Immunostaining of cardiac biopsy specimens in the diagnosis of acute vascular (humoral) rejection: a control study.

The diagnosis of acute vascular (humoral) rejection in heart transplant biopsies is classically based on immunofluorescent studies of frozen tissue that show vascular staining for immunoglobulin and complement. We have noted that some pathologists have used immunostaining of formalin-fixed, paraffin-embedded tissue in testing for vascular rejection. To determine the specificity of immunostaining of heart biopsy specimens in the diagnosis of vascular rejection, we studied tissue from 68 consecutive endomyocardial biopsies from 16 patients without clinical or histologic evidence of vascular rejection. In each case, routinely processed formalin-fixed, paraffin-embedded tissue was stained for immunoglobulin G and immunoglobulin M with an avidin-biotin immunoperoxidase technique. Frozen tissue from each case was also stained for immunoglobulin G, immunoglobulin M, C3, and Clq by immunofluorescence. Immunoperoxidase stains on formalin-fixed tissue showed vascular staining for immunoglobulin in 67 of 68 (99%) of the cases. Staining was ablated if the antibodies were absorbed with their appropriate immunoglobulin. Immunofluorescent studies on frozen tissue showed no vascular staining for immunoglobulin or complement. We conclude that immunoperoxidase studies of routinely processed, formalin-fixed, paraffin-embedded tissues are nonspecific in the diagnosis of heart acute vascular rejection. The positive staining in fixed tissues may be due to labeling of passive immunoglobulins that are "fixed" in the vessels during routine processing but are washed away in techniques using frozen tissue.