Invited article: human natural autoantibodies in the treatment of neurologic disease

Abstract

Naturally occurring autoantibodies are molecules that are part of the normal immunoglobulin repertoire. This review focuses on three distinct groups of human monoclonal antibodies (mAb). These are human natural autoantibodies that, when injected into an animal model of human disease, stimulate remyelination in CNS demyelinating diseases, protect neurons and extend neuronal processes in CNS axonal disorders, and activate immune dendritic cells to produce cytotoxic T cells to clear metastatic tumors. Natural autoantibodies react to self antigens and are of relatively low affinity. They are derived from germline immunoglobulin genes and are usually polyreactive. Our experiments demonstrated CNS entry by autoradiography of labeled mAb and by MRI. Remyelinating mAb rHIgM22 clusters beta-integrin and mouse mAb O4 recognizes sulfatide. Neuronal outgrowth mAbs sHIgM42 and sHIgM12 appear to target carbohydrates on the surface of neurons. The mAb sHIgM12 (B7-DC-Xab) also is promising as therapeutic against metastatic tumors. It functions by binding and cross-linking the antigen B7-DC on dendritic cells, inducing tumor-specific cytotoxic T cells. All these mAbs activate a transient increase in intracellular calcium, signal via NFkappab, and prevent apoptosis. The mAbs engage downstream signaling events that induce the primary function of the cell (that is, remyelination for oligodendrocytes, axonal preservation and neurite extension for neurons, or antigen presentation for dendritic cells). Natural human auto mAbs are a potentially important therapeutic technique in combating a wide spectrum of disease processes.

Figure 1 rHIgM22, a recombinant human monoclonal antibody, promotes remyelination in the spinal cord Light photomicrographs of spinal cord remyelination promoted by rHIgM22, a human immunoglobulin M (IgM) that binds to myelin and oligodendrocytes, in a virus-mediated model of chronic demyelination. An area of nearly complete remyelination is present in the ventral lateral spinal cord (A) of a mouse treated with a single dose of 50 μg of rHIgM22 5 weeks earlier. Remyelination is characterized by densely packed thin myelin sheaths in relation to axon diameter. In contrast, a typical demyelinated lesion with minimal remyelination is observed in the spinal cord of a mouse treated with a control human IgM 5 weeks earlier (B), containing only a few remyelinated axons. Infiltrating macrophages in the demyelinating lesion are characterized by the presence of darkly stained debris-laden vesicles. A and B are at the same magnification.

Figure 2 rHIgM22 binds to the surface of oligodendrocytes The antigen recognized by rHIgM22 is conserved across a number of species. Immunoglobulin M binding was assessed via fluorescently conjugated secondary antihuman μ-chain-specific monoclonal antibodies (mAbs). Panels depict surface immunolabeling of live cells with rHIgM22 on cultures of CNS glia derived from adult mouse (A), rat (B), and human (C) cerebral cortex. (D) Background staining with anti-μ-chain secondary mAbs alone.

Figure 3 A single dose of rHIgM22 combined with methylprednisolone twice a week alters spontaneous activity in mice with demyelinating disease Methylprednisolone (MP) treatment (1 mg 2 times a week) was performed alone or combined with a single initial 500-μg bolus of rHIgM22. Groups of 5 mice 30 days after infection with TMEV were monitored continuously in activity boxes (Accuscan Inc.) which measure spontaneous horizontal and vertical movements by infrared beam breaks. Nocturnal activity, a time when mice are normally active, was used as a measure of a treatment group’s ability and desire to explore their environment. By day 12 of treatment, spontaneous nocturnal horizontal activity of the MP + rHIgM22 group greatly increased and was comparable statistically to the uninfected control group. The nocturnal activity of virus-infected mice treated with MP alone did not change over the course of the study. From day 12 to 24 of treatment, the nocturnal activity of the MP + rHIgM22 group was comparable to the activity of a non–virus-infected group of mice. Statistical comparisons were by Mann-Whitney rank sum test of the values on each day of treatment.

Figure 4 A human immunoglobulin M (IgM) that binds to neurons promotes neurite extension sHIgM42 is a serum-derived human monoclonal IgM that binds to the surface of many types of neurons. sHIgM42 binds to the surface of live rat cerebellar granule cell neurons (A). When rat granule cells are seeded onto surfaces coated with human sHIgM42, long neurite extension is induced within 24 hours (B). During the same time period, minimal neurites are extended by granule cells seeded onto an anti-oligodendrocyte monoclonal antibody (mAb)–coated surface (C) demonstrating the specificity of this natural human mAb. Cell architecture is revealed by Coomassie blue stain.

Figure 5 A single dose of a neurite extension-promoting human monoclonal antibody (mAb) alters spontaneous function of mice with spinal cord pathology Mean hourly spontaneous nocturnal activity is altered within 3 weeks after a single intraperitoneal injection of 100 μg human of sHIgM12. Groups of five mice with chronic TMEV-induced spinal cord demyelination and axon loss were treated with sHIgM12 (black bars) or a control human immunoglobulin M (IgM) that doses not bind to neurons (gray bars). Mice were monitored using AccuScan activity monitoring boxes for 72 consecutive hours per week. The mean ± standard error of hourly horizontal activity over three 12-hour nocturnal periods (when mice are normally active) were compared to the nocturnal activity for the 3 nights before treatment (time 0). A difference in activity compared to pre treatment was present at 3 to 8 weeks after treatment (*p < 0.01) with sHIgM12, but not using a control human IgM mAb. Statistical comparisons of all time points by Kruskal-Wallis one way analysis of variance on ranks indicated a difference in activity. Specific time points with differences in nocturnal activity were then tested by one-way analysis of variance.

Figure 6 Conceptualization of how a human monoclonal antibody (mAb) that binds to B7DC functions in vivo B7-DC-XAb recognizes and binds to the surface of dendritic cells. The pentameric immunoglobulin M (green) mAb clusters portions of the cell membrane, which leads to calcium influx and dendritic cell activation. The activated dendritic cells interact with CD4⁺ T cells, which secrete cytokines to alter the systemic immune response. This mAb-induced change in immune function is sufficient to stimulate cytotoxic CD8⁺ T cells to clear malignant metastatic B16 tumor cells in vivo.