Immunopathogenesis of Multiple Sclerosis
Disha Miglani

Multiple Sclerosis (MS) is a T cell mediated, organ specific autoimmune disease, in which the myelin sheath, which surrounds and protects nerve fibres of the brain, is destroyed. (1) Since the trigger to this disease is yet to be identified, and the initiation mechanism yet to be determined, MS is an interesting disease to investigate from an immunological perspective.

MS is a chronic inflammatory demyelineating disease of the Central Nervous System (CNS). The activation of autoreactive T cells is a central event in the development of an autoimmune response. CD4+ T cells undergo facilitated activation which leads to breakdown of immunologic self-tolerance and the subsequent chronic autoimmune response. (2)

It is thought that damage to brain tissue in MS is initiated by lymphocytes (immune cells) entering the central nervous system (CNS) upon crossing the Blood Brain Barrier (BBB). Under physiological conditions, only activated T cells can pass the BBB. Therefore it is expected that an initial activation process will precede the infiltration of T cells in the CNS. Activation and migration of T cells across the BBB is facilitated by monocytes via cell-cell interactions that subsequently lead to the formation of inflammatory plaques. Aberrations in the expression of MHC or co-stimulatory molecules on professional antigen presenting cells (APCs) like monocytes, might have consequences for the capacity of monocytes to activate T cells. (5)

There is also the possibility that brain cells release cytokines (chemokines) that act as beacons for immune cells directing guided movement towards the brain and thus T cells are activated when they recognise and bind to specific myelin antigen that is presented by professional APCs in the context of major histocompatibility complex II (MHC II).(6)

Markovic-Plese, et al., suggest the following mechanism for the immunopathogenesis of MS, as documented in Experimental Autoimmune Encephlomyelitis (EAE), an animal model:

(1) Myelin specific T cells are activated in the peripheral circulation
(2) Activated T- cells interact with adhesion molecules on endothelial cells and migrate through the BBB into the CNS
(3) CD4+ T cells recognise myelin antigens and initiate chronic inflammatory process within the CNS. Antigens are presented by microglia and astrocytes to myelin-specific memory T cells and local tissue damage is perpetuated
(4) Myelin is destroyed by macrophages, cytokines, oxygen and nitrogen radicals produced by local inflammation. (2)

MS is characterised by infiltration of CD4+ T lymphocytes and other leukocytes into the CNS. Peripheral immunisation with myelin proteins leads to induction of autoimmune Th1 CD4+ T cell response. (1)

MONOCYTES

Circulating monocytes regulate the bursts of intrathecal inflammation observed in MS and are an important source of cytokines. Along side T cells, they are the major cell type involved in the MS characteristic of perivascular infiltration, and in 1998 it was shown that monocytes facilitate the migration of T cells across the BBB. (5)

It is known that activated blood-borne monocytes are abundant in MS lesions, but the phenotype and cytokine profiles were thus far incomplete. Kouwenhoven, et al., investigated the levels of the cytokines IL-6, IL-12, TNF-alpha, and IL-10 that are secreted by monocytes found in MS lesions. They found that patients with untreated MS and other neurological diseases (OND) displayed escalated levels of blood monocytes secreting IL-6 and IL-12 compared to healthy controls, whilst there was no detectable difference between the levels of monocytes secreting TNF-alpha and IL-10. (5)

The results of this investigation, although present a cytokine profile in MS lesions, does not provide any insight into initiation mechanisms of MS. There is the possibility that the balance of cytokine secretions is skewed post MS induction and the changes in expression are actually a consequence of MS.

The blood-borne monocytes of MS patients in the CSF also exhibited increased levels of co-stimulatory molecules CD86 for patients with disease duration of less than 10 years, and higher levels of CD 80 for patients with disease duration greater than 10 years.

The release of IL-12, directs a Th1 immune response due to its potent polarising nature during T cell activation. The presence of MHC molecules in concurrence with costimulatory molecules on the cell surface of these monocytes allows antigen APCs to transduce immunological signals to and activate T cells. The secretion of cytokines IFN-gamma and IL-12 detrimental in MS causing relapses. IL-12 also facilitates the migration of T cells and monocytes across endothelial barriers. This mechanism is important in the development of inflammatory MS lesions.

ANTIGEN PRESENTATION

Local antigen presentation is a critical requirement for the initiation and perpetuation of inflammatory responses within the CNS. However, the CNS is devoid of immunocompetent APCs MHC Class II and co-stimulatory molecules CD80 and CD 86 are upregulated on microgilia and macrophages in the setting of local inflammation and can effectively present antigens. Astrocytes, the CNS resident APCs, present antigen in a co-stimulation \-independent manner and stimulate only memory T cells, which have a lower activation threshold. (2)

Capacity of autoreactive T-cells to recognise many different epitopes reflects the importance of MHC/epitope density on APCs. Mechanisms involved in local antigen presentation plays an important role in the perpetuation of chronic CNS inflammatory responses. (2)

Astrocytes in active lesions of multiple sclerosis express major histocompatibility (MHC) class II molecules and may play an important role in the presentation of antigen to myelin-specific T cells. In 2002, it was postulated that because Astrocytes lack the B7( CD80 and CD 86) co-stimulatory molecules are unable to act as APCs to activate these T-cells. Zeinstra et al., demonstrates that reactive astrocytes in chronic active plaques of multiple sclerosis express the co-stimulatory molecules B7-1 and B7-2, and hence have the necessary attributes to act as APCs. (6)

B7 molecules provide co-stimulation of T cells via two receptors CD28 and CTLA-4. CD28 is constitutively expressed and essential for initiation of the immune response, while the interaction with CTLA-4, which is transcriptionally induced after T-cell activation, is thought to play a role in down-regulating the immune response.

Costimulatory signals regulate T cell activation and maintain the balance between Th1 and Th2 T helper cell differentiation. In the two signal model of T cell activation the first signal is a T cell receptor engagement by MHC expressing antigen, whist the second signal is antigen-independent and relies on costimulatory interactions. Thus for T cells to be activated there must be an interaction of CD80(B7-1) and/or CD86(B7-2) with CD28 expressed on Antigen Presenting Cells (APC). (7)

The second signal of costimulation, i.e. the binding of B7 to CD28 is crucial for T cell activation and proliferation. T cells reactive to myelin antigens including MBP, PLP, and MOG are present in peripheral blood of both MS patients and healthy individuals. Microglia in human CNS constitutively express MHC Class II and, their results show that B7-1 and B7-2 are also constitutively expressed. However, since healthy individuals also express these co-stimulatory molecules without pathological consequences, there is no strong support that microglia are the key players in initiating the autoimmune reactions in the CNS. Zeinstra et al., postulated that astrocytes in active MS lesions are also able to express B7-1/B7-2 molecules. (6) This would challenge the concept that autoreactive T cells that are activated in the periphery can enter into the CNS, and would thus be activated after crossing the BBB, when the myelin antigens are presented with the costimulation signal provided by astrocytes.

However, the results showed that astrocytes in healthy controls, outside the MS lesions and in the astrogliotic centre stained negative for the costimulatory molecules. Only those at the borders of MS plaques stained positive. (6)

Astrocytes in vitro can be induced to express co-stimulatory molecules B7 upon stimulation with IFN-gamma , however since the experimental procedure was adequately controlled, it was found that this was only the case for patients with MS. Those with HSV encephalitis was B7-1 and B7-2 negative. This means that expression of costimulatory molecules on astrocytes in inflammatory diseases other than MS are severely restricted. (6) This is a broad statement that is not validated, as there could be other inducing factors for other inflammatory diseases. The results are also contrary with the work of Wensky et al., who showed that even without IFN-gamma, severe case of MS would be initiated and the IFN-gamma also plays a protective role. This could be true, since the initiation of B7 could lead to interaction with CTLA-4 rather than CD28 and suppress inflammation.

COSTIMULATORY MOLECULES

Inducible costimulatory protein (ICOS) is a member of the CD28-family, and binds ICOS ligand (ICOSL), a member of the B7 family ligands. (7) Wiendl et al., investigated the expression and functional role of ICOS costimulation in healthy donors and MS patients and found that ICOS affects the differentiation of Th1 and Th2 cells after primary activation. (7)

MBP - reactive T cells from MS patients are less dependent on B7/CD28 mediated costimulatory signals than MBP reactive cells from healthy donors. (7) Dysregulation of costimulatory pathways in animal models of autoimmune diseases can lower the T cell activation threshold and lead to a chronic autoimmune response. Therefore the lack of expression of costimulatory molecules also leads to a loss of the CTLA-4 mediated inhibitory signal which controls lymphocyte proliferation. (2)

CD28 costimulation synergises with TCR activation and induces production of multiple cytokines. Following activation, CD4+ cells down-modulate CD28 and express CTLA-4, a structural homologue of CD28. CTLA-4 delivers a negative signal for T-cell activation and terminates the proliferative response. (2)

Wiendl, et al., show that ICOS is upregulated on human T cells after stimulation and can modulate both Th1 and Th2 cytokine production in the absence and presence of B7-costimulation. They also demonstrate the functionality of ICO co-stimulatory pathway in MS patients. (7)

T cell lines showed no constitutive expression of ICOS as measured by flow cytometry, however antigen stimulation induced ICOS expression in all T cell lines in a concentration dependent manner. ICOS is preferentially expressed in IL-4 producing Th2 cells, and therefore an ICOS deficit leads to a Th1 mediated response, a proinflammatory response in the induction of MS. (7)

ICOS deficient mice had a profound defect in isotype switching in T cell dependent B cell responses and were defective in IL-4 and IL-13 production. This leads to low quantities of Th2 cells but dramatic increases in Th1 cytokines increasing the production of Th1 pro-inflammatory CD4 T cells. ICOS deficient mice showed a much more severe case of EAE. (7)

However, blocking ICOSL also led to severe case of EAE, and reduced the production of IFN-beta by 50% with SAg-stimulation (staphylococcal enterotoxin A) and by 60% with GA (Glatiramer acetate), whilst IL-4 reduced by 21% and 27% respectively. (7) This lack of IFN-gamme with more severe form of EAE correlates with the work done by Wensky et al., in which IFN-? may indeed also play a protective role. (3)

CONCLUSION

Though the experiments have been conducted in several different settings, such as in vitro, in animal models and in human clinical trials, they all show that the immunopathogenesis of MS is a complex process. It is very difficult to replicate the ideal physiological conditions for initiation of MS, and consequently results may be skewed by the strong induction methods that override subtle processes of initiation.

Further research should include investigations on the role of chemokines in the initiation of MS, and whether IFN-gamma can even be classified as a treatment of MS. The results thus far are varying and contradictory. This could also lead to the exploration of natural suppression mechanisms of autoimmune responses despite the elevated levels of costimulatory molecules expressed. Further investigation of IFN-gamma should also be made in order to more clearly define the protection mechanism it provides is healthy controls.

The further examination of initiation and suppression mechanisms could lead to the discovery of therapeutic treatments for patients with MS.

Glossary

Demylination: an inflammatory process that disrupts the myelin coating of nervous system structures

Myelin: fat-like substance and a major component of specialised cells Schwann cells that are wrapped around the long fibres (neurites) that transmit signals from nerve cells in the CNS. The myelin actually speeds up the signal transmission, but also protects the nerve cells. Patients with loss of myelin, as in multiple sclerosis, tire easily and movements slow down. Myelin is also the reason for the white appearance of the white brain matter.

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