Venous-plexus-associated lymphoid hubs support meningeal humoral immunity

There is increasing interest in how immune cells in the meninges-the membranes that surround the brain and spinal cord-contribute to homeostasis and disease in the central nervous system1,2. The outer layer of the meninges, the dura mater, has recently been described to contain both innate and adaptive immune cells, and functions as a site for B cell development3-6. Here we identify organized lymphoid structures that protect fenestrated vasculature in the dura mater. The most elaborate of these dural-associated lymphoid tissues (DALT) surrounded the rostral-rhinal confluence of the sinuses and included lymphatic vessels. We termed this structure, which interfaces with the skull bone marrow and a comparable venous plexus at the skull base, the rostral-rhinal venolymphatic hub. Immune aggregates were present in DALT during homeostasis and expanded with age or after challenge with systemic or nasal antigens. DALT contain germinal centre B cells and support the generation of somatically mutated, antibody-producing cells in response to a nasal pathogen challenge. Inhibition of lymphocyte entry into the rostral-rhinal hub at the time of nasal viral challenge abrogated the generation of germinal centre B cells and class-switched plasma cells, as did perturbation of B-T cell interactions. These data demonstrate a lymphoid structure around vasculature in the dura mater that can sample antigens and rapidly support humoral immune responses after local pathogen challenge.

Identification of direct connections between the dura and the brain

The arachnoid barrier delineates the border between the central nervous system and dura mater. Although the arachnoid barrier creates a partition, communication between the central nervous system and the dura mater is crucial for waste clearance and immune surveillance1,2. How the arachnoid barrier balances separation and communication is poorly understood. Here, using transcriptomic data, we developed transgenic mice to examine specific anatomical structures that function as routes across the arachnoid barrier. Bridging veins create discontinuities where they cross the arachnoid barrier, forming structures that we termed arachnoid cuff exit (ACE) points. The openings that ACE points create allow the exchange of fluids and molecules between the subarachnoid space and the dura, enabling the drainage of cerebrospinal fluid and limited entry of molecules from the dura to the subarachnoid space. In healthy human volunteers, magnetic resonance imaging tracers transit along bridging veins in a similar manner to access the subarachnoid space. Notably, in neuroinflammatory conditions such as experimental autoimmune encephalomyelitis, ACE points also enable cellular trafficking, representing a route for immune cells to directly enter the subarachnoid space from the dura mater. Collectively, our results indicate that ACE points are a critical part of the anatomy of neuroimmune communication in both mice and humans that link the central nervous system with the dura and its immunological diversity and waste clearance systems.

Functional characterization of the dural sinuses as a neuroimmune interface

Despite the established dogma of central nervous system (CNS) immune privilege, neuroimmune interactions play an active role in diverse neurological disorders. However, the precise mechanisms underlying CNS immune surveillance remain elusive; particularly, the anatomical sites where peripheral adaptive immunity can sample CNS-derived antigens and the cellular and molecular mediators orchestrating this surveillance. Here, we demonstrate that CNS-derived antigens in the cerebrospinal fluid (CSF) accumulate around the dural sinuses, are captured by local antigen-presenting cells, and are presented to patrolling T cells. This surveillance is enabled by endothelial and mural cells forming the sinus stromal niche. T cell recognition of CSF-derived antigens at this site promoted tissue resident phenotypes and effector functions within the dural meninges. These findings highlight the critical role of dural sinuses as a neuroimmune interface, where brain antigens are surveyed under steady-state conditions, and shed light on age-related dysfunction and neuroinflammatory attack in animal models of multiple sclerosis.

A murine model of Lyme disease demonstrates that Borrelia burgdorferi colonizes the dura mater and induces inflammation in the central nervous system

Lyme disease, which is caused by infection with Borrelia burgdorferi and related species, can lead to inflammatory pathologies affecting the joints, heart, and nervous systems including the central nervous system (CNS). Inbred laboratory mice have been used to define the kinetics of B. burgdorferi infection and host immune responses in joints and heart, however similar studies are lacking in the CNS of these animals. A tractable animal model for investigating host-Borrelia interactions in the CNS is key to understanding the mechanisms of CNS pathogenesis. Therefore, we characterized the kinetics of B. burgdorferi colonization and associated immune responses in the CNS of mice during early and subacute infection. Using fluorescence-immunohistochemistry, intravital microscopy, bacterial culture, and quantitative PCR, we found B. burgdorferi routinely colonized the dura mater of C3H mice, with peak spirochete burden at day 7 post-infection. Dura mater colonization was observed for several Lyme disease agents including B. burgdorferi, B. garinii, and B. mayonii. RNA-sequencing and quantitative RT-PCR showed that B. burgdorferi infection was associated with increased expression of inflammatory cytokines and a robust interferon (IFN) response in the dura mater. Histopathologic changes including leukocytic infiltrates and vascular changes were also observed in the meninges of infected animals. In contrast to the meninges, we did not detect B. burgdorferi, infiltrating leukocytes, or large-scale changes in cytokine profiles in the cerebral cortex or hippocampus during infection; however, both brain regions demonstrated similar changes in expression of IFN-stimulated genes as observed in peripheral tissues and meninges. Taken together, B. burgdorferi is capable of colonizing the meninges in laboratory mice, and induces localized inflammation similar to peripheral tissues. A sterile IFN response in the absence of B. burgdorferi or inflammatory cytokines is unique to the brain parenchyma, and provides insight into the potential mechanisms of CNS pathology associated with this important pathogen.

Idiopathic hypertrophic pachymeningitis with anticardiolipin antibody: A case report

RATIONALE

Idiopathic hypertrophic pachymeningitis (IHP) is a rare neurological disorder without a definite etiology. Diagnosis is mainly based on exclusion of other etiologies.

PATIENT CONCERNS

A 41-year-old male patient presented with insidious onset headache of 3-month duration.

DIAGNOSES

Contrast-enhanced brain magnetic resonance imaging (MRI) revealed diffuse pachymeningeal enhancement over bilateral cerebral hemispheres and the tentorium cerebelli. Lumbar puncture showed increased pressure, lymphocytic pleocytosis, and elevated protein level with normal glucose concentration. Blood tests detected elevated erythrocyte sedimentation rate (ESR) and C-reactive protein. Pathological examination of the dura mater from the right frontal convexity disclosed coarse collagenous deposition with focal lymphoid aggregation. After malignancy and infectious etiologies were excluded, a diagnosis of IHP was made.

INTERVENTIONS

Oral prednisolone and azathioprine followed by methotrexate were administered.

OUTCOMES

During the 7-year follow-up period, although the patient was not totally headache-free, medical therapy significantly reduced the severity of headache. Follow-up MRI studies showed a reduction in meningeal enhancement and serial ESR measurements revealed a trend of improvement.

LESSONS

Methotrexate therapy may be considered in cases of steroid-resistant IHP. In addition to clinical evaluation, serial ESR testing may be considered to guide the treatment strategy and assess the response to therapy.

Aggressive CD4/CD8 Double-Negative Primary Cutaneous T-Cell Lymphoma With Dural Invasion: A Rare Presentation of Mycosis Fungoides?

ABSTRACT

Mycosis fungoides (MF) is primarily characterized by epidermotropic CD3+/CD4+/CD45RO+ memory T cells. CD4/CD8 double-negative MF is an uncommon variant with no presumed prognostic significance. Despite the variability in the clinical course and presentation of MF, most cases behave indolently. About 5% of patients, however, advance to stage IV with visceral organ involvement. Central nervous system metastasis in MF is rare with no known cases of direct central nervous system invasion by MF to date. We report an exceedingly rare locally aggressive case of CD4/CD8 double-negative MF with direct dural invasion and underline pertinent diagnostic challenges encountered in our case.

Mast cell degranulation activates a pain pathway underlying migraine headache

Intracranial headaches such as that of migraine are generally accepted to be mediated by prolonged activation of meningeal nociceptors but the mechanisms responsible for such nociceptor activation are poorly understood. In this study, we examined the hypothesis that meningeal nociceptors can be activated locally through a neuroimmune interaction with resident mast cells, granulated immune cells that densely populate the dura mater. Using in vivo electrophysiological single unit recording of meningeal nociceptors in the rat we observed that degranulation of dural mast cells using intraperitoneal administration of the basic secretagogue agent compound 48/80 (2 mg/kg) induced a prolonged state of excitation in meningeal nociceptors. Such activation was accompanied by increased expression of the phosphorylated form of the extracellular signal-regulated kinase (pERK), an anatomical marker for nociceptor activation. Mast cell-induced nociceptor interaction was also associated with downstream activation of the spinal trigeminal nucleus as indicated by an increase in c-fos expression. Our findings provide evidence linking dural mast cell degranulation to prolonged activation of the trigeminal pain pathway believed to underlie intracranial headaches such as that of migraine.

Late bacterial granuloma at an intrathecal drug delivery catheter

In the case reported, neurological complaints were pain and dysaesthesiae in the lower back and thigh, as well as paresis of the ileopsoas muscle. MRI of the lumbar spine showed an intradural-extramedullary mass at the level of L1 homogeneously enhancing with gadolinium. This mass was situated at the tip of an intrathecal catheter implanted 11 years before for a morphine trial infusion as therapy for phantom pain after amputation of the right arm. Now, removal of the catheter was performed. Cultures of lumbar CSF and the catheter tip demonstrated coagulase negative staphylococcus. Antibiotic medication with cephalosporines was given for 6 weeks. After removal of the catheter, the patient was free of pain and he progressively regained full neurological function. Although most catheter-associated granulomas reported so far were sterile in nature, bacterial infection should still be considered even years after catheter placement.

Subdural hematoma (SDH): assessment of macrophage reactivity within the dura mater and underlying hematoma

OBJECTIVES

Macrophages are an inherent component of the dura mater, and can be characterised in cases of subdural hematoma (SDH) by their progressive and varying accumulation within areas of damage. Gross and histological methods used to determine the age of SDH are inexact. These are in part due to the active nature of such lesions and the diverse manner in which trauma victims respond to injury. Correct diagnosis has obvious medico-legal implications. However, there is as yet no specific diagnostic method that allows the age of SDH to be reliably determined. This study investigated the progressive and orderly pattern of reactivity of resident and infiltrating dural macrophages that occurs in response to injury associated with SDH.

MATERIALS

26 postmortem cases of traumatic SDH were examined with survival times (onset of trauma to death) ranging from a few hours and up to 31 days.

METHODS

Macrophage reactivity associated with the dura mater and the underlying hematoma was determined using CD68 and MHC class II immunohistochemistry and the qualitative and quantitative findings compared with the presence of iron detected using conventional Perl's Prussian blue method.

RESULTS

The results show that CD68 and MHC class II are differentially expressed within the dura mater and hematoma in SDH, and that the expression of MHC class II is markedly upregulated in the inner aspect of the dura mater within the initial 24 hours following injury. CD68 expression can be detected quantitatively in the hematoma, 24-48 hours after SDH, and within the dura following this period. Linear regression analysis further revealed a significant and positive association between the expression of MHC class II or CD68 antigens and the progressive survival of SDH up to 31 days post-injury, which was not seen with Perl's histochemical method. The expression of MHC class II antigen was a distinguishing, and quantifiable feature particularly localized within the inner aspect of the dura from a very early stage in the progression of SDH. Widespread, diffuse and cellular MHC class II reactivity was particularly noted within the inner aspect of the dura mater in cases of SDH with survival > 10 days. Since only a proportion of this widespread immunoreactivity was accounted for by macrophages (considering CD68 immunoreactivity), a large component of this activity was more likely to be due to the reorganisation and activation of fibroblasts within inner dural layers (dural border layer), known to upregulate expression of MHC class II molecules.

CONCLUSIONS

The expression of CD68 and MHC class II antigens provides a more informative picture of the progression of pathology associated with SDH, and may be used in conjunction with other clinicopathological factors, in further investigations that attempt to date SDH according to defined histopathological characteristics.

ABO and Rh phenotyping by absorption-elution technique using cerebral dura maters

Phenotyping of ABO and Rh blood groups was performed by the absorption-elution technique using cerebral dura maters. For the ABO system, the cerebral dura maters extracted from nine autopsied cadavers including two burnt bodies, two putrefied corpses, and one half-mummified and one skeletal structure were tested with commercially available Wako antisera (animal polyclonal antibodies). At the same time, blood, fingernails or bones were sampled. In all the cases, the phenotypes could be typed correctly and more clearly with the use of 2 x 2 x 0.3 mm dura maters (0.6 mm thick dura maters were sliced to 0.3 mm thickness) than the phenotyping using 2 x 2 mm fingernails or 2 x 2 x 1 mm bones. For the Rh system, the cerebral dura maters extracted from eight autopsied cadavers within 2 days after death including two burnt bodies were tested with commercially available Ortho Bioclone anti-C, anti-c, anti-E and anti-e sera (human monoclonal antibodies), and Ortho anti-D serum (human polyclonal antibody). The eluate of anti-D antibody was needed to perform the indirect anti-globulin test (Ortho Coombs serum). At the same time, blood was sampled. In all the cases, the Rh blood groups of cerebral dura maters were in agreement with those of blood.

In vivo biocompatibility testing of peek polymer for a spinal implant system: a study in rabbits

We are developing a new spinal implant system (SIS) without fusion (bone graft). This SIS is made from two materials, metal and polyetheretherketone (PEEK) polymer. The Food and Drug Administration recommended testing in vivo, in an animal model, whether the PEEK polymer could be used in a SIS without any harm of wear debris to the nervous tissue (spinal cord and nerve roots). The objective was to evaluate the biological response of the spinal cord and nerve roots (dura mater) to PEEK polymer particles. Twenty-four female New Zealand white rabbits were used. The rabbits were divided into three groups: test (n = 12), control (n = 9), and sham (n = 3). During the surgery, the test group received the PEEK particle injections (5 x 10(7) particles per site, lumbar and thoracic), while the control group received only the vehicle (0.9% saline solution). The sham group had the same surgical approach without injection. In each group, the rabbits were euthanized at 1, 4, and 12 weeks postsurgery. The macroscopic and semiquantitative histologic analyses of the spinal cords (dura mater) showed normal vascularization and particle adherence to the connective tissue especially at the injection sites. Neither necrosis nor swelling of the dura mater and nerve roots was observed. The PEEK polymer is harmless to the spinal cord; thus it might be used as component in the spinal implant system.

Delayed inflammation in rat meninges: implications for migraine pathophysiology

Nitric oxide (NO) has been implicated in migraine pathogenesis based on the delayed development of typical migraine headache 4-6 h after infusing the NO donor nitroglycerin [glyceryl trinitrate (GTN)] to migraineurs. Furthermore, inhibiting the synthesis of NO by treatment with a NO synthase (NOS) inhibitor attenuates spontaneous migraine headaches in 67% of subjects. Because NO has been linked to inflammation and cytokine expression, we investigated the delayed consequences of brief GTN infusion (30 min) on the development of meningeal inflammation in a rat model using doses relevant to the human model. We found dose-dependent Type II NOS [inducible NOS (iNOS)] mRNA upregulation in dura mater beginning at 2 h and an increase in the corresponding protein expression at 4, 6 and 10 h after infusion. Type II NOS immunoreactivity was expressed chiefly within resident meningeal macrophages. Consistent with development of a delayed inflammatory response, we detected induction of interleukin 1beta in dura mater at 2 and 6 h and increased interleukin 6 in dural macrophages and in rat cerebrospinal fluid at 6 h after GTN infusion. Myeloperoxidase-positive cells were rarely found. Leakage of plasma proteins from dural blood vessels was first detected 4 h after GTN infusion, and this was suppressed by administering a specific Type II NOS inhibitor [L-N(6)-(1-iminoethyl)-lysine (L-NIL)]. In addition to cytokine induction, macrophage iNOS upregulation and oedema formation after GTN infusion, dural mast cells exhibited granular changes consistent with secretion at 4 and 6 h. Because iNOS was expressed in dural macrophages following topical GTN, and in the spleen after intravenous injection, the data suggest that the inflammatory response is mediated by direct actions on the dura and does not develop secondary to events within the brain. Our findings point to the importance of new gene expression and cytokine expression as fundamental to the delayed response following GTN infusion, and support the hypothesis that a similar response develops in human meninges after GTN challenge.

Hypertrophic chronic pachymeningitis as a localized immune process in the craniocervical region

Hypertrophic chronic pachymeningitis (HCP) is a rare disorder that causes intracranial or spinal thickening of the dura mater. This report describes a patient with progressive HCP in the craniocervical region associated with signs of rheumatic disease. A ventricular-atrial shunt had to be inserted because of increased intracranial pressure. The patient improved after suboccipital craniotomy, C1 to C6 laminectomy, and removal of the thickened dura. Additional therapy with methotrexate stopped progression, which was documented by MRI and PET.

Pachymeningitis in microscopic polyangiitis (MPA): a case report and a review of central nervous system involvement in MPA

A case of microscopic polyangiitis (MPA) with pachymeningitis is described. The patient had renal, skin, gallbladder and peripheral nervous system involvement, simultaneously with pachymeningitis. Necrotizing glomerulonephritis with crescent formation, and necrotizing small vessel vasculitis in the kidney and skin were confirmed by biopsy. A highly elevated titer of antineutrophil cytoplasmic antibody for myeloperoxidase (MPO-ANCA) was observed. All of the clinical and laboratory abnormalities improved with high-dose pulse and conventional steroid therapy. The literature on central nervous system involvement in MPA and perinuclear-ANCA (p-ANCA)-related vasculitis is reviewed. This case serves to emphasize that pachymeningitis can occur as one of the features of MPA.

Idiopathic hypertrophic cranial pachymeningitis with perifocal brain edema--case report

A 51-year-old female presented with an extremely rare case of idiopathic hypertrophic cranial pachymeningitis manifesting as markedly thickened frontotemporal meninges with expanding perifocal edema. Magnetic resonance imaging with gadolinium revealed enhancement of the thickened dura mater protruding into the brain parenchyma accompanied by focal edema causing a mass effect. Histological examination of a biopsy specimen revealed thickened dura with infiltrating lymphocytes. Serological and immunological tests were normal. No inflammatory response or evidence of malignant tumors was observed. The patient was treated with predonisolone, resulting in marked improvement of the mass effect. High-dose steroid therapy appears to be effective for intracranial pachymeningitis associated with expanding perifocal brain edema.

Effects of PNU-109,291, a selective 5-HT1D receptor agonist, on electrically induced dural plasma extravasation and capsaicin-evoked c-fos immunoreactivity within trigeminal nucleus caudalis

We studied the effects of PNU-109291 [(S)-(-)-1-[2-[4-(4-methoxyphenyl)-1-piperazinyl]ethyl]-N-methyl-isoc hroman-6-carboxamide], a receptor agonist showing 5000-fold selectivity for primate 5-HT1D versus 5-HT1B receptors (Ennis et al., J. Med. Chem. 41, 2180-2183), on dural neurogenic inflammation and on c-fos like immunoreactivity within trigeminal nucleus caudalis evoked by electrical and chemical activation of trigeminal afferents, respectively. Subcutaneous injection of PNU-109291 in male guinea pigs dose-dependently reduced dural extravasation of [125I]-labeled bovine serum albumin evoked by trigeminal ganglion stimulation with an IC50 of 4.2 nmol kg(-1). A dose of 73.3 nmol kg(-1) blocked the response completely. The selective 5-HT1B/1D receptor antagonist GR-127935 (> or = 2 micromol kg(-1) i.v.) prevented this effect. In addition, the number of c-fos immunoreactive cells within guinea pig trigeminal nucleus caudalis induced by chemical meningeal stimulation (intracisternally administered capsaicin) was reduced by more than 50% with PNU-109291 (> or = 122.2 nmol kg(-1) administered s.c. 45 min before and 15 min after capsaicin). These data indicate that the 5-HT1D receptor subtype plays a significant role in suppressing meningeal neurogenic inflammation and attenuating trigeminal nociception in these guinea pig models. Since 5-HT1D receptor mRNA and protein are expressed in trigeminal ganglia but not vascular smooth muscle, the 5-HT1D receptor subtype may become a useful therapeutic target for migraine and related headaches.

Wegener's granulomatosis with dural involvement as the initial clinical manifestation

We treated a patient with an atypical presentation of Wegener's granulomatosis (WG) with dural involvement as the initial clinical manifestation. A 37-year-old man had a dural lesion without lower respiratory tract or renal manifestations in the initial clinical course. His only initial symptom was headache, and at disease onset computed tomography (CT) and magnetic resonance imaging (MRI) of the head revealed bilateral abnormal subdural masses. The diagnosis of WG was made based on the results of needle biopsy of the nasal polyps and the finding of positive circulating antineutrophil cytoplasmic antibodies (c-ANCA). He achieved remission on daily prednisone and cyclophosphamide with the later addition of sulfamethoxazole-trimethoprim.

Tryptophan hydroxylase can be present in mast cells and nerve fibers of the rat dura mater but only mast cells contain serotonin

Tryptophan hydroxylase-immunopositive (TPH-I) but not serotonin-I nerve fibers were observed in the rat dura mater. This tissue also contained numerous serotonin and TPH-I mast cells. The TPH appeared to be located in granules and/or enclosed in a juxta-nuclear organite. Westernblots showed that the TPH located in the dura mater is similar to the TPH of pineal gland but different from raphe TPH. According to the animal, both nerve fiber and mast cell TPH immunoreactivity was highly variable in intensity and in number of labelled elements. This variability might be due to the complex regulatory mechanisms of TPH as indicated by the presence of two types of mast cells.

Immune response in hosts with cadaveric dural grafts. Report of two cases

The use of cadaveric human dura has been critical in the repair of dural defects since the dawn of neurosurgery. Reports in the literature of immune response to this type of graft have been extremely rare. Two patients are presented who received cadaveric dural implants with resulting meningeal signs and cerebrospinal fluid eosinophilia several weeks after surgery. Peripheral eosinophilia was present in one patient. The signs and symptoms resolved temporarily during corticosteroid therapy and permanently upon removal of the offending grafts. These cases illustrate that an immune-type reaction can occur with significant morbidity in patients receiving cadaveric dural grafts. A proposed mechanism for this response is discussed.

Dural sinus thrombosis and venous infarction associated with antiphospholipid antibodies: MR findings

OBJECTIVE

Our goal was to describe the neuroradiologic findings in hemorrhagic venous infarction related to a hypercoagulable state caused by antiphospholipid antibodies (aPA).

MATERIALS AND METHODS

Magnetic resonance imaging was performed on two patients with superior sagittal thrombosis related to the presence of aPA.

RESULTS

A parenchymal region of hyperintense signal due to hemorrhagic venous infarction was demonstrated in both patients, along with abnormal signal within the thrombosed superior sagittal sinus.

CONCLUSION

Hemorrhagic venous infarction may result from the hypercoagulable state related to aPA. The presence of these antibodies should be considered in the setting of otherwise unexplained dural sinus thrombosis and/or venous infarction.

Antiviral IgM and IgG subclasses in varicella zoster associated neurological syndromes

The varicella zoster virus (VZV) and herpes simplex virus (HSV) IgGl-4 subclasses were compared in serum and cerebrospinal fluid (CSF) of 22 patients with VZV-associated neurological symptoms, 12 patients with HSV-associated neurological symptoms and 14 controls. The clinical syndromes of the VZV-associated diseases comprised meningo-encephalitis, myelitis, myelopathies and polyneuropathies, mostly with a favourable outcome. A characteristic finding was an intrathecal synthesis of VZV IgG1 and HSV-3. Commonly also IgG2 and 4 were seen in CSF of VZV patients. Their intrathecally synthesised HSV IgG was restricted to IgG1. VZV IgG3 occurred in serum and/or CFS together with VZV IgM in 14 cases and may be a marker of recent VZV replication. In patients with HSV-associated neurological disease, a multi-IgG subclass HSV response and concomitant VZV antibodies restricted to IgG1 was found. Intrathecal synthesis of both HSV and VZV IgG occurred in 20 patients. Detection of two or more VZV or HSV specific IgG subclasses synthesised intrathecally identified the aetiological agent in 19 of these 20 cases.

Antigenic properties of fresh and lyophylised human dura mater

The action of human fresh and lyophylised human dura mater on rat, rabbit and human serum complement has been analyzed with the aim of confirming and comparing the species-specific antigenicity of both substances. The results demonstrate that both fresh and lyophylised human dura mater induce a complement consumption on rat and rabbit serum. The phenomenon is related to the species-specific antigenic activity of the tissue, and it is only partially decreased by the lyophylisation procedure. The residual antigenic activity, however, doesn't limit the use of this material as a graft for dural defects repairs in humans.

[Dura transplantation. Multi-sequential transplants of solvent dehydrated dura mater. Animal experiment studies on the question of sensitization]

After the excision of 7 X 5 mm abdominal muscle sections in an experiment using rats, a total of 4 xenogenic, solvent dehydrated dura mater of the same size were implanted at 4 week intervals each. The transplant areas were continuously examined under a light and electron microscope for a period ranging from 7 days up to 3 months after the last transplant. Spontaneous layers of absorbent granulation tissue, rich in cells and blood vessels, surrounded the grafts in the beginning. Subsequently the grafts were gradually decomposed from the periphery to the center through macrophages like biological foreign matter and replaced with endogenous, poorly vascularized, collagenous connective tissue. The tissue reaction to the multi-sequential, xenogenic grafts was consistently the same as after single transplants. Immuno-competent cells do not increase. The solvent preserved dura mater is suited for multisequential transplants due to lacking sensitization, i.e. immunological rejection reaction.