Differential interaction of the two related fungal species Candida albicans and Candida dubliniensis with human neutrophils.

Candida albicans, the most common facultative human pathogenic fungus is of major medical importance, whereas the closely related species Candida dubliniensis is less virulent and rarely causes life-threatening, systemic infections. Little is known, however, about the reasons for this difference in pathogenicity, and especially on the interactions of C. dubliniensis with the human immune system. Because innate immunity and, in particular, neutrophil granulocytes play a major role in host antifungal defense, we studied the responses of human neutrophils to clinical isolates of both C. albicans and C. dubliniensis. C. dubliniensis was found to support neutrophil migration and fungal cell uptake to a greater extent in comparison with C. albicans, whereas inducing less neutrophil damage and extracellular trap formation. The production of antimicrobial reactive oxygen species, myeloperoxidase, and lactoferrin, as well as the inflammatory chemokine IL-8 by neutrophils was increased when stimulated with C. dubliniensis as compared with C. albicans. However, most of the analyzed macrophage-derived inflammatory and regulatory cytokines and chemokines, such as IL-1α, IL-1ß, IL-1ra, TNF-α, IL-10, G-CSF, and GM-CSF, were less induced by C. dubliniensis. Similarly, the amounts of the antifungal immunity-related IL-17A produced by PBMCs was significantly lower when challenged with C. dubliniensis than with C. albicans. These data indicate that C. dubliniensis triggers stronger early neutrophil responses than C. albicans, thus providing insight into the differential virulence of these two closely related fungal species, and suggest that this is, in part, due to their differential capacity to form hyphae.

Role of pH-regulated antigen 1 of Candida albicans in the fungal recognition and antifungal response of human neutrophils.

Candida albicans is an opportunistic human-pathogenic fungus, which can cause superficial but also life-threatening invasive infections. The pH-regulated antigen 1 (Pra1) of C. albicans is a surface-associated and secreted protein highly expressed in the hyphal form. Pra1 can bind to complement receptor 3 (CD11b/CD18) and can mediate adhesion to and migration of human phagocytes. Here, we investigated the role of Pra1 in the activation of human neutrophils. A C. albicans mutant strain lacking Pra1 (pra1Δ) supported neutrophil migration to a lower extent than did the parental wild-type strain. A Pra1-overexpressing C. albicans strain enhanced neutrophil migration and adherence. While inactivated hyphae of the Pra1-overexpressing mutant with surface-associated Pra1 enhanced the production and release of reactive oxygen species, myeloperoxidase, lactoferrin, and interleukin 8 by neutrophils, such responses were reduced when stimulated with inactivated hyphae of the pra1Δ strain. However, Pra1-overexpressing living hyphae, which secrete large amounts of Pra1, also caused a reduced neutrophil activation, indicating that the release of extracellular Pra1 can inhibit the activation of these innate immune cells. Similarly, soluble recombinant Pra1 inhibited the neutrophil responses elicited by cell-wall bound Pra1. Finally, fungal cells lacking Pra1 were more efficiently killed by neutrophils. In conclusion, surface-exposed Pra1 plays a role in the recognition of C. albicans, especially hyphal cells, by human neutrophils and enhances neutrophil antimicrobial responses. However, the fungus can counteract some of these defense mechanisms by releasing soluble Pra1.

Factor H and factor H-related protein 1 bind to human neutrophils via complement receptor 3, mediate attachment to Candida albicans, and enhance neutrophil antimicrobial activity.

The host complement system plays an important role in protection against infections. Several human-pathogenic microbes were shown to acquire host complement regulators, such as factor H (CFH), that downregulate complement activation at the microbial surface and protect the pathogens from the opsonic and lytic effects of complement. Because CFH can also bind to host cells, we addressed the role of CFH and CFH-related proteins as adhesion ligands in host-pathogen interactions. We show that the CFH family proteins CFH, CFH-like protein 1 (CFHL1), CFH-related protein (CFHR) 1, and CFHR4 long isoform bind to human neutrophil granulocytes and to the opportunistic human-pathogenic yeast Candida albicans. Two major binding sites, one within the N-terminus and one in the C-terminus of CFH, were found to mediate binding to neutrophils. Complement receptor 3 (CD11b/CD18; alpha(M)beta2 integrin) was identified as the major cellular receptor on neutrophils for CFH, CFHL1, and CFHR1, but not for CFHR4 long isoform. CFH and CFHR1 supported cell migration. Furthermore, CFH, CFHL1, and CFHR1 increased attachment of neutrophils to C. albicans. Adhesion of neutrophils to plasma-opsonized yeasts was reduced when CFH binding was inhibited by specific Abs or when using CFH-depleted plasma. Yeast-bound CFH and CFHR1 enhanced the generation of reactive oxygen species and the release of the antimicrobial protein lactoferrin by human neutrophils, and resulted in a more efficient killing of the pathogen. Thus, CFH and CFHR1, when bound on the surface of C. albicans, enhance antimicrobial activity of human neutrophils.

Immune evasion of the human pathogen Pseudomonas aeruginosa: elongation factor Tuf is a factor H and plasminogen binding protein.

Pseudomonas aeruginosa is an opportunistic human pathogen that can cause a wide range of clinical symptoms and infections that are frequent in immunocompromised patients. In this study, we show that P. aeruginosa evades human complement attack by binding the human plasma regulators Factor H and Factor H-related protein-1 (FHR-1) to its surface. Factor H binds to intact bacteria via two sites that are located within short consensus repeat (SCR) domains 6-7 and 19-20, and FHR-1 binds within SCR domain 3-5. A P. aeruginosa Factor H binding protein was isolated using a Factor H affinity matrix, and was identified by mass spectrometry as the elongation factor Tuf. Factor H uses the same domains for binding to recombinant Tuf and to intact bacteria. Factor H bound to recombinant Tuf displayed cofactor activity for degradation of C3b. Similarly Factor H bound to intact P. aeruginosa showed complement regulatory activity and mediated C3b degradation. This acquired complement control was rather effective and acted in concert with endogenous proteases. Immunolocalization identified Tuf as a surface protein of P. aeruginosa. Tuf also bound plasminogen, and Tuf-bound plasminogen was converted by urokinase plasminogen activator to active plasmin. Thus, at the bacterial surface Tuf acts as a virulence factor and binds the human complement regulator Factor H and plasminogen. Acquisition of host effector proteins to the surface of the pathogen allows complement control and may facilitate tissue invasion.