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.^[1-3] Commonly assumed to be congenital, postnatal formation may be more prevalent than previously thought,^[4-6] and the etiology of bAVMs still remains unclear. Genetic factors,^[7,8] aberrant vasculogenesis,^[9-11] and inflammation may all play roles in the pathogenesis of bAVMs;^[12] a confluence of these factors has been proposed in a “response-to-injury” paradigm.^[5]

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.^[13-18] 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.^[13] Magnetic resonance imaging has detected hemosiderin deposition in unruptured bAVMs,^[19,20] 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.^[21] Further, histological examination demonstrated that the degree of hemosiderin deposition is positively correlated with the number of macrophages in the lesion.^[21] 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.^[13-15] Shi et al.^[22] 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 (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.

Results

Hemosiderin deposition was present in unruptured brain arteriovenous malformations

Consistent with our published data,^[21] 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 1a]. Prussian blue positive staining was detected in the areas that had hemosiderin deposition [Figure 1d], suggesting the presence of previous micro-hemorrhage.

Figure 1. Hemosiderin deposition in unruptured brain arteriovenous malformations. H and E (a-c) and prussian blue staining (d-f) on the adjacent sections. (b) and (c) are enlarged pictures of the regions in squares (b) and (c) in (a) showing hemosiderin-positive areas. Insert in (b) shows two hemosiderin-laden macrophages. (d) Prussian blue staining of an adjacent section of (a). (e) and (f) are enlarged images of the regions in squares (e) and (f) in (d). Scale bars for a and d: 500 μm; for b, c, e and f: 50 μm

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].

Figure 2. CD3⁺ T-lymphocytes and CD68⁺ macrophages. (a-c) Sections from 3 individual bAVM specimens. (c) Sections from an superficial temporal arteries (STA). Squares in H and E, images are enlarged to show CD68, CD3 and CD20 positive cells in the images next to them. T-lymphocytes and macrophages were detected on the vessel wall (a2 and a3) and between vessels (b2, c2, b3 and c3). Only a few CD20⁺ B-lymphocytes were detected in the lumen (b4) and between vessels (c4). No T- and B-lymphocyte, and macrophage were detected on the wall of STA. Scale bar for a1-d1: 500 μm; scale bar for a2-a4: 100 μm; scale bar for b2-b4, c2-c4, d2-d4: 20 μm

Figure 3. Location of CD3⁺ T-lymphocytes. T-lymphocytes were distributed in the perivascular region (a), in the vessel wall (b), and on the surface of the endothelial lining (c). Scale bar: 50 μm

Compared to the specimens that had no hemosiderin deposition, hemosiderin-positive specimens tended to have more macrophages (478 ± 174 vs. 666 ± 313 cells/mm²; P = 0.11). The T-cell numbers were similar in hemosiderin-positive and hemosiderin-negative samples (147 ± 108 vs. 157 ± 139 cells/mm²; P = 0.88) [Figure 4].

Figure 4. Quantification of inflammatory cells in brain arteriovenous malformation (bAVM). (a) Bar graph shows a trend towards more CD68⁺ cells in hemosiderinpositive (HS+) bAVMs than in hemosiderin-negative samples (HS-). (b) Bar graph shows that the numbers of CD3⁺ T-cells were similar in hemosiderin-positive (HS+) and negative (HS-) samples

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^[24] and irradiated^[25] bAVMs. Our previous study showed that adaptive immune cells were rarely observed in unruptured bAVM.^[13] 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% H₂O₂ in methanol to quench the activity of endogenous peroxidase. However, lymphocyte surface markers have been shown to be sensitive to methanol/H₂O₂ treatment. Treating sections with 0.3% H₂O₂ in methanol can reduce our ability to detect membrane markers on frozen sections,^[26] and thus, we used 0.3% H₂O₂ 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.^[22,27,28] 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.^[11] 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.