Background and objectives Genetic deletion of specific leukocyte lineages strongly contributes to understanding the role of various leukocyte subsets in physiological and pathological conditions. There have been a number of attempts to generate genetically neutrophil-deficient mouse strains. However, all those strains suffer from substantial limitations such as the limited efficiency of neutrophil deletion, the effect of the mutations on other lineages or the limited survival of the mutant mice. We have previously shown that the antiapoptotic Mcl-1 protein is essential for the survival of neutrophils but not macrophages. Therefore, we tested whether myeloid-specific deletion of Mcl-1 could provide a novel and more suitable genetic model of neutrophil deficiency in experimental mice.
Materials and methods LysMCre mice expressing Cre recombinase in the myeloid compartment were crossed with animals carrying the Mcl-1flox mutation to obtain LysMCre/CreMcl-1flox/flox (referred to as Mcl-1ΔMyelo) mice characterised by myeloid-specific conditional deletion of Mcl-1. Leukocyte populations in the peripheral blood, spleen and bone marrow of intact Mcl-1ΔMyelo mice and Mcl-1ΔMyelo bone marrow chimaeras were tested by flow cytometry. K/BxN serum-transfer arthritis was used as a neutrophil-dependent in vivo inflammation model. Arthritis severity was followed by clinical scoring, ankle thickness measurement and assessment of the articular function.
Results Mcl-1ΔMyelo mice are viable and fertile without any gross abnormalities, even in homozygous form. The mice have a strong (>95%) reduction of circulating neutrophil counts and severe deficiency of splenic and bone marrow neutrophils. However, other leukocytes such as circulating eosinophils, monocytes, T- or B cells, or splenic dendritic cells and macrophages are not affected. Intact Mcl-1ΔMyelo mice and Mcl-1ΔMyelo bone marrow chimaeras are completely protected from all signs of arthritis development in the K/BxN serum-transfer model.
Conclusions The Mcl-1ΔMyelo mutation leads to dramatic reduction of the number of circulating and tissue neutrophils without affecting other blood and splenic leukocyte lineages. The Mcl-1ΔMyelo mice are viable and fertile in homozygous form but are completely protected from arthritis development in a known neutrophil-dependent in vivo disease model. Taken together, the Mcl-1ΔMyelo mice may provide a useful novel model of genetically determined neutrophil deficiency.