The olfactory organ is a unique site for resident neutrophils in the brain

Abstract

For decades we have known that the brain "drains" through the subarachnoid space following a route that crosses the cribriform plate to the nasal mucosa and cervical lymph nodes. Yet little is known about the potential role of the olfactory epithelia and associated lymphatic vasculature in the immune response. To better understand the immune response in the olfactory organs we used cell-specific fluorescent reporter lines in dissected, intact adult brains to visualize blood-lymphatic vasculature and neutrophils in the olfactory sensory system. Here we show that the extensive blood vasculature of the olfactory organs is associated with a lymphatic cell type resembling high endothelial venules (HEVs) of the lymph nodes in mammals and a second resembling Mural Lymphatic Endothelial Cells (muLECs) that extended from the brain to the peripheral olfactory epithelia. Surprisingly, the olfactory organs contained the only neutrophil populations observed in the brain. Damage to the olfactory epithelia resulted in a rapid increase of neutrophils within the olfactory organs as well as the appearance of neutrophils in the brain suggesting that neutrophils enter the brain in response to damage. Analysis of cell division during and after damage showed an increase in BrdU labeling in the olfactory epithelia and a subset of the neutrophils. Our results reveal a unique population of neutrophils in the olfactory organs that are associated with an extensive lymphatic vasculature suggesting a dual olfactory-immune function for this unique sensory system.

The Olfactory Tract: Basis for Future Evolution in Response to Rapidly Changing Ecological Niches

“…In spite of these intriguing observations, few studies have examined the potential for a neural-vascular or peripheral “olfactory neurohaemal organ” within the olfactory sensory system. In zebrafish we have recently described the blood/lymphatic vasculature in the developing olfactory organ (Palominos and Whitlock, 2021 ; Palominos et al, 2021 ) that runs adjacent to the region we previously described as containing cell bodies of the TN-GnRH3 neurons (Whitlock et al, 2003 ; Gopinath et al, 2004 ; Whitlock, 2004 ). In zebrafish, the GnRH3 receptor is present in the adult OE as assayed by immunocytochemistry (Whitlock et al, 2006 ) and the isolation of GnRH3-receptor mRNA from olfactory organ tissues (Corchuelo et al, 2017 ).…”

“…(A) Gonadotropin releasing hormone 3 (GnRH3) containing neurons (orange) of the terminal nerve are found intimately associated with the blood/lymphatic vasculature of the olfactory organ. (B) Neutrophils (pink) are associated with the extensive blood/lymphatic vasculature system (green/red) wrapping the olfactory organs as well as OSNs within the olfactory epithelia (purple) in zebrafish (Palominos et al, 2021 ).…”

“…Previously we have shown that zebrafish can make and maintain memories of odors experienced as juveniles and that this “olfactory imprinting” is associated with transcriptional changes within the olfactory organs (Harden et al, 2006 ; Whitlock, 2006 ; Calfun et al, 2016 ). The subsequent analysis of olfactory organ cell types led to the discovery of local neutrophils and macrophages/microglia in the olfactory organs of developing and adult zebrafish (Palominos and Whitlock, 2021 ; Palominos et al, 2021 ; Figure 2B ). These observations build on previous data showing the intimate association between the olfactory sensory and immune systems with macrophages and/or microglia in cultures of olfactory mucosa (Pixley, 1996 ), the major histocompatibility complex class I (MHCI) in the olfactory placode of the mouse (Chacon and Boulanger, 2013 ), and recombination activating gene (RAG) gene expression in the intact OE of zebrafish (Jessen et al, 2001 ).…”

“…Previously copper present in runoff has been shown to affect olfactory sensitivity in salmon (Baldwin et al, 2003 , 2010 ) and trigger an immune response within zebrafish olfactory epithelia (Palominos and Whitlock, 2021 ; Palominos et al, 2021 ). More recently another form of copper, nanoparticles (CuNPs), used in commercial applications such as anti-bacterial and anti-fungal agents in textiles and coatings, has been shown to impair fish olfactory function in rainbow trout.…”

HB-EGF Promotes Horizontal Basal Cell Proliferation and Olfactory Sensory Neuron Regeneration in the Zebrafish Olfactory Epithelium

“…Consistent with the progression of the OSN lineage, the immature OSN markers growth-associated protein 43 ( gap43 ) and LIM homeobox 2a ( lhx2a ) are most highly expressed at 72 ( gap43 : 3.9 ± 0.4-fold, lhx2a : 2.4 ± 0.1-fold) and 120 hpl ( gap43 : 4.3 ±1.8-fold, lhx2a: 1.9 ± 0.2-fold). In contrast, sustentacular glial cells (SCs) appear to undergo an early reactive gliosis as indicated by the strong increase in expression of the marker keratin 8 ( krt8; 5.5 ± 1.1-fold) at the 4 hpl time point despite the small, yet significant, decrease in expression of the alternative zebrafish SC marker six4b ( Palominos et al, 2022 , 0.25 ± 0.01-fold), suggesting that some SCs may also undergo degeneration. Thus, the transcriptome profile of OE cell type-specific marker genes closely reflects the events that occur at the cellular level in response to injury ( Kocagöz et al, 2022 ), which comprise the rapid degeneration of neurons followed by concomitant activation of HBCs and subsequent GBC activity to generate new OSNs.…”