Rapid Magnetic-microbead Method for Efficient Purification of Low-density Neutrophils

Neutrophils, important leukocytes of innate immunity, have traditionally been considered a homogeneous cell population. Nevertheless, recent evidence has shown that neutrophils exist in several subpopulations. One such subpopulation is low-density neutrophils (LDN). LDN are found in small numbers in the blood of healthy individuals, but their numbers increase significantly in diseases such as systemic lupus erythematosus, autoimmune disorders, cancer, and infections. In these cases, LDN may participate in the pathogenesis of the disease. The only way to isolate LDN is through density-gradient centrifugation of peripheral blood. However, after centrifugation, LDN co-purify with mononuclear cells. Thus, studying this neutrophil subpopulation is challenging. There is no standard methodology to separate LDN from mononuclear cells. Typically, LDN are separated by cell sorting in a flow cytometer. However, this method requires long sorting times (hours) to obtain enough pure cells for further functional studies. This seriously affects the viability and function of cells. Here, we propose a practical method to obtain large numbers of pure and viable LDN in a short time. After density-gradient centrifugation, the mononuclear cell fraction is incubated with anti-CD66b magnetic microbeads, and then LDN are separated through magnetic columns in < 30 min. Purified LDN (CD66b⁺ cells) are labelled with monoclonal antibodies against CD10, CD11b, CD14, CD15, CD16b, CD33, CD62L, CD66b, and CD98, and are analyzed by flow cytometry for confirmation. Purified LDN are completely functional as indicated by their capacity to produce reactive oxygen species, and to form neutrophil extracellular traps. This new purification method results in LDN with high purity (more than 90%) and viability (more than 96%) in a short time period. This method can easily be scaled up to obtain large numbers of pure LDN to evaluate LDN functions in different diseases through biochemical or other omics analysis.