Neutrophil homeostasis and inflammation: novel paradigms from studying periodontitis

Abstract

Once viewed as simply antibacterial effector cells packed with antimicrobials, neutrophils are now increasingly appreciated for their regulatory roles in immunity and inflammation. The homeostatic regulation of neutrophils is thus crucial for optimal operation of the immune system. An attractive model to understand mechanistically the role of neutrophils is periodontitis, an oral inflammatory disease that is particularly sensitive to neutrophil alterations in numbers or function. The recruitment and proper activation of neutrophils are largely dependent on leukocyte integrins and complement. This review discusses how these processes are affected by host genetic or microbial factors leading to the development of periodontitis. For instance, both hypo- and hyper-recruitment of neutrophils as a result of deficiencies in the expression of β2 integrins or their negative regulators, respectively, causes unwarranted IL-17-dependent inflammatory bone loss. Moreover, microbial hijacking of C5aR (CD88) signaling in neutrophils impairs their antimicrobial function while promoting destructive inflammatory responses. These studies not only support the concept that neutrophil homeostasis is key to periodontal health but also reveal promising, new therapeutic targets as discussed in the review.

Keywords: Del-1; IL-17; complement; integrins; leukocyte adhesion deficiency.

Figure 1.. Del-1 antagonizes LFA-1 and regulates neutrophil extravasation.

The neutrophil extravasation process is a cascade of low- and high-affinity adhesive interactions between the neutrophils and the vascular endothelium and involves distinct steps, including capturing, rolling, slow rolling, firm adhesion of activated neutrophils, and transmigration [10, 20]. Del-1 binds the LFA-1 integrin and antagonizes its interaction with ICAM-1, thereby blocking LFA-1-dependent leukocyte adhesion onto the vascular endothelium. As firm neutrophil adhesion to the endothelium is essential to subsequent transmigration, Del-1 restrains the migration of neutrophils from the circulation to peripheral tissues [23, 25].

Figure 2.. The neutrophil rheostat (neutrostat) mechanism.

IL-17 promotes granulopoiesis and mobilization of mature neutrophils from the bone marrow by acting through up-regulation of G-CSF. The neutrostat senses neutrophil recruitment and clearance in peripheral tissues and regulates neutrophil production through a negative-feedback loop involving the IL-23–IL-17–G-CSF cytokine cascade. Specifically, after their release from the bone marrow, circulating neutrophils can normally extravasate to tissues in response to inflammation or infection. Upon senescence, neutrophils become apoptotic and are phagocytosed by tissue phagocytes, leading to suppression of their IL-23 production, in turn, down-regulating the IL-17–G-CSF axis for maintaining steady-state neutrophil counts [29].

Figure 3.. Cross-talk and reciprocal recruitment of neutrophils and Th17 cells.

Neutrophils produce CCL2 and CCL20 chemokines, which can selectively recruit Th17 cells by acting on Th17-expressed CCR2 and CCR6. Th17 cells secrete IL-17, which by acting mainly through fibroblast up-regulation of G-CSF and CXC chemokines, can orchestrate bone marrow production and release of neutrophils and their chemotactic recruitment to inflammatory sites [52].

Figure 4.. Disruption of the neutrostat in LAD-I causes IL-17-dependent inflammatory bone loss.

In LAD-I, β₂ integrin-deficient neutrophils fail to extravasate, thereby disrupting the neutrostat circuit. The absence of recruited neutrophils to the periodontal tissue of LAD-I patients leads to unrestrained local production of IL-23 and hence, IL-17 and G-CSF. Whereas increased G-CSF leads to excessive granulopoiesis and blood neutrophilia, elevated IL-17 leads to inflammatory bone loss [47].

Figure 5.. Microbial subversion of neutrophil function leads to periodontal inflammation.

P. gingivalis can manipulate C5aR signaling in neutrophils by means of enzymes (HRgpA and RgpB gingipains) that have C5 convertase-like activity and generate high local concentrations of the C5a ligand. Therefore, the bacterium can coactivate C5aR and TLR2 in neutrophils, and the resulting cross-talk leads to E3 ubiquitin ligase Smurf1-dependent ubiquitination and proteasomal degradation of MyD88, thereby inhibiting a host-protective antimicrobial pathway. Moreover, the C5aR–TLR2 cross-talk activates PI3K, which prevents phagocytosis through inhibition of RhoA and actin polymerization, while promoting an inflammatory response. In contrast to MyD88, Mal is a component of the subversive pathway, acting upstream of PI3K. The integrated, subversive strategy provides "bystander" protection to otherwise susceptible bacterial species and promotes dysbiotic inflammation in vivo; from reference [82] with permission. HRgpA, high molecular mass arginine-specific gingipain A; Pg, P. gingivalis; P.RgpB, arginine-specific gingipain B.