The role of CXCR2-ligands for neutrophil mobilization from the bone marrow to peripheral sites

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The bone marrow is a major reserve for cells of the innate immune system, especially neutrophil granulocytes. From here these cells can be rapidly mobilized to the periphery by action of physiologic stimuli or peripheral inflammation. But they can also cause severe damage, e.g. in the initiation phase of cartilage-destructive diseases such as rheumatoid arthritis. However, how individual neutrophils behave within the bone marrow cavity during the process of mobilization or cartilage destruction and which impact peripheral inflammation has on the process is little understood. Using a newly developed intravital 2-photon microscopy approach in mice carrying a neutrophil-specific allele of EGFP we have shown that one of the major physiologic mobilizers, the hematopoietic cytokine G-CSF, acts by rapidly inducing the motility of individual neutrophils within the marrow cavity of the tibia. On a molecular level we found the increased expression of CXCL1 (KC) and CXCL2 (MIP-2), ligands for the chemokine receptor CXCR2, by marrow-resident megakaryocytes. Further, a blockade of CXCR2 by a specific antibody could block neutrophil mobilization by G-CSF. In a parallel study we have used a newly developed mouse model for the overexpression of IL17A selectively in the skin. This led to a phenotype highly reminiscent of human psoriasis, which was accompanied by massive infiltration of neutrophils into the skin together with strong neutrophilia in the bone marrow. We assume that also here the modulation of CXCL1 and 2 in the bone and also the skin play a major role for the increased frequency of neutrophils in the bone marrow and their recruitment to the skin. To investigate these concepts we propose to generate a novel mouse model in which the alleles for CXCL1 and 2 are made conditional within the same animal. In addition, the fluorescent protein Tomato will be engineered as a knockin into the CXCL1 locus to function as a reporter for promoter activity. Injecting these mice with G-CSF or inducing IL17A overexpression in the skin will show, which cells and with which kinetic produce CXCL1/2 in response to these diverse stimuli. Furthermore, the crossing of these mice to either megakaryocyte- or keratinocyte-specific Cre lines will allow deleting the CXCR2-ligands in a cell type specific manner. This will enable us to clarify, whether physiologic expression of CXCR2 ligands in either bone marrow-resident or peripheral cells is decisive for neutrophil recruitment by systemic G-CSF or skin-derived IL17A. Finally, we will use the mouse model of skin-specific IL17A-overexpression to study bone loss and joint inflammation mediated by excess production of neutrophils in the bone marrow, which are two well known entities accompanying psoriatic skin disease. Collectively this study should provide us with a previously not available knowledge on the physiology and pathophysiology of neutrophils in the bone marrow and the periphery and the role of CXCR2-ligands for these processes, thus a very central mechanism of cellular osteoimmunology.