Localization of Knops system antigens in the long homologous repeats of complement receptor 1

BACKGROUND: The Sl(a) (Knops system) located on complement receptor 1 (CR1) has been associated with malarial rosetting, a process associated with sev ere malarial infections, Moreover, the long homologous repeats (LHRs) B and C of CR1 were implicated in rosette formation. As a step toward mapping th e location of Knops system antigens, truncated CR1 proteins have been expre ssed and their ability to inhibit antibodies to the high-incidence Knops sy stem antigens was assessed. STUDY DESIGN AND METHODS: Individual LHRs (A, B, C, and D) of CRY of the co mmon CR1*1 (F) allotype were expressed as secreted forms in 293T cells. The ir abilities to specifically neutralize Knops system antibodies were tested by both hemagglutination and flow cytometry. RESULTS: Three examples of anti-Kn(a) (n = 6) were almost completely inhibi ted by LHR-C and three by LHR-D. Two examples of anti-McC(a) (n = 2) and se ven examples of anti-Sl(a) (n = 8) were inhibited by LHR-D. Both examples o f anti-Yk(a) (n = 2) were partially inhibited by LHR-D. CONCLUSION: The high-incidence Knops system antigens reside within LHR-D an d to a lesser extent within LHR-C. Because of the role of Sl(a) antigen in malaria rosetting, these results indicate that LHR-D may represent an addit ional malaria interaction region in CR1.