Low-Density Granulocytes Are Elevated in Mycobacterial Infection and Associated with the Severity of Tuberculosis

Abstract

Tuberculosis remains a global health problem caused by infection with Mycobacterium tuberculosis. Numerous studies have established a close correlation between the development of tuberculosis and the roles of neutrophils. Recently, a distinct population of CD15+ granulocytes was found to be present in the peripheral blood mononuclear cell (PBMC) fraction in humans. This population of granulocytes, termed low-density granulocytes (LDGs), was reported to be elevated and associated with disease activity or severity in a number of different conditions including SLE, asthma and HIV infection. However, both the frequency and clinical significance of LDGs associated with tuberculosis are unclear. Here we determined LDG levels and made comparisons between subjects with active pulmonary tuberculosis (PTB) and healthy controls, between PTB patients with mild-to-moderate disease and patients with advanced disease, and among PTB patients following anti-tuberculous therapy of varying durations. The direct correlation between M. tuberculosis infection and LDG levels was confirmed by in vitro infection of whole peripheral blood and isolated granulocytes with mycobacteria. Our results demonstrated that PBMCs in PTB patients contained significantly elevated percentages of LDGs compared with control subjects. LDGs in tuberculosis expressed higher levels of activation markers compared to normal-density granulocytes (NDGs). M. tuberculosis induced the generation of LDGs in both whole blood and isolated NDGs from control subjects, which suggests that LDGs associated with M. tuberculosis infection are likely to originate from in situ activation. Furthermore, our results revealed that the frequency of LDGs is associated with the severity of tuberculosis.

Human neutrophils in the saga of cellular heterogeneity: insights and open questions

“…However, neither LDNs, nor NDNs, from P. vivax‐ infected patients manifested any immunoregulatory properties toward T cells . Finally, LDNs from M. tuberculosis ‐infected patients have been shown to express higher levels of activation markers than autologous NDNs, as well as to manifest enhanced ROS production and M. tuberculosis phagocytosis . Interestingly, the simple incubation of whole blood or NDNs from healthy donors with M .…”

“…Interestingly, the simple incubation of whole blood or NDNs from healthy donors with M . tuberculosis induced the generation of LDNs in vitro suggesting that, in vivo , the latter cell population likely originates as a result of direct neutrophil/pathogen interaction and, consequently, neutrophil activation.…”

“…Whatever the case is, the phenotypic and functional features of the LDNs to date described in various diseases are discussed below and summarized in Tables and . It must be pointed out that, despite the several unanswered questions on their features, there is a considerable clinical interest around LDNs, as their frequency often correlates with disease severity and/or responsiveness to treatment in many pathologies, including cancer , sepsis , HIV‐1 , and mycobacterial infection and autoimmune disorders .…”

Neutrophil heterogeneity: Bona fide subsets or polarization states?

“…However, during acute and chronic inflammation a population of neutrophils with altered buoyancy characteristics colocalizes with the PBMC lower density fraction and as such has been collectively termed low‐density neutrophils (LDNs). Although first described in autoimmunity examples of elevated LDN numbers have spread to a variety of settings including cancer, infection, asthma, sepsis, graft versus host disease, and pregnancy . The overall function of these cells appears to be dependent on inflammatory stimuli and LDN numbers often correlate with disease progression in human patients .…”

Interactions between Type 1 Interferons and the Th17 Response in Tuberculosis: Lessons Learned from Autoimmune Diseases

“…Similar to SLE, the transcriptional T1-IFN signature in TB patients is mostly expressed in PMN (58). Moreover, LDG are also present in TB patients and correlate with disease severity, but it is unclear if these cells also have a similarly increased tendency for NETosis as their SLE counterparts (321). Nevertheless, NETosis does occurs in TB, as Mtb readily induces NETosis itself in PMN in an ESX-1-dependent way and can even stimulate extracellular trap formation in macrophages (204, 322–324).…”