Autoantigens in Addison's disease and associated syndromes

In 1855, when Thomas Addison described the condition which bears his name, tuberculosis was the major etiology, yet he was also able to describe a patient who had associated vitiligo and another in whom adrenal ‘inflammation . finally led to contraction and atrophy’. This autoimmune variant is now the major cause of Addison’s disease in the developed world, but it remains a rare condition, with a prevalence of around 1 in 25 000 in the UK. Around half of such patients have isolated adrenal failure. In the remainder, like Addison’s case VI, there is an associated autoimmune disorder, commonly as part of the autoimmune polyglandular syndrome (APS) type 2, in which there is frequent autoimmune hypothyroidism, Graves’ disease, type 1 diabetes mellitus, pernicious anaemia, vitiligo and other disorders [1]. APS type 1 is rare (except in Finland, presumably as a result of a founder gene effect) but comprises at least two of three major components (Addison’s disease, chronic mucocutaneous candidiasis and hypoparathyroidism) often with associated gonadal failure, ectodermal dysplasia and other features [2]. The demonstration of autoantibodies against the adrenal cortex by complement fixation and immunofluorescence [3,4] was the first major step in defining the autoimmune basis for Addison’s disease, although only 50–60% of patients have adrenal antibodies by such techniques. A subset of patients (around 25% of women and 5% of men) have antibodies with a broader pattern of immunofluorescence, staining steroid-producing cells in the ovary, testis and placental trophoblast, as well as the adrenal cortex. There is a strong correlation between such steroid cell antibodies and premature ovarian failure in women with Addison’s disease [5]. It is only recently that the nature of these steroid cell autoantigens has been elucidated. Initially, a fetal adrenal cDNA library was screened with APS type 1 sera, which were shown to recognize the cytochrome P450 enzyme, 17 -hydroxylase or P450c17 [6]. However, sera from patients with isolated Addison’s disease did not react with P450c17, and the authors pointed out that the fetal library would not express the full array of enzymes needed for steroid synthesis. Sure enough, another cytochrome enzyme, P450c21, was soon identified by immunoblotting as a key autoantigen in isolated Addison’s disease and in the form associated with type 2 APS [7,8]. A third autoantigen, P450scc (side chain cleaving enzyme), was then defined for APS type 1 patients in whom no reactivity to P450c17 (or P450c21) could be identified, but only five such sera were tested [9]. In the same study, 15 sera from Addison’s patients without APS type 1 failed to react with P450scc despite being selected for P450c21 reactivity. It was suggested, first, that this sharp dichotomy in adrenal autoantigen recognition indicated specific, selective autoimmune responses in the two forms of Addison’s disease, and second, that the previous report of P450c17 reactivity in APS type 1 was due to antigenic crossreactivity between this enzyme and P450scc, the latter being the true autoantigen. This proposal was based on weak P450c17 reactivity seen with one serum that was absorbed out with P450scc. Adding to the confusion, others reported that none of five APS type 1 sera reacted with either P450scc or P450c17 expressed as recombinant proteins in Escherichia coli , yet four of the five reacted with recombinant P450c21 [10]. Gradually these muddy waters have cleared. Responding to the failure of others to confirm P450c17 reactivity in APS type 1, Uibo and colleagues used recombinant enzymes in immunoblotting to demonstrate that 81% of 36 sera from APS type 1 patients reacted to one or more of the three contending P450 enzymes, with P450c17 reactivity in 42% [11]. Further proof that this P450c17 reactivity was not cross-reactivity came from immunofluorescence on adrenal sections. Sera that reacted only with P450c17 did not stain the zona glomerulosa, where this enzyme is absent, whereas sera reacting with P450c21 and/or P450scc reacted as expected with all three cortical layers. It should be noted that this study came from Finland, where APS type 1 is relatively common, and that a large number of sera were available. On the other hand, when a small number ( n 17) of Addison’s sera from patients with isolated disease or APS type 2 were tested, only one reacted with P450c17 and none with P450scc [11]. Others have also found a heterogeneous immune response in the various types of Addison’s disease, with an assay based on immunoprecipitation of radiolabelled recombinant P450c21; six of nine APS type 1 sera had antibodies against this enzyme, with comparable frequencies being found in both isolated Addison’s disease and APS type 2 [12]. Two definitive studies to clarify P450 autoreactivity in Addison’s disease have now appeared, with a report in this issue [13] and another recently published [14], based on the use of recombinant enzymes in immunoprecipitation and absorption experiments with sufficient numbers of sera to draw meaningful conclusions. The results with the immunoprecipitation assay of APS type 1 sera compared well with previous immunoblotting studies [11]. Overall, 72% of 46 APS type 1 patients had one or more antibodies against Clin Exp Immunol 1997;107:227–229