PEPTIDE LIBRARIES DEFINE THE FINE SPECIFICITY OF ANTI-POLYSACCHARIDE ANTIBODIES TO CRYPTOCOCCUS-NEOFORMANS

Cryptococcus neoformans is an encapsulated fungus that causes life-thr eatening meningoencephalitis in patients with AIDS. Monoclonal antibod ies to the capsular glucuronoxylomannan can modulate the infection in mice, but the epitopes on this complex polysaccharide recognized by pr otective and non-protective antibodies have not been defined. We have used 2H1, one of our most protective antibodies, to screen phage displ ay peptide libraries for peptide mimotopes that would allow us to expl ore the fine specificity of anti-cryptococcal polysaccharide antibodie s. Hexa- and decapeptides have been identified with sequence homologie s that define four motifs: 1, (E)TPXWM/LM/L; 2, W/YXWM/LYE; 3, DWXDW; and 4, (Ar)WDGQ(Ar). Peptides representing these motifs compete with e ach other for a shared binding site that overlaps the polysaccharide b inding site. Motifs 1 and 2 confer high affinity binding, and PA1, whi ch displays a motif I peptide with the sequence LQYTPSWMLV, binds to 2 H1 with a K-d Of 295 nM. Analysis of the interaction between the 2H1 b inding peptides and 24 structurally related anti-polysaccharide antibo dies reveals a complex pattern of reactivity that strongly suggests bi nding to or close to the complementary determining regions. Furthermor e, those antibodies that have been shown to have different specificity , and in some cases different protective potential, do not bind any of the peptides selected by the protective 2H1 antibody. This study show s that peptide mimotopes for a complex microbial polysaccharide can be identified by screening phage peptide Libraries and demonstrates the usefulness of such peptides in analyzing closely related interactive s ites of proteins in general and of antibodies in particular. (C) 1996 Academic Press Limited