Keywords
multiple sclerosis, chemokines, CXCL13, B cells, tertiary lymphoid organ, meninges
multiple sclerosis, chemokines, CXCL13, B cells, tertiary lymphoid organ, meninges
Although disease modifying therapy (DMT) agents in multiple sclerosis (MS) have contributed to reduction of neuroinflammation, they have not succeeded in the prevention of progression of disease. Inflammation is the appropriate immune response to infection, autoimmunity, cancer, injury and allograft transplantation1. When inflammation does not resolve appropriately, a prolonged immune response persists leading to tissue destruction and loss of function1. Chronic infiltration by immune cells in the meninges is believed to form transitory lymphoid cell aggregates which simulate secondary lymphoid organs (SLO), and are known as meningeal tertiary lymphoid organs (mTLO) which play an important role in the pathogenesis of autoimmunity1,2. The mTLO seem to play a role in the intrathecal activity of immune system cells in MS3. The SLO, such as lymph nodes, show a cellular organization that includes germinal centers (GC) containing antibody secreting and proliferating B-cells with follicular dendritic cells (FDC), a T-cell zone that incorporates naïve cells from the blood stream, high endothelial venules for extravasation of lymphocytes, and a stromal cell network that provides chemokines and extracellular matrix for cell migration and structural integrity1. Chemokines are a family of proteins with the specific property of regulating leukocytes in the immune system and they may play a role in neurotransmission and neuromodulation4. Leukocyte trafficking is mediated by inflammatory chemokines in inflamed tissues and by homeostatic chemokines in lymphoid sites5 (Figure 1). In this review, we focus on the role that CXCL13 (also known as B cell attracting chemokine [BCA-1], C-X-C motif ligand 13, or B lymphocyte chemoattractant [BLC]) plays in the formation of the mTLO in MS.
The SLO have a genetically determined pattern of development and programming that allows trapping and concentration of foreign antigens to initiate an adaptive immune response1. Mucosal associated and non-encapsulated lymphoid tissue (including the Peyer’s patches, adenoid tissue of the nasopharynx, tonsils, and the bronchial associated lymphoid tissue), together with lymphoid nodes and spleen, constitute the SLO7,8. The lymph node cortex contains clusters or primary follicles that include packaged B cells and FDC, whereas the node para-cortex has a lesser number of dendritic cells (DC) and T cells7. Generation of B cells with ability to produce auto antibodies usually occur in physiological conditions9. These auto antibodies are low affinity IgM, which exhibit a wide spectrum of reactivity and strong preference for soluble self-antigens on the cell surface9. Auto reactive low affinity B cells suffer apoptosis being unlikely they represent danger in normal conditions9.
The GC present remarkable lymphocytic mitosis within SLO follicles10. Weyand et al. stated the GC are critical in the development of the B-cell normal immune response by driving-in cell division and maturation, B-cell selection with high affinity for immunoglobulin receptors and differentiation of B-cells and plasma cells (PC)2. Real time imaging technology has allowed visualization of the transit of the B cells from the dark zone to the light zone, and viceversa, during the maturation of the GC10–12. The GC light zone displays a predominance of FDC and follicular T-helper (Tfh) cells, whereas the dark zone contains closely packed lymphocytes and stromal cells10,13. The chemokine receptor CXCR4 is required for the positioning of the B cells in the dark zone where its ligand, CXCL12, is more abundant and is produced by stromal cells14. At the light zone, CXCL13 chemokine is concentrated in the FDC processes and, in conjunction with CXCR5, they contribute to the accumulation of B cells in this zone13,14. T-cells in the GC are essential to maintain signaling and represent approximately 5–20% of cell population10. Tfh cells are characterized by the expression of CXCR5 and ICOS, which is a subtype of Tfh cells10,15. Within the light zone, the three possible different outcomes for the centrocytes include death due to apoptosis; differentiation into memory B-cell or long lived plasma cells (LLPC); and re-entrance to the dark zone for a further round of cell mutation and selection16. The relevant function of the GC is, most likely, the primary production of memory B-cells and LLPC16,17 (Figure 2). Recent studies analyzing IgG heavy chain variable region genes in B cells from MS patients revealed that B cells are able to enter and exit the blood brain barrier in order get exposed to somatic hypermutation at the GC18–22.
The induction of lymphoid chemokines, depends on lymphotoxin β (LT-β) and the tumor necrosis factor α (TNF-α) signaling on stromal cells and FDC23. Lymphotoxin α1β2 (LTα1β2) is expressed in the surface of B and T cells in the adult immune system and ligates to the lymphotoxin β receptor (LTβR) in reticular stromal cells thus inducing expression of lymphoid chemokines, such as CCL19, CCL21 and CXCL1324. These chemokines regulate the homeostatic traffic of lymphocytes in lymphoid organs and their distribution in the GC24. Homeostatic chemokines promote secretion of LTα1β2 by T and B cells, establishing a feedback loop that perpetuates the recruitment of lymphocytes and positional organization in the GC1. The chemokine CXCL13 has the following relevant properties:
1. CXCL13 increases its own production by stimulating the growth of FDC after regulating LTα1β2 on the membrane of B cells5,25.
2. CXCL13 is produced in the SLO by FDC and macrophages and is an important chemoattractant to the CNS26,27.
3. Follicular stromal cells express CXCL13, which is needed for nesting CXCR5+B cells and a subset of T cells in the follicular compartment7.
4. CXCL13 primarily works through CXCR5 expressed in mature B lymphocytes9, CD4+ Tfh28, CD4+ Th17 cells29, minor subset of CD8+ T cells and activated tonsil Treg cells9,29.
5. CXCL13 has no relation with CD138+ and CD38+ plasmablasts, and PC18.
Stromal cells from the T cell zone express the chemokines CCL19 and CCL21, which share the receptor CCR7 that directs naïve, central memory T cells and DC to the T cell compartment7,30. CXCR5 is expressed in 20 to 30% of CD4+T cells in blood and CSF, and virtually in all B cells in blood and the majority of B cells in the CSF compartment31. Mice lacking CXCL13, or its receptor CXCR5, fail to develop peripheral lymph nodes1. Khademi et al. determined the concentration of CXCL13 in CSF of individuals with MS, other neurological diseases including viral and bacterial infection, and healthy controls finding higher levels of the chemokine in subjects with infections followed to a lesser extent by the patients with MS32. The levels of CXCL13 correlated negatively with disease span, concluding that early determination of CXCL13 might predict prognosis of disease32.
By maintaining antibody diversity, B cell differentiation, isotype switching, oligoclonal expansion, and local production of autoreactive PCs, the TLO perpetuate disease in response to environmental inputs33. The processes of biological development involved in lymphoid organogenesis are shared among the secondary and tertiary lymphoid structures2. Lymphoid organogenesis and formation of mTLO may be facilitated by expression of lymphotoxin α (LT-α) at the external layer of meningeal inflamed vessels leading to the compartmentalization of the immune response in MS18,34. The mTLO maintain differentiation and maturation of antigen specific effector lymphocytes which perpetuates inflammation and disease progression27. The TLO, besides SLO, provide a thriving environment where PC differentiate from plasmablasts7,27. In the absence of recirculating immune cells from the periphery, the TLO exerts its remarkable ability to remain active for several weeks35. Therefore, the neutralization of TLO could play a significant role by blocking the re-emergency of auto reactive clones that could be able to drive relapses or resistance to therapy35. Th17 cells, Tfh and a subtype of activated B cells, which are critical in systemic inflammation related with presence of TLO, are strongly associated with MS progression36.
Disorganized B cell follicles in SLO have shown reduced capacity to originate natural antibody responses in CXCL13-/- mice25,37. Deficiency of CXCL13 results in a moderate course of disease characterized by a better recovery with attenuation of white matter inflammation and gliosis during the acute and chronic stage of EAE38. Krumbholz et al. showed there was a direct correlation between CXCL13 levels and the number of B cells, T cells and plasmabasts in the CSF of MS patients5. Clonal expansion and somatic hypermutation of B cells have been observed in the CSF of patients with MS39. CXCL13 was upregulated in active MS lesions but not in chronic inactive lesions and, in a similar range, in the serum of patients with relapsing remitting MS (RRMS) and control subjects indicating the intrathecal production of this chemokine5. CXCL13 was identified by immunohistochemistry in intrameningeal B-cell follicles, but not in the cerebral parenchyma, of chronic active or inactive MS lesions40. Patients with clinically isolated syndrome, who had shown conversion to clinically definitive MS within 2 years, had high levels of CXCL13 in the CSF32,41,42. Elevated levels of CXCL13 in CSF have also been reported in patients with RRMS compared to controls and the CSF levels have been significantly increased during relapses but declining after initiation of B cell depleting therapy23,32,43.
Meningeal infiltrates can be disperse or well organized encompassing mTLO, whose lifespan is unknown27,40. The presence of follicles containing proliferating B cells, T cells, PC and FDC that express CXCL13 in the proximity of inflamed blood vessels in the meninges of patients with secondary progressive MS (SPMS) has been documented40. The mTLO correlated with neuronal loss, adjacent cortical demyelination and a more rapid progression of disease23. Patients with SPMS with positive mTLO have shown wide gray matter demyelination associated with loss of neurons, oligodendrocytes, and astrocytes; cortical atrophy, and microglial activation in the outer layer of the cortex44,45. It remains to be determined whether the formation of mTLO depends on the subtype of disease or it is the result of inflammation or consequence of chronicity35.
A novel therapeutic monoclonal antibody against CXCL13 (Mab 5261 and Mab 5261-muIg) has been shown to induce functional in vitro inhibition of the chemokine in humans and mice9. LT-β receptor blocking immunoglobulin inhibits CXCL13 interactions, suppresses the formation of mTLO in the CNS and ameliorates the symptoms of EAE in rodents24. In the EAE induced by the transfer of myelin-specific Th17 cells (Th17 EAE), Quinn et al. confirmed a role of Tfh cells by blocking Tfh trafficking using antibody against CXCL13 and found that this treatment significantly reduced expression of disease46. Some DMT available for the treatment for MS ameliorate levels of CXCL13, but the mechanisms by which it occurs are not completely understood. In patients with RRMS treated with natalizumab, a significant reduction in CXCL13 in CSF was observed in comparison to β-interferon47. In another study, Novakova et al. evaluated the effect of treatment with fingolimod in CSF biomarkers, including CXCL13, of MS patients who had previously been on β-interferon, glatiramer acetate, teriflunomide (and had to switch therapy because of breakthrough disease activity) or natalizumab (who had to switch due to risk of PML) observing significant reduction of CXCL13 in the CSF of patients in both groups48. Also, Alvarez et al. found that in patients with active RRMS, in spite of treatment with β-interferon or glatiramer acetate, the administration of rituximab led to a normalization of the CSF level of CXCL13 in the majority of patients, thus suggesting that high levels of CXCL13 in CSF at baseline could predict a forthcoming therapeutic response to B cell depletion49. Piccio et al. found that in patients with RRMS treated with IV rituximab, concomitant with either β-interferon or glatiramer acetate, there was a reduction of CXCL13 and CCL19 in CSF, which correlated with significant reduction of B cells (95%) and T cells (50%) in CSF31. Perry et al. found intrathecal reduction of CXCL13 (50.4%) and IgG index (13.5%) resulting from inhibition of development of lymphoid tissue inducer cells in patients with MS treated with daclizumab50. Braendstrup et al. reported the case of a patient with MS who had undergone allogenic hematopoietic stem cells transplant for treatment of follicular lymphoma and who after two years presented negative determination of oligoclonal bands and detectable CXCL13 in CSF51.
A self-sustained intrathecal inflammation fostered by CSF chemokines involved in the traffic and survival of inflammatory cells occurs early in disease and is orchestrated by mTLO3. Studies have shown that lineage of B cells can travel through peripheral blood, cervical lymphoid nodes, and the intrathecal compartment where they can be exposed to somatic hypermutation in the mTLO and return to peripheral blood18. As mentioned above, Piccio et al. found that CSF CXCL13 and CCL19 were decreased at week 24 after IV rituximab31 However, Topping et al. found that therapy with intrathecal rituximab in patients with RRMS and SPMS resulted in no variation of CXCL13 levels in serum and CSF during the period of evaluation52. Bonnan has hypothesized that, in order to prevent an unwanted generalized immune suppression resulting from systemic targeting of resident TLO, intrathecal immune reset should be attempted with a combination of monoclonal antibodies targeting each cell sub-type and aimed at eliminating simultaneously B cells, T cells, PC and FDC, via the intrathecal route. Excepting rituximab, candidate drugs still require preclinical studies for validation3.
An early neutralization of CXCL13 would interfere with the organization and function of the mTLO thus modifying and reducing inflammation in the CNS of patients with MS. Studies in animal models where CXCL13 has been neutralized, or is not expressed (such as the CXCL13-/- mice), confirm its crucial role maintaining, rather than initiating, inflammation and its manipulation could lead to modification of disease in these models37. However, any therapeutic strategy unable to neutralize LLPCs or antibody secreting cells will not be successful in an attempt to impede the chronic progression of disease53. Neutralization of the CXCL13 should be sought as complementary therapy to the DMT in MS.
No data is associated with this article.
CAM is a member of the Data & Safety Monitoring Board for the NINDS/NIH study NS003055-08/NS003056-08. He has received no compensation for his participation in that study. ACL does not report any competing interests.
This work was presented, in preliminary version, at the Third annual Americas Committee for Treatment and Research in Multiple Sclerosis (ACTRIMS Forum 2018) in San Diego, CA, on February 2, 2018. Poster presentation No. P193. Financial support for the on-line publication of this article was provided by The Department of Neurology, MedStar Georgetown University Hospital.
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References
1. Pikor NB, Astarita JL, Summers-Deluca L, Galicia G, et al.: Integration of Th17- and Lymphotoxin-Derived Signals Initiates Meningeal-Resident Stromal Cell Remodeling to Propagate Neuroinflammation.Immunity. 2015; 43 (6): 1160-73 PubMed Abstract | Publisher Full TextCompeting Interests: No competing interests were disclosed.
Is the topic of the review discussed comprehensively in the context of the current literature?
Partly
Are all factual statements correct and adequately supported by citations?
Partly
Is the review written in accessible language?
Yes
Are the conclusions drawn appropriate in the context of the current research literature?
Partly
Competing Interests: No competing interests were disclosed.
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