The assumption that renal allograft histology should be perfectly normal during quiescence in the absence of rejection or nephrotoxic insults has not been adequately investigated. To study this, routine renal allograft biopsies were performed at approximately 1 and 4 weeks, when patients often had normal function or stable acute tubular necrosis (ATN). These were compared with biopsies from other patients during autologous ATN or clinically evident allograft rejection. There were two new findings: (1) Almost all biopsies contained an interstitial infiltrate, so that only the presence of vasculitis provided a clear distinction between rejection and quiescence. Most of the biopsies with infiltrates were from patients who had never received cyclosporine, so that an infiltrate does not necessarily signify toxicity due to this drug. (2) A major proportion of the cells in some biopsies appeared to express both the helper/inducer and the cytotoxic/suppressor phenotype, and a similar finding after in vitro stimulation suggests that this represents a cell population that is activated in some way.
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