Targeting Neutrophil Migration as a Therapeutic Strategy for Chronic Inflammatory Diseases

Abstract:

Neutrophils play a critical role in the innate immune response but their dysregulated recruitment and activation can drive tissue damage in chronic inflammatory conditions. This review examines current understanding of the molecular mechanisms regulating neutrophil migration and discusses emerging approaches to therapeutically modulate this process in inflammatory diseases. We focus on key adhesion molecules, chemoattractant receptors, and signaling pathways involved in neutrophil mobilization, adhesion, and transmigration. Novel strategies to selectively inhibit pathological neutrophil infiltration while preserving host defense functions are highlighted, including glycan-based targeting of CD11b/CD18 integrins. The potential and challenges of neutrophil-directed therapies for conditions such as inflammatory bowel disease and rheumatoid arthritis are evaluated.

Introduction:

Neutrophils are the most abundant circulating leukocytes and serve as a critical first line of defense against invading pathogens. Their rapid recruitment from the bloodstream to sites of infection or injury is essential for effective innate immunity. However, excessive or persistent neutrophil infiltration into tissues can drive chronic inflammation and organ damage in numerous pathological conditions, including inflammatory bowel disease (IBD), rheumatoid arthritis (RA), and chronic obstructive pulmonary disease (COPD) [1,2]. 

The neutrophil migration cascade involves a coordinated series of adhesive interactions with the vascular endothelium, followed by transmigration across the endothelial barrier and chemotaxis within tissues. This process is tightly regulated by adhesion molecules, chemoattractant receptors, and intracellular signaling pathways [3]. Emerging research has revealed substantial heterogeneity and plasticity in neutrophil populations, with distinct phenotypes and functions arising in different tissue microenvironments [4]. 

Given the central role of neutrophils in driving inflammatory pathology, there is significant interest in developing therapeutic approaches to selectively inhibit their aberrant recruitment while preserving beneficial antimicrobial functions. This review examines our current understanding of neutrophil migratory mechanisms and discusses novel strategies to target this process for the treatment of chronic inflammatory diseases.

Molecular Mechanisms of Neutrophil Migration:

Mobilization from Bone Marrow:
Neutrophil mobilization from the bone marrow is regulated by the balance between retention signals (e.g. CXCL12-CXCR4 axis) and mobilization factors (e.g. G-CSF, CXCL1/2) [5]. The transcription factor C/EBPε plays a key role in terminal neutrophil differentiation and egress from the bone marrow [6]. In vivo studies using C/EBPε knockout mice have demonstrated its importance for normal neutrophil maturation and function [7].

Endothelial Adhesion and Rolling:
Initial neutrophil capture and rolling on inflamed endothelium is mediated by selectins interacting with glycosylated ligands. P-selectin glycoprotein ligand-1 (PSGL-1) on neutrophils binds to P- and E-selectin on activated endothelium [8]. L-selectin on neutrophils can also mediate secondary capture of additional neutrophils. In vivo imaging using PSGL-1 knockout mice has revealed the critical importance of this molecule for neutrophil rolling and recruitment in various inflammatory models [9].

Firm Adhesion:
Firm adhesion of neutrophils to the endothelium is primarily mediated by β2 integrins, particularly LFA-1 (CD11a/CD18) and Mac-1 (CD11b/CD18), binding to endothelial ICAM-1 and ICAM-2 [10]. Intravital microscopy studies in CD11a and CD11b knockout mice have demonstrated distinct roles for these integrins, with LFA-1 being crucial for initial arrest while Mac-1 mediates intravascular crawling [11].

Transmigration:
Neutrophil transmigration across the endothelium occurs via paracellular or transcellular routes and involves additional adhesion molecules such as PECAM-1, JAM-A, and CD99 [12]. The junctional adhesion molecule JAM-C has emerged as a key regulator of neutrophil transendothelial migration, with JAM-C knockout mice showing impaired neutrophil recruitment in peritonitis and ischemia-reperfusion injury models [13].

Interstitial Migration:
Once in the tissue, neutrophils migrate along chemokine gradients towards inflammatory foci. This process is less dependent on integrins and instead relies on actin polymerization and myosin-based contraction [14]. The small GTPases Rac and Cdc42 play important roles in regulating neutrophil chemotaxis and polarization [15].

Novel Approaches to Target Neutrophil Migration:

Glycan-Based Targeting of CD11b/CD18:
Recent work has highlighted the potential of targeting specific glycan epitopes on neutrophil integrins to modulate their function. Kelm et al. demonstrated that the N-linked glycans on CD11b/CD18 can be selectively targeted with lectins to inhibit neutrophil transepithelial migration [16]. Specifically, targeting of biantennary galactosylated glycans with the lectin PHA-E blocked neutrophil transmigration and increased adhesion to intestinal epithelial cells. This approach offers the potential for more selective modulation of neutrophil functions compared to global integrin blockade.

Chemokine Receptor Antagonists:
Inhibition of key chemokine receptors involved in neutrophil recruitment, such as CXCR1/2, has shown promise in preclinical models of inflammatory disease. The CXCR1/2 antagonist reparixin reduced neutrophil infiltration and tissue damage in animal models of ischemia-reperfusion injury [17]. However, clinical trials in humans have yielded mixed results, highlighting the need for more selective approaches.

Targeting Neutrophil Subsets:
The identification of distinct neutrophil subpopulations with pro- or anti-inflammatory properties opens up possibilities for more nuanced therapeutic strategies. For example, targeting of the pro-inflammatory CD177+ neutrophil subset using anti-CD177 antibodies reduced disease severity in a mouse model of inflammatory arthritis [18].

Modulation of Neutrophil Reverse Migration:
Recent studies have highlighted the importance of neutrophil reverse migration (retrograde migration from tissues back into the vasculature) in resolving inflammation. Promoting this process through targeting of molecules like JAM-C could represent a novel approach to reduce tissue neutrophil burden [19].

Nanoparticle-Based Delivery:
Nanoparticle-based drug delivery systems offer the potential for more targeted modulation of neutrophil migration. For example, nanoparticles loaded with the neutrophil chemoattractant CXCL1 have been used to redirect neutrophils away from sites of inflammation in a mouse model of inflammatory arthritis [20].

Challenges and Future Directions:
While targeting neutrophil migration holds promise for treating chronic inflammatory diseases, several challenges remain. A key consideration is maintaining an appropriate balance between inhibiting pathological neutrophil recruitment and preserving essential host defense functions. More selective approaches targeting specific neutrophil subsets or functions may help address this issue.

Another challenge is the heterogeneity of neutrophil phenotypes and functions across different tissue environments and disease states. Further characterization of neutrophil subpopulations and their roles in various inflammatory conditions will be crucial for developing tailored therapeutic strategies.

The timing of neutrophil-targeted interventions is also critical, as neutrophils play important roles in both the initiation and resolution of inflammation. Combining neutrophil-directed therapies with other immunomodulatory approaches may be necessary to achieve optimal therapeutic effects.

Conclusion:
Neutrophil migration represents a promising therapeutic target for chronic inflammatory diseases. Advances in our understanding of the molecular mechanisms regulating neutrophil recruitment and function have revealed novel opportunities for intervention. Glycan-based targeting of adhesion molecules, modulation of specific neutrophil subsets, and promotion of inflammation-resolving neutrophil behaviors are among the innovative approaches being explored. As our knowledge of neutrophil biology continues to expand, more selective and effective strategies for modulating neutrophil migration in inflammatory diseases are likely to emerge.

References:

1. Mantovani A, et al. Neutrophils in the activation and regulation of innate and adaptive immunity. Nat Rev Immunol. 2011;11(8):519-531.

2. Soehnlein O, et al. Neutrophils as protagonists and targets in chronic inflammation. Nat Rev Immunol. 2017;17(4):248-261.

3. Ley K, et al. Getting to the site of inflammation: the leukocyte adhesion cascade updated. Nat Rev Immunol. 2007;7(9):678-689.

4. Ng LG, et al. Heterogeneity of neutrophils. Nat Rev Immunol. 2019;19(4):255-265.

5. Eash KJ, et al. CXCR2 and CXCR4 antagonistically regulate neutrophil trafficking from murine bone marrow. J Clin Invest. 2010;120(7):2423-2431.

6. Yamanaka R, et al. Impaired granulopoiesis, myelodysplasia, and early lethality in CCAAT/enhancer binding protein epsilon-deficient mice. Proc Natl Acad Sci USA. 1997;94(24):13187-13192.

7. Lekstrom-Himes JA, et al. Neutrophil-specific granule deficiency results from a novel mutation in the gene encoding transcription factor CCAAT/enhancer binding protein epsilon. J Exp Med. 1999;189(11):1847-1852.

8. McEver RP, Cummings RD. Role of PSGL-1 binding to selectins in leukocyte recruitment. J Clin Invest. 1997;100(11 Suppl):S97-103.

9. Sperandio M, et al. P-selectin glycoprotein ligand-1 mediates L-selectin-dependent leukocyte rolling in venules. J Exp Med. 2003;197(10):1355-1363.

10. Phillipson M, et al. Intraluminal crawling of neutrophils to emigration sites: a molecularly distinct process from adhesion in the recruitment cascade. J Exp Med. 2006;203(12):2569-2575.

11. Henderson RB, et al. The use of lymphocyte function-associated antigen (LFA)-1-deficient mice to determine the role of LFA-1, Mac-1, and alpha4 integrin in the inflammatory response of neutrophils. J Exp Med. 2001;194(2):219-226.

12. Muller WA. Mechanisms of leukocyte transendothelial migration. Annu Rev Pathol. 2011;6:323-344.

13. Woodfin A, et al. JAM-A mediates neutrophil transmigration in a stimulus-specific manner in vivo: evidence for sequential roles for JAM-C and PECAM-1 in neutrophil transmigration. Blood. 2007;110(6):1848-1856.

14. Lämmermann T, et al. Rapid leukocyte migration by integrin-independent flowing and squeezing. Nature. 2008;453(7191):51-55.

15. Szczur K, et al. Rho GTPase CDC42 regulates directionality and random movement via distinct MAPK pathways in neutrophils. Blood. 2006;108(13):4205-4213.

16. Kelm M, et al. Regulation of neutrophil function by selective targeting of glycan epitopes expressed on the integrin CD11b/CD18. FASEB J. 2020;34(2):2326-2343.

17. Bertini R, et al. Noncompetitive allosteric inhibitors of the inflammatory chemokine receptors CXCR1 and CXCR2: prevention of reperfusion injury. Proc Natl Acad Sci USA. 2004;101(32):11791-11796.

18. Wang X, et al. Neutrophil CD177 (human) and Ly6G (mouse) expression distinguishes inflammatory from steady-state neutrophils and reveals a pro-inflammatory phenotype. bioRxiv. 2020. doi: 10.1101/2020.05.29.123307.

19. Colom B, et al. Leukotriene B4-neutrophil elastase axis drives neutrophil reverse transendothelial cell migration in vivo. Immunity. 2015;42(6):1075-1086.

20. Bartneck M, et al. Nanoparticle-based targeting of neutrophils for improved anti-inflammatory therapy. ACS Nano. 2019;13(11):13391-13403.