The Macrophage IL-23/IL-17A Pathway: A New Neuro-Immune Mechanism in Female Mechanical Pain

Considerable clinical evidence has demonstrated that the prevalence and severity of many chronic pain syndromes differ across sex, and are more predominant in women than in men [1]. But most preclinical research on chronic pain mechanisms has been performed primarily in male animals. It is generally understood that inflammatory responses in the peripheral and central nervous systems are critical for chronic pain [2, 3]. Given that sex dimorphism influences immunity in various diseases [4], recent studies have revealed that spinal microglia or dorsal root ganglion (DRG) macrophages regulate neuropathic pain selectively in male mice [1]. However, the specific molecular mechanism underlying the greater occurrence of chronic pain among females than males remains unknown. The interleukin-23 (IL-23)/IL-17 axis is crucial to the progression of multiple inflammatory diseases (such as psoriasis and arthritis) [5], which are more prevalent among females. In line with this idea, a recent study from the RR Ji group provided new insights into the IL-23/IL-17 axis in sex dimorphism arising from macrophage-sensory neuron crosstalk in mechanical pain states [6] (Fig. 1).

Fig. 1.

The macrophage IL-23/IL-17A pathway: a new neuro-immune mechanism in female mechanical pain.

The pro-inflammatory cytokine IL-23 was first reported to have a T cell-independent function in arthritic and inflammatory pain progression and disease [7], but without gender disparity. Here, in the new study by the Ji group [6], the authors designed an elegant series of experiments to elucidate the role of IL-23 in pain in a sex-dimorphic manner. First, intraplantar injections of IL-23 dose-dependently induced mechanical allodynia specifically through IL-23 receptors (IL-23Rs) only in naïve female mice and not in males. Second, the IL-23/IL-23R axis was required for generating female-specific mechanical allodynia (neuropathic pain) and spontaneous pain (inflammatory pain). Third, IL-23 was increased in the serum and DRGs in vivo or peritoneal macrophages in vitro after treatment with the chemotherapeutic agent paclitaxel. More importantly, IL-23 exerted its effects in females via the autocrine and paracrine actions of macrophages and indirectly activating C-fiber nociceptors. These results collectively demonstrated a sex-dimorphic role of IL-23 in mechanical pain development at multiple cellular levels involving macrophages and nociceptors.

The next question is how IL-23 induces mechanical pain in a sex-specific fashion. To address this question, Ji’s study focused on IL-17, which is one of the important effector molecules regulated by IL-23 and displays sex dimorphism in inflammation. IL-17 plays a critical role in driving chronic pain via T cell-dependent or -independent mechanisms, including neuron-glial interactions, central sensitization, and peripheral sensitization [8], but no gender differences were shown in the effect. Here, Luo et al. [6] reported that IL-17A (a subtype of IL-17) was increased in both male and female peritoneal macrophages after paclitaxel treatment, but was greater in females after IL-23 stimulation. Intraplantar IL-17A induced mechanical pain in males only at a high dose (100 ng) but in females at low doses (1 and 10 ng), which was independent of macrophages or IL-23. Moreover, blocking IL-17A/IL-17R signaling suppressed IL-23-induced pain in females. Interestingly, by using Ca²⁺ imaging and whole-cell patch-clamp recording, the authors found that a low dose of IL-17A (1 ng/mL) directly activated DRG nociceptor neurons in females but not in males. These results confirmed that IL-17/IL-17R signaling in sensory neurons is the key downstream effector of IL-23-mediated sex differences in mechanical pain. Previous studies have shown that IL-17 is upregulated in infiltrated T cells within injured nerves, DRG satellite cells, and spinal astrocytes in several types of chronic pain, and its receptor IL-17R is constitutively expressed in nociceptors, various types of DRG cells and spinal cord neurons [8]. Thus, further research is needed to determine whether IL-17 plays a gender-specific role at multiple levels.

To further address how IL-23 and IL-17 facilitate pain in females, the new study investigated whether transient receptor potential vanilloid 1 (TRPV1) or transient receptor potential ankyrin 1 (TRPA1) ion channels are involved. Using gene knockout, the authors found that the effects of IL-23/IL-23R signaling on female-specific mechanical pain were abrogated in TRPV1-knockout (KO) mice, but not in TRPA1-KO mice. Meanwhile, Luo et al. [6] provided evidence that gender differences exist in mechanical pain induced by the TRPV1 agonist capsaicin at a very low dose. More importantly, TRPV1 was not only structurally co-expressed with IL-17Rs in mouse and human nociceptors, but also necessary for IL-17-induced nociceptor hyperexcitability and mechanical allodynia in female mice. Taken together, these results revealed the molecular mechanism of the IL-23/IL-17A axis in mechanical pain in females is mediated by TRPV1.

Sex hormones (especially estrogen) are important factors that modulate the sex dimorphism of pain processing [9], so the new study investigated the impact of sex hormones (estrogen or androgen). The authors found that the sex dimorphism of IL-23/IL-17A axis-mediated mechanical pain was promoted by estrogen in females via estrogen receptor-alpha and suppressed by androgen in males via the androgen receptor. Compared with male macrophages, female macrophages stimulated by PTX in vitro released higher levels of IL-23. In addition, deletion of estrogen receptor-alpha in TRPV1⁺ nociceptors abolished IL-23-, IL-17- and capsaicin-induced mechanical pain in females, suggesting that the direct interaction of IL-17A with TRPV1 is regulated by neuronal estrogen/estrogen receptor. Furthermore, the work extended the mechanisms by which estrogen modulates pain through the IL-23/IL-17 pathway with a cascade-amplification effect. Further studies are warranted to investigate how sex hormones affect the IL-23/IL-17 signaling axis at the macrophage and nociceptor levels.

In summary, this study revealed a novel and clinically relevant role of the IL-23/IL-17A signaling pathway in sex dimorphism in pain processing at the macrophage and nociceptor levels. Clinical transcriptomic evidence showed that IL-17A, IL-23, and TRPV1 are significantly increased in the itchy skin of patients with atopic dermatitis or psoriasis [10]. This axis might underlie sex dimorphism in sensory discomfort such as itching in IL-23-dependent autoimmune and inflammatory diseases. Furthermore, the new study suggests that inhibition of the IL-23/IL-17A axis, such as by the psoriatic biologics secukinumab (an IL-17A inhibitor) and guselkumab (an IL-23 inhibitor), may provide a promising analgesic or antipruritic target for females with chronic pain or inflammatory diseases. As the IL-23/IL-17A axis is associated with T cell-mediated adaptive immune responses and infiltrating T cells in DRGs were involved in mechanical allodynia, this work does not exclude the possibility of T cell-derived IL-17. IL-23/IL-17 mechanisms of sex difference in pain hypersensitivity may vary among different species. For this reason, it will be necessary to replicate these findings in human chronic pain. As noted by the authors, the events underlying sex dimorphism in chronic pain remain to be fully characterized and may result from distinct signaling or multiple cellular levels, such as sensory neurons and immune cells.