[HTML][HTML] Nociceptive neurons regulate innate and adaptive immunity and neuropathic pain through MyD88 adapter

XJ Liu, Y Zhang, T Liu, ZZ Xu, CK Park, T Berta… - Cell research, 2014 - nature.com
XJ Liu, Y Zhang, T Liu, ZZ Xu, CK Park, T Berta, D Jiang, RR Ji
Cell research, 2014nature.com
Neural-immune interaction in health and disease is a hot topic in current biomedical
research [1, 2]. While the central nervous system (CNS) regulates systemic immune
responses through hormonal and neuronal route, the peripheral nervous system (PNS)
appears to modulate local innate immune responses [1]. Our organs are heavily innervated
by the peripheral nerves, yet, it is not fully understood how the PNS controls immunity [2].
Toll-like receptors (TLRs) are traditionally expressed in immune cells to regulate innate …
Neural-immune interaction in health and disease is a hot topic in current biomedical research [1, 2]. While the central nervous system (CNS) regulates systemic immune responses through hormonal and neuronal route, the peripheral nervous system (PNS) appears to modulate local innate immune responses [1]. Our organs are heavily innervated by the peripheral nerves, yet, it is not fully understood how the PNS controls immunity [2]. Toll-like receptors (TLRs) are traditionally expressed in immune cells to regulate innate immunity, but recent studies indicated that TLRs (such as TLR3, 4 and 7) are also expressed in primary sensory neurons, especially nociceptive neurons in dorsal root ganglia (DRGs) and trigeminal ganglia of the PNS to regulate sensory functions such as pain and itch [3-6]. TLR signaling is largely mediated by the myeloid differentiation factor 88 (MyD88) protein (but see [7]), and activation of MyD88 in turn activates the NF-κB and MAP kinase pathways, leading to the production of inflammatory cytokines and chemokines for the initiation of innate immunity [3]. Increasing evidence suggests that nociceptor neurons play a critical role in host defense and inflammation [8, 9]. However, the key signaling molecules in nociceptors for the regulation of immunity remain to be identified. To this end, we generated MyD88-conditional knockout (CKO) mice by deleting MyD88 in nociceptive neurons expressing the sodium channel subunit Nav1. 8 [8, 9]. We first examined the expression of MyD88 in DRG neurons in wild-type (WT) and CKO mice. MyD88 is expressed in majority of DRG neurons, including both small-sized C-fiber and large-sized A-fiber neurons in WT mice. Notably, MyD88 expression was dramatically reduced in small-sized neurons of MyD88-CKO mice (Supplementary information, Figure S1, AC). Loss of MyD88 expression in DRG neurons of CKO mice was also confirmed by single-cell PCR analysis in small-sized neurons (Supplementary information, Figure S1, D and E). Together, we have validated the successful generation of the CKO mice with selective deletion of MyD88 in small-sized nociceptive neurons. Given the well-known role of MyD88 adapter in innate immunity [9], we next investigated if MyD88 in nociceptive neurons would regulate immunity in DRGs by employing fluorescence-activated cell sorting (FACS) analysis. We used CD45 antibody to identify the overall population of infiltrating leukocytes in normal DRGs of C57BL/6 mice. Although at low abundance (0.03% to 0.5% of total cells in DRGs), fourteen different types of immune cells including macrophages, monocytes, neutrophils, helper T cells, cytotoxic T cells and antigen presenting cells (APCs) could be detected in naïve DRGs (Supplementary information, Table S1), suggesting that infiltrating innate and adaptive immune cells are present in low numbers in the normal DRGs where they may play a surveillance role.
Chemotherapy is known to cause peripheral nerve neuropathy and neuropathic pain [5]. Unlike other neuropathic pain models which only affect limited numbers of DRGs of an animal, chemotherapy drugs, such as paclitaxel (PTX) could affect all DRGs following systemic injection. Thus, we collected all DRGs in this neuropathic pain model for FACS analysis. One week after the PTX injection, we found significant increases in infiltrating CD45+ leukocytes, which include innate immune cells such as macrophages (CD11b+), monocytes (CD11b+CD14+, also marked by CD11b+Gr-1medium), and neutrophils (CD11b+Gr-1high) in DRGs. These proinflammatory immune cells such as macrophages are known to promote neuropathic pain [10]. Strikingly …
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