Malou Janssen

Multiple sclerosis (MS), neuromyelitis optica spectrum disorders (NMOSD) and myelin oligodendrocyte glycoprotein (MOG) antibody disease (MOGAD) are demyelinating diseases of the central nervous system (CNS). Although these diseases differ in clinical features, an aberrant response of lymphocytes is thought to be driving the pathogenesis of all three disorders. In this thesis, specificity, local recruitment and development of lymphocyte subsets involved in the pathogenesis of these diseases have been studied.

Chapter 1 outlined the current state of knowledge of MS, NMOSD and MOGAD epidemiology and risk factors. The similarities and differences between diseases are discussed as are the different views on pathogenesis which have changed significantly over the years.

In Chapter 2, we studied the antigen specificity of cerebrospinal fluid (CSF) derived T cell lines in MS. As antigen-presenting cells, we used autologous B cells immortalized with EBV and transfected with a selection of MS-associated antigens (MSAg; contactin-2, inward rectifying potassium channel protein, myelin associated glycoprotein, myelin basic protein, myelin oligodendrocyte glycoprotein, neurofascin, proteolipid protein, protein S100B). This physiologically advantageous model was shown to be useful for triggering both antigen-specific CD4+ and CD8+ T cells. However, no significant responses were detected against the MSAg of choice.

In chapter 3, we found that a functionally distinct Th1-like Th17 subset termed Th17.1 was selectively associated with early disease activity in MS patients. This IFN-γ high GM-CSF high IL-17low CD4+ T cell subset was less present in the peripheral blood of patients who rapidly develop MS, enriched in MS CSF and brain tissues and highly capable of migrating across confluent human brain endothelial monolayers in vitro. Interestingly, in RRMS patients treated with natalizumab, only Th17.1 cells accumulated in the blood of clinical responders. This was not seen in non-responders, supporting the role of this brain-homing T-cell subset in inducing MS disease activity.

Regarding B cells, we showed in Chapter 4 that IFN-γ-related CXCR3+ populations were significantly enriched in MS CSF, meninges and brain compared to paired blood. The brain-infiltrating capacity of CXCR3+ B cells was underlined both in vitro and ex vivo using the same transmigration systems and blood samples from natalizumab-treated patients as in Chapter 3. During TFH-like cultures with naive B-cells IFN-γ mainly triggered plasmablast formation, while the addition of both IFN-γ and Toll like receptor (TLR) 9 ligand (CpG-ODN) resulted in switching to IgG1.

Chapter 5 addressed B-cell differentiation in pregnant MS patients and healthy controls in the context of relapse risk. When comparing B cells between the high relapse risk period (4-8 weeks after delivery) and low relapse risk period (third trimester of pregnancy), memory B cells showed an altered chemokine receptor expression profile related to increased migration into the CNS and preferential localization in the TFH cell-containing germinal center light zone. In addition, we found that postpartum memory B cells are more able to differentiate into plasma cells in a TFH-dependent manner in vitro, which may contribute to their final maturation in the CNS.

In Chapter 6 we observed high frequencies of transitional B cells in treatment naive patients with NMOSD compared to patients with MOGAD, MS and healthy controls. These high frequencies were most prominent in patient samples obtained close to relapse. Steroid treatment reduced proportions of transitional B cells in NMOSD, MOGAD and MS. In vitro, germinal center-like cultures containing IFN-γ induced plasmablast outgrowth. However, only in relapsing NMOSD and MOGAD patients, CpG-ODN synergized with IFN-γ to further enhance plasmablast outgrowth. In patients with stable disease, addition of CpG-ODN to culture medium reduced plasmablast proportions. AQP4 specific IgG could be found in the supernatant of patients with relapsing NMOSD. MOG specific IgG was not found. In conclusion, naive B cell homeostasis is altered in NMOSD and affected by corticosteroids.

Chapter 7 summarized results, compared outcomes between different studies and provided suggestions for future studies.