Histopathology of the ruptured pulmonary artery aneurysm in a patient with Behçet's disease

Objective. Vascular involvement (vasculo-Behçet's disease) is relatively common in Behçet's disease. Although pulmonary artery aneurysm (PAA) is rare, it is the most serious and sometimes fatal complication. However, the mechanism of the development of PAA is unclear. in the present study, we carried out immunohistological examination of the ruptured pulmonary aneurysm in a patient with vasculo-Behçets disease. Methods. Light microscopic examination was carried out on paraffin-embedded sections of autopsied pulmonary arteries of a Behçet's disease patient who died of the rupture of the left pulmonary artery aneurysm. Results. Histopathology of the ruptured aneurysm revealed the formation of thrombus and recanalization. In addition, there was proliferation of small vessels in the vascular wall, as if the recanalization was extended to the vascular wall. Of note, marked perivascular cuffing of mononuclear cells, consisting of CD45RO+ T cells and CD68+ monocytes, were observed around the recanalizing capillaries as well as around the proliferating small vessels in the wall of the pulmonary artery. Of note, pericapillary infiltration of CD20+ B cells was noted exclusively in the vascular wall. The ruptured portion of the aneurysm lacks the lamina elastica, indicating that the aneurysm was so called pseudo-aneurysm. Conclusion. It is likely that the first event might be the formation of inflammatory thrombus in the pulmonary artery. During the process of recanalization of the thrombus, the basic inflammatory process of Behçet's disease caused marked angiogenesis as well as perivascular cuffing of inflammatory cells in the thrombus and the wall, leading to fragility of the wall.