What's the science?
Neuropathic pain can be caused by conditions such as nerve injury, nerve demyelination, spinal cord injury, or stroke, and refers to pain due to injury or disease of the sensory system, according to the International Association for the Study of Pain (IASP). In particular, patients with multiple sclerosis can experience pain which may be due to inflammation of the spinal cord (myelitis). This inflammation may cause an immune response, and a potential mechanism of neuropathic pain could be due to autoantibodies, antibodies within the body’s own immune system which may somehow affect the nerves. This week in Annals of Neurology, Fujii and colleagues assessed autoantibodies in neuropathic pain patients to understand what role they might play in neuropathic pain.
How did they do it?
110 patients with a condition causing probable or definite neuropathic pain (IASP criteria for neuropathic pain) and 50 controls participated. Controls were comprised of 20 healthy individuals, 20 people with a neurodegenerative disease and 10 people with a collagen-vascular disease. Sera was extracted from blood from each individual. Immunofluorescence assays were performed, using sera from human study participants and tissue that had been removed from adult male mice (the dorsal root ganglia, the spinal cord, and skin). Human IgG antibodies bound to the tissue were detected using anti-human IgG antibodies. In the dorsal root ganglia (nerve root near the spinal cord) double immunostaining for both human IgG antibodies and neuronal markers was performed. Participants were classified as seropositive (IgG antibody binding to mouse tissues) or seronegative (no immunoreactivity). Each participant’s IgG subtype was also identified. Western blotting was performed to identify proteins/antigens that the autoantibody was bound to.
What did they find?
There was no immunoreactivity to the dorsal root ganglion neurons amongst controls, but serum IgG binding was positive in 11 neuropathic pain patients (seropositive patients). IgG subclass IgG2 was dominant in these seropositive patients. Using dual immunostaining, the authors identified that IgG antibody binding occurred most frequently at unmyelinated C fiber neurons (most commonly non-peptidergic). C fiber neurons are known to be involved in pain. When the authors performed dual staining for antibodies and two receptors known to be involved in pain (TRPV1 and P2X3), they found that antibodies were partially (TRPV1) or mostly (P2X3) co-localized with the receptors. The stain for IgG antibodies also co-localized with axon terminals in the spinal cord in lamina (layer) I and II (where C fiber neurons are known to terminate). The authors then characterized the autoantigen (that the autoantibody binds to) immunochemically using mass spectrometry, and found that it was likely to be plexin D1 (in mice). To characterize plexin D1 in humans, the authors performed immunostaining in tissue from two deceased human donors and observed co-localization of unmyelinated afferents and plexin D1 in the dorsal horn of the spinal cord. This finding suggests that the autoantibody was specific to neurons important for pain.
The 11 patients with auto-antibodies for small unmyelinated dorsal root ganglion neurons tended to be younger and female, and had burning or tingling pain with sensory impairment. Sera from patients with anti-D1 plexin antibodies was then applied to dorsal root ganglion neurons in mice, and cellular and nuclear swelling and increased permeability of the membrane was observed, suggesting cytotoxicity.
What's the impact?
This is the first study to identify human autoantibodies that bind to neurons known to be involved in pain in neuropathic pain patients. Autoantibodies were found to be specific to the plexin D1 protein, which plays various roles in the immune and nervous systems. This study could have important implications for the study of neuropathic pain and its response to immunotherapy.
Fujii et al. A novel autoantibody against plexin D1 in patients with neuropathic pain. Annals of Neurology. 2018. Access the original scientific publication here.