Chapter 4 : Molecular Mimicry between Streptococcal M Protein and Cardiac Myosin and the Immunopathogenesis of Rheumatic Fever

(A) Hematoxylin-eosin-stained heart tissue section from BALB/c mice immunized with NT4 sequence from the streptococcal M5 protein. Similar cellular infiltrates were observed in mice given NT5, NT6, B1A, and B3B. (B) Control myocardium from a mouse given only phosphate-buffered saline and adjuvant. Reprinted from reference 18 with permission from the American Society for Microbiology.

Antisera against streptococcal M5 peptides reacted with human cardiac myosin in the Western immunoblot. Reprinted from reference 18 with permission from the American Society for Microbiology.

Titers of antistreptococcal M5 peptide against human cardiac myosin in sera determined by reaction of sera with anti-M5 peptide with human cardiac myosin in an enzyme-linked immunosorbent assay. Sera were diluted twofold and were reacted with 10 μg of human cardiac myosin per ml in an enzyme-linked immunosorbent assay. Endpoints were calculated at optical densities of 0.1 and 0.2. The endpoints at an optical density of 0.1 are shown in the figure and were slightly higher but not significantly different from those for the endpoint at an optical density of 0.2. The same relative differences between cardiac and skeletal myosins were observed at both endpoints and in different assays. (A) Titers of the antistreptococcal M5 peptide against human cardiac myosin in sera. (B) Titers of the antistreptococcal M5 peptide against rabbit skeletal myosin in sera. The titers of the antipeptide against cardiac myosin in sera indicated a greater than fourfold increase in reactivity. Reprinted from reference 18 with permission from the American Society for Microbiology.

Responses of myosin-sensitized lymphocytes to streptococcal M5 peptides as determined by stimulation of lymphocytes from BALB/c mice immunized with human cardiac myosin (HCM) with each of 23 overlapping M5 peptides in a tritiated thymidine incorporation assay. Panels A, B, and C each contain data from separate experiments. (A and B) Stimulation of lymphocytes by the M5 peptides at concentrations of 10 and 1 ug/ml, respectively. The data are averages from four separate experiments. The no-antigen control was 1,773 cpm. (C) Data from one experiment in which myosin-immune lymphocytes were stimulated with M5 peptides at a concentration of 0.1 ug/ml. The no-antigen control was 3,720 cpm. Standard deviation bars are shown for the data in panel C. Solid bars represent the counts per minute for myosin-immune lymphocytes, and the open bars represent the counts per minute for lymphocytes from mice immunized with complete Freund's adjuvant (CFA) only. Standard error was calculated for the average of four assays, and stimulation indices (SI) are shown for the dominant peptides. The four individual assays demonstrated similar results and are reflected in panels A and B as an average of the data. The most dominant myosin-cross-reactive site was the B1B2-B2 site, which appears in all three panels. Other dominant peptides included NT5, B3A, and C3. Reprinted from reference 18 with permission from the American Society for Microbiology.

(A) Examples of homology between human cardiac myosin (residues 1313 to 1329) and streptococcal M5 protein peptide B2 (streptococcal M5 protein; residues 150 to 167). Identity (47%) was observed in a 17-amino-acid overlap. The M5 peptides B2, B1B2, and B3A contain large amounts of overlapping sequence. The colons represent identities, and the periods represent conserved substitutions. Three of the residues shown in the myosin sequence are unique to human cardiac myosin. (B) Amino acid sequence identity (LKTEN) between M5 peptide NT4 and human cardiac myosin (LQTEN). NT4 contains residues 40 to 58 of the streptococcal M5 protein. The cardiac myosin sequence shown is found in residues 1279 to 1286, near the beginning of the LMM tail and the end of the S-2 fragment of myosin. The myosin sequence shown in panel B is conserved among cardiac myosins. Reprinted from reference 18 with permission from the American Society for Microbiology. (C) Alignment of streptococcal M5 peptides C2A and C3 with the heavy chain of human cardiac myosin. Identical residues are indicated with colons, while periods indicate conservative substitutions as determined by the FASTA alignment program (Intelligenetics, Mountain View, Calif.). The sequence identified as homologous between C2A (streptococcal M5 protein; residues 254 to 271), C3 (streptococcal M5 protein; residues 393 to 308), and myosin is highly conserved (identical) between skeletal and cardiac myosins. The identical residues are located in the heavy meromyosin fragment of skeletal and cardiac myosin (residues 1172 to 1186). Reprinted from reference 59 with permission from the American Society for Microbiology.

Diagram illustrating the potential mechanism of antibody reactivity in the pathogenesis of rheumatic heart disease. Cross-reactive antibody could bind directly to the endothelium (top diagram) or could bind to the basement membrane of the valve (bottom diagram) if the basement membrane is exposed due to shear stress or damage by antibody and complement. Endothelium is depicted with antibody attached directly in the upper diagram, and then antibody is shown attached to exposed endothelium in the lower diagram.