The Candida antigen CR3-RP (complement receptor 3-related protein) is supposed to be a 'mimicry' protein because of its ability to bind antibody directed against the a subunit of the mammalian CR3 (CD11b/CD18). This study aimed to (i) investigate the specific humoral isotypic response to immunization with CR3-RP in vivo in a rabbit animal model, and (ii) determine the role of CR3-RP in the adherence of Candida albicans in vitro using the model systems of buccal epithelial cells (BECs) and biofilm formation. The synthetic C. albicans peptide DINGGGATLPQ corresponding to 11 amino-acids of the CR3-RP sequence DINGGGATLPQALXQITGVIT, determined by N-terminal sequencing, was used for immunization of rabbits to obtain polyclonal anti-CR3-PR serum and for subsequent characterization of the humoral isotypic response of rabbits. A significant increase of IgG, IgA and IgM anti-CR3-RP specific antibodies was observed after the third (P,0.01) and the fourth (P,0.001) immunization doses. The elevation of IgA levels suggested peptide immunomodulation of the IgA1 subclass, presumably in coincidence with Candida epithelial adherence. Blocking CR3-RP with polyclonal anti-CR3-RP serum reduced the ability of Candida to adhere to BECs, in comparison with the control, by up to 35 % (P,0.001), and reduced biofilm formation by 28 % (P,0.001), including changes in biofilm thickness and integrity detected by confocal laser scanning microscopy. These properties of CR3-RP suggest that it has potential for future vaccine development.
INTRODUCTIONCandida albicans is the most frequently isolated fungal pathogen associated with infection of immunocompromised patients in hospital settings. The high propensity to cause disease is due to the expression of many virulence factors including highly immunogenic cell surface proteins (Alberti-Segui et al., 2004;Jeng et al., 2005;Pietrella et al., 2006) able to trigger cellular and humoral response (Viudes et al., 2001;Ló pez-Ribot et al., 2004;Omaetxebarria et al., 2005;Fukuizumi et al., 2006). Additionally, a variety of cell surface molecules expressed by Candida help it to evade the host immune response by mimicry of host receptors (Gustafson et al., 1991;Phan et al., 2007). The receptor MAC-1 (CD11b/CD18) on lymphocytes is the surface b 2 integrin that mediates lymphocyte adhesion to C. albicans (Forsyth & Mathews, 2002). However, Candida is not only the target for MAC-1, but also CR3-RP identified on the yeast surface might be classified as a member of the integrin family due to its antigenic, structural and functional relation to the a subunit of the mammalian neutrophil receptor CD11b/CD18 (Gilmore et al., 1988;Abbreviations: BEC, buccal epithelial cell; CLSM, confocal laser scanning microscopy; RT, room temperature; XTT, 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide.The GenBank/EMBL/DDBJ accession number for the N-terminal sequence fragment of CR3-RP is P85437. Hostetter et al., 1990;Hostetter, 1996). Binding the human complement fragment iC3b to this receptor results i...