Myeloid-Specific Deletion of Mcl-1 Yields Severely Neutropenic Mice That Survive and Breed in Homozygous Form

Abstract

Mouse strains with specific deficiency of given hematopoietic lineages provide invaluable tools for understanding blood cell function in health and disease. Whereas neutrophils are dominant leukocytes in humans and mice, there are no widely useful genetic models of neutrophil deficiency in mice. In this study, we show that myeloid-specific deletion of the Mcl-1 antiapoptotic protein in Lyz2Cre/CreMcl1flox/flox (Mcl1ΔMyelo) mice leads to dramatic reduction of circulating and tissue neutrophil counts without affecting circulating lymphocyte, monocyte, or eosinophil numbers. Surprisingly, Mcl1ΔMyelo mice appeared normally, and their survival was mostly normal both under specific pathogen-free and conventional housing conditions. Mcl1ΔMyelo mice were also able to breed in homozygous form, making them highly useful for in vivo experimental studies. The functional relevance of neutropenia was confirmed by the complete protection of Mcl1ΔMyelo mice from arthritis development in the K/B×N serum-transfer model and from skin inflammation in an autoantibody-induced mouse model of epidermolysis bullosa acquisita. Mcl1ΔMyelo mice were also highly susceptible to systemic Staphylococcus aureus or Candida albicans infection, due to defective clearance of the invading pathogens. Although neutrophil-specific deletion of Mcl-1 in MRP8-CreMcl1flox/flox (Mcl1ΔPMN) mice also led to severe neutropenia, those mice showed an overt wasting phenotype and strongly reduced survival and breeding, limiting their use as an experimental model of neutrophil deficiency. Taken together, our results with the Mcl1ΔMyelo mice indicate that severe neutropenia does not abrogate the viability and fertility of mice, and they provide a useful genetic mouse model for the analysis of the role of neutrophils in health and disease.

BCL6 modulates tissue neutrophil survival and exacerbates pulmonary inflammation following influenza virus infection

“…Our data and the findings reported by Brandes et al (8) support the view that early engagement of neutrophils instigates a damaging feedforward innate inflammatory circuit responsible for severe disease development following IAV infection. Neutrophils are the first responders to infiltrate the sites of infection to clear bacterial infections (37,38). In this regard, enhanced tissue neutrophil apoptosis caused by BCL6 deficiency may increase pulmonary bacterial burden and worsen outcome during certain respiratory bacterial infections, such as Staphylococcus pneumonia infection (37,38).…”

“…Neutrophils are the first responders to infiltrate the sites of infection to clear bacterial infections (37,38). In this regard, enhanced tissue neutrophil apoptosis caused by BCL6 deficiency may increase pulmonary bacterial burden and worsen outcome during certain respiratory bacterial infections, such as Staphylococcus pneumonia infection (37,38). Conversely, excessive neutrophil recruitment is a hallmark of acute susceptibility to Mycobacterium tuberculosis and is associated with overexuberant inflammatory response and disease progression (39).…”

“…To this end, our data show that BCL6 could bind to Pmaip1 (encoding Noxa) promoter, and that BCL6 deficiency resulted in increased levels of Pmaip1 in lung neutrophils. Noxa is a BH3-only protein that can bind and trigger proteasome-mediated degradation of the prosurvival protein Mcl-1 (myeloid cell factor 1) (40), which plays essential roles in neutrophil survival (38,41). Notably, lung neutrophils express higher Noxa levels than blood and bone marrow neutrophils in WT mice, whereas BCL6 deficiency further increases Noxa expression specifically in the lungs.…”

Mouse Models and Tools for the in vivo Study of Neutrophils

“…Recently G-CSFR −/− mice were shown to exhibit abnormally high responses in distal lymph nodes upon immunization, suggesting that neutrophils play a role in the restriction of the immune response to draining lymph nodes after immunization (108). Overall, however, G-CSFR −/− mice are rarely used, probably due to the already mentioned residual neutrophils (7,104) and the anticipated side effects caused by G-CSFR deficiency. Indeed, G-CSFR plays a major role in endothelial cell regulation (109), in the induction of migration and proliferation (110), in bone regeneration through osteoblast regulation (111) and in sympathetic nerve neurons signaling (112).…”

“…Traditional models for the study of neutrophils, including models of depletion or mutant mice, allowed a global comprehension of neutrophil biology. Nevertheless, these models notably raised questions of specificity, and hence justified the generation of novel models in recent years (6,7). Together, these advances rendered mouse models advantageous for the study neutrophils in health and disease.…”