and activates B-cells to secrete cytokines [25].

Leptin deficiency, starvation and host defence

Much of the evidence examining the role of leptin in relation to pneumococcal infections has used genetically mutated nonobese mice with leptin (ob/ob), leptin receptor (db/db) or leptin signalling (s/s, LysM-LepRb-KO and CPE^fat/fat) deficiencies. Studies using leptin-deficient (ob/ob) mice have consistently shown reduced pulmonary clearance following S. pneumoniae challenge when compared to wild-type mice [27, 28]. Administration of exogenous leptin has improved survival, bacterial clearance and reduced bacteraemia [27]. Following infection with lower doses of S. pneumoniae, ob/ob mice exhibited lower pulmonary levels of pro-inflammatory cytokines such as TNF-α and chemokines. Leptin deficiency has not been shown to affect bacterial growth in the lungs [28]. It is further recognised that leptin receptor deficiencies (db/db) and subsequent signalling (s/s) impairment affects pulmonary host defence in mice [29, 30]. Disruption to different signalling pathways produces opposing effects on the host defence. Disruption to leptin receptor (LepRb) JAK2/STAT3 signalling enhanced leukotriene production and pulmonary bacterial clearance [29]. Whereas ablation of the leptin receptor (LepRb) in myeloid cells impaired S. pneumoniae pulmonary clearance and alveolar macrophage bactericidal function [30]. How this murine work applies to the regulation of bacterial pneumonia inflammation in the human lung needs greater consideration.

A further area of study has been the application of acute starvation models to understand how the metabolic regulatory role of leptin influences the host defence during pneumococcal infection [31]. In studies of nonobese mice, acute starvation during S. pneumoniae infection rapidly decreased leptin levels impairing host defence with associated reduced bronchoalveolar lavage fluid (BALF) neutrophil counts and levels of IL-6 and macrophage inflammatory protein 2. Leptin administration restored bacterial clearance in the lungs and increased levels of BALF neutrophils, cytokines, alveolar macrophages and leukotriene B4 synthesis.

The studies described above focus on leptin deficiency, receptor and signalling defects or conditions of starvation and not obesity per se. These distinctions are important because the pathogenesis of obesity directly influences our understanding of the role of leptin in pneumococcal disease: genetic mutations that cause obesity [27–30] and diet-induced obesity [1, 32], which is the main cause of obesity in humans. New evidence suggests that prolonged overnutrition leading to diet-induced obesity, hyperleptinaemia and leptin resistance, a hallmark of obesity, play a critical role in the immune response to bacterial pathogens [1, 20, 33].

Overnutrition, hyperleptinaemia and host defence

In diet-induced obesity, subjects have sustained higher serum leptin levels (hyperleptinaemia) and leptin resistance from prolonged high fat diet overnutrition than lean humans [34]. One study [1] examining the effect of obesity on the host defence against S. pneumoniae noted that CPE^fat/fat mice (carboxypeptidase E enzyme deficient mice; enzyme critical to processing prohormones and proneuropeptide regulation of appetite and energy expenditure) developed hyperglycaemia, raised serum leptin and triglyceride levels, and increased blood neutrophil count prior to S. pneumoniae infection. These findings, in conjunction with emerging evidence of hospitalised patients with severe pneumonia, suggest that it is not body weight per se but chronic hyperleptinaemia that is the link between obesity and increased risk of pulmonary infection [20].

Therefore, leptin resistance and the impact of altered inflammatory signalling in diet-induced obesity [35] need further consideration. Mutation in the leptin gene or the leptin receptor as described in the murine studies above are not the main causes of leptin resistance [35]. Instead, leptin resistance is induced by altered leptin transportation across the blood–brain barrier, deterioration in function of the leptin receptors accompanied by hypothalamic inflammation, endoplasmic reticulum stress and defective autophagy [34, 36, 37]. In high fat diet fed mice, leptin transport across the blood–brain barrier is substantially decreased [38]. In obese humans with severe hyperleptinaemia, leptin levels in the cerebrospinal fluid are only slightly increased [36]. Therefore, it is possible that pulmonary host defence may be impaired differently, in various models of obesity, such as diet-induced obesity with or without leptin resistance.

Clinical studies

There are very few clinical studies examining the relationship between leptin levels in obesity and S. pneumoniae infections in humans. Díez et al. [32] specifically examined the relationship between leptin and the outcomes of hospitalised patients with confirmed diagnoses of CAP. This study showed no difference in the serum leptin levels between hospitalised CAP patients and the healthy control group, when adjusting for body mass index. At the time of admission, approximately 5 days from symptom onset, a linear relationship between leptin levels and body mass index, body fat and muscle mass was observed suggesting that leptin acts as a nutritional marker, rather than an inflammatory reactant. However, the role of leptin in the host defence of pneumococcal disease and pneumococcal nasopharyngeal colonisation, in the absence of infection-induced starvation, remains unknown.

Determining the susceptibility or risk of infection in relation to obesity requires carefully controlled human infection studies that investigate pre and post exposure to S. pneumoniae. Similarly, the timing of infection in regards to leptin levels is critical to understand the severity of illness. In mice, leptin levels rise immediately after infection and decline after 24 h [31]. In humans, the time between infection and hospital admission is likely to be greater than 24 h and vary considerably between patients.

Often clinical studies involve obese patients who have multiple comorbidities, known to increase susceptibility to pneumococcal disease, thus making direct links with obesity difficult. Clinical data supports the notation that type 2 diabetes is a risk factor for pneumococcal and other types of CAP [39, 40]. Since approximately 90% of patients with type 2 diabetes are overweight or obese [41], it is difficult to determine if obesity or diabetes is responsible for impaired host defence in humans. Research that includes normal weight patients with diabetes is needed to address this issue. Likewise, the question of greater rates of colonisation in humans with obesity requires carefully controlled human infection studies. Patients with and without type 2 diabetes would need to be included in these studies since this is an important distinction that will affect understandings of host defence.

Body mass index measurements are frequently used in clinical studies as a proxy for adiposity despite not directly measuring fat mass. Given that leptin is primarily produced in adipocyte cells, collecting data on waist circumference, bioelectric impedance and dual energy X-ray absorptiometry scans will provide more accurate data. Furthermore, 10–30% of people with obesity have normal metabolic parameters (metabolically healthy obesity) [42]. Recording metabolic syndrome or measures of metabolic status will provide a more accurate assessment of metabolic health and assist in the distinction between the host defence mechanisms resulting from obesity and diabetes.

Conclusion

Collectively, the evidence suggests that there are both metabolic and immune responses that influence the role of leptin in the host defence of S. pneumoniae in obese subjects; the effects of these responses being intricately related to the pathogenesis of obesity (genes or diet), the acute phase of illness, nutritional status, and the severity of the disease. Furthermore, the evidence suggests that not only leptin deficiency but prolonged hyperleptinaemia and leptin resistance may impair the pulmonary host defence as observed in diet-induced obesity. As most of our understanding about the impact of obesity on the host defence to pneumococcal disease comes from research in genetically mutated mice models, direct comparisons of murine models with diet-induced obesity in humans may not be feasible. There is a paucity of clinical research specifically examining pneumococcal nasopharyngeal colonisation, the prerequisite for pneumococcal infection, in obesity. Further research is required to understand how S. pneumoniae colonisation is regulated in obese populations and informs our understanding of susceptibility to subsequent pneumococcal disease; and how hyperleptinaemia in diet-induced obesity influences nasopharyngeal colonisation and lung bacterial burden. The distinction between the influence of obesity and type 2 diabetes on host defence to S. pneumoniae exposure, colonisation and pneumococcal disease needs further exploration. The use of human challenge models [43–45] in people with an extreme body mass index could enable a better understanding of the interactions between S. pneumoniae exposure, colonisation and subsequent mucosal and systemic immunity in humans with obesity.