Morphological aspects of giant cells in giant cell arteritis: an electron-microscopic and immunocytochemical study

To compare the morphology of foreign body and Langhans giant cells in the two different inflammatory phases of giant cell arteritis (GCA).

Electron microscopy was performed on 6 positive temporal arterial biopsies. Light microscopy and immunocytochemistry for macrophage-associated antigen (KP1) and alpha-smooth muscle actin (alpha-SMA) were performed on 16 positive biopsies.

A focal granulomatous reaction with foreign body giant cells was found only in association with the internal elastic membrane (IEM) in atrophic arterial segments, which often displayed calcification of the IEM. Diffuse invasion of lymphocytes and monocytes/macrophages affected non-atrophic as well as atrophic arterial segments. Within such segments Langhans giant cells were found in all layers of the wall. Electron microscopy of biopsies displaying the focal foreign body reaction revealed that large cells devoid of lysosomes but with cytoplasmic densities, tightly packed cytoplasmic filaments and numerous micropinocytotic vesicles formed clusters close to calcified parts of the internal elastic membrane. Furthermore, foreign body giant cells were surrounded by large cells devoid of lysosomes. Lysosomes tended to concentrate in central parts of the foreign body giant cells. In the diffusely inflamed arteries, the Langhans giant cells were surrounded by mononuclear cells rich in lysosomes. The lysosomes in the Langhans giant cells were more evenly distributed than in foreign body giant cells. Immunocytochemistry of biopsies displaying the focal granulomatous reaction revealed an uneven, often central immunoreactivity for the macrophage marker (KP1) in the foreign body giant cells, and immunostaining for alpha-smooth muscle antigen (alpha-SMA) showed their poor delineation from the surrounding vascular smooth muscle cells. The Langhans giant cells in the diffusely inflamed arteries displayed a strong even cytoplasmic immunoreactivity for KP1 and a distinct delineation from the smooth muscle cells in the alpha-SMA staining.

Differences in terms of distribution, light microscopy, immunocytochemistry and electron microscopy between the two types of giant cells in GCA indicate a difference in their function as well as their histogenesis.