Type-specific antibodies to structurally defined fragments of streptococcal M proteins in patients with acute rheumatic fever.

Group A streptococci of M protein type 5 have been epidemiologically related to acute rheumatic fever in a number of reported outbreaks. Preliminary bacteriological evidence suggests that M5 may be an important "rheumatogenic" type in Santiago, Chile. To assess further the relationship of this streptococcal serotype to rheumatic fever in Chile, sera of 34 patients with rheumatic fever and an equal number of age-, sex-, and race-matched controls were assayed for antibodies to types 5, 6, and 24 in an enzyme-linked immunosorbent assay with purified pepsin extracts of the respective M proteins as solid-phase antigens. Sera of 11 rheumatic fever patients (32%) were positive (titer greater than 1:800) for type 5 antibodies, but only 1 (3%) of the matched controls was positive (P less than 0.01). Neither the patients nor the controls had antibodies to type 24. Although 38% of the patient sera contained antibodies to type 6, 29% of the control sera also had such antibodies (P greater than 0.20). The enzyme-linked immunosorbent assay served as an accurate predictor of which sera contained type 5 opsonic antibodies as measured by the opsonophagocytic test. Although antigenic cross-reactivity exists between M protein type 5 and type 6 group A streptococci, this phenomenon is unlikely to have accounted for the preferential occurrence of type 5 antibodies in rheumatic fever sera. The enzyme-linked immunosorbent assay and opsonic antibody results suggest that M5 is an important rheumatogenic type in Chile.