Morphological evidence for chronic mast cell activation after prolonged exposure with supernatants from chronic graft-versus-host splenocytes.

Chronic graft-vs.-host disease (cGVHD) includes a syndrome of inflammatory and fibrotic changes in some respects resembling scleroderma. In the present study we have quantitated the number of peritoneal mast cells (MC) in mice with cGVHD induced across minor histocompatibility barriers. MC were evaluated by staining with toluidine blue. The number of MC decreased significantly (by 25%) at the onset of the cGVHD fibrosis (day 12). Around day 35, MC were virtually undetectable, and started to reappear on day 130. Upon clinical recovery (day 200) a dramatic increase in MC numbers was found (about 8-fold). In addition, we evaluated by electron microscopy the morphology of peritoneal MC, obtained from normal mice and rats, that had been co-cultured on 3T3 fibroblast monolayer in the presence of splenocyte supernatants from mice with cGVHD or from control mice. After 6-8 days of continuous incubation with the cGVHD splenocyte supernatant, MC appeared to be activated, since they displayed an array of heterogenous granules. Few of the granules were dense; many were swollen and pale. Rare granule extrusion was evident. This would indicate that MC underwent a slow activation process due to a factor(s) present in the cGVHD supernatant, different from the classical acute anaphylactic activation.