Neutrophils are known to play an important role in inflammatory responses by virtue of their ability to perform a series of effector functions that collectively represent a major mechanism of innate immunity against injury and infection. In recent years, however, it has become obvious that the contribution of neutrophils to host defence and natural immunity extends well beyond their traditional role as professional phagocytes. Indeed, neutrophils can be induced to express a number of genes whose products lie at the core of inflammatory and immune responses. These include not only Fc receptors, complement components, cationic antimicrobial and NADPH oxidase proteins, but also a variety of cytokines (including tumour necrosis factor-alpha, interleukin (IL)-1beta, IL-1R alpha, IL-12 and vascular endothelial growth factor), and chemokines such as IL-8, growth-related gene product, macrophage inflammatory protein (MIP)-1alpha, MIP-1beta, interferon-gamma-inducible protein of 10 kDa and monokine induced by interferon-gamma. Because these chemokines are primarily chemotactic for neutrophils, monocytes, immature dendritic cells and T-lymphocyte subsets, a potential role for neutrophils in orchestrating the sequential recruitment of distinct leukocyte types to the inflamed tissue is likely to occur. The purpose of this review is to summarize the essential features of the production of chemokines by polymorphonuclear neutrophil leukocytes and the contribution that we have made to characterize some aspects of this newly discovered crucial function of neutrophils.

Myeloid leukocyte recruitment into the lung in response to environmental cues represents a key factor for the induction of lung damage. We report that Hck and Fgr-deficient mice show a profound impairment in early recruitment of neutrophils and monocytes in response to bacterial lipopolysaccharide (LPS). The reduction in interstitial and airway neutrophil recruitment was not due to a cell-intrinsic migratory defect, because Hck and Fgr-deficient neutrophils were attracted to the airways by the chemokine CXCL2 as wild type cells. However, early accumulation of chemokines and TNFα in the airways was reduced In hck−/−fgr−/− mice. Considering that chemokine and TNFα release into the airways was neutrophil-independent, as suggested by a comparison between control and neutrophil-depleted mice, we examined LPS-induced chemokine secretion by neutrophils and macrophages in wild type and mutant cells. Notably, mutant neutrophils displayed a marked deficit in their capability to release the chemokines CXCL1, CXCL2, CCL3 and CCL4, and TNFα, in response to LPS. However, intracellular accumulation of these chemokines and TNFα in, as well as secretion of a wide array of cytokines, including IL-1α, IL-1β, IL-6 and IL-10, by hck−/−fgr−/− neutrophils was normal. Intriguingly, secretion of CXCL1, CXCL2, CCL2, CCL3, CCL4, RANTES and TNFα, but not IL-1α, IL-1β, IL-6, IL-10 and GM-CSF, was also markedly reduced in bone marrow-derived macrophages (BMDM). Consistently, the Src kinase inhibitors PP2 and dasatinib reduced chemokine secretion by neutrophils and BMDM. These findings identify Src kinases as critical regulator of chemokine secretion in myeloid leukocytes during lung inflammation.

Using a mouse model of allergic lung inflammation, we found that mice deficient of Fgr, a Src family tyrosine kinase highly expressed in myelomonocytic cells, fail to develop lung eosinophilia in response to repeated challenge with aerosolized OVA. Both tissue and airway eosinophilia were markedly reduced in fgr−/− mice, whereas mice with the sole deficiency of Hck, another Src family member, responded normally. Release of allergic mediators, such as histamine, IL-4, RANTES/CCL5, and eotaxin/CCL11, in the airways of OVA-treated animals was equal in wild-type and fgr−/− mice. However, lung eosinophilia in Fgr-deficient mice correlated with a defective accumulation of GM-CSF and IL-5 in the airways, whereas secretion of these cytokines by spleen cells in response to OVA was normal. Examination of mRNA expression in whole lung tissue allowed us to detect comparable expression of transcripts for eotaxin/CCL11, macrophage-inflammatory protein-1α/CCL3, macrophage-inflammatory protein-1β/CCL4, monocyte chemoattractant protein-1/CCL2, TCA-3/CCL1, IL-4, IL-10, IL-2, IL-3, IL-9, IL-15, and IFN-γ in OVA-sensitized wild-type and fgr−/− mice. In contrast, the increase in IL-5 and IL-13 mRNA expression was lower in fgr−/− compared with wild-type mice. These findings suggest that deficiency of Fgr results in a marked reduction of lung eosinophilia and the establishment of a positive feedback loop based on autocrine secretion of eosinophil-active cytokines. These results identify Fgr as a novel pharmacological target to control allergic inflammation.