Prevalence of Anti-Myeloperoxidase Antibodies in Methimazole-Treated Hyperthyroid Cats
M.E. Robarge; K.A. Anderson; B.M. Pressler
As with cats, people with hyperthyroidism are frequently treated with oral antithyroidal drugs. A significant percentage of these patients develop anti-neutrophil cytoplasmic autoantibodies (ANCA), most commonly specific for the neutrophil granule protein myeloperoxidase (MPO). Cats with hyperthyroidism are often treated with the same antithyroidal drugs, and may develop clinical signs that mimic those of people with ANCA-associated diseases. Normal cats administered propylthiouracil develop anti-MPO antibodies (using a recombinant human MPO [hMPO] ELISA), and our laboratory has determined that the feline and human MPO sequences are 85% homologous. We therefore hypothesized that a subset of cats with hyperthyroidism treated with methimazole develop anti-MPO ANCA.
Aliquots were collected from all serum samples submitted to the Purdue University Clinical Pathology Laboratory for T4 measurement. Aliquots were maintained at -80 C until testing. Samples included those from newly diagnosed hyperthyroid cats, cats currently or historically treated for hyperthyroidism, and cats suspected of being hyperthyroid but whose T4 serum concentrations were within the reference range. Presence of anti-hMPO antibodies was evaluated by ELISA. In brief, plates were coated with hMPO, blocked with a commercial blocking agent (Superblock T20; Pierce), and incubated with 1:50 serum dilutions in the blocking agent. Alkaline phosphatase-conjugated goat-anti-cat antibody (KPL Laboratories) was added, and optical density was measured at 1 hr after administration of substrate. Background reactivity versus coat buffer alone-coated wells was subtracted for each patient sample, and an internal control of anti-hMPO primary antibody (Dako) served as both a positive control and to adjust for inter-plate variability. Immunoreactivity was defined as an optical density above the mean plus two standard deviations of the non-hyperthyroid cat patients.
Samples from 112 hyperthyroid (treated and untreated) and 85 non-hyperthyroid cats were evaluated. 7 of 112 hyperthyroid cats (6.3%) vs. 5 of 85 (5.9%) non-hyperthyroid cats were immunoreactive against hMPO; these results were not statistically different (two-sample Wilcoxon rank-sum test, p>0.05). When hyperthyroid cats were subcategorized based on treatment modality all cats with anti-hMPO immunoreactivity were found to have been treated with methimazole (4 of 39; 10.3%), although insufficient numbers of cats have been tested to reach statistical significance.
These preliminary results suggest that a subset of hyperthyroid cats do develop anti-hMPO antibodies, although whether this is a unique feature of hyperthyroidism or treatment with methimazole is unclear. However, because human and feline MPO are only 85% homologous, a significant number of antibodies may not be detected using the assay reported here. Future studies are being developed using a recombinant feline MPO protein.