Pathogenesis

KD is a self-limited vasculitis that affects medium-sized, extraparenchymal muscular arteries and is now the most common cause of acquired heart disease in the pediatric age group (10,11). The systemic vasculitis is associated with elevated levels of proinflammatory cytokines including interleukin (IL)-6 and tumor necrosis factor-a and a marked acute phase response (11). Most of our knowledge of the evolution of the vasculitis in KD is derived from autopsy data (12,13). The earliest changes involve the endothelium of the musculoelastic arteries and include swelling, proliferation, enlarged nuclei, and frank degeneration with adherent platelets entrapped in fibrin (14). The endothelium of the vaso vasorum of larger arteries can be similarly involved. More advanced lesions show edema and inflammatory cell infiltrate in the subendothelial space. This progresses to destruction of the media with frank necrosis associated with infiltration of monocytes and lymphocytes of the memory T-cell (CD45RO+) and cytotoxic/suppressor (CD8+) phenotypes from both the lumen and adventitia, which results in a transmural vasculitis (15). Replacement of the intima and media with fibrous connective tissue, thinning of the media with aneurysm formation, scarring, and stenosis complete the progression of the vasculitis. The development of stenotic lesions occurs over a period of months to years, and these lesions may remain silent until the moment of acute thrombotic occlusion, often decades after the initial acute illness (16).

The finding of IgA-secreting plasma cells infiltrating into the trachea and small airways coupled with molecular evidence of an oligoclonal IgA response suggest a pathogen with a respiratory portal of entry (17,18). In addition, a synthetic antibody produced using reverse genetics and molecular methods stains intracytoplasmic inclusions in tingible macrophages, which are found infiltrating various tissues (19). Histologic examination of lymph nodes biopsied from acute KD patients and described in Kawasaki's original report revealed nonspecific changes including swelling of endothelial cells in postcapillary venules and hyperplasia of reticulum cells (20,21). Subsequent reports described focal areas of necrosis associated with microthrombi in adjacent vessels, hyperplasia of the T-cell zones of lymphoid follicles, and macrophage infiltration of B-cel! zones (22,23).

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