Low density neutrophils as a potential biomarker of leprosy severity

Erythema Nodosum Leprosum (ENL) is a recurrent acute inflammatory complication of leprosy affecting up to 50% of patients with multibacillary (MB) leprosy, and is treated with thalidomide. Although ENL is described as an immune reaction mediated by neutrophils, studies showing the direct role of neutrophils in ENL are still neglected. One subpopulation of low-density neutrophils (LDNs), present within the fraction of peripheral blood mononuclear cells (PBMC), is associated in the pathogenesis and severity of diseases like sepsis, lupus and tuberculosis. In this study, we characterized circulating LDNs in leprosy, highlighting their involvement in the development of ENL. First, we observed segmented/hypersegmented cells morphologically compatible to neutrophils in PBMC of nonreactional and ENL patients. By flow cytometry, we confirmed the elevated frequency of circulating LDNs (CD14−CD15+) in ENL patients compared to nonreactional. Also, the pattern of expression of neutrophil activation markers suggests a greater activation of LDNs during ENL. Treatment with thalidomide did not interfered the frequency of LDNs, but reduced their activation. Serum levels of MMP-9 (neutrophilic degranulation marker) were correlated with the frequency of LDNs. Finally, we revealed that Mycobacterium leprae (ML) induce generation of LDNs in whole blood and purified normal density neutrophils of healthy individuals in vitro. In brief, we showed that activated LDNs are elevated in the circulation of ENL patients, and may be originated by degranulation caused by ML stimulus. These results point the neutrophilic involvement in leprosy especially on ENL favoring the systemic inflammation and suggest LDNs as a biomarker in ENL development.