B-cell survival factors in multiple sclerosis and myasthenia gravis

Abstract

Multiple sclerosis (MS) is a chronic, progressive neurological disease in which death of myelin-producing oligodendrocytes and axonal damage are prominent pathological features. The cause of MS is unknown but genetic and environmental factors have been proposed to be important. There is a paradigm shift in our understanding that demyelination may be the primary insult with secondary influx of inflammatory cells into the central nervous system (CNS) in MS.

The function of B cells can be categorized into one of the four: antibody-production, antigen-presentation, cytokine production, and regulation of other immune cells. Except for antibody-production, none of the other functions are exclusive to B cells. Therefore, whether B cells are of any relevance in the pathogenesis of MS has been questioned. By contrast, the pathogenic potential of anti-acetylcholine receptors (AChR) antibodies is well understood in myasthenia gravis (MG). Binding of anti-AChR antibodies to AChR receptors present at the neuromuscular end-plate result in muscle weakness.

The aim of this thesis was to understand whether B cells contribute to tissue healing or tissue injury, or do both in MS. Two molecules indispensable for B-cell survival - a proliferation-inducing ligand (APRIL) and B-cell activating factor (BAFF) - were studied. BAFF also rescues self-reactive B cells from apoptosis. APRIL and BAFF are secreted by monocytes and macrophages; they share two receptors, B-cell maturation antigen (BCMA) and transmembrane activator and calcium modulating and cyclophilin ligand interactor (TACI) whereas BAFF exclusively binds BAFF-receptor (BAFF-R). All three receptors are expressed by B cells.

APRIL and BAFF mRNA were quantified in monocytes and T cells of MS patients and healthy controls (HC) using real-time PCR. BAFF-R, TACI, and BCMA mRNA in B cells were also quantified. The mRNA levels of all the molecules studied were increased in patients compared to levels in HC. In a second dataset of MS patients and age- and gender-matched headache controls (HEC), APRIL and BAFF protein, measured in plasma and cerebrospinal fluid (CSF) by ELISA, were comparable between patients and HEC. The surface expression of BAFF-R, TACI, and BCMA on B cells of HC, MS, and MG patients was studied using flow cytometry. The intensity of expression of these molecules was comparable between the three groups. Although mRNA levels of B-cellsurvival factors and the receptors that bind them are increased in patients than levels in controls, whether B cells do survive longer is unknown.

Sensitization of AChR expressed by myoid cells in the thymus may underlie MG pathogenesis. The novel finding of APRIL and BAFF expression in the thymus - a T-cell organ - may explain that autoreactive B cells may use the thymus as a survival niche. Antibodies in the CSF can be used as a biomarker to predict disease outcome. Analysis of CSF and plasma of MS patients and controls revealed that lipid-reactive immunoglobulin (Ig) M in CSF was detectable in one-third of MS patients. Lipid-specific IgM predicted an adverse outcome, as defined by conversion to secondary progressive disease phase.

B cells as agents of tissue injury or tissue healing is still not resolved. But antibodies are useful as biomarkers to predict disease outcome. In the course of this thesis work, it became evident that APRIL and BAFF are also expressed by astrocytes and is increased in MS plaques. The expression of B-cell-survival factors by CNS resident cells and their functional relevance in disease promises to be an exciting area of research.