An unappreciated role for neutrophil-DC hybrids in immunity to invasive fungal infections

Abstract

Neutrophils are classically defined as terminally differentiated, short-lived cells; however, neutrophils can be long-lived with phenotypic plasticity. During inflammation, a subset of neutrophils transdifferentiate into a population called neutrophil-DC hybrids (PMN-DCs) having properties of both neutrophils and dendritic cells. While these cells ubiquitously appear during inflammation, the role of PMN-DCs in disease remains poorly understood. We observed the differentiation of PMN-DCs in pre-clinical murine models of fungal infection: blastomycosis, aspergillosis and candidiasis. Using reporter strains of fungal viability, we found that PMN-DCs associate with fungal cells and kill them more efficiently than undifferentiated canonical neutrophils. During pulmonary blastomycosis, PMN-DCs comprised less than 1% of leukocytes yet contributed up to 15% of the fungal killing. PMN-DCs displayed higher expression of pattern recognition receptors, greater phagocytosis, and heightened production of reactive oxygen species compared to canonical neutrophils. PMN-DCs also displayed prominent NETosis. To further study PMN-DC function, we exploited a granulocyte/macrophage progenitor (GMP) cell line, generated PMN-DCs to over 90% purity, and used them for adoptive transfer and antigen presentation studies. Adoptively transferred PMN-DCs from the GMP line enhanced protection against systemic infection in vivo. PMN-DCs pulsed with antigen activated fungal calnexin-specific transgenic T cells in vitro and in vivo, promoting the production of interferon-γ and interleukin-17 in these CD4+ T cells. Through direct fungal killing and induction of adaptive immunity, PMN-DCs are potent effectors of antifungal immunity and thereby represent innovative cell therapeutic targets in treating life-threatening fungal infections.

Neutrophil Diversity in Health and Disease

“…Moreover, retro-transmigrated neutrophils express CD54 and have been associated with secondary organ damage [14], as well as with effective bacterial phagocytosis during sepsis [95] (Figure 2). Overall, although the specific neutrophil subpopulations found in mouse models do not completely mirror their human counterparts (i.e., from cell-surface markers), important similarities are shared in terms of functionality (i.e., immunosuppressive activity, APC functions), as well as their influence on the outcomes of infections [84,[86][87][88]93,94]. As a result, mouse models are invaluable in that they can shed light on the importance of specific pathogen infections as possible sources of neutrophil heterogeneity; this information might in turn help in the development of tailored therapeutic strategies to improve host immune defense.…”

“…After encountering pathogens in the tissue, a subpopulation of CD11b high CD62L low C-X-C chemokine receptor type 2 (CXCR2) low neutrophils can migrate to the lymph node to promote adaptive immunity [91]. Populations of APC-like neutrophil-DC hybrids have also been identified in different mouse models of infection [92,93]. Moreover, after infection or sterile injury, CD54 + neutrophils can retro-transmigrate from the affected tissue to the bloodstream, exerting deleterious actions on secondary organs but also exhibiting an enhanced ability to fight pathogens [95].…”

“…Migrating neutrophils have been characterized by enhanced expression of surface receptors associated with antigen-presenting cell (APC) functions (i.e., MHC class II, CD80, or CD86) relative to circulating neutrophils (Figure 2). Similarly, populations of 'neutrophil-dendritic cell (DC) hybrids' expressing markers of both neutrophils and DCs and displaying APC activity, have been identified in mice with acute peritonitis, Escherichia coli chronic skin inflammation, and fungal infections such as Blastomyces dermatitidis, A. fumigatus, or C. albicans [92,93] (Figure 2). Of note, similar populations of neutrophil-DC hybrids have been isolated from patients with lung cancer [64] (see below).…”

Recent advances on the crosstalk between neutrophils and B or T lymphocytes

“…While the studies reported above describe APC‐like functions acquired by peripheral neutrophils, populations of so‐called ‘neutrophil–DC hybrids’, expressing markers of both neutrophils and DCs, have also been obtained from in vitro differentiation of murine and human neutrophil precursors, as well as isolated in vivo . These neutrophil–DC hybrids retain intrinsic functional abilities of neutrophils (including the capacity to capture exogenous material, extrude neutrophil extracellular traps and kill bacteria), but also exhibit several DC properties (including dendritic morphology, podosome formation and presentation of various forms of foreign protein antigens to naive CD4 + T cells) .…”

“…These neutrophil–DC hybrids retain intrinsic functional abilities of neutrophils (including the capacity to capture exogenous material, extrude neutrophil extracellular traps and kill bacteria), but also exhibit several DC properties (including dendritic morphology, podosome formation and presentation of various forms of foreign protein antigens to naive CD4 + T cells) . Given these unique features, a potential role of neutrophil–DC hybrids as potent effectors in anticancer immunity in humans, and antifungal defence in mice, has been recently proposed. In the former case, neutrophil–DC hybrids have been described as a population of tumour‐associated neutrophils (TANs) that infiltrate the tumour tissue in early‐stage lung cancer .…”