Identification of cardiac autoantigens in human heart cDNA libraries using acute rheumatic fever sera

Antigenic mimicry or cross-reactivity between Group A streptococcal antigens and cardiac autoantigens may initiate an autoimmune response resulting in cardiovascular damage in acute rheumatic fever. This study describes a molecular biological approach to the identification of such cross-reactive cardiac antigens. Two human heart cDNA libraries were constructed in the expression vector lambda gt11 and screened with patient sera, monoclonal antibodies and rabbit immune sera cross-reactive with streptococcal and cardiac antigens. Using the serum of a patient with a recurrent acute attack of rheumatic fever containing high titres of antibodies cross-reactive with both sets of antigens, we were able to identify three positive clones with insert sizes of 1.0 kb, 1.4 kb and 0.9 kb in these libraries. Acute rheumatic fever (ARF) sera reacted more strongly with these autoantigen clones than did normal sera. Autoantibodies eluted from the purified plaques of all three clones displayed different patterns of cross-reactivity against immunoblots of streptococcal M5, M6, M19 and M24 protein extracts. The cDNA inserts were sequenced and compared with known sequences in the EMBL and Genbank databases. One clone was 98% homologous with human cytokeratin 8 and showed homologies of 40 to 50% with human cardiac heavy chain myosin, tropomyosin and streptococcal M5 protein--all members of the alpha-helical coiled-coil family of proteins. Another clone was completely homologous to the G-protein alpha-subunit of adenyl cyclase, whilst the sequence of the third clone was not found in any of the data banks.