Tuesday, 29 December 2015

The neuroinflammatory story in MS - astrocytes and B cells

van Luijn MM, van Meurs M, Stoop MP, Verbraak E, Wierenga-Wolf AF, Melief MJ, Kreft KL, Verdijk RM, 't Hart BA, Luider TM, Laman JD, Hintzen RQ.

Elevated Expression of the Cerebrospinal Fluid Disease Markers Chromogranin A and Clusterin in Astrocytes of Multiple Sclerosis White Matter Lesions.

J Neuropathol Exp Neurol. 2015. pii: nlv004. [Epub ahead of print]

Using proteomics, we previously identified chromogranin A (CgA) and clusterin (CLU) as disease-related proteins in the cerebrospinal fluid (CSF) of patients with multiple sclerosis (MS). CgA and CLU are involved in cell survival and are implicated in neurodegenerative disorders and may also have roles in MS pathophysiology. We investigated CgA and CLU expression in lesions and nonlesional regions in postmortem brains of MS patients and controls and in the brains of marmosets with experimental autoimmune encephalomyelitis. By quantitative PCR, mRNA levels of CgA and CLU were elevated in white matter but not in grey matter of MS patients. In situ analyses showed greater expression of CgA and CLU in white matter lesions than in normal-appearing regions in MS patients and in the marmosets, primarily in or adjacent to perivascular spaces and inflammatory infiltrates. Both proteins were expressed by glial fibrillary acidic protein-positive astrocytes. CgA was more localized in astrocytic processes and endfeet surrounding blood vessels and was abundant in the superficial glia limitans and ependyma, 2 CSF-brain borders. Increased expression of CgA and CLU in reactive astrocytes in MS white matter lesions supports a role for these molecules as neuro-inflammatory mediators and their potential as CSF markers of active pathological processes in MS patients.

Wuhrer M, Selman MH, McDonnell LA, Kümpfel T, Derfuss T, Khademi M, Olsson T, Hohlfeld R, Meinl E, Krumbholz M.

Pro-inflammatory pattern of IgG1 Fc glycosylation in multiple sclerosis cerebrospinal fluid.J Neuroinflammation. 2015;12(1):235.

BACKGROUND:Immunoglobulin G (IgG) effector functions are regulated by the composition of glycans attached to a conserved N-glycosylation site in the Fc part. Intrathecal production of IgG, especially IgG1, is a hallmark of multiple sclerosis (MS), but nothing is known about IgG Fc glycosylation in MS and in cerebrospinal fluid (CSF) in general.

METHODS:We applied mass spectrometry of tryptic Fc glycopeptides to analyze IgG Fc glycosylation (sialylation, galactosylation, fucosylation, and bisecting N-acetylglucosamine (GlcNAc)) in 48 paired CSF and serum samples from adult patients with MS or a first demyelinating event highly suggestive of MS (designated as MS cases), and from healthy volunteers and patients with other non-inflammatory diseases (control group). p values were adjusted for multiple testing.

RESULTS:Our experiments revealed four main results. First, IgG1 glycosylation patterns were different in CSF vs. serum, in the MS group and even in control donors without intrathecal IgG synthesis. Second, in MS patients vs. controls, IgG1 glycosylation patterns were altered in CSF, but not in serum. Specifically, in CSF from the MS group, bisecting GlcNAc were elevated, and afucosylation and galactosylation were reduced. Elevated bisecting GlcNAc and reduced galactosylation are known to enhance IgG effector functions. Third, hypothesis-free regression analysis revealed that alterations of afucosylation and bisecting GlcNAc in CSF from MS cases peaked 2-3 months after the last relapse. Fourth, CSF IgG1 glycosylation correlated with the degree of intrathecal IgG synthesis and CSF cell count.

CONCLUSIONS:The CNS compartment as well as the inflammatory milieu in MS affect IgG1 Fc glycosylation. In MS, the CSF IgG1 glycosylation has features that enhance Fc effector functions.

Figure: Astrocytes hold a strategic position in the CNS, being in close contact to neurons, microglia, oligodendrocytes and other astrocytes and with blood vessels. Following activation (for instance during infection), astrocytes secrete factors that activate bystander cells and increase BBB permeability, resulting in recruitment of immune cells into the brain.

There is more to the inflammatory process than meets the eye. Meet three of the front line contenders: T and B cells with their global response capability and astrocytes which were initially felt to be gap fillers in the brain neural network, but now are understood to be active players in CNS innate immunity. The astrocytes, however are the only resident myeloid cells in the nervous system, and form a glia limitans around blood vessels restricting access of immune cells to the brain (see figure above).

It is possible that astrocytes are locally activated, as demonstrated in the first abstract with increased expression of both chromogranin and clusterin in MS white matter lesions and not grey matter. This may have something to do with the location of perivascular spaces, which are compartments that surround small blood vessels as they penetrate the brain, and are a site for immune cell accumulation during CNS inflammation.

The B/plasma cells on the other hand, among other things are defined by their antibody production. As I've alluded to in previous blog posts, we do not know what the oligoclonal bands are directed at. Maybe, it doesn't really matter?! In the second abstract, the authors point out that MS CSF contains oligoclonal bands with a glycan pattern that enhances its effector function. It's conceivable that the glycosylation process itself is what makes the unsuspecting immunoglobulin more pro-inflammatory than it intended to be? In fact, this process itself is not unique to MS and has been described in rheumatoid arthritis and neuromyelitis optica. Moreover, they also found that alterations in glycosylation patterns in the CSF occured shortly after a relapse. See a common theme here? As Carl von Clausewitz's famous treatise On War goes: "War is fighting and operates in a peculiar element-danger. But war is served by many activities quite different from it, all of which concern the maintenance of the fighting forces. These preparatory activities are excluded from the narrower meaning of the art of war-the actual conduct of war, because they are concerned only with the creation, training, and maintenance of the fighting forces."

1 comment:

  1. It probably still matters what the "recognition" sections of IgGs bind to - but it is really intriguing to read that altered glycosylation of the Fc regions (which are otherwise constant in different antibodies) is found in MS. A possible scenario then would be normally glycosylated IgGs in healthy CNS, but alternatively glycosylated IgGs, with the same targets, in pathology. The question still remains on what the "normal" IgGs do in the brain, and why this normal function has to be modified in disease.


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