Distinctive distribution of lymphocytes in unruptured and previously untreated brain arteriovenous malformation

Introduction

Human brain arteriovenous malformations (bAVMs) are tangles of abnormal vessels between arteries and veins and lack of capillary bed. Brain AVM is the most common cause of hemorrhagic stroke in young adults and children.  Commonly assumed to be congenital, postnatal formation may be more prevalent than previously thought,  and the etiology of bAVMs still remains unclear. Genetic factors, aberrant vasculogenesis, and inflammation may all play roles in the pathogenesis of bAVMs;  a confluence of these factors has been proposed in a "response-to-injury" paradigm. Evidence indicating the involvement of inflammation in bAVM pathogenesis includes neutrophil and macrophage infiltration, and increased expression of various inflammatory signals, such as matrix metalloproteinase-9, interleukin-6, myeloperoxidase and adhesion molecules. About half of bAVMs cases present with an intracranial hemorrhage (ICH), which itself can induce inflammation. However, even in unruptured and untreated AVMs, substantial infiltration of inflammatory cells has been detected in the vascular wall and intervening stroma. Magnetic resonance imaging has detected hemosiderin deposition in unruptured bAVMs,  consistent with episodes of clinically silent intralesional micro-hemorrhage.We recently described a strong association between imaging evidence of old silent hemorrhage and the risk of clinically symptomatic ICH.  Further, histological examination demonstrated that the degree of hemosiderin deposition is positively correlated with the number of macrophages in the lesion.  It is not clear, however, whether the macrophage response is specific or whether other inflammatory cells are also correlated with hemosiderin deposition and macrophage. Our previous studies demonstrated that both macrophage and neutrophil may play roles in bAVM pathogenesis.  Shi et al. described evidence of adaptive immunological responses in cavernous malformation.  Although bAVM tissue was used as control in Shi's study and while no oligoclonal response was observed, bAVM had a higher polyclonal response compared to normal brain tissue, suggesting that lymphocytes may also play a role in bAVM.In this study, we analyzed lymphocytes in addition to macrophages, and tested the hypothesis that, unlike the innate immune cells (macrophages), adaptive immune cell (lymphocytes)-infiltration is not associated with micro-hemorrhage and iron deposition.

Methods

All studies involving patients were approved by the Institutional Review Board of the University of California, San Francisco (UCSF), and patients gave informed consent.PatientsPatients with AVMs evaluated at UCSF have been entered into an ongoing prospective registry since 2000. We identified 24 unruptured brain AVMs from patients who did not undergo preoperative embolization or radiosurgery with frozen tissue available in our database; 19 samples were located and used in this study [Table 1]. Three superficial temporal arteries (STA), obtained from autopsies of patients who died from nonbrain-related diseases, were used as control.

Histology

Prussian blue staining was performed using Accustain Iron Stain kit (Sigma-Aldrich, St. Louis, MO) according to the manufacturer's protocol.For immunohistochemistry, adjacent sections were used to stain different surface markers. CD68, CD3, CD20 and CD138-specific antibodies were purchased from Abcam (Abcam, Cambridge, MA). Brain AVM specimens were embedded in optimum cutting temperature, sectioned into 8 μm sections, and fixed with 4% paraformaldehyde. Endogenous peroxidase activity was quenched by incubating slides in 0.3% H 2 O 2 in phosphate-buffered saline (PBS) for 15 min. After blocking with 10% normal donkey serum, sections were incubated at 4 °C overnight with primary antibodies diluted in PBS with 1% BSA in the following concentrations: mouse monoclonal antihuman CD68, 1:1000; rabbit monoclonal antihuman CD3, 1:400; rabbit monoclonal antihuman CD20, 1:200; mouse monoclonal antihuman CD138, 1:800. After washing in PBS, the sections were incubated with horseradish peroxidase labeled antimouse or antirabbit IgG (vector labs) for 1 h at room temperature. The positive staining was visualized using 3, 3-diaminobenzidene. Negative controls were performed by omitting the primary antibodies during immunostaining.The criteria for identifying hemosiderin were birefringent or brownish particles seen in the vascular wall or interstitial tissue between vessels, and were confirmed by Prussian blue staining on adjacent sections. CD68 + , CD3 + , CD20 + , CD138 + cells were quantified by counting the positively stained cells using stereological microscopy (Olympus, Japan).Statistical analysisAll data are expressed as mean ± standard deviation the differences of means were analyzed using unpaired Student's t-test. Exact binomial 95% confidence intervals (CIs) for proportions are reported. P < 0.05 was considered as statistically significant.

Results

Hemosiderin deposition was present in unruptured brain arteriovenous malformationsConsistent with our published data,  hemosiderin deposition was found in 8 out of 19 specimens (42%; 95% CI: 20-67%) [Figure 1]. Hemosiderin positive cells were scattered mainly around the abnormal vessels [Figure 1]a. Prussian blue positive staining was detected in the areas that had hemosiderin deposition [Figure 1]d, suggesting the presence of previous micro-hemorrhage.

T-lymphocytes and macrophages were detected in unruptured brain arteriovenous malformations

To analyze whether the lymphocytes were present in unruptured bAVM and whether their location was associated with macrophages and iron deposition, we analyzed T- and B-lymphocytes, plasma cells and macrophages. We found that T-lymphocyte was the predominant type of lymphocytes present in unruptured bAVM. Whereas the macrophages were scattered mostly in the vessel walls and intervening stromal regions [Figure 2], T-lymphocytes were clustered on the luminal side of the endothelial surface, in the vascular wall, and in the tissue between abnormal vessels [Figure 3]. Few B-lymphocytes were detected; they were mostly present in samples that had a large number of T-lymphocytes, and were co-localized with the T-lymphocytes [Figure 2]. In addition, a few plasma cells were identified in 5 samples, of which 4 had hemosiderin deposition (data not shown). No lymphocytes and macrophages were detected in STA [Figure 2].

Compared to the specimens that had no hemosiderin deposition, hemosiderin-positive specimens tended to have more macrophages (478 ± 174 vs. 666 ± 313 cells/mm 2 ; P = 0.11). The T-cell numbers were similar in hemosiderin-positive and hemosiderin-negative samples (147 ± 108 vs. 157 ± 139 cells/mm 2 ; P = 0.88) [Figure 4].

Discussion

We found in this study that T-cells are the predominant lymphocytes in unruptured bAVMs. Few B-lymphocytes and plasma cells were detected. Unlike macrophages, the number and location of T-lymphocytes did not correlate with hemosiderin, suggesting an independent cell-mediated immunological mechanism in bAVM pathogenesis.Previously, immune cells were mostly analyzed in ruptured and irradiatedbAVMs. Our previous study showed that adaptive immune cells were rarely observed in unruptured bAVM. We found in this study that many T-lymphocytes were present in unruptured, previously untreated bAVMs. The possible reason for the discrepancy is that we used a different immunohistochemical staining procedure in this study. Previously, we incubated sections in 0.3% H 2 O 2 in methanol to quench the activity of endogenous peroxidase. However, lymphocyte surface markers have been shown to be sensitive to methanol/H 2 O 2 treatment. Treating sections with 0.3% H 2 O 2 in methanol can reduce our ability to detect membrane markers on frozen sections,  and thus, we used 0.3% H 2 O 2 in PBS in this study. The case selection could also be responsible for the discrepancy.Humoral immunity has been reported to play an important role in cerebral cavernous malformation, which might be due to chronic deposition of iron and blood degradation products. Consistent with this view, we found that plasma cells were present mainly in specimens that had hemosiderin deposition. However, we cannot draw any conclusion regarding adaptive immune responses to the presence of iron from our small descriptive study.Our study was underpowered to detect a difference in macrophage loads between hemosiderin-positive and negative specimens, although our data show a strong trend toward that hemosiderin-positive specimens having more macrophages (P = 0.11). The most important finding, however, was that macrophages were present even in the hemosiderin-negative specimens, suggesting that the presence of macrophages is not merely a response to hemorrhage and iron deposition. What remains to be determined is whether the baseline level of macrophage load is causally related to the formation of micro-hemorrhage (e.g. will bAVM with high macrophage burden develop micro-hemorrhage?). This will be difficult to test in human studies, and would probably be best addressed in an animal model. An animal study has shown that in bAVM, vessel integrity is impaired. Therefore, the macrophages in bAVM could also be a response to the extravasation of blood content.One limitation of the study is that we only used one marker for each cell-type. Adding additional markers, including positive and negative controls, would make our data more convincing. However, the markers we used in this study are the most commonly used for macrophages, total lymphocytes, T- and B-lymphocytes, and plasma cells. A future study will employ more markers to confirm the cell-types we have identified here, and to define the subtypes of T- or B-lymphocytes or other inflammatory cells.In summary, we found that the load and location of T-lymphocytes were not associated with hemosiderin and macrophages. Macrophages are present in unruptured and previously untreated bAVMs, and their load was greater when hemosiderin is present. However, the presence of macrophages is not uniquely driven by hemosiderin, because they were also found in hemosiderin-negative specimens. Future studies need to be conducted to determine (1) how macrophages and lymphocytes contribute to the pathogenesis and progression of the disease, and (2) whether the burden of these cell loads is causally related to the development of micro-hemorrhage, and ultimately, clinically symptomatic hemorrhage.

 Acknowledgments

The authors would like to thank Voltaire Gungab for assistance with manuscript preparation, and members of the UCSF bAVM Study Project (http://avm.ucsf.edu.) for their support.

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