Of interest in this context is that another protective antibody, which is directly candidacidal, has been reported to bind to the N-terminus region of Als 3. Overall, both Hyr1 and, more evidently, Als3 play important roles in C. albicans virulence properties such as hyphal growth and adherence which are both inhibited by our protective IgG mAb which recognizes the two proteins. In contrast, neither hyphal growth nor adherence are affected by the non protective IgM mAb which does not recognize the two proteins. Nonetheless, it remains possible that the protective Butenafine hydrochloride antibody interacts with, and inhibits the function of other unidentified b1,3 glucan constituents exerting a role in fungal virulence or other critical biological properties in vivo. It is rather surprising that neither the Hyr1 nor the Als3 proteins are bound by the promiscuous IgM mAb which recognizes different b-linked saccharide molecules, including b1,3 glucose sequences. However, it should be considered that mAb epitope specificity data shown in this study have been obtained using polysaccharides, free oligosaccharides and lipid-linked oligosaccharide probes. It is possible that the b-glucan antigen on native and secreted proteins is presented in a form that can be bound by the IgG but not by the IgM. In principle, the low specific activity of the IgM mAb for the b1,3 glucan, as assessed with laminarin or oligosaccharides used in ELISA and microarrays, could be even lower for the b1,3 glucan sequences of the secreted native proteins. Extensive mannosylation of the two proteins could also exert steric hindrance toward the access of the large IgM to the b-glucan epitope. As outlined above, the two mAbs used throughout this study have different isotypes but identical amino acid sequence of both light and heavy chain CDRs. A few examples of mAbs with the same CDR but different isotypes were reported to have different binding specificities and avidities toward polysaccharides, because of the influence exerted on binding by structural elements of the antibody distant from the antigen-binding site. These factors may also influence the biological properties of the antibody, including its protective ability against infections. It is well known that antibody isotype influences the avidity whereby the antibody binds its cognate antigen, as well as a number of Fc-dependent, host effector functions, such as, for instance, complement activation and interaction with activating or inhibitory receptors on various immune cells. However, other mechanisms translating fine differences in antigen binding of antibodies sharing the same CDR into profound differences in their biological properties have not been established. We show here for the first time that the isotype contributes to the selectivity and intensity of binding to particular, structurally defined, bglucan sequences as well as to defined cell wall molecules of the fungus. As an important consequence, we show here that antibodies with different isotypes may differ in their ability to inhibit critical virulence properties of a fungal pathogen such as hyphal growth and adherence in vitro. It remains to be Gentamycin Sulfate determined whether the isotype-dependent protective properties of the IgG mAb are also independent of Fcmediated antibody effector mechanisms in vivo. Opsonisation is one of the main mechanisms whereby the host can eliminate or control Candida in vivo. Our anti-b-glucan IgG mAb did not show opsonic potential in the macrophage model system tested. The lack of opsonisation appears to be in keeping with the discontinuous nature of b-glucan expression at the fungal cell surface.