Anti-aquaporin-4 antibodies in neuromyelitis optica: how to prove their pathogenetic relevance?

The discovery of a specific autoantibody response in neuromyelitis optica (NMO) patients, which selectively targets astrocytic end feet at the glia limitans1 and which is directed against the water channel aquaporin-4 (AQP-4)2 was a milestone in defining this disease entity and profoundly changed our view regarding its pathogenesis.3 Indirect evidence, coming from clinical observations and pathology strongly suggest that these autoantibodies play a major role in driving the disease process. The pathological hallmark of NMO lesions is a very selective and characteristic deposition of immunoglobulins and complement on astrocytes at the glia limitans, which is associated with destruction and loss of glial fibrillary acidic protein and AQP-4 positive astrocytes in fresh lesions4 to 6 followed by demyelination and global tissue destruction. As in other antibody-mediated diseases, granulocytes, and in particular eosinophils, are a major component of the inflammatory infiltrates.4 Furthermore, the distribution of the lesions in the brain and spinal cord of NMO patients correlates with the extent of regional AQP-4 expression.6,7 Most importantly, therapies, which eliminate antibodies (plasma exchange),8 or which target Blymphocytes (anti-CD20 antibodies),9 are at least in part effective in NMO patients. Based on this evidence NMO is now considered an antibody-mediated autoimmune disease, however, direct proof of the pathogenic potential of AQP-4 antibodies is so far lacking.

Molecular pathogenesis of neuroinflammation

The past few years have seen significant progress towards understanding the mechanisms of immune surveillance and inflammation in the nervous system. In this review, the milestones of scientific discovery in this field are discussed, and the strengths and limitations of the different ways of examining the molecular pathogenesis of neuro-inflammation examined. The review is limited to the inflammatory reactions of the central nervous system that occur in multiple sclerosis and experimental autoimmune encephalomyelitis.

New tools to trace populations of inflammatory cells in the CNS

Cells of the central nervous system (CNS) and immune system communicate regularly. There is a constant surveillance of the intact, healthy CNS by activated T-cells, and massive infiltration of the CNS by immune cells under pathological conditions such as neurodegeneration or neuroinflammation. Labeling CNS-infiltrating T-cells is an essential tool to identify the signals and mechanisms, which mediate the interaction between immune cells and cells of the CNS. In this article, we will present an overview describing currently used cellular markers and demonstrate how these markers have contributed to our current knowledge of CNS inflammation and immune surveillance.

Transformation of donor-derived bone marrow precursors into host microglia during autoimmune CNS inflammation and during the retrograde response to axotomy

Macrophages in the brain can have a triple source. They may originate from recently blood-derived precursors, from the largely resident perivascular cell population (perivascular macrophages and related cells), and from intrinsic parenchymal as well as perivascular microglia. Although continuous exchange of part of the perivascular cell population with bone marrow-derived precursors is now accepted, the turnover of adult parenchymal microglia has remained enigmatic. Using bone-marrow chimeras carrying an unexpressed marker gene and carbon labeling of peripheral monocyte/macrophages in a combined model of facial nerve axotomy and transfer experimental autoimmune encephalitis, we demonstrate for the first time that there is an easy to induce exchange between parenchymal central nervous system (CNS) microglia and the macrophage precursor cell pool of the bone marrow. Furthermore, very low level infiltration of the CNS parenchyma by recently bone marrow-derived microglia could be observed after simple peripheral nerve axotomy that is followed by neuronal regeneration. Thus, microglial cells can be considered wanderers between the peripheral immune system and the CNS where they may act as a "Trojan horse" in infections. The fact that recently bone marrow-derived parenchymal microglia fully integrate into a regenerating brain nucleus' architecture encourages entirely new approaches for delivering genes into the adult CNS.

Transgenic Lewis rats overexpressing the proteolipid protein gene: myelin degeneration and its effect on T cell-mediated experimental autoimmune encephalomyelitis

Transgenic Lewis rats overexpressing proteolipid protein (PLP) genes in peripheral and central nervous myelin were produced by microinjecting murine genomic PLP sequences into fertilized eggs. The mouse PLP gene shares 98.7% homology in the nucleotide sequence with its rat counterpart, but both are fully identical on protein level. Homozygous rats show tremors early in postnatal life, eventually develop seizures, and die before they reach weaning age, while hemizygous animals are phenotypically normal and have a normal life expectancy. Transgene expression in the central nervous system (CNS) has profound consequences for myelin formation and maintenance: approximately twofold overexpression of PLP/DM-20, as seen in homozygotes, results in apoptosis of mature, and a developmental arrest of the remaining immature oligodendrocytes. Severe dysmyelination ensues, associated with reactive astrogliosis and microglia activation/proliferation. Activation of microglia is also prominent in hemizygous rats with low levels of transgene overexpression. In these animals, myelin sheaths remain intact, but there is low-grade myelin degeneration throughout life witnessed by myelin uptake and activation of microglia and astrocytes, in the absence of the expression of major histocompatibility complex class II gene products. There were no spontaneous lymphocytic infiltrates in areas of myelin degeneration. However, hemizygous LEW.PLP rats were more sensitive to experimental autoimmune encephalomyelitis mediated by T cells specific for PLP, but not another encephalitogenic myelin protein, MBP.

Myelin dysfunction/degradation in the central nervous system: why are myelin sheaths susceptible to damage?

In the central nervous system, myelin sheaths are produced to electrically insulate axons and to increase the velocity of axonal conduction. They are highly complex structures, which are often destructed in neurological disorders. One possible reason for the vulnerability of myelin sheaths to damage became apparent from analyses of animals with altered amounts of otherwise normal myelin components: Due to limited redundance in function between different myelin proteins, dysfunction or loss of one protein may cause loss of function and instability of the entire myelin sheath.

T-cell apoptosis in inflammatory brain lesions: destruction of T cells does not depend on antigen recognition

Elimination of inflammatory T cells by apoptosis appears to play an important role in the down-regulation of inflammation in the central nervous system. Here we report that apoptosis of T lymphocytes occurs to a similar extent in different models of autoimmune encephalomyelitis. Apoptosis is restricted to cells located in the neuroectodermal parenchyma, thereby leaving T cells present in the brain's connective tissue compartments unharmed. Death of T cells in the parenchyma does not depend on antigen presentation by resident microglial cells or astrocytes. Adoptive transfer experiments with T lymphocytes carrying a specific genetic marker revealed that in the central nervous system these cells are destroyed regardless of their antigen specificity or state of activation. Although many of both antigen-dependent and -independent mechanisms in the induction of T-cell apoptosis may act simultaneously, our results suggest that the nervous system harbors a specific, currently undefined, mechanism that effectively eliminates infiltrating T lymphocytes.

The myelin basic protein-specific T cell repertoire in Lewis rats: T cell receptor diversity is influenced both by intrathymic milieu and by extrathymic peptide presentation

In the Lewis rat, the T lymphocyte response to guinea pig myelin basic protein (MBP) is focused almost exclusively on epitopes nested in the MBP peptide sequence p68-88, and is dominated by T cell receptors (TCR) using Vbeta8.2 gene elements, together with short N(D)N regions. Here we analyzed MBP-specific TCR from Lewis T cells differentiating in chimeric thymuses of Lewis rat/SCID mouse chimeras, in the absence of an intact rat thymic microenvironment (SCID(FL) mice). In these T cells, the TCR Vbeta repertoire is broad, N(D)N regions are significantly longer, and contain regular rates of template-independent N nucleotides. In striking contrast, a Vbeta8.2 biased TCR repertoire and few N-region inserts are seen in p68-88-specific, Lewis rat-derived T cells differentiating in the complete rat thymic microenvironment provided by chimeric SCID mice bearing embryonic Lewis thymus grafts (SCID(FL/FT) mice). A T cell repertoire resembling the one in SCID(FL) mice is used by T cells of intact Lewis rats following immunization with a truncated epitope of MBP, p69-86. Also this selection generates a broad TCR Vbeta pattern with long N(D)N regions, and higher numbers of N nucleotides. These results show that both intrathymic repertoire selection, and extrathymic peptide priming exert profound effects on the TCR usage in the anti-MBP response of Lewis rats.

Animal models of demyelination

Demyelination is a pathological feature that is characteristic of many diseases of the central nervous system (CNS) including multiple sclerosis (MS), sub-acute sclerosing panencephalomyelitis (SSPE), metachromatic leukodystrophy and Pelizaeus-Merzbacher disease. While demyelination is a pathological end-point that is common to all of these diseases, the cellular and molecular mechanisms responsible for this pathology are very different . These range from genetic defects that affect lipid metabolism in the leukodystrophies, cytopathic effects of viral infection in SSPE to the action of immunological effector mechanisms in MS and the viral encephalopathies. Irrespective of the initial cause of myelin degradation, many of these disorders are associated with some degree of CNS inflammation, as indicated by the local activation of microglia, recruitment of macrophages or the intrathecal synthesis of immunoglobulin. Many of these phenomena are now being duplicated in animal models, providing not only new insights into the pathogenesis of human demyelinating diseases , but also unexpected interrelationships between the immune response in the CNS and the pathogenesis of diseases such as Alzheimers disease and HIV encephalopathy. Autoimmune mediated models of inflammatory demyelinating CNS disease have proved particularly valuable in this respect as they allow the effects of defined immune effector mechanisms to be studied in the absence of CNS infection.

The myelin basic protein-specific T cell repertoire in (transgenic) Lewis rat/SCID mouse chimeras: preferential V beta 8.2 T cell receptor usage depends on an intact Lewis thymic microenvironment

In the Lewis rat, myelin basic protein (MBP)-specific, encephalitogenic T cells preferentially recognize sequence 68-88, and use the V beta 8.2 gene to encode their T cell receptors. To analyze the structural prerequisites for the development of the MBP-specific T cell repertoire, we reconstituted severe-combined immunodeficient (SCID) mice with fetal (embryonic day 15-16) Lewis rat lymphoid tissue, and then isolated MBP-specific T cell lines from the adult chimeras after immunization. Two types of chimera were constructed: SCID mice reconstituted with rat fetal liver cells only, allowing T cell maturation within a chimeric SCID thymus consisting of mouse thymic epithelium and rat interdigitating dendritic cells, and SCID mice reconstituted with rat fetal liver cells and rat fetal thymus grafts, allowing T cell maturation within the chimeric SCID and the intact Lewis rat thymic microenvironment. Without exception, the T cell lines isolated from MBP-immunized SCID chimeras were restricted by MHC class II of the Lewis rat (RT1.B1), and none by I-Ad of the SCID mouse. Most of the T cell lines recognized the immunodominant MBP epitope 68-88. In striking contrast to intact Lewis rats, in SCID mice reconstituted by rat fetal liver only, MBP-specific T cell clones used a seemingly random repertoire of V beta genes without a bias for V beta 8.2. In chimeras containing fetal Lewis liver plus fetal thymus grafted under the kidney capsule, however, dominant utilization of V beta 8.2 was restored. The migration of liver-derived stem cells through rat thymus grafts was documented by combining fetal tissues from wild-type and transgenic Lewis rats. The results confirm that the recognition of the immunodominant epitope 68-88 by MBP-specific encephalitogenic T cells is a genetically determined feature of the Lewis rat T cell repertoire. They further suggest that the formation of the repertoire requires T cell differentiation in a syngeneic thymic microenvironment.

The shaping of the brain-specific T lymphocyte repertoire in the thymus

We have shown in several distinct experimental systems that the immune system of intact Lewis rats contains T cells which, upon activation, are able to mediate autoimmune brain inflammation. These T cells seem to differentiate within the thymus although the autoantigens are produced (and presumably expressed in a recognizable fashion) within the thymic medulla. Furthermore, an intact fully MHC compatible thymic microenvironment seems to be required for the development of all features of the autoimmune TCR repertoire. Biased utilization of V beta 8.2 gene for the TCR, a hallmark of the Lewis rat T cell response to MBP, is only seen in T cells having matured in thymuses entirely composed of stroma elements of rat origin. It seems that the thymus contains a large spectrum of protein structures, which hitherto had been considered autoantigens specific for "peripheral" tissues, and, most surprisingly, components of the CNS, the classical "sequestered" organ. Deletion of autoreactive T cell clones by many local intrathymic autoantigens is leaky, at best. The reduced expression of CD4 on thymus-derived autoreactive T cells may be construed to reflect abortive efforts of negative selection. Alternatively, however, it may be worthwhile to consider a positive role for intrathymic autoantigens and their complementary T cells clones. It is possible that the requirement of an intact thymus milieu for the typical, V beta 8.2 dominated MBP specific T cell repertoire in the Lewis rat could reflect self peptide presentation by thymus epithelium cells in positive selection stages. In that case, the unusual diversity of thymic autoantigens could indeed have a role in shaping the immune system's TCR diversity, possible in the sense of an "immunological homunculus" as postulated by Cohen (Cohen 1992). Finally, there is a need to explain the mechanisms that in the healthy organism prevent the numerous, potentially autoaggressive T cell clones from attacking the body's own tissues. This is especially important, as T cells reactive against potentially pathogenic autoantigens, e.g. MBP (Ota et al. 1990, Pette et al. 1990b) and acetylcholine receptor (Salvetti et al. 1991, Sommer et al. 1991), are seen at especially high frequency in the human immune repertoire. Clearly, in all experimental paradigms investigated, activation of self-reactive T cells was the critical prerequisite for induction of autoimmune disease. Thus, in principle, prevention of such activation would be one way to maintain self tolerance. The mechanisms that achieve this goal in most individuals remain to be elucidated.

Melanocyte culture lines from Tyr-SV40E transgenic mice: models for the molecular genetic evolution of malignant melanoma

Transgenic Tyr-SV40E mice previously produced on the C57BL/6 inbred-strain background, with SV40 oncogenic sequences specifically expressed in pigment cells, are predisposed to melanoma [Bradl, M., Klein-Szanto, A., Porter, S. & Mintz, B. (1991). Proc. Natl. Acad. Sci. USA, 88, 164-168]. Separate lines of these animals differ genetically only in the number of copies and chromosomal site of integration of the transgene. Skin melanocytes from young mice with no apparent skin lesions were established in continuous culture from hemizygous donors with low, medium and high numbers of transgene copies, and from a homozygous offspring of the low-copy mouse line. The standard culture conditions enable C57BL/6 wild-type melanocytes to become stably immortalized without transformation. The transgenic cell lines all changed over time in an orderly progression. However, with greater numbers of transgene copies, the cells more rapidly displayed shorter doubling times, increased anchorage independence, reduced serum and growth factor requirements, decreased tyrosinase expression and melanin content, increased oncogene expression, and capacity to form malignant melanomas when tested by grafting. Melanocytes with the lowest number of transgene copies were of special interest. They grew more rapidly than the wild-type cells from the outset, but did not become tumorigenic until an apparently small number of still-unknown genetic changes had spontaneously occurred, or until the number of transgene copies was increased slightly by homozygosity. In contrast to the hemizygous low-copy cells, the homozygous counterparts underwent striking and rapid transformational changes and early conversion to malignancy. Thus such low-copy transgenic melanocyte lines afford an exceptional opportunity for molecular analysis of somatic genetic evolution toward malignant melanoma.

Mosaic expression of a tyrosinase fusion gene in albino mice yields a heritable striped coat color pattern in transgenic homozygotes

Genetically albino mouse eggs were injected with an inducible transgene comprising the wild-type tyrosinase (monophenol, L-dopa: oxygen oxidoreductase, EC 1.14.18.1) cDNA and the metallothionein gene promoter in the expectation that the transgene would be expressed to different extents in the various developing pigment cell clones of at least some individuals, thereby leading to patterned coats. This proved to be the case. Five transgenic mice had lightly pigmented patterns of transverse stripes visualizing melanoblast proliferation and migration dorsoventrally on each side. Similar patterns have been seen in genetically mosaic mouse models produced from conjoined blastomeres of different color genotypes and in many naturally patterned genotypes of mice. Four of the transgenics had heritable patterns and autosomal transgene integration. Their homozygous descendants were darker than hemizygotes and transmitted the basic pattern through many generations. Eyes were also pigmented, with clonal patches of melanized cells in the retinal pigment epithelium. The skin was dark due to many pigmented dermal melanocytes, whereas relatively few were in the hair follicles. This "inversion" is attributable to precocious maturation and migratory arrest of many melanoblasts during passage through the dermis en route to the hair bulbs. Patterning in these mice is considered in light of the view, previously proposed, that phenotypically different clones, or phenoclones, may exist in virtually all mammalian cell types and that many genes may be associated with cis-acting control regions causing variations in their expression that are mitotically perpetuated. We point out that mosaic expression has many implications for development as well as neoplasia. In the latter case, the potential for tumor susceptibility may be affected by clonal variation without further gene mutations or deletions. Thus, mice with variegating transgenes can provide molecular access to gene control mechanisms and to their consequences in development and disease.

Clonal coat color variation due to a transforming gene expressed in melanocytes of transgenic mice

Transgenic mice of an inbred black strain were previously produced with the Tyr-SV40E transgene, comprising simian virus 40 transforming sequences driven by the tyrosinase promoter, in order to obtain melanomas; the animals were found to be lighter than normal in coat color, to various degrees. As described here, hypopigmentation resulted from diminished differentiation of melanized pigment granules in the melanocytes of the hair bulbs in vivo and occurred autonomously in cultured melanocytes. Whereas some of the mice had single-color coats, most (7/13) had coats of two or three colors; in addition, one single-color founder produced a two-color descendant. These eight mice had patterns seen in natural genotypes; the most striking were transversely striped to various extents, with regions of left-right asymmetry on either side of the dorsal midline. The patterns visualized the same clonal developmental territories of coat melanocytes displayed in allophenic mice that are formed from conjoined early embryo cells of different color genotypes. Some of the Tyr-SV40E transgenics were also cellular genotypic mosaics, probably arising by late integration of the transgene. However, one transgenic founder with a completely striped coat proved to be true-breeding, with autosomal inheritance of the pattern. The inherited striped pattern thus exemplifies the formation of phenotypically different but genetically identical developmental clones, or phenoclones, among cells of the same type. This line of transgenic mice provides exceptional material for experimental analysis of the molecular basis for clonal variation in gene expression and of the fate of oncogenic phenoclones of melanocytes occurring in the same individual.

Malignant melanoma in transgenic mice

Ocular and cutaneous melanomas arose in new inbred lines of transgenic mice having an integrated recombinant gene comprised of the tyrosinase promoter, expressed in pigment cells, and the simian virus 40 early-region transforming sequences. The tumors were hypomelanotic and were histopathologically similar to corresponding human melanomas. Eye melanomas often originated at a young age, chiefly from the retinal pigment epithelium, also from the choroid, and rarely from the ciliary body. The eye tumors grew aggressively, were highly invasive, and metastasized to local and distant sites. The earliest formation of these tumors was associated with higher copy numbers of the transgene; mice of different single-copy lines varied greatly in age of onset and frequency of eye tumors. Coat pigmentation was reduced in almost all lines, to various extents. Primary skin melanomas arose later and less frequently than eye melanomas. Hence they were at early stages and of unknown long-range incidence in this investigation, in which autopsies covered the first half-year of life. For both ocular and cutaneous melanomas, the transgenic mice offer numerous possibilities for experimental study of mechanisms underlying formation and spread of melanomas.

Melanosis and associated tumors in transgenic mice

Melanosis was found to various extents in a wide array of tissues of all 23 autopsied mice whose transgene consisted of the tyrosinase promoter fused to the simian virus 40 early-region oncogenic sequences. Pigmentation in a given animal was attributable to any or all of the following; an increase in numbers of some normally pigmented cells of neural crest origin (a result compatible with early stages of transformation); elicitation of melanin synthesis in some cells that normally have little melanin, or none at all (the latter possibly signaling metaplasia); unusual intercellular transfer of pigment granules from melanocytes into certain normally unpigmented epithelia and endothelia; and profusion of melanin-phagocytizing cells. Neoplasms, occasionally also containing melanin, arose in association with some of these melanotic tissues and included three choroid plexus tumors, three endocardial tumors, two peripheral nerve sheath tumors (schwannomas), two cochlear tumors, two pineal gland tumors, one salivary gland tumor, and one nasal mucosa tumor. These apparently originated independently of the ocular and cutaneous melanomas found in the same animals. The events involved in melanosis may thus contribute to neoplastic conversion.

Clonotypic chromosomal aberrations in long-term lines of myelin-specific rat T lymphocytes

A panel of 16 long-term rat T lymphocyte lines and clones were screened for cytogenetical abnormalities using chromosomal banding techniques. All T lines were CD4+, recognizing the relevant antigen in the molecular context of major histocompatibility complex (MHC) class II determinants. With one exception (an ovalbumin-specific line), all lines were specific for myelin proteins, and apart of one BS rat-derived T line and its clones, all lines were selected from the Lewis strain of rat. After in vitro culture of more than 1 year, all lines and clones exhibited subtle but definite chromosomal aberrations, which included deletions, enlargement, translocations and formation of isochromosomes. All lines were near diploid, structural chromosomal changes being more frequent than numerical abnormalities. Each T line investigated had an individual pattern of chromosomal changes. In our analysis, 16 of the 22 different chromosomes had changes in at least one line. Chromosome 9 and the X chromosome appeared to have an enhanced susceptibility of alterations. In two cases, chromosomal markers could be traced through different stages of in vitro culture of the T lines.

Augmentation of demyelination in rat acute allergic encephalomyelitis by circulating mouse monoclonal antibodies directed against a myelin/oligodendrocyte glycoprotein

In this study the authors have developed a model with which can be studied directly the influence of circulating anti-myelin antibody on the clinical and pathologic course of inflammatory T-cell-mediated experimental allergic encephalomyelitis (EAE) in the rat. EAE was induced by passive transfer of either myelin basic protein (MBP)-activated spleen cells derived from sensitized donors or long-term-cultured MBP-specific T-cell lines. At the onset of the disease, monoclonal antibodies against a myelin/oligodendrocyte glycoprotein (MOG) were injected intravenously. This antigen is exposed on the surface of central nervous system myelin and oligodendrocytes. Intravenous injection of the antibody in the course of T-cell-mediated transfer EAE augmented the severity and duration of clinical signs and resulted in the formation of large, confluent demyelinated plaques.

Experimental allergic encephalomyelitis: the balance between encephalitogenic T lymphocytes and demyelinating antibodies determines size and structure of demyelinated lesions

The effect of a circulating monoclonal antibody recognizing an antigen located on the surface of myelin sheaths (myelin/oligodendroglia glycoprotein, MOG) on clinical and histopathological expression of experimental allergic encephalomyelitis (EAE) was tested in a model of EAE passively transferred by monospecific T lymphocytes. Intravenous injection of anti-MOG antibody at the onset of the disease massively augmented clinical impairment as well as primary demyelination. The structure of the CNS lesions depended on the balance between encephalitogenic T cells and anti-MOG antibody: when EAE was induced with high numbers of T cells, circulating anti-MOG antibody resulted in ubiquitous perivenous demyelination in the spinal cord and medulla oblongata. On the contrary, focal confluent demyelinated lesions were observed in animals injected with low numbers of T cells (even as few as 10(4] and anti-MOG antibody. Our studies, thus, indicate that the formation of inflammatory demyelinating lesions may be due to a synergistic action of cellular and humoral immune mechanisms.