Detection of interstitial increase in macrophages, characteristic of acute interstitial rejection, in routinely processed renal allograft biopsies using the monoclonal antibody KP1.

Acute interstitial rejection (AIR) of renal allografts is accompanied by a characteristic peritubular increase in macrophages, which can be identified with the CD14 monoclonal antibody (mAb) WT14 in cryostat sections. Since frozen tissue is not always available, we tested whether this increase can also be demonstrated in Bouin-fixed, paraffin-embedded biopsies, using the CD68 antimacrophage mAb KP1, which can also be applied to paraffin sections. Sections of 16 biopsies with AIR and 11 controls were stained with KP1. In 25 of the 27 biopsies, macrophages were strongly positive for KP1. Two AIR biopsies were completely negative, probably due to prolonged fixation. In the remaining 14 AIR biopsies, the number of KP1-positive cells was significantly higher than in the controls [1184 +/- 410 per mm2 (mean +/- SD) vs 112 +/- 126 per mm2]. We conclude that, especially in cases in which frozen tissue is not available, the demonstration of increased numbers of monocytes/macrophages with mAb KP1 can be a helpful adjunct in the histological diagnosis of AIR in routinely processed renal biopsies.